Submitted to: Plant Health Progress
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/10/2008
Publication Date: 6/18/2008
Citation: Wrather, A., Shannon, G., Carter Jr, T.E., Bond, J., Rupe, J., Almeida, A. 2008. Reaction of Drought Tolerant Soybean Genotypes to Macrophomina phaseolina. Plant Health Progress.Online. doi:10.1094/PHP-2008-0618-01-RS. Interpretive Summary: The USDA-ARS unit at Raleigh, NC has been researching various aspects of drought tolerance in soybean. Slow wilting drought tolerant types have been identified. However, these slow wilting types are not well characterized for disease resistance. One disease is of particular importance because it occurs almost exclusively under drought situations. That disease is charcoal rot. To date, no immunity to charcoal rot has been found in soybean cultivars and germplasm. Only a few partially resistant genotypes have been identified. It occurred to us that the slow wilting types we have discovered, by virtue of their favorable performance under drought, might also be resistant to charcoal rot and that disease resistance might be, in fact, a part of the drought resistance.
Technical Abstract: Charcoal rot caused by Macrophomina phaseolina is a common disease of soybean, and resistant genotypes are not available. Level of soybean genotype resistance and susceptibility to M. phaseolina is most frequently measured by determining colony forming units of M. phaseolina/g root, but using this method is labor intensive. This method also has been used to determine charcoal rot resistance among common bean genotypes. However, resistance in common bean to M. phaseolina was associated with drought tolerance, and selection for drought tolerant common bean genotypes was less labor intensive. Our objective was to determine if there were differences in root colonization by M. phaseolina between drought tolerant and susceptible soybean genotypes. Genotype colonization by M. phaseolina at the R6 and R8 stages of growth were not related to drought tolerance in this study. Some drought-tolerant soybean genotypes may be resistant to M. phaseolina, but our results suggest that selection of drought-tolerant soybean genotypes as a means for selecting M. phaseolina-tolerant genotypes is not practical. We tested this hypothesis in the field from 2003-2005, testing the slow wilting types for colonization by the disease organism in comparison to check cultivars. Results indicated that the slow wilting types were no different from normal cultivars in terms of charcoal rot susceptibility. Thus,the slow wilting drought-tolerant nature of our lines was not due to disease resistance. We infer that the mechanism of resistance may be physiological in nature.