Correlation between mycelium-inoculated detached leaf and field assessments of resistance to Cercospora canescens in mungbean

Abstract

Cercospora leaf spot (CLS) caused by Cercospora canescens is a serious disease of mungbean (Vigna radiata) in Thailand. For the selection of resistant varieties in a breeding program, a rapid screening of numerous genotypes for CLS resistance is necessary. Most screening methods use spores as inoculum, but their productivity on culture media is low. The objective was to develop an efficient laboratory inoculation method using mycelium. We compared resistance/susceptibility levels of 19 mungbean genotypes from detached leaf inoculation using mycelium from the most virulent C. canescens isolates (SUT1, SUT4, PAK1, and PAK2) with field inoculations. The resistance levels of mungbean genotypes evaluated by the detached leaf inoculations were comparable to the field inoculations when comparing genotypes with a correlation coefficient of 0.822 (p< 0.01). In addition, the paired t-test revealed no statistically significant difference (p> 0.05) between the average disease severity scores obtained from the detached leaf assay and those from field evaluation. Both inoculation methods consistently identified mungbean genotypes V4718, V4785, V4758, and SUPER5 as resistant to CLS, which will be useful for future breeding programs. These results suggest that the mycelium-inoculated detached leaf assay was as effective as field inoculation for inducing disease symptoms and identifying resistant genotypes. Moreover, mycelium inoculum exhibited much simpler and faster to prepare in large quantities than spore inoculum, allowing effective and economical large-scale screening with multiple isolates to ensure reliability.