First Report of Charcoal Rot Epidemics Caused by Macrophomina phaseolina in Soybean in Iowa.

Macrophomina phaseolina, the causal agent of soybean charcoal rot, is widely present in soil and has been reported to cause yield losses of 30 to 50%, mainly in southern soybean production regions of the United States. (2). Charcoal rot was first reported in North Dakota during 2002, suggesting a range expansion for M. phaseolina (1). Charcoal rot has been occasionally observed in Iowa in individual soybean plants, but epidemics of the disease have not been recorded. During the 2003 growing season, a severe epidemic of charcoal rot was observed throughout the state. Diseased plants were first noticed in late July and by late August, patches of diseased plants wilted and died prematurely in many fields. The pith of diseased plants had a brown discoloration in taproots and lower stems. Symptoms were observed up to the 4th or 5th nodes, typical of charcoal rot. In some plants, no discoloration was evident. In discolored plants, microsclerotia of M. phaseolina were commonly observed in the epidermis, just beneath the epidermis, and inside taproots and lower stems of wilted plants. A systematic survey was conducted between late August and early September, 2003 to determine the prevalence and severity of charcoal rot in Iowa. The disease was observed in 60% of fields surveyed in northern Iowa (north of latitude 42.5°N), 90% in central Iowa (latitude 41.6 to 42.5°N), and 20% in southern Iowa (south of latitude 41.6°N). Incidence in surveyed fields ranged from 10 to 80%. One 80-ha field in Hampton (northern Iowa) had 50% disease incidence. An incidence of 80%, with extensive premature death prior to growth stage R6 (full seed), was observed in a 16-ha field east of Huxley (central Iowa). The fungus was isolated by splitting open the tap roots of a few representative symptomatic plants from 18 fields. Tissue colonized with microsclerotia of M. phaseolina was scraped, collected in sterile petri dishes, surface sterilized in 1% sodium hypochlorite, washed in distilled sterile water, and transferred to potato dextrose agar. The plates were incubated for 2 weeks at 22 ± 1°C on laboratory benches with a 12-h photoperiod. All resulting cultures produced abundant microscleorotia of M. phaseolina similar to those described by Smith and Wyllie (3). Pycnidia were observed in two cultures. August 2003 was the driest month on record in Iowa, which may have contributed to the outbreaks of this disease. Statewide, yield in Iowa soybean during 2003 was 1,976.1 kg/ha (2,798 kg/ha in 2002). The largescale epidemic of charcoal rot may have contributed to the overall reduction in soybean productivity in Iowa in 2003 (4). Since M. phaseolina also infects corn (2), and corn/soybean rotation is the most common cropping system in Iowa, efforts are needed to address the future risk of M. phaseolina to corn and soybean. References: (1) C. A. Bradley and L. E. del Rio. Plant Dis. 87:601, 2003. (2) D. C. McGee. Soybean Diseases: A Reference Source for Seed Technologists. The American Phytopathological Society, St. Paul, MN, 1992; (3) G. S. Smith and T. D. Wyllie. Charcoal rot. Pages 29-31 in: Compendium of Soybean Diseases. 4th ed. G. L. Hartman et al., eds. The American Phytopathological Society, St. Paul, MN, 1999. (4). X. B. Yang et al. Biology and management of soybean charcoal rot. Pages 55-60. in: Proc. 15th Integrated Crop Management Conf. Iowa State University, Ames. 2003.