Carmona Marcelo A. 1*, Sautua Francisco J. 1, Reis Erlei M. 2
1 Department of Plant Pathology. Faculty of Agronomy, University of Buenos Aires, Av. San Martín 4453, C1417DSE, Buenos Aires, Argentina. 2 OR Melhoramento de Sementes Ltda, Passo Fundo, RS, Brazil.
Frogeye leaf spot of soybean caused by Cercospora sojina is an economically important foliar disease in warm and humid regions. In Argentina, the severity and prevalence of FLS has increased during the last years. The main control strategies include the use of tolerant or resistant cultivars, seed and foliar fungicide treatment and crop rotation. Currently, the chemical control is the most frequent available strategy to avoid damages. In the present study, FLS epidemic progress was evaluated in a field trial with a natural intensity gradient created by combining dose of fungicide, number of applications and different application timings. The experimental design was Randomized Complete Block with four repetitions. Weekly assessments of disease severity was estimated as the average number of spots per central leaflet (NL) during eight evaluations. Area under the disease progress curves (AUDPC) and soybean grain yield were estimated. Damage functions were determined by linear regression analysis between grain yield as dependent variable (y) and FLS NL as independent variable (x) for growth stages R3, R4 and R5, for lower (L), upper (U) half of canopy and average between them (M). Equations were expressed on the basis of grain yield (Y) normalized to 1 ton/ha in the form Y = 1000 – a (NL). Where (a) is the damage coefficient (ton/lesion.ha) (Cd); Y is the grain yield normalized to 1 ton/ha, and NL is the FLS intensity (average number of lesions/central leaflet). For example, the equation obtained for R3 growth stage, Y = 1 -0,009 NL, represents a reduction of approximately 9 kg/ha per every average spot per central leaflet and for each ton of expected grain yield. The 9 equations obtained may be used to calculate the EDT for FLS in susceptible soybean cultivars. This is the first report of an EDT for FLS.