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 varyin g amounts of yellow rust (Puccinia striiformis (Westend.)) were direct ly proportional to laboratory measured green leaf area index (R-2 = 0. 75). The field technique depended on shoot counts and a leaf form fact or (F = 0.83) which was derived from 20 varieties of winter wheat by r elating the product of their leaf lengths and widths to leaf areas mea sured by a planimeter (R-2 = 0.95). In two experiments at ADAS Terring ton, UK, on the susceptible winter wheat variety Slejpner, epidemics o f yellow rust ranged from nil to severe with 60 (1994) and 52 (1995) d ifferent combinations of fungicide dose and timing. Assessments of dis ease severity (%) integrated as the area under the disease progress cu rve accounted for yield differences within each season, but the relati onship differed markedly between seasons. In-field assessments of gree n leaf area index integrated over time, or healthy area duration, show ed 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 yea r 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 fro m field-measured green leaf area index and total incident radiation (i .e., healthy area absorption), accounted for more variation in grain y ield (1994, R-2 = 0.80; 1995, R-2 = 0.92). There was no seasonal diffe rence in the conversion coefficient between grain dry matter and the a mount of incident radiation absorbed by green leaf tissue (1.4 g/MJ) b ut the intercepts of the relationships were sensitive to the date from which integration began. It is suggested that in-field green leaf are a index assessments, interpreted through a simple model which provides estimates of differences in intercepted light energy, may prove usefu l in the analysis of experiments on disease control. (C) 1997 Elsevier Science B.V.