Evaluation of Soybean Entries in the Pan-African Trials for Response to Coniothyrium glycines, the Cause of Red Leaf Blotch

Red leaf blotch (RLB), caused by the fungus Coniothyrium glycines, is an important disease of soybean known to cause yield losses across soybean-growing regions in Africa. Fungicides are one option to manage this disease, but utilization of host resistance may be a better option suited for smallholder soybean farmers in Africa. Fifty-nine soybean entries were evaluated for RLB severity in nine field locations in Ethiopia, Kenya, Uganda, and Zambia. Disease incidence was 100% and disease severity differed (P < 0.01) among entries at eight of the nine locations. Mean severity ratings ranged from 1.4 to 3.2 based on a 0-to-5 scale, with higher disease severities recorded in Ethiopia followed by Zambia. Eight of the 59 entries were common to all nine locations and had severity ratings ranging from 1.6 to 2.9. The cultivar SC Signal had the lowest RLB severity ratings in the combined analysis over locations. Based on correlations of weather variables to RLB severity, mean rainfall from planting to 30 days before assessment date had a positive correlation (r = 0.70; P = 0.035), as did mean morning maximum wind speed (r = 0.88; P = 0.016). Other variables, such as temperature and relative humidity, did not correlate to RLB severity. This is the most comprehensive report to date on the occurrence of RLB in the region, which for the first time demonstrates an association between rainfall and wind speed with RLB severity. It also represents the first extensive report evaluating soybean genotypes for resistance against RLB in multiple environments.

Automated summary

Red leaf blotch (RLB), caused by the fungus Coniothyrium glycines, is an important disease of soybean in Africa. Fungicides can be used to manage the disease, but host resistance may be a better option. This study evaluated 59 soybean entries in nine locations and found a correlation between rainfall, wind speed, and RLB severity.