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Disease-induced losses in winter wheat in England and Wales 1985–1989
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Disease-induced losses in winter wheat in England and Wales 1985-1989 R. J. Cook*, R. W. Polley t and M. R. Thomas t ADAS, Ministry of Agriculture, Fisheries and Food, Lawnswood, Leeds LS16 5PY, UK and tMinistry of Agriculture, Fisheries and Food, Central Science Laboratory, Hatching Green, Harpenden, Hertfordshire AL5 2BD, UK
Abstract
Keywords
Annual surveys ofwinter wheat diseases in England and Wales provide information on the relative severity of the leaf and stem-base disease complex. Septoria tritici was the most damaging disease during the 5-year period 1985-1989 causing annual losses estimated to be 0.329 Mt compared with 0.251 Mt and 0.231 Mt for eyespot and mildew, the next most damaging winter wheat diseases. Annual losses during the period averaged 1.078Mt excluding losses attributable to the fusarium diseases, for which no satisfactory yield loss relationships are available. Sowing date and length of break from cereals exert a strong influence on incidence ofeyespot and losses attributable to the disease and, on the basis of these surveys, growing all winter wheat crops after a l-year break would prevent national losses of ~ I00 kt annually. Wheat, winter; diseases; crop damage; England; Wales
Introduction ADAS surveys of cereal crops in England and Wales monitor leaf and stem-base diseases to provide an annual 'snapshot' o f disease incidence in relation to cultural practices. The winter wheat surveys are based on random samples collected at the milky ripe growth stage (GS 71-73; Tottman, 1987) from crops throughout England and Wales. Annual reports (King, 1985; Thomas, 1986, 1987, 1988, 1989) summarize disease incidence and fungicide use. The data have been used to rank individual diseases by their relative order of importance (Polley and Thomas, 1990) after application of fungicides. The surveys also provide data that can be used to estimate the influence of cultural practices on diseases. This paper considers some of these factors in relation to the economics of disease for the five years 1985-1989. During this period there was a general stability of fungicide use and cereal husbandry in England and Wales. Surveys and loss estimates
Husbandry factors were considered only where earlier scrutiny (Polley and Thomas, 1'991) had indicated significant differences in amounts of disease between factors. Disease incidence, based on the percentage of stems with moderate and severe eyespot (Pseudocercosporella herpotrichoMes) and sharp eyespot (Rhizoctonia cerealis), and percentage infection on leaf 2 [leaf 1 for rusts (Pucchlia spp.)] was used to estimate the percentage yield loss for each disease. *Present address, and address for correspondence: Morley Research Centre, Wymondharn, Norfolk, NR18 9DB, UK 0261-2194/9110610504-05 © 1991 Butterworth-Heinemann Ltd
The following basic assumptions were made concerning the data.
ADAS regions. These were defined by current regional boundaries and represent the appropriate geographical areas of England and Wales. Wales has been omitted from the regional estimates owing to the small proportion of winter wheat grown there.
Prices. These were assumed to be £105 t Yields. An estimate of the potential average ~,ield of winter wheat in England and Wales was derived from ADAS fungicide experiments. This was taken as 8.8 t h a - 1, being the yield o f plots receiving a three-spray fungicide programme (GS 3 1 + 3 9 + 5 9 ) (Cook and Thomas, 1990). This compares with an actual national average yield of -,, 6.1 t h a - 1 ( M A F F statistics). National production. The total area of wheat has been assumed to be 1.9 Mha ( M A F F statistics). Where proportions of the crop area are subject to different cultural or fungicide regimens, relative proportions have been takeh from the 1989 survey report (Thomas, 1989). Estimates ofpotential national grain production used to calculate financial values were based on the treated yield from ADAS experiments multiplied by the relevant proportion of the national crop area. It was accepted that this would provide an estimate of potential yield and, therefore, would maximize potential losses. YieMIosses. Yield losses caused by disease were estimated by using published formulae, derived from yield loss
Disease levels in winter wheat: R. J. Cook et aL
505
Table 1. Formulae used to estimate yield loss in ADAS winter wheat disease surveys a Disease
Relationship I'
Powdery mildew Septoria tritici Yellow and brown rust Eyespot Sharp eyespot
Reference
y = 3 xi y = 0.42 xii y = 0.4 xiii y=0.1 xiv+0.36 xv y = 0.05 xiv + 0.26 xv
Thomas (unpublished") Thomas, Cook and King, 1989 King, 1976 Clarkson, 1981 Clarkson and Cook, 1983
°Note: There are no satisfactory formulae by which to estimate losses caused by fusarium disease, at present; '~y,percentage loss in grain yield; xi, percentage disease on leaf2 (flag- I)~in the range 0-10%; xii, percentage disease leaf2 (flag- I), in the range 0-45*/,; xiii, percentage disease leaf I (flag); xiv and xv, percentage of stems with moderate and severe lesions respectively; ¢derived from ADAS experiments designed to study disease progress in sequential epidemics
Table2. Estimated annual national losses caused by winter wheat stem-base and leaf disease, 1985-1989
Disease
Loss (%)
Eyespot Sharp eyespot Mildew Septoria tritici S. nodorum Yellow rust Brown rust
1.5 0.6 1.4 2.0 0.9 <0.1 < 0.1
250.8 96.1 231.2 328.9 149.8 11.4 9.8
26.3 10.1 24.3 34.5 15.7 1.2 1.0
--
1078.0
113. I
Total
Loss (kilotonnes)
Cost (£ × 106)
experiments, which were assumed to be equally valid for fungicide-treated and untreated crops. The formulae used are represented in Table l. The surveys have also estimated incidence of take-all (Gaeumannomyces gramhlis) based on frequency of aboveground symptoms. It is not possible to use these assessments to provide direct loss estimates and a 6% yield reduction has been assumed for second and third wheat crops following a non-cereal break (D. J. Yarham, ADAS, Cambridge; personal communication, 1990). This agrees well with the yield difference noted by Cook and Thomas (1990) for second and successive wheat crops receiving a full three-spray fungicide programme.
Growth stages. The growth stages defined in the tables represent a range of stages identified on replies in questionnaires. GS 31 represents the period between GS 30 and 33, GS 39 represents GS 37-45 and GS 59 the period between GS 47 and GS 69. Yield loss estimates
Regional losses Loss estimates (Table3) were made for those diseases that showed significant regional variations (Polley and Thomas, 1991). Yellow rust (Puccinia striiformis) is included owing to the variations between Eastern and Northern England, where the disease was significantly more severe than in the remainder of the country in 1988. Data for S. tritici are based on the 4-year period starting in 1986, because of very low levels in South West England and a very high incidence in Eastern England in 1985, which distorted the long-term distribution pattern for this disease. In general, losses from this disease are largest in the South West, whereas brown rust (Pucchlia recondita) causes most severe damage in the warmer East and South East regions. Total financial losses attributable to disease would, however, be greatest in the Eastern region, owing to the high proportion (43%, MAFF statistics) of the national wheat crop in that area.
Sowing date Potential yield losses per hectare for both eyespot and sharp eyespot decrease as sowing date is delayed from September until November (Table4). However, the high proportion of the national crop sown in October results in higher absolute losses for October-sown crops.
Cropping sequence Eyespot is a disease of intensive cereals and survey data show a consistent positive relationship between incidence and the same previous crop type. Table5 estimates losses for this disease in relation to length of break from cereals, suggesting that substantial potential savings could be hchieved by 1-year or 2-year breaks.
Overall losses Mean annual losses due to disease total 1.1Mt (Table2) and this represents a maximum of 6.5% of current potential wheat production. The losses represent 8% of national average yields after application of fungicides which annually cost -,-£113 x 106. Eyespot, Septoria tritici and mildew (Erysiphe gramhds) were the most damaging diseases of winter wheat. Losses caused by take-all on winter wheat are difficult to estimate. However, assuming a 6% yield reduction in second and third wheats, there are additional annual losses of £43.2x l06 caused by this disease. Fusarium diseases have been omitted.
Table 3. Regional estimates of percentage yield losses for winter" wheat, 1985-1989
Region East Midlands and West South East South West North °1986-1989
Septoria triticia 0.9 0.8 0.8 1.7 0.5
Brown rust 0.l <0.1 0.1 < 0.1 0.0
Yellow Sharp rust eyespot 0.1 <0.1 <0.1 0.0 0. I
0.5 0.8 0.5 0.8 0.5
506
Disease levels in winter wheat: R. J. Cook et aL
Table 4. Influence of delayed sowing date on losses due to eyespot and sharp eyespot on winter wheat, 1985-1989 a Estimated losses Sowing date
Disease
Number o f crops
Mean percentage
Kilotonnes h
£ x I06h
September
Eyespot Sharp eyespot
290 290
2.1 0.9
5 ! .0 21.4
5.4 2.2
October
Eyespot Sharp eyespot
1189 1189
1.5 0.6
158.2 61.8
16.6 6.5
November
Eyespot Sharp eyespot
206 206
0.9 0.2
25.6 6.6
2.6 0.7
"Losses are based on infection levels for crops sown in each month; ~corrected for different crop area sown in each month and different potential yields
Table 5. Effect of length of break from cereals on losses caused by eyespot in winter wheat Length of break (years) None 1 2
Estimated loss b
Number of crops
Mean loss 1985-1989" (%)
Kilotonnes
£ x 106
1052 478 69
!.9 1.2 0.4
170.7 69.6 6.4
17.9 7.3 0.7
"Based on infection in crops sown in each situation; ~corrected for crop area sown in each situation
Effects of spray prograrnmes on winter wheat The survey data indicate differences between disease levels in those crops subject to different fungicide regimens. Recent studies on eyespot (ADAS, unpublished) have shown that yield benefits arising from control ofthe disease with fungicides are similar whether sprays are applied at GS31 or GS32. On average, GS32 normally occurs in early May. Experiments have also indicated (Thomas, Cook and King, 1989) that the most effective control orS. tritici is achieved when fungicide treatment coincides with the heavy rainfall needed to spread the disease in early to mid-May. Table6 illustrates the benefit of a GS 39 treatment for control orS. tritici in relation to the start date of the spray programme. It also confirms that prochloraz provides some control of eyespot compared with other fungicides used at GS 31.
Table 7 gives the estimated yield losses caused by leaf diseases despite application of the spray programmes used in Table 6. Most effective control of S. tritici (as measured by estimated yield loss) is provided by spray programmes starting in early May rather than in April. These figures are estimates of national losses. The average national loss caused by S. tritici, shown in Table2, is £34 x 106 compared with the mean on untreated crops from Table 7 of£49 x 106. On the assumption that there is no loss in eyespot control from delaying the first spray until early May, Table7 suggests that there are substantial national losses associated with the use of inappropriate timings of two-spray programmes.
Fungicide group used at GS31 The need to control eyespot is the primary justification for fungicide application at GS31. However, despite the widespread occurrence of resistance of the eyespot fungus to carbendazim (MBC) and related fungicides (King and Griffin, 1985) these products are still commonly used in fungicide treatments aplblied at GS31 (Thomas, 1989). The additional financial cost of using these fungicides on crops where there is a high risk of eyespot is --,£15 x 106 (Table8), assuming that the entire crop area is treated with each fungicide group. This cost decreases to £5 x 106 on the basis of the smaller area treated in 1989. There is also no evidence ofcontrol of eyespot associated with the use of MBC fungicides.
Table 6. Effect of fungicide programme on S. triticiand eyespot infection Percentage o f stems affected by eyespot (moderate + severe) Percentage o f
Septoria tritici Spray p r o g r a m m e
Programme start date
Untreated 31 + 3 9 31 + 3 9 31 + 59 31 + 5 9 31 + 39 + 59 31 + 3 9 + 5 9
-April 1-15 M a y April 1-15 M a y April 1-15 M a y
N u m b e r o f crops
(leaf 2)
N o prochloraz
121 145 17 246 37 138 20
6.7 4.7 0.9 5.6 2.0 3.2 3.7
16.9 I 1. ! 16.3 18.9 16.6 15.0
- Prochloraz 16.29 6.9 I 1.0 9.5 7.7 10.8 ! 1.5
Disease levels in w i n t e r wheat: R. J. Cook et aL
507
Table 7. Effect of spray programme on yield losses (£ x 10~) caused by leaf diseases, assuming that the entire national crop is treated with each programme Estimated yield loss caused by: Spray programme
P r o g r a m m e start date
Untreated 31 + 3 9 31 + 3 9 31 + 59 31 + 5 9 31 + 39 + 59 31 + 3 9 + 5 9
-April 1-15 M a y April !-15 M a y April 1-15 M a y
Number ofcrops
S. tritici
Mildew
Total"
121 145 17 246 37 138 20
49.1 33.5 6.5 39.8 14.7 22.9 26.8
36.4 16.4 23.2 22.2 15.3 19.0 25.3
89.6 51,6 30.5 71.2 30.2 43.2 54.9
"Includes yellow rust and brown rust which are not shown in the Table Table 8. Effect of fungicide applied at GS 31 on subsequent eyespot and losses caused by the disease, 1985-1989 a
Fungicide group M BC (excluding prochloraz) Prochloraz Prochloraz + M BC N o fungicide
N u m b e r of crops
Stems affected by eyespot (%)
Estimated yield losses (%)
£ x 106h
128 94 107 86
38.7 24.0 25.2 38.2
3.3 1.2 ! .6 2.4
24.5 8.8 1 ! .4 ! 7.9
"High-risk crops only, i.e. second or subsequent wheats sown before I I October; t'assumingall the high-risk crop area treated with relevant fungicide groups
Costs and benefits of national disease control Yield losses deduced from ADAS experiment results (Cook and Thomas, 1990) suggest a potential gross benefit of fungicide application (i.e. the yield difference between those plots receiving a full three-spray treatment intended to control all diseases and untreated plots) of 12%, worth £217 x 10 6. The net financial losses, after allowing for the costs of fungicides and their albplication, are £87 x 106, representing5% of wheat production. By contrast, losses estimated from the survey data total £113 x 106 ( 8 % of production). Table 7 suggests that some fungicide regimens provide more effective control of S. tritici than others and that two-spray programmes starting in early May provide better control of this disease than regimens that start in April, especially when the second fungicide application is delayed until GS 59. For example, the difference between losses caused by programmes starting in April or May, with a second spray at GS 39 or GS 59, suggests that losses of --,£40 x 106 may be ascribed to the use of inappropriate programmes (Table7) and that national losses of controllable diseases could be reduced to 5% (£73 x 106) by adoption of more suitable timing regimens. This figure is in good agreement with the net benefit of fungicides derived from experimental evidence.
Discussion and conclusions It is difficult to obtain precise values for the losses caused by cereal diseases. The estimates presented here are based on samples collected in surveys where > 90% ofcrops were treated with fungicides. Yield losses have been calculated
on the assumption that the formulae used are equally valid on fungicide-treated and untreated crops. This is not necessarily the case but in the absence ofmore information they provide the best estimates currently available. Most formulae assume a straight-line relationship, which may not necessarily be valid. Additionally, in some toxigenic fungi (e.g.S. triticO there is evidence that, above certain threshold disease levels, further infection causes no additional losses (M. R. Thomas, unpublished). There may also be physiological effects that influence dry-matter accumulation and distribution or variations in the tolerance of cultivars to diseases. The apparent increase in eyespot noted in MBC-freated crops, compared with prochloraz-treated crops (Table8), has been noted elsewhere. Griffin and King (1985) reported a similar effect in fungicide experiments and the effect has often been noted in experiments testing eyespot fungic!des (ADAS, unpublished data). The apparent yield penalty of using these fungicides may, therefore, be real. There are other difficulties in interpreting the survey data. Surprisingly, disease levels in crops receiving the three-spray programme referred to in Table 7 were higher than in some crops receiving other programmes. This isillustrated in Table 7as an apparent, anomalous increase in yield losses in crops receiving a three-spray programme starting in May, compared with those receiving only two sprays. Sample sizes within each programme group vary from yea[ to year and the growth stages at sampling are means of a range. It is possible that mistiming of sprays (Thomas, 1986) results in poor disease control as sprays are applied when disease is well established in crops,~so that disease are 'chased' by sequential sprays being applied to already infected crops.
508
Disease levels in winter wheat: R. J. Cook et aL
ADAS experiments have consistently shown that the period GS 37-39 (flag leaf emergence) is a critical growth stage for fungicide treatment on wheat (e.g. Cook and Thomas, 1990) and the highest-yielding programmes in fungicide experiments always include a spray at this time. Survey data have shown that the proportion of farm crops receiving treatment at GS 39 varies, presumably because of farmer perceptions that ear sprays are an essential element ofcrop husbandry. These data illustrate that ADAS advice to apply the main spray to wheat at GS 39 is fullyjustified by survey results. The importance of disease control during this period of growth relates in part to control of S. tritici(Thomas, Cook and King, 1989). Some of the fungicide sprays applied between 1 and 15 May (Table 7) will have been applied to crops at GS 32 or GS 33 when the third or second leaves below the flag leaf will have emerged from the leaf sheath and will be expanding. Application of certain fungicides during this period will enhance control of S. tritici and is partly responsible for the improved disease control and reduced losses associated with these spray sequences. The surveys are made on randomly selected crops so that there is no control over either the products used or spray timing. Data from individual surveys suggest that not al~ crops are treated with the most appropriate chemical or at the most appropriate time. It is possible that the disease incidence recorded on treated crops in the survey represents late infections which would have little effect on yield. However, data from fungicide experiments (Thomas, Cook and King, 1989; ADAS, unpublished) show that some simple one- or two-spray programmes with effective fungicides will keep crops free of disease throughout the growing season. Survey losses may, therefore, be interpreted as representing residual losses persisting after treatment and are thus an avoidable consequence of inappropriate fungicide use. Comparisons of the survey data with results of ADAS field experiments suggest that the yield loss formulae used in the survey are not as erratic as might be thought. In the case of eyespot, for example, ADAS experiments suggest that a mean yield loss of 0.6 t ha- i is caused by moderatesevere eyespot (Jones, 1988), equivalent to £50 x 10 6 each year on the basis of assumptions in this article. This is approximately double the value of losses shown in Table2, based on treated crops in the survey. However, there is a similar proportional relationship between losses in treated crops (£8 x 106) and untreated crops in Table8, suggesting that treatment with current fungicides reduces losses by 50%. There are many dangers inherent in estimation of yield losses from survey data. In particular, fungicides are most likely to be applied to those crops with the highest disease risk, so that differences in disease incidence between crops receiving different spray regimens will not be due solely to the effects of fungicides. In addition, the calculations used in this paper are based on the potential yield of winter wheat in England and Wales rather'than on the actual yield recorded by MAFF. This is .justified because actual recorded yields include losses attributable to disease, in addition to the range of management, husbandry and climate factors that influence
yields. The potential yield which provided the basis for these estimates was derived from well-managed crops maintained in as disease-free a state as possible by fungicides, but otherwise influenced by the normal range of climatic and geographical variables. Thus, the data presented here do provide valid estimates of residual yield losses caused by wheat diseases after application of fungicides in England and Wales. The estimated losses agree with estimates derived from fungicide experiments and also with losses estimated by different methods (Cook and Jenkins, 1988). These estimates can, in the authors' opinion, be used with reasonable confidence to monitor the impact of annual changes in disease incidence, farm practice and fungicide on the economics of wheat production in England and Wales. References Clarkson, J. D. S. (1981) Relationship between eyespot severity and yield loss in winter wheat. Plant PathoL 30, 125-131 Clarkson, J. D. S. and Cook, R. J. (1983) Effect of sharp eyespot (Rhizoctonia cerealis) on yield loss in winter wheat and of some agronomic factors on disease incidence. Plant PathoL 32, 421-428 Cook, R. J. and Jenkins, J. E. E. (1988) The contribution and value of pesticides to disease control in cereals. In: Control of Plant Diseases: Costs and Benefits, Blackwell Scientific Publications, Oxford Cook, R. J. and Thomas, M. R. (1990) Effect of agronomic factors on yield response of winter wheat to fungicide programmes. Plant Pathol. 39, 548-557 Griffin, M. J. and King, J. E. (1985) Benzimidazole resistance in Pseudocercosporella herpotrichoides: Results of ADAS random surveys and fungicide trials in England and Wales, 1982-1984. EPPO Bull. 15, 485-494 Jones, D. R. (1988) IVinter IVheat Timing of Fungicides for Eyespot Control 1988. CSG Commissioned R&D Report, Agricultural Development and Advisory Service, MAFF King, J. E. (1976) Relationship between yield loss and severity of yellow rust recorded on a large number of single stems of winter wheat. Plant PathoL 25, 172-177 King, J. E. E. (1985) Winter IVl!eat Survey 1985. Agricultural Development and Advisory Service, MAFF, Harpenden Laboratory, St Albans AL5 2BD, UK King, J. E. and Griffin, M. J. (1985) Survey of benomyl resistance in Pseudocercosporella herpotrichoides on winter wheat and barley in England and Wales in 1983. Plant PathoL 34, 272-283 Policy, R. W. and Thomas M. R. (1991) Surveys of disease of winter wheat in England and Wales, 1976-88. Ann. Appl. BioL 119, 1-20 Thomas, M. R. (1986) Winter Wheat Surrey 1986. Agricultural Development and Advisory Service, MAFF, Harpenden Laboratory, St Albans AL5 2BD, UK Thomas, M. R. (1987) tVinter Wheat Survey 1987. Agricultural Development and Advisory Service, MAFF, Harpenden Laboratory, St Albans AL5 2BD, UK Thomas, M. R. (1988) IVinter Wheat Survey 1988. Agricultural Development and Advisory Service, MAFF, Harpenden Laboratory, St Albans AL5 2BD, UK Thomas, M. R. (1989) Winter IVheat Surve)" 1989. Agricultural Development and Advisory Service, MAFF, Harpenden Laboratory, St Albans AL5 2BD, UK Thomas, M. R., Cook, R. J. and King, J. E. (1989) Factors affecting development ofSeptoria tritici on winter wheat and its effect on yield. Plant Pathol. 38, 246-257 Tnttman, D. R. (1987) The decimal code for the growth stages ofcereals, with illustrations. Ann. AppL Biol. 110, 441-454 Received 8 November 1990 Revised 1 May 1991 Accepted 24 June 1991
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Disease-induced losses in winter wheat in England and Wales 1985-1989 R. J. Cook*, R. W. Polley t and M. R. Thomas t ADAS, Ministry of Agriculture, Fisheries and Food, Lawnswood, Leeds LS16 5PY, UK and tMinistry of Agriculture, Fisheries and Food, Central Science Laboratory, Hatching Green, Harpenden, Hertfordshire AL5 2BD, UK
Abstract
Keywords
Annual surveys ofwinter wheat diseases in England and Wales provide information on the relative severity of the leaf and stem-base disease complex. Septoria tritici was the most damaging disease during the 5-year period 1985-1989 causing annual losses estimated to be 0.329 Mt compared with 0.251 Mt and 0.231 Mt for eyespot and mildew, the next most damaging winter wheat diseases. Annual losses during the period averaged 1.078Mt excluding losses attributable to the fusarium diseases, for which no satisfactory yield loss relationships are available. Sowing date and length of break from cereals exert a strong influence on incidence ofeyespot and losses attributable to the disease and, on the basis of these surveys, growing all winter wheat crops after a l-year break would prevent national losses of ~ I00 kt annually. Wheat, winter; diseases; crop damage; England; Wales
Introduction ADAS surveys of cereal crops in England and Wales monitor leaf and stem-base diseases to provide an annual 'snapshot' o f disease incidence in relation to cultural practices. The winter wheat surveys are based on random samples collected at the milky ripe growth stage (GS 71-73; Tottman, 1987) from crops throughout England and Wales. Annual reports (King, 1985; Thomas, 1986, 1987, 1988, 1989) summarize disease incidence and fungicide use. The data have been used to rank individual diseases by their relative order of importance (Polley and Thomas, 1990) after application of fungicides. The surveys also provide data that can be used to estimate the influence of cultural practices on diseases. This paper considers some of these factors in relation to the economics of disease for the five years 1985-1989. During this period there was a general stability of fungicide use and cereal husbandry in England and Wales. Surveys and loss estimates
Husbandry factors were considered only where earlier scrutiny (Polley and Thomas, 1'991) had indicated significant differences in amounts of disease between factors. Disease incidence, based on the percentage of stems with moderate and severe eyespot (Pseudocercosporella herpotrichoMes) and sharp eyespot (Rhizoctonia cerealis), and percentage infection on leaf 2 [leaf 1 for rusts (Pucchlia spp.)] was used to estimate the percentage yield loss for each disease. *Present address, and address for correspondence: Morley Research Centre, Wymondharn, Norfolk, NR18 9DB, UK 0261-2194/9110610504-05 © 1991 Butterworth-Heinemann Ltd
The following basic assumptions were made concerning the data.
ADAS regions. These were defined by current regional boundaries and represent the appropriate geographical areas of England and Wales. Wales has been omitted from the regional estimates owing to the small proportion of winter wheat grown there.
Prices. These were assumed to be £105 t Yields. An estimate of the potential average ~,ield of winter wheat in England and Wales was derived from ADAS fungicide experiments. This was taken as 8.8 t h a - 1, being the yield o f plots receiving a three-spray fungicide programme (GS 3 1 + 3 9 + 5 9 ) (Cook and Thomas, 1990). This compares with an actual national average yield of -,, 6.1 t h a - 1 ( M A F F statistics). National production. The total area of wheat has been assumed to be 1.9 Mha ( M A F F statistics). Where proportions of the crop area are subject to different cultural or fungicide regimens, relative proportions have been takeh from the 1989 survey report (Thomas, 1989). Estimates ofpotential national grain production used to calculate financial values were based on the treated yield from ADAS experiments multiplied by the relevant proportion of the national crop area. It was accepted that this would provide an estimate of potential yield and, therefore, would maximize potential losses. YieMIosses. Yield losses caused by disease were estimated by using published formulae, derived from yield loss
Disease levels in winter wheat: R. J. Cook et aL
505
Table 1. Formulae used to estimate yield loss in ADAS winter wheat disease surveys a Disease
Relationship I'
Powdery mildew Septoria tritici Yellow and brown rust Eyespot Sharp eyespot
Reference
y = 3 xi y = 0.42 xii y = 0.4 xiii y=0.1 xiv+0.36 xv y = 0.05 xiv + 0.26 xv
Thomas (unpublished") Thomas, Cook and King, 1989 King, 1976 Clarkson, 1981 Clarkson and Cook, 1983
°Note: There are no satisfactory formulae by which to estimate losses caused by fusarium disease, at present; '~y,percentage loss in grain yield; xi, percentage disease on leaf2 (flag- I)~in the range 0-10%; xii, percentage disease leaf2 (flag- I), in the range 0-45*/,; xiii, percentage disease leaf I (flag); xiv and xv, percentage of stems with moderate and severe lesions respectively; ¢derived from ADAS experiments designed to study disease progress in sequential epidemics
Table2. Estimated annual national losses caused by winter wheat stem-base and leaf disease, 1985-1989
Disease
Loss (%)
Eyespot Sharp eyespot Mildew Septoria tritici S. nodorum Yellow rust Brown rust
1.5 0.6 1.4 2.0 0.9 <0.1 < 0.1
250.8 96.1 231.2 328.9 149.8 11.4 9.8
26.3 10.1 24.3 34.5 15.7 1.2 1.0
--
1078.0
113. I
Total
Loss (kilotonnes)
Cost (£ × 106)
experiments, which were assumed to be equally valid for fungicide-treated and untreated crops. The formulae used are represented in Table l. The surveys have also estimated incidence of take-all (Gaeumannomyces gramhlis) based on frequency of aboveground symptoms. It is not possible to use these assessments to provide direct loss estimates and a 6% yield reduction has been assumed for second and third wheat crops following a non-cereal break (D. J. Yarham, ADAS, Cambridge; personal communication, 1990). This agrees well with the yield difference noted by Cook and Thomas (1990) for second and successive wheat crops receiving a full three-spray fungicide programme.
Growth stages. The growth stages defined in the tables represent a range of stages identified on replies in questionnaires. GS 31 represents the period between GS 30 and 33, GS 39 represents GS 37-45 and GS 59 the period between GS 47 and GS 69. Yield loss estimates
Regional losses Loss estimates (Table3) were made for those diseases that showed significant regional variations (Polley and Thomas, 1991). Yellow rust (Puccinia striiformis) is included owing to the variations between Eastern and Northern England, where the disease was significantly more severe than in the remainder of the country in 1988. Data for S. tritici are based on the 4-year period starting in 1986, because of very low levels in South West England and a very high incidence in Eastern England in 1985, which distorted the long-term distribution pattern for this disease. In general, losses from this disease are largest in the South West, whereas brown rust (Pucchlia recondita) causes most severe damage in the warmer East and South East regions. Total financial losses attributable to disease would, however, be greatest in the Eastern region, owing to the high proportion (43%, MAFF statistics) of the national wheat crop in that area.
Sowing date Potential yield losses per hectare for both eyespot and sharp eyespot decrease as sowing date is delayed from September until November (Table4). However, the high proportion of the national crop sown in October results in higher absolute losses for October-sown crops.
Cropping sequence Eyespot is a disease of intensive cereals and survey data show a consistent positive relationship between incidence and the same previous crop type. Table5 estimates losses for this disease in relation to length of break from cereals, suggesting that substantial potential savings could be hchieved by 1-year or 2-year breaks.
Overall losses Mean annual losses due to disease total 1.1Mt (Table2) and this represents a maximum of 6.5% of current potential wheat production. The losses represent 8% of national average yields after application of fungicides which annually cost -,-£113 x 106. Eyespot, Septoria tritici and mildew (Erysiphe gramhds) were the most damaging diseases of winter wheat. Losses caused by take-all on winter wheat are difficult to estimate. However, assuming a 6% yield reduction in second and third wheats, there are additional annual losses of £43.2x l06 caused by this disease. Fusarium diseases have been omitted.
Table 3. Regional estimates of percentage yield losses for winter" wheat, 1985-1989
Region East Midlands and West South East South West North °1986-1989
Septoria triticia 0.9 0.8 0.8 1.7 0.5
Brown rust 0.l <0.1 0.1 < 0.1 0.0
Yellow Sharp rust eyespot 0.1 <0.1 <0.1 0.0 0. I
0.5 0.8 0.5 0.8 0.5
506
Disease levels in winter wheat: R. J. Cook et aL
Table 4. Influence of delayed sowing date on losses due to eyespot and sharp eyespot on winter wheat, 1985-1989 a Estimated losses Sowing date
Disease
Number o f crops
Mean percentage
Kilotonnes h
£ x I06h
September
Eyespot Sharp eyespot
290 290
2.1 0.9
5 ! .0 21.4
5.4 2.2
October
Eyespot Sharp eyespot
1189 1189
1.5 0.6
158.2 61.8
16.6 6.5
November
Eyespot Sharp eyespot
206 206
0.9 0.2
25.6 6.6
2.6 0.7
"Losses are based on infection levels for crops sown in each month; ~corrected for different crop area sown in each month and different potential yields
Table 5. Effect of length of break from cereals on losses caused by eyespot in winter wheat Length of break (years) None 1 2
Estimated loss b
Number of crops
Mean loss 1985-1989" (%)
Kilotonnes
£ x 106
1052 478 69
!.9 1.2 0.4
170.7 69.6 6.4
17.9 7.3 0.7
"Based on infection in crops sown in each situation; ~corrected for crop area sown in each situation
Effects of spray prograrnmes on winter wheat The survey data indicate differences between disease levels in those crops subject to different fungicide regimens. Recent studies on eyespot (ADAS, unpublished) have shown that yield benefits arising from control ofthe disease with fungicides are similar whether sprays are applied at GS31 or GS32. On average, GS32 normally occurs in early May. Experiments have also indicated (Thomas, Cook and King, 1989) that the most effective control orS. tritici is achieved when fungicide treatment coincides with the heavy rainfall needed to spread the disease in early to mid-May. Table6 illustrates the benefit of a GS 39 treatment for control orS. tritici in relation to the start date of the spray programme. It also confirms that prochloraz provides some control of eyespot compared with other fungicides used at GS 31.
Table 7 gives the estimated yield losses caused by leaf diseases despite application of the spray programmes used in Table 6. Most effective control of S. tritici (as measured by estimated yield loss) is provided by spray programmes starting in early May rather than in April. These figures are estimates of national losses. The average national loss caused by S. tritici, shown in Table2, is £34 x 106 compared with the mean on untreated crops from Table 7 of£49 x 106. On the assumption that there is no loss in eyespot control from delaying the first spray until early May, Table7 suggests that there are substantial national losses associated with the use of inappropriate timings of two-spray programmes.
Fungicide group used at GS31 The need to control eyespot is the primary justification for fungicide application at GS31. However, despite the widespread occurrence of resistance of the eyespot fungus to carbendazim (MBC) and related fungicides (King and Griffin, 1985) these products are still commonly used in fungicide treatments aplblied at GS31 (Thomas, 1989). The additional financial cost of using these fungicides on crops where there is a high risk of eyespot is --,£15 x 106 (Table8), assuming that the entire crop area is treated with each fungicide group. This cost decreases to £5 x 106 on the basis of the smaller area treated in 1989. There is also no evidence ofcontrol of eyespot associated with the use of MBC fungicides.
Table 6. Effect of fungicide programme on S. triticiand eyespot infection Percentage o f stems affected by eyespot (moderate + severe) Percentage o f
Septoria tritici Spray p r o g r a m m e
Programme start date
Untreated 31 + 3 9 31 + 3 9 31 + 59 31 + 5 9 31 + 39 + 59 31 + 3 9 + 5 9
-April 1-15 M a y April 1-15 M a y April 1-15 M a y
N u m b e r o f crops
(leaf 2)
N o prochloraz
121 145 17 246 37 138 20
6.7 4.7 0.9 5.6 2.0 3.2 3.7
16.9 I 1. ! 16.3 18.9 16.6 15.0
- Prochloraz 16.29 6.9 I 1.0 9.5 7.7 10.8 ! 1.5
Disease levels in w i n t e r wheat: R. J. Cook et aL
507
Table 7. Effect of spray programme on yield losses (£ x 10~) caused by leaf diseases, assuming that the entire national crop is treated with each programme Estimated yield loss caused by: Spray programme
P r o g r a m m e start date
Untreated 31 + 3 9 31 + 3 9 31 + 59 31 + 5 9 31 + 39 + 59 31 + 3 9 + 5 9
-April 1-15 M a y April !-15 M a y April 1-15 M a y
Number ofcrops
S. tritici
Mildew
Total"
121 145 17 246 37 138 20
49.1 33.5 6.5 39.8 14.7 22.9 26.8
36.4 16.4 23.2 22.2 15.3 19.0 25.3
89.6 51,6 30.5 71.2 30.2 43.2 54.9
"Includes yellow rust and brown rust which are not shown in the Table Table 8. Effect of fungicide applied at GS 31 on subsequent eyespot and losses caused by the disease, 1985-1989 a
Fungicide group M BC (excluding prochloraz) Prochloraz Prochloraz + M BC N o fungicide
N u m b e r of crops
Stems affected by eyespot (%)
Estimated yield losses (%)
£ x 106h
128 94 107 86
38.7 24.0 25.2 38.2
3.3 1.2 ! .6 2.4
24.5 8.8 1 ! .4 ! 7.9
"High-risk crops only, i.e. second or subsequent wheats sown before I I October; t'assumingall the high-risk crop area treated with relevant fungicide groups
Costs and benefits of national disease control Yield losses deduced from ADAS experiment results (Cook and Thomas, 1990) suggest a potential gross benefit of fungicide application (i.e. the yield difference between those plots receiving a full three-spray treatment intended to control all diseases and untreated plots) of 12%, worth £217 x 10 6. The net financial losses, after allowing for the costs of fungicides and their albplication, are £87 x 106, representing5% of wheat production. By contrast, losses estimated from the survey data total £113 x 106 ( 8 % of production). Table 7 suggests that some fungicide regimens provide more effective control of S. tritici than others and that two-spray programmes starting in early May provide better control of this disease than regimens that start in April, especially when the second fungicide application is delayed until GS 59. For example, the difference between losses caused by programmes starting in April or May, with a second spray at GS 39 or GS 59, suggests that losses of --,£40 x 106 may be ascribed to the use of inappropriate programmes (Table7) and that national losses of controllable diseases could be reduced to 5% (£73 x 106) by adoption of more suitable timing regimens. This figure is in good agreement with the net benefit of fungicides derived from experimental evidence.
Discussion and conclusions It is difficult to obtain precise values for the losses caused by cereal diseases. The estimates presented here are based on samples collected in surveys where > 90% ofcrops were treated with fungicides. Yield losses have been calculated
on the assumption that the formulae used are equally valid on fungicide-treated and untreated crops. This is not necessarily the case but in the absence ofmore information they provide the best estimates currently available. Most formulae assume a straight-line relationship, which may not necessarily be valid. Additionally, in some toxigenic fungi (e.g.S. triticO there is evidence that, above certain threshold disease levels, further infection causes no additional losses (M. R. Thomas, unpublished). There may also be physiological effects that influence dry-matter accumulation and distribution or variations in the tolerance of cultivars to diseases. The apparent increase in eyespot noted in MBC-freated crops, compared with prochloraz-treated crops (Table8), has been noted elsewhere. Griffin and King (1985) reported a similar effect in fungicide experiments and the effect has often been noted in experiments testing eyespot fungic!des (ADAS, unpublished data). The apparent yield penalty of using these fungicides may, therefore, be real. There are other difficulties in interpreting the survey data. Surprisingly, disease levels in crops receiving the three-spray programme referred to in Table 7 were higher than in some crops receiving other programmes. This isillustrated in Table 7as an apparent, anomalous increase in yield losses in crops receiving a three-spray programme starting in May, compared with those receiving only two sprays. Sample sizes within each programme group vary from yea[ to year and the growth stages at sampling are means of a range. It is possible that mistiming of sprays (Thomas, 1986) results in poor disease control as sprays are applied when disease is well established in crops,~so that disease are 'chased' by sequential sprays being applied to already infected crops.
508
Disease levels in winter wheat: R. J. Cook et aL
ADAS experiments have consistently shown that the period GS 37-39 (flag leaf emergence) is a critical growth stage for fungicide treatment on wheat (e.g. Cook and Thomas, 1990) and the highest-yielding programmes in fungicide experiments always include a spray at this time. Survey data have shown that the proportion of farm crops receiving treatment at GS 39 varies, presumably because of farmer perceptions that ear sprays are an essential element ofcrop husbandry. These data illustrate that ADAS advice to apply the main spray to wheat at GS 39 is fullyjustified by survey results. The importance of disease control during this period of growth relates in part to control of S. tritici(Thomas, Cook and King, 1989). Some of the fungicide sprays applied between 1 and 15 May (Table 7) will have been applied to crops at GS 32 or GS 33 when the third or second leaves below the flag leaf will have emerged from the leaf sheath and will be expanding. Application of certain fungicides during this period will enhance control of S. tritici and is partly responsible for the improved disease control and reduced losses associated with these spray sequences. The surveys are made on randomly selected crops so that there is no control over either the products used or spray timing. Data from individual surveys suggest that not al~ crops are treated with the most appropriate chemical or at the most appropriate time. It is possible that the disease incidence recorded on treated crops in the survey represents late infections which would have little effect on yield. However, data from fungicide experiments (Thomas, Cook and King, 1989; ADAS, unpublished) show that some simple one- or two-spray programmes with effective fungicides will keep crops free of disease throughout the growing season. Survey losses may, therefore, be interpreted as representing residual losses persisting after treatment and are thus an avoidable consequence of inappropriate fungicide use. Comparisons of the survey data with results of ADAS field experiments suggest that the yield loss formulae used in the survey are not as erratic as might be thought. In the case of eyespot, for example, ADAS experiments suggest that a mean yield loss of 0.6 t ha- i is caused by moderatesevere eyespot (Jones, 1988), equivalent to £50 x 10 6 each year on the basis of assumptions in this article. This is approximately double the value of losses shown in Table2, based on treated crops in the survey. However, there is a similar proportional relationship between losses in treated crops (£8 x 106) and untreated crops in Table8, suggesting that treatment with current fungicides reduces losses by 50%. There are many dangers inherent in estimation of yield losses from survey data. In particular, fungicides are most likely to be applied to those crops with the highest disease risk, so that differences in disease incidence between crops receiving different spray regimens will not be due solely to the effects of fungicides. In addition, the calculations used in this paper are based on the potential yield of winter wheat in England and Wales rather'than on the actual yield recorded by MAFF. This is .justified because actual recorded yields include losses attributable to disease, in addition to the range of management, husbandry and climate factors that influence
yields. The potential yield which provided the basis for these estimates was derived from well-managed crops maintained in as disease-free a state as possible by fungicides, but otherwise influenced by the normal range of climatic and geographical variables. Thus, the data presented here do provide valid estimates of residual yield losses caused by wheat diseases after application of fungicides in England and Wales. The estimated losses agree with estimates derived from fungicide experiments and also with losses estimated by different methods (Cook and Jenkins, 1988). These estimates can, in the authors' opinion, be used with reasonable confidence to monitor the impact of annual changes in disease incidence, farm practice and fungicide on the economics of wheat production in England and Wales. References Clarkson, J. D. S. (1981) Relationship between eyespot severity and yield loss in winter wheat. Plant PathoL 30, 125-131 Clarkson, J. D. S. and Cook, R. J. (1983) Effect of sharp eyespot (Rhizoctonia cerealis) on yield loss in winter wheat and of some agronomic factors on disease incidence. Plant PathoL 32, 421-428 Cook, R. J. and Jenkins, J. E. E. (1988) The contribution and value of pesticides to disease control in cereals. In: Control of Plant Diseases: Costs and Benefits, Blackwell Scientific Publications, Oxford Cook, R. J. and Thomas, M. R. (1990) Effect of agronomic factors on yield response of winter wheat to fungicide programmes. Plant Pathol. 39, 548-557 Griffin, M. J. and King, J. E. (1985) Benzimidazole resistance in Pseudocercosporella herpotrichoides: Results of ADAS random surveys and fungicide trials in England and Wales, 1982-1984. EPPO Bull. 15, 485-494 Jones, D. R. (1988) IVinter IVheat Timing of Fungicides for Eyespot Control 1988. CSG Commissioned R&D Report, Agricultural Development and Advisory Service, MAFF King, J. E. (1976) Relationship between yield loss and severity of yellow rust recorded on a large number of single stems of winter wheat. Plant PathoL 25, 172-177 King, J. E. E. (1985) Winter IVl!eat Survey 1985. Agricultural Development and Advisory Service, MAFF, Harpenden Laboratory, St Albans AL5 2BD, UK King, J. E. and Griffin, M. J. (1985) Survey of benomyl resistance in Pseudocercosporella herpotrichoides on winter wheat and barley in England and Wales in 1983. Plant PathoL 34, 272-283 Policy, R. W. and Thomas M. R. (1991) Surveys of disease of winter wheat in England and Wales, 1976-88. Ann. Appl. BioL 119, 1-20 Thomas, M. R. (1986) Winter Wheat Surrey 1986. Agricultural Development and Advisory Service, MAFF, Harpenden Laboratory, St Albans AL5 2BD, UK Thomas, M. R. (1987) tVinter Wheat Survey 1987. Agricultural Development and Advisory Service, MAFF, Harpenden Laboratory, St Albans AL5 2BD, UK Thomas, M. R. (1988) IVinter Wheat Survey 1988. Agricultural Development and Advisory Service, MAFF, Harpenden Laboratory, St Albans AL5 2BD, UK Thomas, M. R. (1989) Winter IVheat Surve)" 1989. Agricultural Development and Advisory Service, MAFF, Harpenden Laboratory, St Albans AL5 2BD, UK Thomas, M. R., Cook, R. J. and King, J. E. (1989) Factors affecting development ofSeptoria tritici on winter wheat and its effect on yield. Plant Pathol. 38, 246-257 Tnttman, D. R. (1987) The decimal code for the growth stages ofcereals, with illustrations. Ann. AppL Biol. 110, 441-454 Received 8 November 1990 Revised 1 May 1991 Accepted 24 June 1991