Diversity and longevity of the soybean debris mycobiota in a no-tillage system

The survival of the mycobiota on pod and stem debris of soybean produced in a no-tillage system with cover crops of alfalfa, canola, rye, or wheat or with no cover was studied during 1994 and 1995. Fiberglass mesh bags containing pods and stems were assayed every 28 to 31 days to determine the isolation frequency of fungi. Over 90% of the 11,906 isolates obtained were members of the Deuteromycotina. The most common genera isolated were Alternaria, Cercospora, Colletotrichum, Epicoccum, Fusarium, and Phoma. Alternaria spp. had the greatest isolation frequencies and constituted 40% of the total cultures. Numbers of total fungi (all fungi isolated) on sampling dates in 1994 were similar to the totals in 1995. In May 1994, the mean isolation rates for many of the fungal species were significantly lower (P = 0.05) in several of the cover crops, but no consistent pattern could be determined. Common soybean pathogens isolated included Colletotrichum spp., Diaporthe spp., and Cercospora kikuchii. Fusarium graminearum, which is responsible for several diseases of maize and wheat, was commonly isolated during this study. Of the Diaporthe spp. (anamorph Phomopsis spp.), 87% were identified as D. phaseolorum var. sojae. Colletotrichum spp. were identified as C. truncatum in 85% of the isolates, C. destructivum (teleomorph Glomerella glycines) in 12%, and both species in 3%. Cercospora kikuchii was more commonly isolated from pods than from stem tissue, and Colletotrichum spp. occurred more frequently on stems. Isolation frequencies of Diaporthe spp. were greater in May of both years than in the preceding months. These results show that no-tillage soybean debris harbors numerous fungi pathogenic to soybean, and producers who grow soybeans continuously may find more disease in this crop and lower yields. Fungi that attack crops such as maize and wheat were commonly isolated from soybean debris in both years, and a no-tillage rotation which includes maize or wheat could result in increased disease in these crops. Isolation frequencies of the fungi from cover crops varied with the sampling date, but no consistent patterns could be determined for a particular cover crop or fungal species. This is the first detailed study of survival rates of soybean, maize, and wheat pathogens that overwinter on soybean debris in a no-tillage system.