1a. Objectives (from AD-416):
Obj 1: Evaluate soybean germplasm in MG I, II, and early III for resistance to charcoal rot under greenhouse conditions. Obj. 2: Assess variability in aggressiveness among soybean isolates of Macrophomina phaseolina from geographically diverse areas. Obj. 3: Assess variability in aggressiveness among isolates of M. phaseolina from diverse hosts against soybean.
1b. Approach (from AD-416):
Obj 1: Soybean accessions (USDA-ARS Soybean Germplasm Collection) and commercial soybean varieties in MG I, II, and early III, will be evaluated under greenhouse conditions for charcoal rot development using previously reported methods. Obj 2: Establish a collection of isolates of M. phaseolina from geographically diverse regions of the US and from multiple host (primarily soybean, corn, and alfalfa). Isolates from the southern US will be obtained from existing collections while plant expressing symptoms of charcoal rot or from areas with a history of the disease, will be collected to obtain northern US isolates. Isolates will be evaluated for aggressiveness towards susceptible and moderately resistant soybean genotypes, representing both early and late MG, under greenhouse conditions by using methods employed for Obj 1.
3. Progress Report:
Objective 1. Thirty and 70 commercial soybean varieties submitted to the 2012 Wisconsin and Indiana, Variety trials, respectively, were evaluated in replicated experiments for resistance to charcoal rot using the cut-stem technique developed by ARS scientists at Urbana, IL. Three varieties in MG group I-II and five in MG II-III, were identified as having resistance to charcoal rot equal to or better than the resistant check, DT97-4290. Objectives 2 and 3. Six isolates of Macrophomina phaseolina, the causal agent of charcoal rot, three from the southern U.S. and three from the north, were evaluated at six temperatures to determine the influence of temperature on growth and development. Results from these preliminary studies indicated that northern isolates had more growth at temperature extremes (15°C and 38°C) than southern isolates. In addition, overall production of microsclerotia (inoculum of M. phaseolina) was greater for southern isolates than for northern isolates. Additional isolates of M. phaseolina from these two geographical regions were obtained through collaborations with scientists at Purdue University, University of Wisconsin-Madison, and Michigan State University, as well as ARS scientists in Jackson, TN. A total of 42 isolates, 21 from each geographic region, are currently being evaluated at three temperatures (15°C, 30°C, and 40°C) in repeated experiments. Based on results obtained so far, isolates from the northern U.S. appear to be better adapted to differences in temperature than isolates from the southern U.S. The same set of 42 isolates is also being used to determine differences in aggressiveness as well as the influence of host x pathogen interactions on disease development.