Produção de esporos e inoculação de Corynespora cassiicola em soja

Abstract

The target spot is found in most soybean producing areas of Brazil. The name of the disease is due to the concentric rings, with center and darker yellow halos present in the patches, which resemble the shape of a target. Premature defoliation may occur in susceptible cultivars, and the decay of many spots on pods and stems. The objectives of this study were to morphologically characterize the causal agent of an isolated target spot of soybean, quantify the sporulation of Corynespora cassiicola on different substrates; evaluate the effect of temperature on germination of conidia; assess the reaction of soybean cultivars to the pathogen and to quantify the effect of different concentrations of inoculum on the intensity of the disease. The experiments were conducted at the Laboratory of Plant, home-de-vegetation and climate chambers of the School of Agronomy and Veterinary Medicine, University of Passo Fundo - RS. Comparing the symptoms of the disease with the descriptions in literature and the reisolations of the pathogen, followed by their morphological characterization, it is confirmed the identification and pathogenicity of C. cassiicola isolated from leaves of soybean. Assessing the sporulation of C. cassiicola on different substrates (Potato Dextrose Agar, Czapek Solution Agar, infant food, Malt Agar, Oat Meal Agar and V8 juice), Czapek solution agar in the presence of light photoperiod 12 hours, plus filter paper showed greater sporulation of C. cassiicola. Among the tested temperatures (6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 36, 37, 38, 39, 40 and 41° C) to assess the germination of conidia of C. cassiicola, temperatures to 23° C was the lower thermal threshold was 7° C and higher than 39° C. The cultivar most susceptible among the tested cultivars (Apollo BMX, BMX Power, BRS 243 RR, BRS 255 RR, RR 214 CD, RR 219 CD, Fundacep 56 RR, 59 RR Fundacep, P98C11 Pioneer, Pioneer P98R31) the number of injuries by leaflet was BRX Power, and the diameter of lesion, BRX Apollo. Fundacep 56 and 59 were the most resistant cultivars to the pathogen, both in number and to diameter of lesion. It was demonstrated that the concentration of 35x10³ conídios.mL-¹ causes a level of disease that can be used in assessing the reaction of soybean cultivars