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Effect of controlling Colletotrichum leaf fall of rubber tree on epidemic development and rubber production
Crop Protection 20 (2001) 581}590
E!ect of controlling Colletotrichum leaf fall of rubber tree on epidemic development and rubber production Jean Guyot *, Edith Ntawanga Omanda, Auguste Ndoutoume, Abd-Allah Mba Otsaghe, Frank Enjalric , Henri-GreH goire Ngoua Assoumou CIRAD-CP, TA 80/PS3, Boulevard de la Lironde 34 398, Montpellier Ce& dex 5, France Centre d+Appui Technique a% l+He& ve& aculture (CATH), B.P. 643, Libreville, Gabon Received 20 December 1999; received in revised form 30 November 2000; accepted 2 February 2001
Abstract This paper studies the impact on phenology, epidemiology and subsequently on production of artici"al defoliation for the control of Colletotrichum leaf fall of Hevea brasiliensis for the susceptible rubber tree clone GT 1. Large-scale observations of tree phenology over 4 years show the impact of the treatment on foliage behaviour. These results provide on understanding how the trees escape from the disease and how the occurence of the an epidemic is prevented. A 50% increase in foliage density led to around a 40% increase in yields on the susceptible clone GT 1. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Hevea brasiliensis; Colletotrichum gloeosporioides; Avoidance control; Colletotrichum leaf fall; Epidemiology; Production
1. Introduction In Gabon, the secondary leaf fall of rubber tree caused by Colletotrichum gloeosporioides Penz. (Melanconiales) occurs after tree wintering, when trees are producing new foliage. Tree wintering takes place in March}April, during the rainy season, a period favourable for fungus development. On susceptible clones, the consequences of the disease are leaf necrosis and deformation, and even secondary fall of the youngest leaves a!ected at an early stage of their development. Consequently, new leaves produced throughout the rainy season are systematically destroyed by the disease. The trees become less vigorous as the #ushes succeed each other, and the initial canopy density is therefore never re-established. The disease epidemic causes a substantial drop in foliage density and death of terminal branches, known as die-back. Only at the start of the dry season (June}July) the most vigorous trees succeed in regenerating a part of their canopy. The Mitzic estate, in Gabon, was planted mostly with suscep-
* Corresponding author. Present address: DeH leH gation CIRAD, BP701, 97 387 Kourou Cedex, France. Tel.: #33-594-32-73-57; fax: #33-594-32-73-51. E-mail address: [email protected] (J. Guyot).
tible clones and foliage density was very substantially reduced by the disease since the estate was established and has adversely a!ected latex production. The economic impact of severe and repeated attacks is di$cult to evaluate with any accuracy but several authors consider that they slow down growth and lead to production losses (Rao, 1970; Peries, 1966; Wastie and Mainstone, 1969; Wimalajeewa and Lloyd, 1963). Necrosis also provide access for other infections, such as die-back by Botryodiplodia theobromae, and increased light under the rubber crop favours weed growth which necessitates frequent and prolonged weeding. Furthermore, prolonged production of new #ushes from March to the end of June/beginning of July exposes rubber trees to attacks by On( dium heveae which "nds suitable conditions for its development on young leaves of susceptible clones, such as PB 235, at the start of the dry season. Control of Colletotrichum leaf fall in mature rubber plantings is di$cult because of the size of the trees. To be e$cient, aerial treatment is usually required, but frequency of treatments may be limited. E$cient fungicide application is therefore very di$cult. In Malaysia, arti"cial defoliation was successfully tested in 1968 and has been developed on a large scale since 1974 to control secondary leaf fall caused by On( dium heveae and Colletotrichum gloeosporioides, by applying herbicides such as
0261-2194/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 1 - 2 1 9 4 ( 0 1 ) 0 0 0 2 7 - 8
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MSMA (Anon, 1977), merphos (Anon, 1978) or sodium cacodylate (Azaldin and Rao, 1974; Mabbett, 1982). This technique results in early refoliation occuring before the begining of the rainy season, allowing Hevea trees to avoid fungal attacks. This control method has been also adopted succesfully in Cameroon on the HEVECAM estate at Kribi, using ethephon, an ethylene generator, to speed up leaf senescence and fall and thus promote rapid refoliation (Senechal, 1986). Control of Colletotrichum leaf fall by arti"cial defoliation has now been developed in Gabon. This paper describes its impact on rubber tree phenology, epidemic development and rubber production in relation to the susceptible clone GT 1.
The percentages of stages B and C observed in the "eld enabled calculation of a maximum theoretical leaf area (LAth) for clone GT 1, estimating, from our observations, that a stage B leaf represented of the area of a stage D leaf, at full development, and that a stage C leaf had an area one third that of a stage D leaf. Thus LAth"0.17%B#0.33%C#%D,
(1)
where %B"percentage of stage B leaves, %C"percentage of stage C leaves, %D"percentage of stage D leaves (2)
"100!%B!%C. 2. Material and method 2.1. Treatment Trials were conducted during "ve arti"cial defoliation operations at HEVEGAB's Mitzic estate using a Pawnee Aircraft to apply the defoliant (Table 1). An adjuvant was used to add weight to spray droplets to increase they downward velocity into the tree foliage and reduce downwind dispersion of the spray. It also reduced evaporation of liquid before droplets reached the canopy in the higher temperatures.
This maximum theoretical leaf area di!ered in most cases from the observed leaf density (LD), since rarely all the branches of a tree bear leaves. The calculation of the leaf area for stages B and C therefore had to account for this di!erence and led to the following relations: Area of stage B leaves "AB"(0.17;%B) (LD/LAth),
(3)
Area of stage C leaves "AC"(0.33;%C) (LD/LAth).
2.2. Leaf density assessment
(4)
The total susceptible leaf area (SA) was therefore Leaf density was estimated visually on a scale of 11 levels between 0 and 100; the level 0 indicates the total absence of foliage while level 100 corresponds to a tree with all branches completely covered by mature leaves; 2.3. Leaf stages The presence of leaf stages A, B, C, and D (Halle and Martin, 1968) was noted in the large-scale observations. For the small-scale observations, the respective percentages of each stage in place was recorded, except for A stage (budding stage) with a nil leaf area.
Susceptible area"SA"AB#AC.
(5)
2.4. Disease score The level of disease, or disease score was recorded only for the small-scale observations and for each tree was an average score for susceptible stages B and C. The score comprised six levels: 0"absence of disease, 1"a few necrosis without leaf deformation, 2"leaf deformation not resulting in leaf fall, 3"moderate leaf fall (less than a third of lea#ets), 4"average leaf fall (one to two thirds
Table 1 Treatment conditions Treatment period
Litres of mixture per hectare
Commercial product (CP)
Active ingredient
Litres of CP/ha
Grammes of active ingredient/ha
Weighting agent
Millilitres of weighting agent per hectare
January 94 January 95 December 95} January 96 January 97 January 98
37 43 40
Ethrel Callel Almephon
Ethephon (480 g/l) Ethephon (480 g/l) Ethephon (480 g/l)
2.75 3.25 3
1320 1560 1440
CA 844 CA 844 CA 844
11.7 13.9 12.8
40 40
Almephon Almephon
Ethephon (480 g/l) Ethephon (480 g/l)
3 3
1440 1440
Extravon Extravon
40 40
J. Guyot et al. / Crop Protection 20 (2001) 581}590
of lea#ets), 5"severe leaf fall (more than two thirds of lea#ets). Based on the disease scores recorded separately for stages B and C, a diseased leaf area was evaluated for stage B leaves (DAB) and for stage C leaves (DAC). A correlation was established between the disease score and the diseased unit leaf area for stages B and C leaves remaining in place on the tree (Table 2). The diseased area of susceptible stages therefore amounted to: Diseased area of stage B leaves (6)
"DAB"AB;DUAB, Diseased area of stage C leaves
(7)
"DAC"AC;DUAC, Diseased area of susceptible stages
(8)
"DAS"DAB#DAC.
Lastly, the concept of a percentage of susceptible leaf stages, which will be used below, was obtained as follows: Percentage of susceptible leaf stages (9)
"%SS"%B#%C. 2.5. Experimental designs
2.5.1. Large-scale phenological observation Large-scale observation of treatment e!ects was carried out on trees planted in 1984. It involved four groups
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of "ve 12.5-ha plots (500 m;250 m), each representing a replicate (Table 3). Observations were carried out in each plot from once a week to once a month, depending on the defoliation operations. 100 trees spread over the 20 central rows, by series of 5 consecutive trees along two half-diagonals were observed (Fig. 1). In the 1994 operation, data for the untreated control (group 4) were obtained by fortnightly observation of 1069 trees representing all the trees in three adjacent 250-m long rows in the centre of "ve plots next to the "ve treated plots. For each observation round and for each tree, leaf density and stages were noted. 2.5.2. Small-scale epidemiological observation A small-scale epidemiological trial was carried out in two 8-year-old plots E a 50-ha plot having received no treatment, in which observations were made on 15 trees located in the most central zone, in three distinct rows spaced 75 m apart (10 interrows). Every 15th tree along each row was observed, i.e. every 36 m. E a 50-ha plot subjected to arti"cial defoliation on 8th January 1997 (fourth operation), for which observations were carried out on four distinct groups of three neighbouring trees, each selected in a central zone of the plot selected as there was very good defoliation and the beginning of refoliation coincident with the start of the trial.
Table 2 Correspondence between disease score and diseased unit leaf area for stages B and C Disease score
Diseased unit leaf area for stage B leaves (DUAB) (%)
Diseased unit leaf area for stage C leaves (DUAC) (%)
0 1 2 3 4 5
0 10 30 50 70 80
0 10 20 70 7 70
Fig. 1. Diagrammatic representation of the leaf density monitoring design over a large area (plot groups 1, 2 and 3).
Table 3 Dates of treatments of each group of plots used for large-scale observation Group 1
Group 2
Group 3
Group 4
First operation
Treated on 4th and 5th January 1994
Not treated not observed
Not treated not observed
Second operation
Treated on 7th and 8th January 1995 Treated on 29th and 30th December 1995
Treated on 7th and 8th January 1995 Treated on 29th and 30th December 1995
Not treated, observed as untreated control Treated on 29th and 30th December 1995
Not treated observed as untreated control Not treated not observed Not treated not observed
Third operation
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In this trial, leaf density, stage percentages and disease score were recorded for each tree every three days. Rainfall was also recorded daily with a pluviometer. 2.5.3. Assessment of the impact on rubber yield The trial was conducted in a 13-year-old 25-ha plot. The trees were in their "fth tapping year at the start of the trial (20th November 1995). Tapping was in a downward half-spiral every 5 days, 6 days out of 7 (from Monday to Saturday). Every day, one replicate was tapped. It was tapped again 5 days later (or 6 days later if there is a Sunday between the two tapping days). Stimulation was carried out with ELS 50 (480 g of ethephon per litre#additive) 11 times a year. ELS 50 was applied on the tapping cut. Tapping was interrupted each year for one month (in February, March or April) so as to follow the tapping programme of the Mitzic estate. A split-plot design was used, with two factors and "ve replicates. The main factor was the existence or not of arti"cial defoliation (two treatments). The secondary factor involved the tapper (two tappers). So, each sub-plot with six or seven 250-m long rows represented a half task for the tapper over 1.13 or 1.26 ha. Thus, two tappers could tap four sub-plots in a day. The rubber was collected as cup lumps "ve or six days after tapping (just before the following tapping) and weighed immediately at the edge of the "eld. The measured weight was assigned a coe$cient of transformation of 0.5, to evaluate the dry weight of the rubber taken into account in the analyses described in this article. The number of tapped trees was checked every 3 months. Leaf density was observed on 10 trees per sub-plot every fortnight during leaf emission periods, and around once a month for the rest of the year. Data analysis was by the Newman and Keuls test. Leaf density data collected in August were considered. The yield analysis was on the total production collected from the beginning of the tapping season (end of the previous tapping break) to when the defoliant was applied in the following year (annual total production). Thus, the immediate stimulant e!ect of the treatment, as will be demonstrated later, was avoided. Production per hectare was evaluated for 306 tapped trees per hectare, the mean for the whole trial.
3. Results 3.1. Phenological and epidemiological observations 3.1.1. Large-scale phenological survey Fig. 2 illustrates the results obtained in the di!erent groups of plots over the four defoliation operations. Figs. 2A, C, E and G indicate the percentage of trees that had started or "nished their refoliation (refoliation rate) and
Figs. 2B, D, F and H illustrate the variation in the average leaf density. With arti"cial defoliation, leaf density dropped rapidly and reached its minimum (between 0% and 5%) at the end of January or beginning of February (Figs. 2B, D, F). Refoliation began in the second half of January (Figs. 2A, C, E). Refoliation rates of 50% and 80% were reached before 20th February and 10th March, respectively. At the beginning of the rainy season, which usually occurs around 15th March, the refoliation of almost all the trees had started or was completed. C. gloeosporioides attacks can cause serious damage only on stage A and B leaves (i.e. leaves less than 21 days old). So, all the trees that began their refoliation before 23rd February can be considered as protected. In our trials those trees represented 60% to 80% of the trees. The refoliation rate levelled o! for 5}10 days between 40% and 60% depending on the plots. This revealed the existence of two populations of trees, one precocious, the other slightly later. The presence of those two populations of trees had already been reported by Senechal (1986) who observed that the younger a plot was, the more heterogeneous was its refoliation. Guyot (1996) found on 6-year-old GT 1 that only 15% of trees reacted to defoliant treatment by early refoliation, whereas 5% had intermediate refoliation and 80% at the same time as untreated trees. In the same GT 1 plot at eight years, the same defoliant treatment led to early refoliation in 40% of trees, whereas 15% had intermediate refoliation and 45% late refoliation, at the same time as untreated trees (Guyot, 1998b). Without defoliant treatment, leaf density gradually decreased, reaching its minimum around 20th March (Figs. 2B and D). Refoliation began in the second half of February (Figs. 2A, C, G), i.e. around a month later than in the treated plots. The earliest trees had already refoliated when the later trees were still undergoing refoliation. The 50% refoliation rate was only reached in the second half of March and the percentage of protected trees was almost nil. That led to a much lower leaf density than that obtained with defoliant treatment. A phenology survey in year 4 showed that halting treatment, even after three consecutive arti"cial defoliations (group 1) led to the resumption of a defoliation}refoliation cycle identical to that of plots that had never been treated (Figs. 2G and H). The same applies to groups 2 and 3. It showed that there is no capitalization of the bene"t of the previous arti"cial defoliation e!ects for phenology and leaf density. 3.1.2. Small-scale epidemiological observation 3.1.2.1. Untreated plot (Figs. 3A and B) Refoliation of the earliest trees started at the beginning of March, when leaf density was 40%, indicating very partial natural
J. Guyot et al. / Crop Protection 20 (2001) 581}590
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Fig. 2. Observation of leaf density and refoliation rate depending on the presence or absence of arti"cial defoliation. (A, B) Year 1, treatment on 4th and 5th January 1994. (C, D) Year 2, Treatment on 7th and 8th January 1995. (E, F) Year 3, Treatment on 29th and 30th December 1995. (G, H) Year 4, no treatment.
defoliation. The "rst disease symptoms observed on 18th March were very likely linked to a 22.3 mm rainfall on 12th March between 8:30 am and midday. Its e!ect was probably heightened by morning fog on 13th, 14th, 15th
and 17th March. These "rst signs of C. gloeosporioides attacks corresponded to the start of the rainy season. Then, the frequent rainy periods following those "rst attacks prevented any disease remission. The average
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Fig. 3. Variation of leaf density, proportion of susceptible stages, disease score for stages B and C, total and diseased susceptible area on untreated and treated plots. (A, B) Untreated plot. (C, D) Treated plot.
disease score increased steadily and reached level 3 (secondary fall of young leaves) on 8th April. At that time, refoliation was at its peak: susceptible stages accounted for 40}50% of the foliage, i.e. a leaf area of 15%. The earliest leaves were at the end of their susceptible stages and could escape the disease, allowing an approximate 15-point increase in leaf density in the sample. Secondary leaf fall then increased rapidly on youngest leaves. Both maturation of the earliest leaves and secondary fall of the youngest leaves were responsible for the quick decrease of the proportion and area of susceptible leaf stages. A second phase of leaf production was recorded from 23rd April onwards, corresponding to that of the trees that stated refoliation later, at the same time as they defoliated. The young leaves, which developed in the midst of the rainy season, when the disease was already well-established, were severely attacked immediately and most of them fell. Thus, late trees virtually did not refoliate at all and that led to a drop in overall leaf density.
together to provide conditions suitable for infection. The absence of rainfall between 16th and 28th was an essential factor in the low disease pressure. Thus, the young foliage was only slightly a!ected (disease score of less than 3) and secondary leaf fall was nonexistent. The leaf density rose very rapidly and reached its maximum (92%) as early as 24th March. The disease score exceeded 3 (secondary leaf fall) at the beginning of April, as in the untreated plot. But in this case, only a few residual leaf shoots were involved and average leaf density of the plot was not a!ected.
3.1.2.2. Treated plot (Figs. 3C and D). Refoliation started at the beginning of February, i.e. one month before the untreated plot. Refoliation was massive and primarily concentrated in 1 month. The rainy events occurring during this refoliation (0.8 mm in the evenings of 10th and 11th February and 17.3 mm on 15th February between 7:00 pm and 8:00 pm) were not close enough
3.2.1. Comparison of the short-term ewect of natural and artixcial defoliation Fig. 5A shows that defoliant treatment alone led to an increase in latex production of around 50 g per tree on the "rst tapping, as opposed to 25}35 g for tapping panel stimulation. This direct and immediate e!ect of treatment on yields was also found by Senechal (1986) who
3.2. Ewect of controlling C. gloeosporioides on rubber production During this trial, there was a signi"cant e!ect of the tappers but there was no interaction between the tappers' e!ect and the presence or absence of defoliant treatment. Inter-replicates e!ect was not signi"cant.
J. Guyot et al. / Crop Protection 20 (2001) 581}590
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also noted the superiority of stimulation induced by defoliant treatment over that obtained with tapping panel stimulation using 2.5% ethephon. Thereafter, the start of the dry season and leaf fall led to a period of lower yields, which continued into the start of refoliation (Fig. 5B). Natural defoliation also was coincident with lower yields per tree, but it was less marked than with arti"cial defoliation (Fig. 5C). Indeed, in the case of untreated plots, defoliation and refoliation did not a!ect all the trees at the same time, and the e!ect on yield was therefore more gradual. On the other hand, once the canopy was well established and the di!erence in leaf density between treated and untreated plots signi"cant, the di!erence in yields was very rapidly clearly marked and lasted throughout the trial. 3.2.2. Long-term ewect of artixcial defoliation Fig. 4 illustrates changes in leaf density (4A) and production (4B) in the treated and untreated plots during the three tapping seasons of the trial. It represents also the di!erences in yields between the two types of plots expressed as g per tree (4C) and as a percentage of the untreated plots production (4D). Table 4 summarizes, for the average leaf density (observed in August) and the annual total production per tree, the di!erences between treated and control plots. 3.2.2.1. 1996}1997 production season. Arti"cial defoliation in January 1996 did not prove very e!ective and leaf density stabilized at 62% in the treated plots, not signi"catively di!erent from 57% in the untreated plots (P"0.05). During the 1996}1997 production year, the two production curves were very similar. From the beginning of November onwards, yields per tree became consistently higher in the treated trees than in the untreated trees. However, over the 48 tappings during the year, the di!erence in yields per tree was not signi"cant (0.4%). 3.2.2.2. 1997}1998 production season. Defoliant treatment proved to be very e!ective in this case. The gain in leaf density was highly signi"cant (#47%) and the yield per tree became very signi"cantly higher in the treated trees during the 1997}1998 season (#35%). The di!erence was around 349 kg per hectare over nine and a half months' tapping. The gain was steady throughout the season, with surplus production per tree per tapping remaining at between 20 and 40 g, i.e. an extra yield of 30}50%. 3.2.2.3. 1998}1999 production season. As during the previous production season, defoliant treatment proved to be very e!ective and resulted in a signi"cant gain in leaf density (#56%). The e!ect on yields was even more marked than previously. The di!erence in yields per tree was continually over 40 g per tapping up to 22nd October 1998 (between 50% and 70% extra production
Fig. 4. Monitoring of mean leaf density and dry rubber production for the "ve replicates during three years of tapping with and without defoliant treatment. (A) Variation in leaf density. (B) Variation in production. (C) Treated-untreated di!erences in production. (D) Treated-untreated di!erence in production per tree (% of untreated).
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with defoliant treatment). Then it decreased slightly over the following three months, whilst remaining between 20 and 40 g per tapping (20}50% extra production). From 3rd June to 22nd January, the di!erence in yield per tree was 1614 g, i.e. 494 kg per hectare over seven and a half months. Between 10th January 1997 and 22nd January 1999, including two successful arti"cial defoliation operations and a 52-day halt to tapping, i.e. 23 months' e!ective tapping, the gain in production between treated and untreated trees was 970 kg per hectare, amounting to an extra yield of 506 kg over 12 months' tapping obtained by controlling C. gloeosporioides: 1274 kg for untreated trees and 1780 kg for treated trees. The yield increase compared to the untreated trees was therefore 40%.
4. Discussion
Fig. 5. Short-term comparison of defoliant treatment and natural defoliation e!ects on yield. (A) Comparison of yields in treated and untreated plots. (B) Evolution of yield and leaf density in treated plants. (C) Evolution of yield and leaf density in untreated plants.
The arti"cial defoliation technique, used to control secondary leaf fall of rubber tree, was developed in Asia, then in Cameroon, and proved appropriate for the Mitzic estate in Gabon. It enabled almost total defoliation of the trees, with refoliation concentrated in time, and, as in Malaysia (Rao and Azaldin, 1973), around a month sooner than in untreated plots. Thus, most of the young foliage was produced in February, a relatively dry period, during which the fungus did not have the possibility of completing several successive epidemic cycles and of causing serious damage. Without treatment, natural defoliation was only partial and prolonged, and refoliation began in March, at the same time as the rainy season. Refoliation was also more staggered in time and the C. gloeosporioides bene"tted from more frequent rainfall and wetness duration periods. Thus, infection could occur and the fungus could complete several epidemic cycles, thereby increasing its pressure on young leaves. Only the earliest leaves, emitted right at the beginning of March, could reach maturity without being too seriously a!ected by the disease. The last trees to defoliate were almost totally unable to refoliate. The leaf density in untreated plots therefore remained low and tended to decrease over the years. When treatment was halted, the trees returned immediately to their former natural
Table 4 E!ect of arti"cial defoliation on leaf density and dry rubber production observed over three operations Leaf density
Yield (g per tree)
Operation
Untreated
Treated
Di!erence
Untreated
Treated
Di!erence
1996}1997 (20/3/96}8/1/97) 1997}1998 (21/3/97}11/1/98) 1998}1999 (3/6/98}22/01/99)
58 58 50
62 85 78
4,1 (#7%) 27* (#47%) 28* (#56%)
4475 3280 2910
4493 4422 4524
18,1 (#0.40%) 1142* (#35%) 1614* (#55%)
NS: not signi"cant; *: signi"cant at 1%.
J. Guyot et al. / Crop Protection 20 (2001) 581}590
defoliation}refoliation cycle, exposing them again to attacks by the fungus. Leaf density was a!ected right from the "rst year. Annual treatment is therefore necessary on the most susceptible clones, notably GT 1, the most widely planted clone on the estate. Some clones that are less susceptible to the disease (PB 260), or clones for which the canopy can be re-established with a single treatment operation (PB 217, PB 235) may be treated only as needed, in the most severely defoliated plots. Arti"cial defoliation applied to old enough plots can be considered a reliable method, considering the regularity of the results obtained over "ve treatment operations at the Mitzic estate. Bad results, restricted to a few plots, can sometimes occur, either because of unusual rainfall in February, or problems associated with application of the defoliant. Nevertheless, defoliant treatment never resulted in reduced foliage density compared to neighbouring untreated plots. Rao and Azaldin (1973) also noted that even moderate results of arti"cial defoliation led to less secondary leaf fall than in untreated zones. Arti"cial defoliation also remains more e!ective than fungicide on rubber tree and it is less expensive since only one treatment is required. The increase in leaf density is greater than with fungicide treatment, as observed by Rao and Azaldin (1973) and Azaldin and Rao (1974) in On( dium heveae control, and by Guyot (1998b) for C. gloeosporioides control. This di!erence in e$ciency has a direct impact on yield increases (Rao and Azaldin, 1973; Azaldin and Rao, 1974). Chemical control also involves many more constraints: the leaf area has to be su$cient to enable e!ective fungicide action; the epidemic must not be too advanced when treatment is initiated; intervention needs to be very fast; since the number of treatments has to be limited to one or two at the most, for technical and economic reasons, refoliation in the plots has to be as uniform as possible, which is not the case for certain clones such as GT 1. Consequently, fungicide control of Colletotrichum gloeosporioides in rubber plantations is very di$cult to implement (Guyot, 1998b), though it is sometimes highly e!ective (Guyot, 1997). Controlling Colletotrichum leaf fall also helps in preventing serious On( dium heveae damage, which occurs at the end of June/beginning of July, because the fungus only "nds mature foliage on which its attacks are inconsequential. The situation is therefore similar to the one found by Wastie and Mainstone (1969), who noted that Oidium control considerably reduced the risks of secondary leaf fall caused later in the year by C. gloeosporioides. Since 1994, arti"cial defoliation has resulted in a clear increase in the average leaf density at the Mitzic estate, evaluated in 33 reference plots (Guyot, 1998a). Defoliant treatment has a dual e!ect on production. The "rst e!ect is an immediate increase due to the stimulant action of the treatment: by penetrating the rubber tree and acting systemically, ethephon acts upon the tapped
589
latex-bearing tissues. The second e!ect is a longer-term e!ect induced by the better phytosanitary condition of the trees. The increase in production can be estimated at between 450 and 500 kg per hectare over 12 months. Our production results in the "rst year were similar to those obtained in Malaysia on trees half-spiral tapped every 2 days. Indeed, Azaldin and Rao (1974) recorded around an extra 30 g per tree per tapping throughout the year on arti"cially defoliated trees compared to the untreated control. That amounted to a 33% increase in production over 12 months. Data obtained by RRIM (Anon, 1990) indicate a more than 30% increase in production after three consecutive years of arti"cial defoliation. In Cameroon, in a 9-year-old GT 1 plot tapped for 3 years and arti"cially defoliated, Senechal (1986) estimated the production increase at 115 kg/ha over a year compared to a control plot whereas for a 6-year-old GT 1 plot tapped for a year, the production increase was 48 kg per hectare. The consequences of a decrease or increase in the amount of foliage are almost immediate, but there also appears to be a delayed action, which is re#ected in a di!erence in production between treated and untreated plots that is greater in the second year than in the "rst. These observations support those by Azaldin and Rao, who found a greater yield increase in plots treated 4 years running than in plots treated for the "rst time. Senechal (1986) came to the same conclusion on clone PB 5/51. He obtained 33% higher production than the control for a plot during the 12 months following its fourth consecutive year of treatment. In a plot arti"cially defoliated for the "rst time, he found only 12.5% increase of yield for 12 months. Our results agree with those of the previous authors, and clearly con"rm the impact of leaf density on rubber yield. Azaldin and Rao (1974), working on Oidium control, already established a relation between the di!erence in e$cacy, in terms of leaf density and in terms of yield, between a defoliant treatment, a treatment with sulphur and untreated plots. According to the results obtained in Gabon, the production increase induced by defoliant treatment is su$cient to justify the implementation of this control method. The operation cost between US$50 and US$58, so a yield increase of 108 kg per hectare was necessary to cover costs at a rubber price of US$0.5/kg, and 81 kg for a price US$0.67/kg. In this trial, such production increases were easily reached and exceeded in one production season. The economic bene"ts of controlling Colletotrichum gloeosporioides by arti"cial defoliation are therefore unquestionable, since the production increase easily covers the cost of treatment. By protecting against this pathogen, On( dium heveae attacks can also be avoided. Gains in growth and bark regeneration, combined with reduced weeding costs, as reported by Wastie and Mainstone
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(1969), are a further bene"t that increases the merits of such control method. Arti"cial defoliation also reduces the consequences for the environment by limiting the spread of chemicals.
Acknowledgements The authors would like to thank HEVEGAB, and particularly its Agriculture Service at the Mitzic estate, for its support in conducting these trials.
References Anon, 1977. On( dium and Colletotrichum secondary leaf fall. Rubber Research Institute, Malaya Annual Report, p. 149. Anon, 1978. On( dium and Colletotrichum secondary leaf fall. Rubber Research Institute, Malaya Annual Report, pp. 169}170. Anon, 1990. Major leaf diseases of rubber and their management. RRIM Planters' Bulletin 204, 67}79. Azaldin, M.Y., Rao, B.S., 1974. Practicability and economics of largescale arti"cial defoliation for avoiding secondary leaf fall. Proceedings of the Rubber Research Institute, Malaya Planters' Conference, pp. 161}170. Guyot, J., 1996. Etude du comportement des clones PB 235, PB 260 et GT 1 a( geH s de six ans face a` un traitement deH foliant. In: Rapport Annuel d'ActiviteH 1995 du Centre d'Appui Technique a` l'HeH veH aculture Gabonaise, pp. 25}26.
Guyot, J., 1997. Essai fongicide en compleH ment de la deH foliation. In: Rapport Annuel d'ActiviteH 1996 du Centre d'Appui Technique a` l'HeH veH aculture Gabonaise, p. 18. Guyot, J., 1998a. Suivi foliaire de la plantation de Mitzic. In: Rapport Annuel d'ActiviteH 1997 du Centre d'Appui Technique a` l'HeH veH aculture Gabonaise, p. 2}3. Guyot, J., 1998b. Essai de lutte contre les maladies de feuilles par deH foliation arti"cielle et traitement fongicide sur GT 1 a` l'anneH e de son ouverture. In: Rapport Annuel d'ActiviteH 1997 du Centre d'Appui Technique a` l'HeH veH aculture Gabonaise, pp. 9}11. Halle, F., Martin, R., 1968. Etude de la croissance rythmique chez l'heH veH a. Adansonia 2 (8), 475}503. Mabbett, T., 1982. Some practical aspects in rubber disease control. World Crops 34 (5), 162}178. Peries, O.S., 1966. The economics of disease control in Hevea. Bull. Rubb. Res. Inst. Ceylan 1, 27. Rao, B.S., 1970. Controlled wintering of Hevea brasiliensis for avoiding secondary leaf fall. In: Wastie, R.L., Wood, B.J. (Eds.), Crop Protection in Malaysia, Kuala Lumpur, pp. 204}212. Rao, B.S., Azaldin, M.Y., 1973. Progress Towards Recommending Arti"cial Defoliation for Avoiding Secondary Leaf Fall. Proceedings of the Rubber Research Institute, Malaya Planters' Conference, pp. 267}280. Senechal, Y., 1986. L'anthracnose de l'heH veH a, eH tude eH pideH miologique et recherche de moyen de lutte. The`se de Docteur IngeH nieur, UniversiteH Paris XI, Orsay. Wastie, R.L., Mainstone, B.J., 1969. Economics of controlling secondary leaf fall of Hevea caused by On( dium heveae Steinm. J. Rubb. Res. Inst. Malaya 21 (1), 64}72. Wimalajeewa, D.L.S., Lloyd, J.H., 1963. The incidence and economic importance of the Gloeosporium leaf disease of Hevea in Ceylon. Rubb. Res. Inst. Ceylan Quat. J. 39, 18}24.
E!ect of controlling Colletotrichum leaf fall of rubber tree on epidemic development and rubber production Jean Guyot *, Edith Ntawanga Omanda, Auguste Ndoutoume, Abd-Allah Mba Otsaghe, Frank Enjalric , Henri-GreH goire Ngoua Assoumou CIRAD-CP, TA 80/PS3, Boulevard de la Lironde 34 398, Montpellier Ce& dex 5, France Centre d+Appui Technique a% l+He& ve& aculture (CATH), B.P. 643, Libreville, Gabon Received 20 December 1999; received in revised form 30 November 2000; accepted 2 February 2001
Abstract This paper studies the impact on phenology, epidemiology and subsequently on production of artici"al defoliation for the control of Colletotrichum leaf fall of Hevea brasiliensis for the susceptible rubber tree clone GT 1. Large-scale observations of tree phenology over 4 years show the impact of the treatment on foliage behaviour. These results provide on understanding how the trees escape from the disease and how the occurence of the an epidemic is prevented. A 50% increase in foliage density led to around a 40% increase in yields on the susceptible clone GT 1. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Hevea brasiliensis; Colletotrichum gloeosporioides; Avoidance control; Colletotrichum leaf fall; Epidemiology; Production
1. Introduction In Gabon, the secondary leaf fall of rubber tree caused by Colletotrichum gloeosporioides Penz. (Melanconiales) occurs after tree wintering, when trees are producing new foliage. Tree wintering takes place in March}April, during the rainy season, a period favourable for fungus development. On susceptible clones, the consequences of the disease are leaf necrosis and deformation, and even secondary fall of the youngest leaves a!ected at an early stage of their development. Consequently, new leaves produced throughout the rainy season are systematically destroyed by the disease. The trees become less vigorous as the #ushes succeed each other, and the initial canopy density is therefore never re-established. The disease epidemic causes a substantial drop in foliage density and death of terminal branches, known as die-back. Only at the start of the dry season (June}July) the most vigorous trees succeed in regenerating a part of their canopy. The Mitzic estate, in Gabon, was planted mostly with suscep-
* Corresponding author. Present address: DeH leH gation CIRAD, BP701, 97 387 Kourou Cedex, France. Tel.: #33-594-32-73-57; fax: #33-594-32-73-51. E-mail address: [email protected] (J. Guyot).
tible clones and foliage density was very substantially reduced by the disease since the estate was established and has adversely a!ected latex production. The economic impact of severe and repeated attacks is di$cult to evaluate with any accuracy but several authors consider that they slow down growth and lead to production losses (Rao, 1970; Peries, 1966; Wastie and Mainstone, 1969; Wimalajeewa and Lloyd, 1963). Necrosis also provide access for other infections, such as die-back by Botryodiplodia theobromae, and increased light under the rubber crop favours weed growth which necessitates frequent and prolonged weeding. Furthermore, prolonged production of new #ushes from March to the end of June/beginning of July exposes rubber trees to attacks by On( dium heveae which "nds suitable conditions for its development on young leaves of susceptible clones, such as PB 235, at the start of the dry season. Control of Colletotrichum leaf fall in mature rubber plantings is di$cult because of the size of the trees. To be e$cient, aerial treatment is usually required, but frequency of treatments may be limited. E$cient fungicide application is therefore very di$cult. In Malaysia, arti"cial defoliation was successfully tested in 1968 and has been developed on a large scale since 1974 to control secondary leaf fall caused by On( dium heveae and Colletotrichum gloeosporioides, by applying herbicides such as
0261-2194/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 1 - 2 1 9 4 ( 0 1 ) 0 0 0 2 7 - 8
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MSMA (Anon, 1977), merphos (Anon, 1978) or sodium cacodylate (Azaldin and Rao, 1974; Mabbett, 1982). This technique results in early refoliation occuring before the begining of the rainy season, allowing Hevea trees to avoid fungal attacks. This control method has been also adopted succesfully in Cameroon on the HEVECAM estate at Kribi, using ethephon, an ethylene generator, to speed up leaf senescence and fall and thus promote rapid refoliation (Senechal, 1986). Control of Colletotrichum leaf fall by arti"cial defoliation has now been developed in Gabon. This paper describes its impact on rubber tree phenology, epidemic development and rubber production in relation to the susceptible clone GT 1.
The percentages of stages B and C observed in the "eld enabled calculation of a maximum theoretical leaf area (LAth) for clone GT 1, estimating, from our observations, that a stage B leaf represented of the area of a stage D leaf, at full development, and that a stage C leaf had an area one third that of a stage D leaf. Thus LAth"0.17%B#0.33%C#%D,
(1)
where %B"percentage of stage B leaves, %C"percentage of stage C leaves, %D"percentage of stage D leaves (2)
"100!%B!%C. 2. Material and method 2.1. Treatment Trials were conducted during "ve arti"cial defoliation operations at HEVEGAB's Mitzic estate using a Pawnee Aircraft to apply the defoliant (Table 1). An adjuvant was used to add weight to spray droplets to increase they downward velocity into the tree foliage and reduce downwind dispersion of the spray. It also reduced evaporation of liquid before droplets reached the canopy in the higher temperatures.
This maximum theoretical leaf area di!ered in most cases from the observed leaf density (LD), since rarely all the branches of a tree bear leaves. The calculation of the leaf area for stages B and C therefore had to account for this di!erence and led to the following relations: Area of stage B leaves "AB"(0.17;%B) (LD/LAth),
(3)
Area of stage C leaves "AC"(0.33;%C) (LD/LAth).
2.2. Leaf density assessment
(4)
The total susceptible leaf area (SA) was therefore Leaf density was estimated visually on a scale of 11 levels between 0 and 100; the level 0 indicates the total absence of foliage while level 100 corresponds to a tree with all branches completely covered by mature leaves; 2.3. Leaf stages The presence of leaf stages A, B, C, and D (Halle and Martin, 1968) was noted in the large-scale observations. For the small-scale observations, the respective percentages of each stage in place was recorded, except for A stage (budding stage) with a nil leaf area.
Susceptible area"SA"AB#AC.
(5)
2.4. Disease score The level of disease, or disease score was recorded only for the small-scale observations and for each tree was an average score for susceptible stages B and C. The score comprised six levels: 0"absence of disease, 1"a few necrosis without leaf deformation, 2"leaf deformation not resulting in leaf fall, 3"moderate leaf fall (less than a third of lea#ets), 4"average leaf fall (one to two thirds
Table 1 Treatment conditions Treatment period
Litres of mixture per hectare
Commercial product (CP)
Active ingredient
Litres of CP/ha
Grammes of active ingredient/ha
Weighting agent
Millilitres of weighting agent per hectare
January 94 January 95 December 95} January 96 January 97 January 98
37 43 40
Ethrel Callel Almephon
Ethephon (480 g/l) Ethephon (480 g/l) Ethephon (480 g/l)
2.75 3.25 3
1320 1560 1440
CA 844 CA 844 CA 844
11.7 13.9 12.8
40 40
Almephon Almephon
Ethephon (480 g/l) Ethephon (480 g/l)
3 3
1440 1440
Extravon Extravon
40 40
J. Guyot et al. / Crop Protection 20 (2001) 581}590
of lea#ets), 5"severe leaf fall (more than two thirds of lea#ets). Based on the disease scores recorded separately for stages B and C, a diseased leaf area was evaluated for stage B leaves (DAB) and for stage C leaves (DAC). A correlation was established between the disease score and the diseased unit leaf area for stages B and C leaves remaining in place on the tree (Table 2). The diseased area of susceptible stages therefore amounted to: Diseased area of stage B leaves (6)
"DAB"AB;DUAB, Diseased area of stage C leaves
(7)
"DAC"AC;DUAC, Diseased area of susceptible stages
(8)
"DAS"DAB#DAC.
Lastly, the concept of a percentage of susceptible leaf stages, which will be used below, was obtained as follows: Percentage of susceptible leaf stages (9)
"%SS"%B#%C. 2.5. Experimental designs
2.5.1. Large-scale phenological observation Large-scale observation of treatment e!ects was carried out on trees planted in 1984. It involved four groups
583
of "ve 12.5-ha plots (500 m;250 m), each representing a replicate (Table 3). Observations were carried out in each plot from once a week to once a month, depending on the defoliation operations. 100 trees spread over the 20 central rows, by series of 5 consecutive trees along two half-diagonals were observed (Fig. 1). In the 1994 operation, data for the untreated control (group 4) were obtained by fortnightly observation of 1069 trees representing all the trees in three adjacent 250-m long rows in the centre of "ve plots next to the "ve treated plots. For each observation round and for each tree, leaf density and stages were noted. 2.5.2. Small-scale epidemiological observation A small-scale epidemiological trial was carried out in two 8-year-old plots E a 50-ha plot having received no treatment, in which observations were made on 15 trees located in the most central zone, in three distinct rows spaced 75 m apart (10 interrows). Every 15th tree along each row was observed, i.e. every 36 m. E a 50-ha plot subjected to arti"cial defoliation on 8th January 1997 (fourth operation), for which observations were carried out on four distinct groups of three neighbouring trees, each selected in a central zone of the plot selected as there was very good defoliation and the beginning of refoliation coincident with the start of the trial.
Table 2 Correspondence between disease score and diseased unit leaf area for stages B and C Disease score
Diseased unit leaf area for stage B leaves (DUAB) (%)
Diseased unit leaf area for stage C leaves (DUAC) (%)
0 1 2 3 4 5
0 10 30 50 70 80
0 10 20 70 7 70
Fig. 1. Diagrammatic representation of the leaf density monitoring design over a large area (plot groups 1, 2 and 3).
Table 3 Dates of treatments of each group of plots used for large-scale observation Group 1
Group 2
Group 3
Group 4
First operation
Treated on 4th and 5th January 1994
Not treated not observed
Not treated not observed
Second operation
Treated on 7th and 8th January 1995 Treated on 29th and 30th December 1995
Treated on 7th and 8th January 1995 Treated on 29th and 30th December 1995
Not treated, observed as untreated control Treated on 29th and 30th December 1995
Not treated observed as untreated control Not treated not observed Not treated not observed
Third operation
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In this trial, leaf density, stage percentages and disease score were recorded for each tree every three days. Rainfall was also recorded daily with a pluviometer. 2.5.3. Assessment of the impact on rubber yield The trial was conducted in a 13-year-old 25-ha plot. The trees were in their "fth tapping year at the start of the trial (20th November 1995). Tapping was in a downward half-spiral every 5 days, 6 days out of 7 (from Monday to Saturday). Every day, one replicate was tapped. It was tapped again 5 days later (or 6 days later if there is a Sunday between the two tapping days). Stimulation was carried out with ELS 50 (480 g of ethephon per litre#additive) 11 times a year. ELS 50 was applied on the tapping cut. Tapping was interrupted each year for one month (in February, March or April) so as to follow the tapping programme of the Mitzic estate. A split-plot design was used, with two factors and "ve replicates. The main factor was the existence or not of arti"cial defoliation (two treatments). The secondary factor involved the tapper (two tappers). So, each sub-plot with six or seven 250-m long rows represented a half task for the tapper over 1.13 or 1.26 ha. Thus, two tappers could tap four sub-plots in a day. The rubber was collected as cup lumps "ve or six days after tapping (just before the following tapping) and weighed immediately at the edge of the "eld. The measured weight was assigned a coe$cient of transformation of 0.5, to evaluate the dry weight of the rubber taken into account in the analyses described in this article. The number of tapped trees was checked every 3 months. Leaf density was observed on 10 trees per sub-plot every fortnight during leaf emission periods, and around once a month for the rest of the year. Data analysis was by the Newman and Keuls test. Leaf density data collected in August were considered. The yield analysis was on the total production collected from the beginning of the tapping season (end of the previous tapping break) to when the defoliant was applied in the following year (annual total production). Thus, the immediate stimulant e!ect of the treatment, as will be demonstrated later, was avoided. Production per hectare was evaluated for 306 tapped trees per hectare, the mean for the whole trial.
3. Results 3.1. Phenological and epidemiological observations 3.1.1. Large-scale phenological survey Fig. 2 illustrates the results obtained in the di!erent groups of plots over the four defoliation operations. Figs. 2A, C, E and G indicate the percentage of trees that had started or "nished their refoliation (refoliation rate) and
Figs. 2B, D, F and H illustrate the variation in the average leaf density. With arti"cial defoliation, leaf density dropped rapidly and reached its minimum (between 0% and 5%) at the end of January or beginning of February (Figs. 2B, D, F). Refoliation began in the second half of January (Figs. 2A, C, E). Refoliation rates of 50% and 80% were reached before 20th February and 10th March, respectively. At the beginning of the rainy season, which usually occurs around 15th March, the refoliation of almost all the trees had started or was completed. C. gloeosporioides attacks can cause serious damage only on stage A and B leaves (i.e. leaves less than 21 days old). So, all the trees that began their refoliation before 23rd February can be considered as protected. In our trials those trees represented 60% to 80% of the trees. The refoliation rate levelled o! for 5}10 days between 40% and 60% depending on the plots. This revealed the existence of two populations of trees, one precocious, the other slightly later. The presence of those two populations of trees had already been reported by Senechal (1986) who observed that the younger a plot was, the more heterogeneous was its refoliation. Guyot (1996) found on 6-year-old GT 1 that only 15% of trees reacted to defoliant treatment by early refoliation, whereas 5% had intermediate refoliation and 80% at the same time as untreated trees. In the same GT 1 plot at eight years, the same defoliant treatment led to early refoliation in 40% of trees, whereas 15% had intermediate refoliation and 45% late refoliation, at the same time as untreated trees (Guyot, 1998b). Without defoliant treatment, leaf density gradually decreased, reaching its minimum around 20th March (Figs. 2B and D). Refoliation began in the second half of February (Figs. 2A, C, G), i.e. around a month later than in the treated plots. The earliest trees had already refoliated when the later trees were still undergoing refoliation. The 50% refoliation rate was only reached in the second half of March and the percentage of protected trees was almost nil. That led to a much lower leaf density than that obtained with defoliant treatment. A phenology survey in year 4 showed that halting treatment, even after three consecutive arti"cial defoliations (group 1) led to the resumption of a defoliation}refoliation cycle identical to that of plots that had never been treated (Figs. 2G and H). The same applies to groups 2 and 3. It showed that there is no capitalization of the bene"t of the previous arti"cial defoliation e!ects for phenology and leaf density. 3.1.2. Small-scale epidemiological observation 3.1.2.1. Untreated plot (Figs. 3A and B) Refoliation of the earliest trees started at the beginning of March, when leaf density was 40%, indicating very partial natural
J. Guyot et al. / Crop Protection 20 (2001) 581}590
585
Fig. 2. Observation of leaf density and refoliation rate depending on the presence or absence of arti"cial defoliation. (A, B) Year 1, treatment on 4th and 5th January 1994. (C, D) Year 2, Treatment on 7th and 8th January 1995. (E, F) Year 3, Treatment on 29th and 30th December 1995. (G, H) Year 4, no treatment.
defoliation. The "rst disease symptoms observed on 18th March were very likely linked to a 22.3 mm rainfall on 12th March between 8:30 am and midday. Its e!ect was probably heightened by morning fog on 13th, 14th, 15th
and 17th March. These "rst signs of C. gloeosporioides attacks corresponded to the start of the rainy season. Then, the frequent rainy periods following those "rst attacks prevented any disease remission. The average
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Fig. 3. Variation of leaf density, proportion of susceptible stages, disease score for stages B and C, total and diseased susceptible area on untreated and treated plots. (A, B) Untreated plot. (C, D) Treated plot.
disease score increased steadily and reached level 3 (secondary fall of young leaves) on 8th April. At that time, refoliation was at its peak: susceptible stages accounted for 40}50% of the foliage, i.e. a leaf area of 15%. The earliest leaves were at the end of their susceptible stages and could escape the disease, allowing an approximate 15-point increase in leaf density in the sample. Secondary leaf fall then increased rapidly on youngest leaves. Both maturation of the earliest leaves and secondary fall of the youngest leaves were responsible for the quick decrease of the proportion and area of susceptible leaf stages. A second phase of leaf production was recorded from 23rd April onwards, corresponding to that of the trees that stated refoliation later, at the same time as they defoliated. The young leaves, which developed in the midst of the rainy season, when the disease was already well-established, were severely attacked immediately and most of them fell. Thus, late trees virtually did not refoliate at all and that led to a drop in overall leaf density.
together to provide conditions suitable for infection. The absence of rainfall between 16th and 28th was an essential factor in the low disease pressure. Thus, the young foliage was only slightly a!ected (disease score of less than 3) and secondary leaf fall was nonexistent. The leaf density rose very rapidly and reached its maximum (92%) as early as 24th March. The disease score exceeded 3 (secondary leaf fall) at the beginning of April, as in the untreated plot. But in this case, only a few residual leaf shoots were involved and average leaf density of the plot was not a!ected.
3.1.2.2. Treated plot (Figs. 3C and D). Refoliation started at the beginning of February, i.e. one month before the untreated plot. Refoliation was massive and primarily concentrated in 1 month. The rainy events occurring during this refoliation (0.8 mm in the evenings of 10th and 11th February and 17.3 mm on 15th February between 7:00 pm and 8:00 pm) were not close enough
3.2.1. Comparison of the short-term ewect of natural and artixcial defoliation Fig. 5A shows that defoliant treatment alone led to an increase in latex production of around 50 g per tree on the "rst tapping, as opposed to 25}35 g for tapping panel stimulation. This direct and immediate e!ect of treatment on yields was also found by Senechal (1986) who
3.2. Ewect of controlling C. gloeosporioides on rubber production During this trial, there was a signi"cant e!ect of the tappers but there was no interaction between the tappers' e!ect and the presence or absence of defoliant treatment. Inter-replicates e!ect was not signi"cant.
J. Guyot et al. / Crop Protection 20 (2001) 581}590
587
also noted the superiority of stimulation induced by defoliant treatment over that obtained with tapping panel stimulation using 2.5% ethephon. Thereafter, the start of the dry season and leaf fall led to a period of lower yields, which continued into the start of refoliation (Fig. 5B). Natural defoliation also was coincident with lower yields per tree, but it was less marked than with arti"cial defoliation (Fig. 5C). Indeed, in the case of untreated plots, defoliation and refoliation did not a!ect all the trees at the same time, and the e!ect on yield was therefore more gradual. On the other hand, once the canopy was well established and the di!erence in leaf density between treated and untreated plots signi"cant, the di!erence in yields was very rapidly clearly marked and lasted throughout the trial. 3.2.2. Long-term ewect of artixcial defoliation Fig. 4 illustrates changes in leaf density (4A) and production (4B) in the treated and untreated plots during the three tapping seasons of the trial. It represents also the di!erences in yields between the two types of plots expressed as g per tree (4C) and as a percentage of the untreated plots production (4D). Table 4 summarizes, for the average leaf density (observed in August) and the annual total production per tree, the di!erences between treated and control plots. 3.2.2.1. 1996}1997 production season. Arti"cial defoliation in January 1996 did not prove very e!ective and leaf density stabilized at 62% in the treated plots, not signi"catively di!erent from 57% in the untreated plots (P"0.05). During the 1996}1997 production year, the two production curves were very similar. From the beginning of November onwards, yields per tree became consistently higher in the treated trees than in the untreated trees. However, over the 48 tappings during the year, the di!erence in yields per tree was not signi"cant (0.4%). 3.2.2.2. 1997}1998 production season. Defoliant treatment proved to be very e!ective in this case. The gain in leaf density was highly signi"cant (#47%) and the yield per tree became very signi"cantly higher in the treated trees during the 1997}1998 season (#35%). The di!erence was around 349 kg per hectare over nine and a half months' tapping. The gain was steady throughout the season, with surplus production per tree per tapping remaining at between 20 and 40 g, i.e. an extra yield of 30}50%. 3.2.2.3. 1998}1999 production season. As during the previous production season, defoliant treatment proved to be very e!ective and resulted in a signi"cant gain in leaf density (#56%). The e!ect on yields was even more marked than previously. The di!erence in yields per tree was continually over 40 g per tapping up to 22nd October 1998 (between 50% and 70% extra production
Fig. 4. Monitoring of mean leaf density and dry rubber production for the "ve replicates during three years of tapping with and without defoliant treatment. (A) Variation in leaf density. (B) Variation in production. (C) Treated-untreated di!erences in production. (D) Treated-untreated di!erence in production per tree (% of untreated).
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J. Guyot et al. / Crop Protection 20 (2001) 581}590
with defoliant treatment). Then it decreased slightly over the following three months, whilst remaining between 20 and 40 g per tapping (20}50% extra production). From 3rd June to 22nd January, the di!erence in yield per tree was 1614 g, i.e. 494 kg per hectare over seven and a half months. Between 10th January 1997 and 22nd January 1999, including two successful arti"cial defoliation operations and a 52-day halt to tapping, i.e. 23 months' e!ective tapping, the gain in production between treated and untreated trees was 970 kg per hectare, amounting to an extra yield of 506 kg over 12 months' tapping obtained by controlling C. gloeosporioides: 1274 kg for untreated trees and 1780 kg for treated trees. The yield increase compared to the untreated trees was therefore 40%.
4. Discussion
Fig. 5. Short-term comparison of defoliant treatment and natural defoliation e!ects on yield. (A) Comparison of yields in treated and untreated plots. (B) Evolution of yield and leaf density in treated plants. (C) Evolution of yield and leaf density in untreated plants.
The arti"cial defoliation technique, used to control secondary leaf fall of rubber tree, was developed in Asia, then in Cameroon, and proved appropriate for the Mitzic estate in Gabon. It enabled almost total defoliation of the trees, with refoliation concentrated in time, and, as in Malaysia (Rao and Azaldin, 1973), around a month sooner than in untreated plots. Thus, most of the young foliage was produced in February, a relatively dry period, during which the fungus did not have the possibility of completing several successive epidemic cycles and of causing serious damage. Without treatment, natural defoliation was only partial and prolonged, and refoliation began in March, at the same time as the rainy season. Refoliation was also more staggered in time and the C. gloeosporioides bene"tted from more frequent rainfall and wetness duration periods. Thus, infection could occur and the fungus could complete several epidemic cycles, thereby increasing its pressure on young leaves. Only the earliest leaves, emitted right at the beginning of March, could reach maturity without being too seriously a!ected by the disease. The last trees to defoliate were almost totally unable to refoliate. The leaf density in untreated plots therefore remained low and tended to decrease over the years. When treatment was halted, the trees returned immediately to their former natural
Table 4 E!ect of arti"cial defoliation on leaf density and dry rubber production observed over three operations Leaf density
Yield (g per tree)
Operation
Untreated
Treated
Di!erence
Untreated
Treated
Di!erence
1996}1997 (20/3/96}8/1/97) 1997}1998 (21/3/97}11/1/98) 1998}1999 (3/6/98}22/01/99)
58 58 50
62 85 78
4,1 (#7%) 27* (#47%) 28* (#56%)
4475 3280 2910
4493 4422 4524
18,1 (#0.40%) 1142* (#35%) 1614* (#55%)
NS: not signi"cant; *: signi"cant at 1%.
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defoliation}refoliation cycle, exposing them again to attacks by the fungus. Leaf density was a!ected right from the "rst year. Annual treatment is therefore necessary on the most susceptible clones, notably GT 1, the most widely planted clone on the estate. Some clones that are less susceptible to the disease (PB 260), or clones for which the canopy can be re-established with a single treatment operation (PB 217, PB 235) may be treated only as needed, in the most severely defoliated plots. Arti"cial defoliation applied to old enough plots can be considered a reliable method, considering the regularity of the results obtained over "ve treatment operations at the Mitzic estate. Bad results, restricted to a few plots, can sometimes occur, either because of unusual rainfall in February, or problems associated with application of the defoliant. Nevertheless, defoliant treatment never resulted in reduced foliage density compared to neighbouring untreated plots. Rao and Azaldin (1973) also noted that even moderate results of arti"cial defoliation led to less secondary leaf fall than in untreated zones. Arti"cial defoliation also remains more e!ective than fungicide on rubber tree and it is less expensive since only one treatment is required. The increase in leaf density is greater than with fungicide treatment, as observed by Rao and Azaldin (1973) and Azaldin and Rao (1974) in On( dium heveae control, and by Guyot (1998b) for C. gloeosporioides control. This di!erence in e$ciency has a direct impact on yield increases (Rao and Azaldin, 1973; Azaldin and Rao, 1974). Chemical control also involves many more constraints: the leaf area has to be su$cient to enable e!ective fungicide action; the epidemic must not be too advanced when treatment is initiated; intervention needs to be very fast; since the number of treatments has to be limited to one or two at the most, for technical and economic reasons, refoliation in the plots has to be as uniform as possible, which is not the case for certain clones such as GT 1. Consequently, fungicide control of Colletotrichum gloeosporioides in rubber plantations is very di$cult to implement (Guyot, 1998b), though it is sometimes highly e!ective (Guyot, 1997). Controlling Colletotrichum leaf fall also helps in preventing serious On( dium heveae damage, which occurs at the end of June/beginning of July, because the fungus only "nds mature foliage on which its attacks are inconsequential. The situation is therefore similar to the one found by Wastie and Mainstone (1969), who noted that Oidium control considerably reduced the risks of secondary leaf fall caused later in the year by C. gloeosporioides. Since 1994, arti"cial defoliation has resulted in a clear increase in the average leaf density at the Mitzic estate, evaluated in 33 reference plots (Guyot, 1998a). Defoliant treatment has a dual e!ect on production. The "rst e!ect is an immediate increase due to the stimulant action of the treatment: by penetrating the rubber tree and acting systemically, ethephon acts upon the tapped
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latex-bearing tissues. The second e!ect is a longer-term e!ect induced by the better phytosanitary condition of the trees. The increase in production can be estimated at between 450 and 500 kg per hectare over 12 months. Our production results in the "rst year were similar to those obtained in Malaysia on trees half-spiral tapped every 2 days. Indeed, Azaldin and Rao (1974) recorded around an extra 30 g per tree per tapping throughout the year on arti"cially defoliated trees compared to the untreated control. That amounted to a 33% increase in production over 12 months. Data obtained by RRIM (Anon, 1990) indicate a more than 30% increase in production after three consecutive years of arti"cial defoliation. In Cameroon, in a 9-year-old GT 1 plot tapped for 3 years and arti"cially defoliated, Senechal (1986) estimated the production increase at 115 kg/ha over a year compared to a control plot whereas for a 6-year-old GT 1 plot tapped for a year, the production increase was 48 kg per hectare. The consequences of a decrease or increase in the amount of foliage are almost immediate, but there also appears to be a delayed action, which is re#ected in a di!erence in production between treated and untreated plots that is greater in the second year than in the "rst. These observations support those by Azaldin and Rao, who found a greater yield increase in plots treated 4 years running than in plots treated for the "rst time. Senechal (1986) came to the same conclusion on clone PB 5/51. He obtained 33% higher production than the control for a plot during the 12 months following its fourth consecutive year of treatment. In a plot arti"cially defoliated for the "rst time, he found only 12.5% increase of yield for 12 months. Our results agree with those of the previous authors, and clearly con"rm the impact of leaf density on rubber yield. Azaldin and Rao (1974), working on Oidium control, already established a relation between the di!erence in e$cacy, in terms of leaf density and in terms of yield, between a defoliant treatment, a treatment with sulphur and untreated plots. According to the results obtained in Gabon, the production increase induced by defoliant treatment is su$cient to justify the implementation of this control method. The operation cost between US$50 and US$58, so a yield increase of 108 kg per hectare was necessary to cover costs at a rubber price of US$0.5/kg, and 81 kg for a price US$0.67/kg. In this trial, such production increases were easily reached and exceeded in one production season. The economic bene"ts of controlling Colletotrichum gloeosporioides by arti"cial defoliation are therefore unquestionable, since the production increase easily covers the cost of treatment. By protecting against this pathogen, On( dium heveae attacks can also be avoided. Gains in growth and bark regeneration, combined with reduced weeding costs, as reported by Wastie and Mainstone
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(1969), are a further bene"t that increases the merits of such control method. Arti"cial defoliation also reduces the consequences for the environment by limiting the spread of chemicals.
Acknowledgements The authors would like to thank HEVEGAB, and particularly its Agriculture Service at the Mitzic estate, for its support in conducting these trials.
References Anon, 1977. On( dium and Colletotrichum secondary leaf fall. Rubber Research Institute, Malaya Annual Report, p. 149. Anon, 1978. On( dium and Colletotrichum secondary leaf fall. Rubber Research Institute, Malaya Annual Report, pp. 169}170. Anon, 1990. Major leaf diseases of rubber and their management. RRIM Planters' Bulletin 204, 67}79. Azaldin, M.Y., Rao, B.S., 1974. Practicability and economics of largescale arti"cial defoliation for avoiding secondary leaf fall. Proceedings of the Rubber Research Institute, Malaya Planters' Conference, pp. 161}170. Guyot, J., 1996. Etude du comportement des clones PB 235, PB 260 et GT 1 a( geH s de six ans face a` un traitement deH foliant. In: Rapport Annuel d'ActiviteH 1995 du Centre d'Appui Technique a` l'HeH veH aculture Gabonaise, pp. 25}26.
Guyot, J., 1997. Essai fongicide en compleH ment de la deH foliation. In: Rapport Annuel d'ActiviteH 1996 du Centre d'Appui Technique a` l'HeH veH aculture Gabonaise, p. 18. Guyot, J., 1998a. Suivi foliaire de la plantation de Mitzic. In: Rapport Annuel d'ActiviteH 1997 du Centre d'Appui Technique a` l'HeH veH aculture Gabonaise, p. 2}3. Guyot, J., 1998b. Essai de lutte contre les maladies de feuilles par deH foliation arti"cielle et traitement fongicide sur GT 1 a` l'anneH e de son ouverture. In: Rapport Annuel d'ActiviteH 1997 du Centre d'Appui Technique a` l'HeH veH aculture Gabonaise, pp. 9}11. Halle, F., Martin, R., 1968. Etude de la croissance rythmique chez l'heH veH a. Adansonia 2 (8), 475}503. Mabbett, T., 1982. Some practical aspects in rubber disease control. World Crops 34 (5), 162}178. Peries, O.S., 1966. The economics of disease control in Hevea. Bull. Rubb. Res. Inst. Ceylan 1, 27. Rao, B.S., 1970. Controlled wintering of Hevea brasiliensis for avoiding secondary leaf fall. In: Wastie, R.L., Wood, B.J. (Eds.), Crop Protection in Malaysia, Kuala Lumpur, pp. 204}212. Rao, B.S., Azaldin, M.Y., 1973. Progress Towards Recommending Arti"cial Defoliation for Avoiding Secondary Leaf Fall. Proceedings of the Rubber Research Institute, Malaya Planters' Conference, pp. 267}280. Senechal, Y., 1986. L'anthracnose de l'heH veH a, eH tude eH pideH miologique et recherche de moyen de lutte. The`se de Docteur IngeH nieur, UniversiteH Paris XI, Orsay. Wastie, R.L., Mainstone, B.J., 1969. Economics of controlling secondary leaf fall of Hevea caused by On( dium heveae Steinm. J. Rubb. Res. Inst. Malaya 21 (1), 64}72. Wimalajeewa, D.L.S., Lloyd, J.H., 1963. The incidence and economic importance of the Gloeosporium leaf disease of Hevea in Ceylon. Rubb. Res. Inst. Ceylan Quat. J. 39, 18}24.