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First Report of Mycoleptodiscus terrestris Infecting Forage Legumes and Soybeans in Wisconsin

Soilborne plant pathogens are regarded as important causes of failures of newly established and mature stands of forage legumes and reduced yield of soybean (Glycine max (L.) Merr.) in the North Central Region of the U.S. Birdsfoot trefoil (Lotus corniculatus L.) plants exhibited decaying roots and stems in yield trials in 1994 and 1995 at the Arlington Agricultural Research Station, Arlington, WI. After first harvest, over 40% of the plants failed to produce regrowth or regrowth was extremely slow. Dark brown lesions were evident on root and crown tissue and occasionally spread to newly initiated stem tissue. A fungus resembling Mycoleptodiscus terrestris was isolated from birdsfoot trefoil by placing surface-disinfested plant tissue on potato dextrose agar (PDA) amended with 10 ppm chlorotetracycline or tetracycline and 100 μl liter^-1 of tergitol. Isolates of M. terrestris produced dark green mycelium and black scerotia on PDA. These same isolates also produced conidia with characteristics similar to those described by Ostazeski (1) for M. terrestris when cultures were grown on gamma ray-sterilized chrysanthemum leaf pieces positioned on the surface of 2% water agar on petri dishes. The setaed conidia were generally two-celled and oval, and measured 24 to 30 × 4.5 to 7.4 μm. Isolates were also obtained from soybean plants that expressed a premature decline symptom at the R6 to R7 growth stage. Soybean plants in two commercial fields initially expressed curled leaves followed by defoliation. Lower stems expressed a gray-tan discoloration of the cortical tissue. Alfalfa (Medicago sativa L.), red clover (Trifolium pratense L.), birdsfoot trefoil, and soybean were inoculated with isolates of M. terrestris. All crop species expressed symptoms and M. terrestris was recovered from symptomatic tissue. M. terrestris isolates have also been baited from other soils from southern Wisconsin with red clover, birdsfoot trefoil, and soybean. Although recognized in states south of Wisconsin, M. terrestris has not been implicated previously in poor health of forage legumes and soybeans in Wisconsin. This report provides evidence that M. terrestris inhabits agricultural soils farther north than previously recognized.

References: (1) S. A. Ostazeski. 1967. Mycologia 59:970-975.