Use of in-field measurements of green leaf area and incident radiation to estimate the effects of yellow rust epidemics on the yield of winter wheat

In-field estimates of green leaf area index for treatments with varying amounts of yellow rust (Puccinia striiformis (Westend.)) were directly proportional to laboratory measured green leaf area index (R-2 = 0.75). The field technique depended on shoot counts and a leaf form factor (F = 0.83) which was derived from 20 varieties of winter wheat by relating the product of their leaf lengths and widths to leaf areas measured by a planimeter (R-2 = 0.95). In two experiments at ADAS Terrington, UK, on the susceptible winter wheat variety Slejpner, epidemics of yellow rust ranged from nil to severe with 60 (1994) and 52 (1995) different combinations of fungicide dose and timing. Assessments of disease severity (%) integrated as the area under the disease progress curve accounted for yield differences within each season, but the relationship differed markedly between seasons. In-field assessments of green leaf area index integrated over time, or healthy area duration, showed a curvilinear relationship with grain yield (1994, R-2 = 0.63; 1995, R-2 = 0.73), but any healthy area duration value in the brighter year of 1995 related to larger yields than the equivalent value in 1994. Intercepted radiation by green leaf tissue accumulated after flowering (20 June in both years), estimated through the Beer's Law analogy from field-measured green leaf area index and total incident radiation (i.e., healthy area absorption), accounted for more variation in grain yield (1994, R-2 = 0.80; 1995, R-2 = 0.92). There was no seasonal difference in the conversion coefficient between grain dry matter and the amount of incident radiation absorbed by green leaf tissue (1.4 g/MJ) but the intercepts of the relationships were sensitive to the date from which integration began. It is suggested that in-field green leaf area index assessments, interpreted through a simple model which provides estimates of differences in intercepted light energy, may prove useful in the analysis of experiments on disease control. (C) 1997 Elsevier Science B.V.