Development of an Inoculation Technique and the Evaluation of Soybean Genotypes for Resistance to Coniothyrium glycines.

Red leaf blotch, caused by Coniothyrium glycines, is a serious disease affecting soybean in several African countries but has not yet appeared in the United States. The fungal pathogen is listed as a Select Agent by the Federal Select Agent Program because of potential consequence to U.S. agriculture if introduced. Sources of resistance to red leaf blotch have not yet been identified in spite of extensive field testing in Zambia and Zimbabwe. A seedling inoculation protocol was used to investigate the effects of inoculum density, temperature (20 and 25°C), and dew chamber incubation period (2 to 5 days) on disease development in 'Williams 82' soybean. Inoculum density and incubation period had significant effects on disease severity but no significant difference was observed at 20 and 25°C. Highest levels of disease were obtained using 1.94 cm of mycelial inoculum per 10 ml of water and incubating plants for 5 days at 100% relative humidity. Significant differences were not observed between isolates from Zambia and Zimbabwe at 25°C. In total, 23 soybean genotypes that represent nearly 90% of the genes present in U.S. soybean were evaluated and found to be susceptible, which is consistent with previous field evaluations. This method could be useful in identifying new sources of resistance to red leaf blotch.