|Stetina, K - MISS STATE UNIV, DREC|
|Russin, J - SOUTHERN ILLINOIS UNIV|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 11, 2005
Publication Date: January 3, 2006
Citation: Stetina, K.C., Stetina, S.R., Russin, J.S. 2006. Comparison of Severity Assessment Methods for Predicting Yield Loss to Rhizoctonia Foliar Blight in Soybean. Plant Disease. 90(1):39-43. Interpretive Summary: Rhizoctonia foliar blight, caused by the fungus Rhizoctonia solani, is a disease that causes significant yield loss in soybean in the Gulf Coast states, especially Louisiana, Arkansas, and Mississippi. Normally, testing soybean lines for resistance is done when disease pressure is very high, but this may not occur in the field every year. A disease assessment method that would allow reliable evaluation of soybean lines under low to moderate disease pressure conditions could speed up the variety development process. Yield loss in this study was primarily due to loss of entire pods. Disease ratings based specifically on pod damage successfully identified differences between known resistant and susceptible varieties and were better than conventional foliar ratings at detecting these differences under low-disease conditions. The pod-based disease rating described in this study will allow soybean breeders to reliably test varieties for resistance to this disease even in years when disease pressure is low. The resulting timely release of resistant soybean varieties will benefit soybean growers in this region of the United States.
Technical Abstract: Rhizoctonia foliar blight, caused by Rhizoctonia solani Kühn anastomosis group 1, causes rapid and severe destruction of soybean foliage and pods during hot, wet weather. The objectives of this study were to determine the mechanism(s) by which the disease limits yield and whether rating pod damage or foliar damage provides a more reliable method of assessing disease severity. Disease severity in the moderately resistant variety NK S57-11 and the susceptible variety Buckshot 723 was assessed in field plots in 1996 and 1997 using ratings (0 to 10 scale) of both foliar and pod disease. Numbers of full and partially filled pods, number of seeds, total seed weight, and weight of 100 seeds were determined. Regression results showed that pod number, seed number, and yield decreased as disease severity increased, while the proportion of partially filled pods and the weight of 100 seeds were not affected. Data suggest that yield loss was primarily due to loss of entire pods. Foliar and pod assessments of disease severity correlated positively in 1996 (r=0.8343) and 1997 (r=0.5958) across both varieties, which suggests that either method can be used to identify relative differences among varieties. However, pod assessments explained more variability than did foliar assessments under low-disease conditions. Plants that showed moderate to severe symptoms of Rhizoctonia foliar blight also retained green stems and pods at harvest, which was evidence of delayed maturity.