Varietal response to soybean target spot (Corynespora cassiicola) in the Brazilian Neotropical Savannah

Brazilian soybean is mainly grown in the Cerrado, the South American Savannah. We report epidemiological studies of soybean target spot (STS), caused by Corynespora cassiicola in two Cerrado regions in the Federal District (DF) and the state of Bahia (BA), relating the environmental parameters with disease intensities. Here we: i) characterize the field response of soybean cultivars to STS and compute their respective latent periods; ii) relate STS severity to yield; and iii) verify the cross-reaction of C. cassiicola isolates of soybean and cotton to each of these hosts. Ten commercial soybean cultivars were cultivated in DF and BA fields. Additionally, the cv. reactions were studied in greenhouse assays in the spring/summer and autumn/winter seasons. Precipitation and relative humidity were higher in DF than in BA, whereas the temperature was cooler with fewer extremely hot days. Greater STS intensity was observed in DF, reaching 48% incidence and 19% severity. Fungicide applications reduced disease only in the DF trial. Cultivars were separated into four groups: highly susceptible, susceptible, moderately resistant, and resistant. Higher disease severities were recorded in the spring/summer than in the autumn/winter. Length of latent periods correlated with the cv. response to STS and climate conditions. Environmental differences were the probable causes for the lower disease levels recorded in the BA fields and in the autumn/winter trial. A negative relationship was established between severity and yield. In the cross-inoculation studies, isolates of soybean and cotton were able to infect both hosts, but aggressiveness correlated with the host of origin.