- Email: [email protected]
Components of Septoria nodorum infection in winter wheat: Lesion number and lesion size
Trans. Br. mycol. Soc. 73 (1) 57-63 (1919)
Printed in Great Britain
COMPONENTS OF SEPTORIA NODORUM INFECTION IN WINTER WHEAT: LESION NUMBER AND LESION SIZE By ELIZABETH A. BAKER*
AND
1. M. SMITH*
Department of Botany, Imperial College, London S.W.7
Plots of three winter wheat cultivars differing in reaction to Septaria nodorum Berk. (resistant, intermediate and susceptible) were inoculated with this pathogen at G.S. 10. Increases in infection, numbers of lesions initiated and the proportion of these exceeding 3 rom in length were observed on the flag and second leaves. Temperatures within the crop canopy, leaf surface wetness and prevailing temperatures, humidity and rainfall were recorded. The resistant cultivar showed little increase in lesion number and size, while the intermediate and susceptible ones showed steep rises in lesion number at end of season. Second leaves, in particular, of the susceptible cultivar showed a marked increase in lesion number 5 weeks before the other two cultivars. The more susceptible cultivars differed much more with respect to lesion number than lesion size, although both differed significantly from the resistant variety in the latter parameter. The canopy of the most susceptible cultivar tended to be uniformly cooled and denser. There have been a number of investigations into field infection of wheat by Septaria nodorum Berk. (Bronniman, 1968; Cooke & Fozzard, 1973; Cooke & Jones, 1971; Jones & Odebunmi, 1971; Scott, 1973; Sharp, Bronniman & McNeal 1972; Williams & Jones, 1972). In these studies, infection was assessed on the basis of the total percentage infection by S. nodorum of the leaf area, and was based on only one to four assessments during the season. In the investigation presented in this paper, an attempt was made to look at the components of infection: lesion size and lesion number, in wheat cultivars resistant and susceptible to the pathogen, in order to assess the relative importance of the number of initial infections and their subsequent increase in size throughout the season. MATERIALS AND METHODS
Experimental design
The winter wheat cultivars grown, Maris Huntsman, Cappelle Desprez and Maris Ranger, were chosen because of their reaction in detached leaf tests, the former, on inoculation with S. nodorum, giving a resistant limited necrotic reaction, Maris Ranger giving a spreading necrotic and chlorotic lesion, and Cappelle Desprez being intermediate, giving a spreading, chlorotic lesion with less necrosis (Baker, 1975). Seed was sown in 17 ern spaced rows, in early Oct. 1974, in a trial at Silwood Park Field Station, Ascot, Berks. The design was a split plot, the three main varietal plots being divided into four 0001-1536/19/2828-5020
$01.00
replicate 3'6 x 3'6 m square subplots. All subplots were separated from each other by 60 em guard rows of winter oats, variety Peniarth. Two clumps of five plants were randomly chosen within each subplot. One clump was artificially inoculated (as below) and one was an uninoculated control. Spore production and inoculation
A 1 X 106 spores/ml inoculum suspension was prepared by flooding sporulating 7- to ro-day-old oatmeal agar cultures of S. nodorum (obtained from the Plant Breeding Institute, Cambridge; S20/72) with sterile distilled water. Cultures were maintained in an illuminated Gallenkamp IH280 incubator at 20°C. The selected clumps of five plants at G.S. 10 were spray-inoculated with an atomiser sprayer until run-off, and then covered with a polythene bag for a period of 96 h. The control clumps were sprayed with distilled water before bagging. Parameters recorded
Just before, and at weekly intervals after inoculation up to complete senescence of the canopy, the following assessments were made on the flag and second leaves: Length and breadth (at the midpoint) of the leaf, until no further increase. Proportion of leaf area affected by S. nodorum, according to the keys of Bronniman (1968) is referred to as 'percentage infection'. * Present address: EPPO, 1, rue le Notre, 75016 Paris, France.
© 1979The British Mycological Society
50
(c)
R 40
~
C
30
c
:§ OJ
~
.s
20
10
0
E
RIC
(d l
0-4 N
0·3
OJ
~ 0·2
s 0· 1 "
....i
H
0 I J une R,H
C
1 Jul y Inoculatio n da te s
Fig.
1. Increase in percent infection and lesion number for control (a, b) and Sep taria nodorum inoculated (c, d) flag leaves of winter wheat cultivars Maris Huntsman (H), Maris Ranger (R) and Cappelle Desprez (C).
Elizabeth A. Baker and I. M. Smith Total numbers of lesions caused by S. nodorum. Number of lesions longer than 3 rom. Coalesced lesions were scored, as far as possible, as the number of original lesions contributing. An approximation of the leaf area was made from the product of the length and breadth measurements, and then the number of lesions per unit area calculated . Temperatures at three levels within the crop canopy and in the open air were recorded hourly for certain periods throughout the season, using a Grant Miniature Temperature recorder, Model D, fitted with type B thermistor probes (ground level, midway between the second and third leaves, level with the flag leaf). A leaf surface-wetness recorder (Meteorological Office) and a thermohygrograph (Cassella, London) were also placed adjacent to the experimental plots. RESULTS
Leaf infection It can be seen (Figs. 1, z s, b) that there was some natural infection of the control plants, possibly due to seed-borne contamination or, more probably, to spread from infected plants within the subplots. With the exception of one anomalously high percentage infection value for cv. Maris Ranger second leaves, the control plants showed a slower and later development of infection than the inoculated plants. The relative behaviour of the cultivars was similar in control and inoculated treatments and further comparisons will be based essentially on the results obtained with inoculated plants . The cultivars differed very significantly (P < 0'01) in percentage infection , with cv. Maris Ranger always showing most infection and cv. M aris Huntsman least. This was in good agreement with the results of laboratory tests on detached leaves. Second leaves were, on average, significantly more infected than flag leaves (P < o·OS). Lesion number The number of lesions per em" on flag leaves and second leaves of the three varieties is shown in Figs . 1 and 2. There was a highly significant difference between the cultivars (P < 0-01), with cv. Maris Ranger always having more lesions, especially on the second leaf, and also earlier in the season. The cv. Cappelle Desprez showed a slower, but steady, increase in lesion number through the season, while cv. Maris Huntsman showed very little increase, especially in the case of flag leaves. It was thus clear that a large part of the difference in percentage infection between the
59
cultivars could be attributed to this difference in lesion number. A similar analysis was made for total lesion number, unrelated to leaf area, and the same pattern obtained. On all cultivars, second leaves had significantly more lesions than flag leaves (P < o-oy).
Lesion size The estimate of lesion size used was the percentage of lesions longer than 3 rom (Table 1). The cultivars also differed highly significantly in this respect (P < 0 '01). The cv. Maris Huntsman had few or no such lesions until quite late in the season, and then showed a slow increase to less than half of the lesions longer than 3 rom at the end of the season. Maris Ranger and Cappelle Desprez cvs. both had many more large lesions than cv. Maris Huntsman but differed relatively little from each other through most of the season (40-60 % except at the very beginning and end). Flag leaves and second leaves did not differ significantly in this index of lesion size. Prevailing meteorological conditions, 1975 The season was generally rather dry, with little rain between mid-May and mid-July. Temperatures in May were relatively low, the minimum usually below 7° and the maximum not exceeding 20°, while in June and July the maximum temperatures were mainly around 20° and the minimum not often below 10°. Maximum humidity remained fairly constant around 90 %, rather higher in May and July, but very low in June, around 30-S0 %. In general, the increase in lesion number seen to occur at the end of the season (Figs. 1, 2) in mid-July coincided with a z-week period with some hours of leaf surface-wemess and with relatively high minimum temperatures. Increase in the infection indices was relatively slow in June, when only short periods of leaf surface-wetness occurred and the relative humidity was low. Nevertheless, cv. Maris Ranger showed a substantial increase in lesion number on second leaves during this period, under conditions far from optimal for S. nodorum. Microclimate of the crop canopy The mean hourly day-time temperatures were recorded for a week early (29 May-S June) and late (2S August-e August) in the season. The data for the earlier week are presented in Fig. 3. There was apparently less variation in temperature within the canopy of the susceptible cv. Maris
60
Septoria infection of winter wheat Table
1,
Percentage of S, nodorum lesions exceeding 3 mm in length on inoculated wheat plants Cultivar Time*
Leaf
Maris Ranger
Cappelle Desprez
Maris Huntsman
0 39'0 42,6 46'3 59'5 83'8
0 20'6 59'2 57"7 38'6 45'0
0 0 25'0 31'0 36'6 45'0
4'0 46-6 33'8 57'4
10'6 54'3 49'1 43'3 63'6
0 0 0 29'3 38'0 41'3
Flag -3 3 10 17 24 31
Second
-3 3 10 17 24 31
78'2
* Days after inoculation.
Ranger, and temperatures were slightly lower in the canopy than for the resistant cv. Maris Huntsman. The cv. Cappelle Desprez was intermediate. The difference in temperature was most apparent in the mornings around 11.00, when there was about 4 0 difference between the ground and flag leaf level in the cv. Maris Huntsman canopy, but only about 1 0 for cv. Cappelle Desprez, and less than 1 0 for cv. Maris Ranger. Later in the season, when the canopy was senescing, the difference was diminished, there being less than 1 0 variation within all the canopies. DISCUSSION
Some inaccuracies in the measurement of parameters in this study were unavoidable. Lesions coalesced, affecting estimates of lesion size and number; necrotic flecks were not always due to S. nodorum infection, affecting estimates of lesion number. However, highly significant differences were observed between cultivars and these inaccuracies may be regarded therefore as of relatively little importance. The ranking order of the cultivars in detached leaf tests (Baker, 1975) was maintained with respect to percentage infection in the field, although symptoms were slightly different. Thus, cv. Maris Huntsman, which showed least infection in both cases, gave some chlorosis and lesion spread in the field but never in the laboratory. The cv. Maris Ranger was very susceptible in the field and differed rather more from cv. Cappelle Desprez than in the laboratory.
The attempt to partmon the differences in percentage infection into two components - lesion size and lesion number - showed that both were involved. The cv, Maris Huntsman had far fewer lesions than the other two cultivars and showed very little increase in lesion number with time. It did, however, show a slow increase in lesion size. In other words, the lesions established by the inoculation slowly spread but few new lesions were initiated. Further work would be needed to show whether this arose from a difference in pycnidial formation, sporulation, spore dispersal or penetration, possibly also affected by microclimate. The cv. Maris Ranger had many more lesions per em", earlier in the season, than cv, Cappelle Desprez, especially on second leaves. These cultivars did not, however, greatly differ in the proportion of lesions longer than 3 mm. The cv. Maris Ranger's greater susceptibility thus seemed to arise largely from having more lesions rather than bigger lesions. Further research would again be needed to establish the cause of this apparent readier dissemination, although the results discussed below on microclimatic effects are suggestive. It was also possible to use the data to compare the development of infection on flag leaves and second leaves. Second leaves had more, but not larger, lesions. Since both were fully expanded at the time of inoculation, this difference may again be due to microclimatic effects later in the season. In relating the development of infection to the prevailing meteorological conditions, the optimum provisions for the fungus should be considered.
Elizabeth A. Baker and I . M. Smith R
60
(a)
so 40
~ c
.s <>
30
~
.s 20
10
0 0·4 ~E
(b )
0·3
..2. r= 0·2 .~
....." 0· 1 0
I J une
30
~ c s
1 Jul y
(c )
R
20
<>
~
.s
c
10
0 0·6
(d)
R
0'5 ~E
0·4
..2. r= 0·3 .~
....,'!
C
0·2 0·1 0
1 J une R,H
C
I Ju ly Inoculation dates
Fig. 2. Increase in percent infection and lesion number for control (a, b) and Septorianodorum inoculated (c, d) second leaves of winter wheat cultivars Maris Huntsman (H), Maris Ranger (R) and Cappelle Desprez (C).
61
Septoria infection of winter wheat
62
Cappello Desprez
Maris Ranger
20
18
P
16
12
10 0900
1300
1700
0900
1300
1700
Time
Time
Maris Huntsman
Open air
20
18
G
o
~
16
E
e '"
0.
E 14 ~
12
10 0900
1300
1700
Time
0900
1700
1300 Time
Fig. 3. Temperatures at three levels within the canopy (.-., 2 em above ground level; .--e, midway; A-A, near top) of wheat cultivarsMaris Ranger (a), CappelleDesprez (b), Maris Huntsman (c) and in the open air (t!). These may be taken as those conditions in which the length of the latent period (the time from infection to pycnidiospore release) is minimized. Investigations (Scharen, 1964; Shearer & Zadoks, 1972, 1974; Holmes & Colhoun, 1974) have shown that both temperature and wetness need to be considered. A period of 6 wet days at 20-22° was sufficient for pycnidial formation in second and
third leaves of the winter wheat cv. Felix, while a decrease in the duration of leaf surface-wetness of 12 h (from 17 to 5 h) prolonged the latent period by 7 days (Shearer & Zadoks, 1972, 1974). Although infection will occur in dry conditions, no pyenidia will form. Also, the minimum temperature is of importance, 7° being a threshold (for Felix) below which pycnidia do not form.
Elizabeth A. Baker and 1. M. Smith The period of time for the establishment of a successful infection varies with cultivars, tending to be shorter for the more susceptible varieties (Bronniman, Sally & Sharp, 1972) . In the present experiment, the presence of polythene bags for 96 h following inoculation provided ample time for the spores to infect even the resistant variety. The prevailing weather conditions following removal of the bags must have affected the development of infection, especially with respect to lesion number rather than lesion size . However, the conditions within the crop may have been buffered to some extent by the crop canopy. The more uniform microclimate of cv. Maris Ranger may be a reflexion of canopy structure since this variety had rather larger leaves (average leaf area == 32 em "), forming a denser canopy, than cv, Maris Huntsman (average leaf area == 26 em "). The more uniform, slightly cooler and probably more humid canopy of cv. Maris Ranger may have contributed to the greater increases in lesion number and lesion size on this variety. However, this effect could partly account only for the difference between varieties, since differences in the rate of lesion spread were clearly seen on detached leaves under standard laboratory conditions (Baker, 1975). The results given here are for the 1974/5 season, when there were no periods of continuous leaf surface-wetness and minimum temperatures early in the season were below 7°. Conditions were much drier than in 1973/4 and amounts of infection correspondingly lower than in that season (Baker, 1975). However, the percentage increase in size of existing lesions was greater. This may be a result of the inability of the fungus to produce pycnidia in dry conditions, and consequent tendency for increased mycelial growth. REFERENCES
BAKER, E. A. (1975). Resistance of wheat to S eptoria nodorum Berk. Ph.D. Thesis, London Uni versity. BRONNIMAN, A. (1968). Zur Kenntnis von Septaria nodorum Berk., dem Erreger der Spelzenbraune und der Blattdure des Weizens. Phytopathologische Zeitschrift 61, 101-146.
BRONNIMAN, A., SALLY, B. K. & SHARP, E. L. (1972). In vestigations on Septaria nodorum in spring wheat in Montana. Plant Di sease Reporter 56, 188-191. COOKE, B. M. & FOZZARD, J. T. F. (1973). Development, assessment and seed transmission of Septaria nodorum, Transactions of the British Mycolog ical Society 60, 211-222. COOKE, B. M. & JONES, D. G. (1971). The epidemiology of S eptoria tritici and Septaria nodorum, III. The
reaction of spring and winter wheat varieties to infection by Septaria. tritici and Septaria nodorum. Transactions of the Br itish Mycological Society 56, 121-135 · HOLMES, S. J. I. & COLHOUN, J. (1974). Infection of wheat by Septoria nodorum and Septaria tritici in relation to plant age, air temperature and relative humidity. Transactions of the British Mycological Soci ety 63, 329-338. JONES, D. G. & ODEBUNMI, K. (1971). The epidemiology of Septoria tritici and Septaria nodorum, IV. The effect of inoculation at different growth stages and on different plant parts. Transactions of the British Mycological Society 56, 281-288. SCHAREN, A. L. (1964). Environmental influences on development of glume blotch in wheat. Phytopathology 54, 300-303. SCOTT, P. R. (1973). Incidence and effects of Septoria nodorum on wheat cultivars. Annals of Applied Biology 75, 321-329. SHARP, E. L., BRONNIMAN, A. & McNEAL, F. H. (1972). Reaction of selected spring wheat varieties to infection by Septoria nodorum, Plant Disease Reporter 56, 761-764. SHEARER, B. L. & ZADOKS, J. C. (1972). The latent period of Septoria nodorum in wheat. I. The effect of temperature and moisture treatments under controlled conditions. Netherlands Journal of Plant Pathology 78, 231-241. SHEARER, B. I. & ZADOKS, J. C. (1974). The latent period of Septora nodorum in wheat. II . The effect of temperature and moisture under field conditions. Ne therlands Journal of Plant Pathology 80,48-60. WILLIAMS, J. R. & JONES, D. G. (1972). The epidemiology of Sep taria tritici and Septaria nodorum, VI. Effect of time of initial infection on disease development and grain yield in spring wheats. Tran sactions of the British Mycological Society 59, 273-283.
(Accepted for publication 6 October 1978)
Printed in Great Britain
COMPONENTS OF SEPTORIA NODORUM INFECTION IN WINTER WHEAT: LESION NUMBER AND LESION SIZE By ELIZABETH A. BAKER*
AND
1. M. SMITH*
Department of Botany, Imperial College, London S.W.7
Plots of three winter wheat cultivars differing in reaction to Septaria nodorum Berk. (resistant, intermediate and susceptible) were inoculated with this pathogen at G.S. 10. Increases in infection, numbers of lesions initiated and the proportion of these exceeding 3 rom in length were observed on the flag and second leaves. Temperatures within the crop canopy, leaf surface wetness and prevailing temperatures, humidity and rainfall were recorded. The resistant cultivar showed little increase in lesion number and size, while the intermediate and susceptible ones showed steep rises in lesion number at end of season. Second leaves, in particular, of the susceptible cultivar showed a marked increase in lesion number 5 weeks before the other two cultivars. The more susceptible cultivars differed much more with respect to lesion number than lesion size, although both differed significantly from the resistant variety in the latter parameter. The canopy of the most susceptible cultivar tended to be uniformly cooled and denser. There have been a number of investigations into field infection of wheat by Septaria nodorum Berk. (Bronniman, 1968; Cooke & Fozzard, 1973; Cooke & Jones, 1971; Jones & Odebunmi, 1971; Scott, 1973; Sharp, Bronniman & McNeal 1972; Williams & Jones, 1972). In these studies, infection was assessed on the basis of the total percentage infection by S. nodorum of the leaf area, and was based on only one to four assessments during the season. In the investigation presented in this paper, an attempt was made to look at the components of infection: lesion size and lesion number, in wheat cultivars resistant and susceptible to the pathogen, in order to assess the relative importance of the number of initial infections and their subsequent increase in size throughout the season. MATERIALS AND METHODS
Experimental design
The winter wheat cultivars grown, Maris Huntsman, Cappelle Desprez and Maris Ranger, were chosen because of their reaction in detached leaf tests, the former, on inoculation with S. nodorum, giving a resistant limited necrotic reaction, Maris Ranger giving a spreading necrotic and chlorotic lesion, and Cappelle Desprez being intermediate, giving a spreading, chlorotic lesion with less necrosis (Baker, 1975). Seed was sown in 17 ern spaced rows, in early Oct. 1974, in a trial at Silwood Park Field Station, Ascot, Berks. The design was a split plot, the three main varietal plots being divided into four 0001-1536/19/2828-5020
$01.00
replicate 3'6 x 3'6 m square subplots. All subplots were separated from each other by 60 em guard rows of winter oats, variety Peniarth. Two clumps of five plants were randomly chosen within each subplot. One clump was artificially inoculated (as below) and one was an uninoculated control. Spore production and inoculation
A 1 X 106 spores/ml inoculum suspension was prepared by flooding sporulating 7- to ro-day-old oatmeal agar cultures of S. nodorum (obtained from the Plant Breeding Institute, Cambridge; S20/72) with sterile distilled water. Cultures were maintained in an illuminated Gallenkamp IH280 incubator at 20°C. The selected clumps of five plants at G.S. 10 were spray-inoculated with an atomiser sprayer until run-off, and then covered with a polythene bag for a period of 96 h. The control clumps were sprayed with distilled water before bagging. Parameters recorded
Just before, and at weekly intervals after inoculation up to complete senescence of the canopy, the following assessments were made on the flag and second leaves: Length and breadth (at the midpoint) of the leaf, until no further increase. Proportion of leaf area affected by S. nodorum, according to the keys of Bronniman (1968) is referred to as 'percentage infection'. * Present address: EPPO, 1, rue le Notre, 75016 Paris, France.
© 1979The British Mycological Society
50
(c)
R 40
~
C
30
c
:§ OJ
~
.s
20
10
0
E
RIC
(d l
0-4 N
0·3
OJ
~ 0·2
s 0· 1 "
....i
H
0 I J une R,H
C
1 Jul y Inoculatio n da te s
Fig.
1. Increase in percent infection and lesion number for control (a, b) and Sep taria nodorum inoculated (c, d) flag leaves of winter wheat cultivars Maris Huntsman (H), Maris Ranger (R) and Cappelle Desprez (C).
Elizabeth A. Baker and I. M. Smith Total numbers of lesions caused by S. nodorum. Number of lesions longer than 3 rom. Coalesced lesions were scored, as far as possible, as the number of original lesions contributing. An approximation of the leaf area was made from the product of the length and breadth measurements, and then the number of lesions per unit area calculated . Temperatures at three levels within the crop canopy and in the open air were recorded hourly for certain periods throughout the season, using a Grant Miniature Temperature recorder, Model D, fitted with type B thermistor probes (ground level, midway between the second and third leaves, level with the flag leaf). A leaf surface-wetness recorder (Meteorological Office) and a thermohygrograph (Cassella, London) were also placed adjacent to the experimental plots. RESULTS
Leaf infection It can be seen (Figs. 1, z s, b) that there was some natural infection of the control plants, possibly due to seed-borne contamination or, more probably, to spread from infected plants within the subplots. With the exception of one anomalously high percentage infection value for cv. Maris Ranger second leaves, the control plants showed a slower and later development of infection than the inoculated plants. The relative behaviour of the cultivars was similar in control and inoculated treatments and further comparisons will be based essentially on the results obtained with inoculated plants . The cultivars differed very significantly (P < 0'01) in percentage infection , with cv. Maris Ranger always showing most infection and cv. M aris Huntsman least. This was in good agreement with the results of laboratory tests on detached leaves. Second leaves were, on average, significantly more infected than flag leaves (P < o·OS). Lesion number The number of lesions per em" on flag leaves and second leaves of the three varieties is shown in Figs . 1 and 2. There was a highly significant difference between the cultivars (P < 0-01), with cv. Maris Ranger always having more lesions, especially on the second leaf, and also earlier in the season. The cv. Cappelle Desprez showed a slower, but steady, increase in lesion number through the season, while cv. Maris Huntsman showed very little increase, especially in the case of flag leaves. It was thus clear that a large part of the difference in percentage infection between the
59
cultivars could be attributed to this difference in lesion number. A similar analysis was made for total lesion number, unrelated to leaf area, and the same pattern obtained. On all cultivars, second leaves had significantly more lesions than flag leaves (P < o-oy).
Lesion size The estimate of lesion size used was the percentage of lesions longer than 3 rom (Table 1). The cultivars also differed highly significantly in this respect (P < 0 '01). The cv. Maris Huntsman had few or no such lesions until quite late in the season, and then showed a slow increase to less than half of the lesions longer than 3 rom at the end of the season. Maris Ranger and Cappelle Desprez cvs. both had many more large lesions than cv. Maris Huntsman but differed relatively little from each other through most of the season (40-60 % except at the very beginning and end). Flag leaves and second leaves did not differ significantly in this index of lesion size. Prevailing meteorological conditions, 1975 The season was generally rather dry, with little rain between mid-May and mid-July. Temperatures in May were relatively low, the minimum usually below 7° and the maximum not exceeding 20°, while in June and July the maximum temperatures were mainly around 20° and the minimum not often below 10°. Maximum humidity remained fairly constant around 90 %, rather higher in May and July, but very low in June, around 30-S0 %. In general, the increase in lesion number seen to occur at the end of the season (Figs. 1, 2) in mid-July coincided with a z-week period with some hours of leaf surface-wemess and with relatively high minimum temperatures. Increase in the infection indices was relatively slow in June, when only short periods of leaf surface-wetness occurred and the relative humidity was low. Nevertheless, cv. Maris Ranger showed a substantial increase in lesion number on second leaves during this period, under conditions far from optimal for S. nodorum. Microclimate of the crop canopy The mean hourly day-time temperatures were recorded for a week early (29 May-S June) and late (2S August-e August) in the season. The data for the earlier week are presented in Fig. 3. There was apparently less variation in temperature within the canopy of the susceptible cv. Maris
60
Septoria infection of winter wheat Table
1,
Percentage of S, nodorum lesions exceeding 3 mm in length on inoculated wheat plants Cultivar Time*
Leaf
Maris Ranger
Cappelle Desprez
Maris Huntsman
0 39'0 42,6 46'3 59'5 83'8
0 20'6 59'2 57"7 38'6 45'0
0 0 25'0 31'0 36'6 45'0
4'0 46-6 33'8 57'4
10'6 54'3 49'1 43'3 63'6
0 0 0 29'3 38'0 41'3
Flag -3 3 10 17 24 31
Second
-3 3 10 17 24 31
78'2
* Days after inoculation.
Ranger, and temperatures were slightly lower in the canopy than for the resistant cv. Maris Huntsman. The cv. Cappelle Desprez was intermediate. The difference in temperature was most apparent in the mornings around 11.00, when there was about 4 0 difference between the ground and flag leaf level in the cv. Maris Huntsman canopy, but only about 1 0 for cv. Cappelle Desprez, and less than 1 0 for cv. Maris Ranger. Later in the season, when the canopy was senescing, the difference was diminished, there being less than 1 0 variation within all the canopies. DISCUSSION
Some inaccuracies in the measurement of parameters in this study were unavoidable. Lesions coalesced, affecting estimates of lesion size and number; necrotic flecks were not always due to S. nodorum infection, affecting estimates of lesion number. However, highly significant differences were observed between cultivars and these inaccuracies may be regarded therefore as of relatively little importance. The ranking order of the cultivars in detached leaf tests (Baker, 1975) was maintained with respect to percentage infection in the field, although symptoms were slightly different. Thus, cv. Maris Huntsman, which showed least infection in both cases, gave some chlorosis and lesion spread in the field but never in the laboratory. The cv. Maris Ranger was very susceptible in the field and differed rather more from cv. Cappelle Desprez than in the laboratory.
The attempt to partmon the differences in percentage infection into two components - lesion size and lesion number - showed that both were involved. The cv, Maris Huntsman had far fewer lesions than the other two cultivars and showed very little increase in lesion number with time. It did, however, show a slow increase in lesion size. In other words, the lesions established by the inoculation slowly spread but few new lesions were initiated. Further work would be needed to show whether this arose from a difference in pycnidial formation, sporulation, spore dispersal or penetration, possibly also affected by microclimate. The cv. Maris Ranger had many more lesions per em", earlier in the season, than cv, Cappelle Desprez, especially on second leaves. These cultivars did not, however, greatly differ in the proportion of lesions longer than 3 mm. The cv. Maris Ranger's greater susceptibility thus seemed to arise largely from having more lesions rather than bigger lesions. Further research would again be needed to establish the cause of this apparent readier dissemination, although the results discussed below on microclimatic effects are suggestive. It was also possible to use the data to compare the development of infection on flag leaves and second leaves. Second leaves had more, but not larger, lesions. Since both were fully expanded at the time of inoculation, this difference may again be due to microclimatic effects later in the season. In relating the development of infection to the prevailing meteorological conditions, the optimum provisions for the fungus should be considered.
Elizabeth A. Baker and I . M. Smith R
60
(a)
so 40
~ c
.s <>
30
~
.s 20
10
0 0·4 ~E
(b )
0·3
..2. r= 0·2 .~
....." 0· 1 0
I J une
30
~ c s
1 Jul y
(c )
R
20
<>
~
.s
c
10
0 0·6
(d)
R
0'5 ~E
0·4
..2. r= 0·3 .~
....,'!
C
0·2 0·1 0
1 J une R,H
C
I Ju ly Inoculation dates
Fig. 2. Increase in percent infection and lesion number for control (a, b) and Septorianodorum inoculated (c, d) second leaves of winter wheat cultivars Maris Huntsman (H), Maris Ranger (R) and Cappelle Desprez (C).
61
Septoria infection of winter wheat
62
Cappello Desprez
Maris Ranger
20
18
P
16
12
10 0900
1300
1700
0900
1300
1700
Time
Time
Maris Huntsman
Open air
20
18
G
o
~
16
E
e '"
0.
E 14 ~
12
10 0900
1300
1700
Time
0900
1700
1300 Time
Fig. 3. Temperatures at three levels within the canopy (.-., 2 em above ground level; .--e, midway; A-A, near top) of wheat cultivarsMaris Ranger (a), CappelleDesprez (b), Maris Huntsman (c) and in the open air (t!). These may be taken as those conditions in which the length of the latent period (the time from infection to pycnidiospore release) is minimized. Investigations (Scharen, 1964; Shearer & Zadoks, 1972, 1974; Holmes & Colhoun, 1974) have shown that both temperature and wetness need to be considered. A period of 6 wet days at 20-22° was sufficient for pycnidial formation in second and
third leaves of the winter wheat cv. Felix, while a decrease in the duration of leaf surface-wetness of 12 h (from 17 to 5 h) prolonged the latent period by 7 days (Shearer & Zadoks, 1972, 1974). Although infection will occur in dry conditions, no pyenidia will form. Also, the minimum temperature is of importance, 7° being a threshold (for Felix) below which pycnidia do not form.
Elizabeth A. Baker and 1. M. Smith The period of time for the establishment of a successful infection varies with cultivars, tending to be shorter for the more susceptible varieties (Bronniman, Sally & Sharp, 1972) . In the present experiment, the presence of polythene bags for 96 h following inoculation provided ample time for the spores to infect even the resistant variety. The prevailing weather conditions following removal of the bags must have affected the development of infection, especially with respect to lesion number rather than lesion size . However, the conditions within the crop may have been buffered to some extent by the crop canopy. The more uniform microclimate of cv. Maris Ranger may be a reflexion of canopy structure since this variety had rather larger leaves (average leaf area == 32 em "), forming a denser canopy, than cv, Maris Huntsman (average leaf area == 26 em "). The more uniform, slightly cooler and probably more humid canopy of cv. Maris Ranger may have contributed to the greater increases in lesion number and lesion size on this variety. However, this effect could partly account only for the difference between varieties, since differences in the rate of lesion spread were clearly seen on detached leaves under standard laboratory conditions (Baker, 1975). The results given here are for the 1974/5 season, when there were no periods of continuous leaf surface-wetness and minimum temperatures early in the season were below 7°. Conditions were much drier than in 1973/4 and amounts of infection correspondingly lower than in that season (Baker, 1975). However, the percentage increase in size of existing lesions was greater. This may be a result of the inability of the fungus to produce pycnidia in dry conditions, and consequent tendency for increased mycelial growth. REFERENCES
BAKER, E. A. (1975). Resistance of wheat to S eptoria nodorum Berk. Ph.D. Thesis, London Uni versity. BRONNIMAN, A. (1968). Zur Kenntnis von Septaria nodorum Berk., dem Erreger der Spelzenbraune und der Blattdure des Weizens. Phytopathologische Zeitschrift 61, 101-146.
BRONNIMAN, A., SALLY, B. K. & SHARP, E. L. (1972). In vestigations on Septaria nodorum in spring wheat in Montana. Plant Di sease Reporter 56, 188-191. COOKE, B. M. & FOZZARD, J. T. F. (1973). Development, assessment and seed transmission of Septaria nodorum, Transactions of the British Mycolog ical Society 60, 211-222. COOKE, B. M. & JONES, D. G. (1971). The epidemiology of S eptoria tritici and Septaria nodorum, III. The
reaction of spring and winter wheat varieties to infection by Septaria. tritici and Septaria nodorum. Transactions of the Br itish Mycological Society 56, 121-135 · HOLMES, S. J. I. & COLHOUN, J. (1974). Infection of wheat by Septoria nodorum and Septaria tritici in relation to plant age, air temperature and relative humidity. Transactions of the British Mycological Soci ety 63, 329-338. JONES, D. G. & ODEBUNMI, K. (1971). The epidemiology of Septoria tritici and Septaria nodorum, IV. The effect of inoculation at different growth stages and on different plant parts. Transactions of the British Mycological Society 56, 281-288. SCHAREN, A. L. (1964). Environmental influences on development of glume blotch in wheat. Phytopathology 54, 300-303. SCOTT, P. R. (1973). Incidence and effects of Septoria nodorum on wheat cultivars. Annals of Applied Biology 75, 321-329. SHARP, E. L., BRONNIMAN, A. & McNEAL, F. H. (1972). Reaction of selected spring wheat varieties to infection by Septoria nodorum, Plant Disease Reporter 56, 761-764. SHEARER, B. L. & ZADOKS, J. C. (1972). The latent period of Septoria nodorum in wheat. I. The effect of temperature and moisture treatments under controlled conditions. Netherlands Journal of Plant Pathology 78, 231-241. SHEARER, B. I. & ZADOKS, J. C. (1974). The latent period of Septora nodorum in wheat. II . The effect of temperature and moisture under field conditions. Ne therlands Journal of Plant Pathology 80,48-60. WILLIAMS, J. R. & JONES, D. G. (1972). The epidemiology of Sep taria tritici and Septaria nodorum, VI. Effect of time of initial infection on disease development and grain yield in spring wheats. Tran sactions of the British Mycological Society 59, 273-283.
(Accepted for publication 6 October 1978)