Suppression of alternaria leaf spot in pima cotton by systemic fungicides

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Suppression of Alternaria leaf spot in Pima cotton by systemic fungicides D. S h t i e n b e r g * and J. D r e i s h p o u n t

*Department of Plant Pathology and Microbiology, The Hebrew University of Jerusalem, Faculty of Agriculture, PO Box 12, Rehovot 76100, and t Ministry of Agriculture, Extension Service, Department of Plant Protection, Hakirya, TeI-Aviv, Israel

Abstract

Keywords

The effects of difenoconazole, tebuconazole (systemic fungicides) and maneb (a protectant fungicide) on Altcrnaria leaf spot (Ahernaria macrospora) and on yields of Pima cotton were determined in five field trials conducted in 1989 and 1990. Disease-induced defoliation was calculated as the sum of weighted estimates of the disease severity of attached leaves and the proportion ofshed leaves. In all trials, difenoconazole at a rate of 0.125kg a.i. ha-i and tebuconazole at 0.187kg a.i. h a - ' suppressed disease to a significant extent (p<0.05) relative to untreated plots, and differences in yield (15.6-39.0°/'o increase) between treated and untreated plots were significant. Although maneb (at 2.0 kg a.i. ha- t) also suppressed disease significantly, in four of the trials the yield differences (6.3-26.0°/'0 increase) between treated and untreated plots were insignificant. In general, defoliation and yields did not differ significantly among the different fungicide treatments. The systemic fungicides exhibited some curative properties because of their effects in suppressing previous infections. Alternaria macrospora; control; Gossypium barbadense; yield loss; fungicides; Alternaria leaf spot

Introduction

Alternaria leaf spot in cotton (Gossyphtm barbadense L.) is caused by the fungus Alternaria macrospora Zimm. and occurs in most cotton-growing areas ofthe world (Ellis and Holliday, 1970). In Israel, A. macrospora appeared soon after the introduction of cotton production in the 1950s, and by the 1970s had started to cause economic losses (Bashi, Sachs and Rotem, 1983). AIternaria macrospora can cause leaf spot, stem cankers, defoliation and boll shedding in cotton (Ellis and Holliday, 1970). Infected leaves become yellow and eventually drop off (shed). Leaf shedding is induced by spots occupying an average of 1-3% of the leaf area. Shed leaves are rapidly replaced, but because the new leaves are small the leaf area in infected plants is smaller than in uninfected plants. Reduction in the photosynthesizing leafarea was the main reason for the 20-30% difference in yields recorded for fungicide-treated and untreated crops (Bashi et aL, 1983; Rotem et al., 1988). For suppression of the pathogen and disease, fungicides are commonly applied to the foliage. Only protectant fungicides are currently registered in Israel for Alternaria leaf spot suppression, and include tin and dithiocarbamate fungicides. Growers are advised to initiate spraying when an average of one lesion per 10 m row of plants is detected. On the coastal plain of Israel this occurs usually around early June. Further sprays should be applied when new lesions are observed on plant tops. In a typical growing *To whom correspondence should be addressed 0261-2194•91 t05/0381-05 © 1991 Butterworth-HeinemannLtd

season, a Pima cotton field is sprayed with fungicide 5-10 times. Despite this intensive chemical protection, epidemics of Alternaria leaf spot developed in 1989 and 1990 in commercial fields, resulting in 5-15% yield losses (D. Shtienberg, unpublished data). Difenoconazole and tebuconazole are systemic fungicides now available for commercial use in other crops. Both belong to the triazole group of fungicides, which alter the pathway ofsterol biosynthesis of the fungus pathogen. Tebuconazole was recently found to induce larger" yield increments than chlorothalonil (a protectant fungicide) when used for suppression of A. solani, the agent causing early blight in potato (Shtienberg et aL, 1989). The objective of this study was to determine the effectiveness of difenoconazole and tebuconazole in comparison with maneb in suppression of Alternaria leaf spot in Pima cotton in Israel.

M a t e r i a l s and methods Cultural practices

The effectiveness of difenoconazole and tebuconazole in the suppression of AIternaria leaf spot in Pima cotton was determined in five field trials. One trial was planted in a commercial field in early April 1989 and four in 1990. Cultural methods were those recommended in Israel. In .t 1989 the cultivar Pima F-27 was sown in Kvutzat Yavne (trial I). In 1990, trials were located in Kibbutz Tzora (cv. Pima BF-19, trial 2), Kvutzat Yavne (cv. Pima S-5, trial 3),

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Suppression of AIternaria leaf spot: D. Shtienberg and J. Dreishpoun

Table 1. Application rates and dates of spraying of fungicides in five field trials conducted in 1989 and 1990

Trial number

Fungicide and application rate (kg a.i. ha- ~)

Date of spraying initiation

Spraying intervals (days)

5 5 5 0

5 July 5 July 5 July

9-19 9-19 9-19

Number of sprays

I

Difenoconazole, 0.075 Difenoconazole, 0.125 Maneb, 2.00 Untreated

2

Tebuconazole, 0.125 Tebuconazole, 0.187 Maneb, 2.00 Untreated

6 6 I0 0

24 June 24 June 24 June

14-19 14-19 7-12

3

Difenoconazole, 0.125 Difenoconazole, 0.187 Tebuconazole, 0.125 Tebuconazole, 0.187 Tebuconazole, 0.250 Maneb, 2.00 Untreated

7 7 7 7 7 13 0

10 June 10 June 10 June 10 June 10 June 10 June

14 14 14 14 14 7

4

Difenoconazole, 0.125 Difenoconazole, 0.125 Maneb, 2.00 Maneb, 2.00 Untreated

7 4 10 7 0

4 June 16 July 4 June 16 July

14 14 7 7

5

Tebuconazole, 0.250 Tebuconazole, 0.250 Maneb, 2.00 Maneb, 2.00 Untreated

7 4 10 7 0

6 June 17 July 6 June 17 July

14 14 7 7

Kibbutz Hama 'apil (cv. Pima F-177, trial 4) and Moshav Bnei-Darom (cv. Pima S-5, Trial 5). All experimental sites were located on the coastal or the inland plains of Israel, and all Pima cultivars are susceptible to A. macrospora. The relative susceptibility of the cultivars is still unknown. Top sprinkler irrigation was used in trials 1 and 2 and trickle irrigation systems in the others. Experimental plots in each trial were arranged in complete blocks (trials 1, 2, 4 and 5) or in a completely randomized design (trial 3), with four replicates per treatment. The size of each experimental plot varied according to the mode of fungicide application and the size of the fields in which the trials were carried out. Plot sizes were 18 × 400 m in trial 1, 18 × 250 m in trial 2, 18 x 40 m in trial 3, and 4 x 12m in trials 4 and 5. Sprays were applied by a tractor-mounted boom sprayer at 90-1001 water h a - J in trials 1-3, and by a motorized back sprayer at 180-2001 water h a - l in trials 4 and 5. Spreader, sticker or adjuvant were not included. Treatments consisted of different fungicidal sprays applied at different rates and starting dates. The fungicides used were difenoconazole (Score, 25% a.i., Ciba-Geigy Ltd, Switzerland), tebuconazole (Folicur, 25% a.i., Bayer AG, Germany) and maneb (Manebgan, 50% a.i., Agan Ltd, Israel). Application rates and spraying details are listed in Table 1. Unless otherwise indicated, spraying was initiated at the recommended action threshold level, i.e. when an average of one lesion was detected per 10 m row of plants. In trials 4 and 5, sprayings were also initiated after an interval of 6 weeks from the time of this observation, in order to test the effectiveness of the fungicides in suppressing previous infections.

Yields (fibres and seeds) were collected from the middle part o f each experimental plot in early October. Cotton was picked with a mechanical harvester in trials I and 3, and by hand in the other trials. The harvested areas were 800m 2 in trial 1, four randomly chosen samples each of 2 m 2 per replicate in trial 2, 80m 2 in trial 3, and one randomly chosen sample of 2 m 2 per replicate in trials 4 and 5. Yields were recorded and expressed as t h a Disease assessment Disease was assessed visually in all trials, but the assessment procedure was modified from year to year. In 1989 the method of scoring changed as the epidemic became more severe. On 17 July the number of lesions in all plants in five samples of 10 m row chosen at random was counted in each experimental plot. On 31 July the number oflesions in 30 randomly chosen plant tops (height < 8 1 c m ) was counted in each experimental plot. On 30 August, leaf shedding was noticed and assessed according to a scale of 0 (no leaf shedding) to 5 (100% shedding), as described by Bashi et al. (1983). In 1990 the disease severity of attached leaves and the proportion of shed leaves were assessed separately for the low ( < 4 0 c m ) , middle (41-80cm) and upper ( > 8 1 c m ) levels of the canopy. Disease was assessed every 10-14 days, starting in mid-June and ending in mid-September, The disease severity of fully expanded leaves (leaf area > 100 cm 2) was assessed using the disease assessment key ii published by Ephrath et aL (1989). Twenty (trials 2 and 3) or ten (trials 4 and 5) randomly chosen leaves were assessed at each of the three height levels in each experimental plot.

Suppression of A I t e r n a r i a leaf spot: D. Shtienberg and J. Dreishpoun

383

initiated according to the disease action threshold (Tables 2-4). Treated and untreated plots also differed significantly (p_< 0.05) with respect to A U D P C (Tables3, 4). There were,

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however, slight differences in disease suppression among the different fungicides and application rates. In general, application ofdifenoconazole at a rate of 0.125 kg a.i. h a and tebuconazole at a rate of 0.187 kg a.i. h a - i resulted in similar or significantly lower defoliation and A U D P C values compared with application of maneb (Tables 2-4). At these rates the effectiveness of the two systemic fungicides did not differ significantly, but both were less effective than difenoconazole at a rate of 0.187 kg a.i. h a - t (trial 3,

Table 3). Figure 1. Scale for assessment of leaf shedding induced by Alternaria macrospora in Pima cotton. Each graph corresponds to the indicated proportion of shed leaves. Dots on stems and branches represent the sites of abscised leaves. Flowers and bolls are not indicated for the sake of simplicity. Arrows indicate the lower (<40cm), middle (41-80cm) and upper (>81cm) levels of the canopy. A separate assessment was carried out at each level of the canopy

The proportion of shed leaves was assessed on the basis of number of the easily distinguished sites of abscission on the stem and branches, with the aid of the scale illustrated in Figure 1. In constructing the scale, consideration was given to the tendency of different leaves to shed and to the relative areas of shed and attached leaves in each canopy layer. In each experimental plot, leaf shedding was assessed separately by two individuals for each height level of the canopy often randomly chosen plants. The assessments of disease severity and proportion of shed leaves were used to calculate crop defoliation (De%), as follows. Defoliation at each height level (h, h = 1,2,3) was calculated as the sum of the proportion of shed leaves (SLh) and weighted estimates of the disease severity of attached leaves (S Vh). The means of these separate estimates corresponded to the crop defoliation score: Oc%

=

3 ISLh+(I-SLh/IOO)SVh1 h=~

3

As an example, if mean severity readings were 15.4% for the lower, 5.2% for the middle and 2.4% for the upper canopy levels, and their corresponding proportions of shed leaves were assessed as 40, 20 and 0% (Figure 2), then the crop defoliation score was calculated as: Dc%=((40-1(1 - 40/100) 15.4) + (20 + (I - 20/100)5.2) + 2.4)/3 = 25.25 %. For some analyses, the area under the defoliation progress curve (AUDPC) as calculated by Shaner and Finney (1977) was used. The period over which A U D P C was calculated was the date of the first disease assessment until the end of the growing season. A U D P C units are proportion days. All data were subjected to analyses of variance, and means were compared by the F test.

Results Disease was significantly suppressed by all three fungicides at all rates of application, provided that sprayings were

Yields were affected to different extents by the three fimgicides. Plots treated with maneb yielded 6.3-26.0% more than untreated plots; these differences were insignificant (p=0.05) in four out of the five trials (Tables 2-4). Plots treated with difenoconazole or tebuconazole yielded 15.6-39.0% more (p<0.05) than untreated plots in all trials. In most cases, there were no significant differences among yields of fungicide-treated plots (Tables 2-4). The effectiveness of the fungicides in suppressing previous infections was examined in trials 4 and 5, in which sprayings were initiated in mid-July. Significant differences in final defoliation between treated and untreated plots were observed only for the systemic fungicides. Application o f maneb was not effective. A U D P C values were

20

~0

Figure 2. Example depicting leaf shedding induced by Alternaria macrospora in Pima cotton. About 40% of leaves were shed at the lower level of the canopy (>40cm), 20% at the middle level (41-80 cm) and none at the upper level ( > 81 cm). Dots on ~stems and branches represent the sites of abscised leaves. Flowers and bolls are not indicated for the sake of simplicity. Canopy levels are indicated by arrows.

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S u p p r e s s i o n of A l t e r n a r i a leaf spot: D. Shtienberg and J. Dreishpoun

Table2. Etfects of fungicides on Alternaria leaf spot and on yields of Pima cotton in trial 1 conducted in 1989 No. of lesions Fungicide and application rate (kg a.i. h a - t)

per 10 m row (17 July)

Difenoconazole, 0.075 Difenoconazole, 0.125 Maneb, 2.00 Untreated

3.9 b b 2.1 b 3.1 b 10.0a

per plant top (31 July)

Disease index" (30 August)

4.7 bc 2.4c 6.1 b 15.0a

!.5 1.1 2.0 3.8

Yield (t h a - ~) 5.07 ab 5.63 b 4.69a 4.41 a

"According to a scale of leaf shedding where 0 = no leaf shedding and 5 = 100% leaf shedding (Bashi et al., 1983); bnumbers followed by different letters differ significantly fp <0.05) according to the Ftest

Table 3. Effects of fungicides and application rates on Alternaria leaf spot and on yields of Pima cotton in two field trials conducted in 1990

Trial number 2

Fungicide and application rate (kg a.i. h a - l)

Final defoliation"

AUDPC h

Yield (t h a - t)

Tebuconazole, 0.125 Tebuconazole, 0.187 Maneb, 2.00 Untreated

56.7 c c 42.0 c 76.3 b 97.0 a

9.2 bc 6.7 c 12.4 b 19.6 a

4.17 4.59 4.16 3.30

ab b ab a

Difenoconazole, 0.125 Difenoconazole, 0.187 Tebuconazole, 0.125 Tebuconazole, 0_187 Tebuconazole, 0.250 Maneb, 2,00 Untreated

75.2 55.1 83.3 74.4 71.2 71.2 98.0

10.9 bc 8.4 d 12.8 b 11.1 be 9.7 cd 9.9 cd 25.3 a

5.92 5.87 5.48 5.76 6.12 5.80 4.98

b b ab b b b a

b c b b b b a

"Defoliation was assessed in mid-September as described in the text; harea under the defoliation progress curve (AUDPC units are proportion days); Cnumbersfollowed by different letters differ significantly (p <0.05) according to the F test.

Table 4. Effects of fungicides and the date of spraying initiation on Alternaria leaf spot and on yields of Pima cotton in two field trials conducted in 1990

Trial number

Fungicide" and date of spraying initiation

Final defoliation r'

AUDPC"

Yield (t h a - ~)

4

Difenoconazole, 4 June Difenoconazole, 16 July Maneb, 4 June Maneb, 16 July Untreated

33.5 56.2 79.1 88.5 96.6

da c b a a

6.0 e 9.5 d 12.1 c 14.9 b 21.0 a

6.71 6.53 6.13 5.88 5.28

c be abc ab a

5

Tebuconazole, 6 June Tebuconazole, 17 July Maneb, 6 June Maneb, 17 July Untreated

50.0 59.2 58.7 66.8 87.3

b b b ab a

8.0 d 12.5 c 10.2 d 17.1 b 20.8 a

5.13 4.47 4:49 4.05 3.79

b ab ab a a

~Difenoconazole was applied at a rate of 0.125 kg a.i. ha - ', tebuconazole at 0.250 kg a .i. ha - i and maneb at 2.00 kg a.i. ha - t; *defoliation was assessed in mid-September as described in the text; earea under the defoliation progress curve (AUDPC units are proportion days); anumbers followed by different letters differ significantly (p<0.05) according to the F test

significantly decreased by all three fungicides, but these effects were less pronounced than if sprayings were initiated in early June. Yield increments relative to untreated plots were significant in plots treated with the systemic fungicides on both initiation dates, with the exception of thelate initiation of tebuconazole (Table 4).

Discussion A basic requirement in the investigation of host-parasite relationships is that disease assessments should consider the effect of pathogen and disease on the host. Expression of disease intensity in terms of the proportion of lesioned foliage is misleading in the case of Alternaria leaf spot in

cotton, because of the tendency of infected leaves to shed and their replacement by new leaves (Bashi et aL, 1983). Assessment of defoliation by means of an arbitrary rating scale (0-5), as carried out here in trial 1, was subjective and yielded inaccurate results. However, calculation of defoliation as the snm of disease severity in attached leaves and the proportion of shed leaves, as proposed here, seems to reflect more accurately the effects of disease. This conclusion is supported by the fact that within each trial the yields and the corresponding AUDPC values were highly correlated (D. Shtienberg, unpublished data). This method is also less subjective than the one based on the arbitrary rating scale. In 1990, disease was assessed independently by several persons in each experimental plot, with only small variations obtained in defoliation scoring.

Suppression of Alternaria leaf spot: D. Shtienberg and J. Dreishpoun

Over the recent period of low overall cotton prices, the high-quality long-staple Pima cotton has become increasingly favoured by growers because of its relatively high value per acre (Cotty, 1987). Consequently, 30-70% of Israel's cotton-growing area was planted with Pima cotton in 1988-1990, compared with an area of 5-10% in the early 1980s. At ,the same time, there was a corresponding increase in' importance of Alternaria leaf spot as a yieldrestricting agent in Pima crops, and it became common practice to apply fungicides to suppress the disease and minimize yield reductions. Our study supports observations by cotton growers (J. Dreishpoun, unpublished data) that protectant fungicides are not adequately effective in suppressing Alternaria leaf spot if applied according to the current guidelines. The effectiveness of maneb was only moderate in crops irrigated by top sprinkler (trials I and 2), as well as in crops irrigated by a trickle system (trials 3-5). Moreover, yield increments relative to untreated crops in plots sprayed with maneb were insignificant in most trials. On the other hand, the effects ofdifenoconazole (at a rate of 0.125 kg a.i. ha- t) and tebuconazole (at 0.187 kg a.i. ha- l) on A. macrospora and on yields were significant in all trials (Tables 2-4). The different results obtained for the two types of fungicides may be attributable not only to differences in the effectiveness of the compounds themselves but also to the more rapid weathering and loss of the protectant fungicides than of the systemic fungicides. In commercial cropping, spraying programmes are often initiated after disease has already become established. As demonstrated in Table 4, the suppression ofdisease in such situations, although less effective, was still adequate if difenoconazole or tebuconazole was applied. Maneb, however, was not effective in suppressing such epidemics. Differences in the effects of systemic and protectant fungicides have been described also for other host-parasite systems (e.g. by Fry, Bruck and Mundt, 1979). A number of factors may induce Pima growers to apply difenoconazole and tebuconazole rather than protectant fungicides for suppression of Alternaria leaf spot. These systemic fungicides may be applied at 14-day intervals, whereas the recommended interval for the protectants is 7 days. The dosage of the active ingredient in the systemic fungicides is 6-9% of an equivalently effective dosage of maneb; thus, use of the former is associated with less contamination of the environment. The systemic fungicides, unlike the protectant, also exhibit some curative properties. Finally, relative to untreated plots the yield increments in plots treated with difenoconazole and tebuconazole were significant in all trials. Despite these promising results, growers are obliged to await the completion of registration procedures before using these products commercially in cotton fields. The existence of systemic fungicides that are highly effective against A. macrospora is of great interest, because only protectants are currently registered in Israel for the

385

suppression of Alternaria species in all crops. However, caution should be exercised in the use of the systemic fungicides. Difenoconazole and tebuconazole inhibit the C t4 demethylation of lanosterol, one of the steps in the pathway of sterol biosynthesis of the fungus. Thus, one cannot ignore the possibility that resistance to these compounds could emerge. Such an eventuality may be minimized by adoption of one of the following strategies: (I) manufacture of the systemic compounds in a mixture with a broad-spectrum protectant; (2) alternately applying fungicides with different modes of action; or (3) limiting the number of applications of the systemic compounds at risk. As the last strategy is also the least hazardous to the environment and may have added advantages of reducing inputs in cotton cropping, experiments focusing on this alternative are currently in progress.

Acknowledgements This research was supported in part by the Valtzi-Pikovski foundation, by the Israeli Cotton Council, by Lidor Ltd and by C. T. Z. Ltd. The authors gratefully acknowledge the assistance of R. Epshtein and Y. Baum ofC. T. Z. Ltd and A. Yaniv of Lidor Ltd and are grateful to the members of the Kibbutzim for providing space for the field trials.

References Bashi, E., Sachs, Y. and Rotem, J. (1983) Relationships between disease and yield in cotton fields affected by Alternaria macrospora. Phytoparasitical 1, 89-97 Cotty, P. J. (1987) Evaluation of cotton cultivars susceptibility to Alternaria leaf spot. Plant Dis. 71, 1082-1084 Ellis, M. B. and llolliday, B..P. (1970) Alternaria macrospora. In: Description of Pathogenic Fungi. No. 246. Commonwealth Mycological Institute, Kew, Surrey, England Ephrath, J. E., Shticnberg, D., Drieshpoun, J., Dinoor, A. and Mal'ani, A. (1989) Ahernaria ahernata in cotton (Gossyphmt hisrsutum) cv. Akala: effects on gas exchange, yield components and yield accumulation. lVeth. J. Plant PathoL 95, 157-166 Fry, W. E., Bruek, R. l. and Mundt, C. C. (1979) Retardation of potato late blight epidemics by fungicides with eradicant and protectant properties. Plant Dis. Repr, 63, 970-974 Rotcm, J., Eidt, J., Went, U. and Kranz, J. (1988) Relative effects of Alternaria alternata and A. macrospora on cotton crops in Israel. Plant PathoL 37, 16-19 Shaner, G. and Finney, R. E. (1977) The effect of nitrogen fertilization on the expression of slow-mildewing resistance in Knox wheat. Phytopathology 67, 1051-1056 Shticnbcrg, D., Bergeron, S. N., Nicholson, A. G., Fry, W. E. and E~ing, E. E. (1990) Development and evaluation of a general model for yield loss assessment in potatoes. Phytopathology 80, 466-472 Received 10 December 1990 Revised 29 April 1991 Accepted 29 April 1991