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 plants 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 using methods employed for Obj 1.
3. Progress Report:
Objective 1. Three protocols for evaluating resistance to charcoal rot were evaluated using a standard maturity group (MG) IV resistant and susceptible soybean variety (courtesy of ARS scientists in Stoneville, MS) under greenhouse conditions. The method developed by ARS scientists at Urbana, Illinois provided the most consistent results and was best suited for efficient screening of a large number of soybean varieties. Commercial soybean varieties submitted to the 2012 Wisconsin and Indiana Variety Trials were obtained from collaborators at the University of Wisconsin-Madison and Purdue University, respectively. All lines have been evaluated in at least one trial using the previously mentioned protocol and replicated experiments are ongoing. Results to date suggest that resistance to charcoal rot is available in some commercial varieties. Objectives 2 and 3. Isolates of Macrophomina phaseolina, the causal agent of charcoal rot, were obtained through collaborations with ARS scientists at Jackson, Tennessee and from soybean plants collected in Wisconsin. These isolates were evaluated at six temperatures from 15°C- 38°C, to determine the influence of temperature on growth and development. Isolates from Wisconsin showed more growth at 15° C and 38° C than those from the southern U.S. Southern isolates showed more growth at 20° C and 25° C than Wisconsin isolates, while no difference in growth was observed among isolates at 30° C and 35° C. Overall, production of microsclerotia (inoculum of M. phaseolina) was greater for southern isolates than those from Wisconsin. In collaboration with scientists at Mississippi State University, the collection of isolates of M. phaseolina has expanded and now includes isolates from diverse hosts and states throughout the U.S. These isolates will be used to further study the influence of temperature on growth and development as it relates to the isolates geographic origin, as well as for assessing variability in aggressiveness.