Identification of Soybean Accessions Resistant to Macrophomina phaseolina by Field Screening and Laboratory Validation

Charcoal rot of soybean, caused by Macrophomina phaseolina (Tassi) Goidanich, has been a problem for soybean farmers in the United States for many years. However, recently its incidence and severity has increased in midwestern and north-central states. Most management strategies, including chemical and biological methods and other cultural options, have been ineffective in controlling this disease. Furthermore, soybean is often planted every other year or in monoculture. Although genetic resistance would be the most sustainable control strategy, resistant commercial soybean cultivars are presently unavailable. The objective of this test was to identify resistant accessions from the USDA soybean germplasm collection in Maturity Groups (MG) 00 to VII by field screening in five environments. A total of 628 accessions were evaluated using established methods for reaction to this disease in Missouri and Illinois (2008 to 2009), and in Tennessee (2009). In 2010, accessions with disease severity reactions of 1 to ≤ 2 were selected for further evaluation in Tennessee. Among the 45 accessions identified as resistant and moderately resistant (1 to ≤ 2), ten were further assessed using colony forming units recovered from ground root and stem tissues which was then converted to colony forming unit index (CFUI) and validated their resistance using this bioassay. All ten had CFUI levels ranging from 0 to 18.2, which were significantly lower (P ≤ 0.05) than the moderately resistant accession, DT97-4290, with 31.8. Four selected accessions, PI594302 (MG VII), PI567562A (MG IV), PI506764 (MG VII), and PI567334 (MG VI), had significantly lower (P ≤ 0.05) CFUI of 0, 0.8, 0.8, and 1.5, respectively. This is the first report of soybean accessions with better levels of resistance than the standard released germplasm, DT97-4290. These accessions can serve as additional sources of charcoal rot resistance in future breeding programs. Accepted for publication 20 January 2013. Published 18 March 2013.