Novel seed inoculation method for early selection of soybean genotypes resistant to target spot

Abstract

Target spot, caused by Corynespora cassiicola, is a major soybean disease that may lead to early defoliation and yield loss. This study aimed to develop and validate a diagrammatic scale for assessing target spot on soybean cotyledons; compare seed and spray inoculation methods in V4 stage plants under greenhouse conditions; and correlate the severity of artificial inoculations with natural field conditions. Cotyledons with varying severity levels were photographed and analyzed using the Assess® software. The resulting scale included five severity levels based on the Weber-Fechner law: 0.3, 2.9, 20.8, 70.0, and 95.4 %. The use of this scale improved the precision and accuracy, even for inexperienced evaluators. Thirty-five genotypes (26 lines and nine cultivars) were tested under controlled conditions and later in field trials carried out in Lucas do Rio Verde and Campo Novo do Parecis (Mato Grosso state, Brazil). Both inoculation methods differentiated the genotype responses, with seed inoculation showing a higher repeatability. The trial in Lucas do Rio Verde was more informative because of the greater disease severity and showed a positive correlation with the greenhouse results.

KEYWORDS: Corynespora cassiicola, Glycine max, genetic resistance.