1a.Objectives (from AD-416):
To idenify new sources of charcoal resistance in adapted and exotic germplasm.
1b.Approach (from AD-416):
A uniform cultivar screening protocol will be developed and implemented. A set of standard susceptible cultivars will be used. These standard susceptible cultivars will represent early and late cultivars with in maturity groups 3 through 5. Tests will be established in naturally infested fields supplemented with adding inoculum at planting. Disease will be evaluated by rating the development of foliar symptoms of charcoal rot, rate of premature plant death, and by splitting the stems at harvest, visually determining the extent of stem colonization and quantifying colonization by determining the colony forming unit of M. phaseolina. Yields will be taken. Test cultivars will be selected based on previous observations and from other charcoal rot screening tests in the field and in greenhouse and laboratory assays. Soil temperature and moisture and rainfall data will be taken and related to disease development.
This project will identify resistance to charcoal rot through a multi-state evaluation program and refine and develop effective field and greenhouse screening methods for this disease. Charcoal rot is one of the most yield limiting soybean diseases in the U.S. and around the world, and there are no known resistant commercial cultivars yet available for growers. Progress in identifying charcoal rot resistance has been slow because the disease is sporadic from year to year, results from individual states are difficult to compare because of differences in experimental approaches, and the reaction of resistant cultivars identified in greenhouse assays are often not confirmed under field conditions. To address these issues, ARS scientists have developed a regional field screening protocol that will allow us to evaluate cultivars in standardized, multi-state field trials. The main objective for this study is to evaluate breeding and exotic soybean lines and confirm cultivars thought to be resistant to charcoal rot. Seventeen accessions consisting of maturity group (MG) III, MG IV, and MG V were evaluated for resistance against the charcoal rot fungus in Tennessee over the last two years and will continue to be evaluated over the next several years. These genotypes were grown, and disease assessments were made in no-till and non-irrigated environments. The results show that among the 17 lines tested, there was no genotype that was immune to Macrophomina (M). phaseolina, but there were genotypes with moderate levels of resistance. Five of the genotypes, namely DT97-4290, DT991755, DT991686, DT987553 and DT991748, had the lowest colony forming unit (CFU) values. This result validates previous research by ARS scientists. DT97-4290 had a higher yield with 49 bushels per acre when tested under non-irrigated environment. The genotypes identified as having moderate resistance to M. phaseolina across years could be useful as a source of resistance in soybean breeding programs.