FIRST REPORT OF MYCOLEPTODISCUS TERRESTRIS INFECTING FORAGE LEGUMES AND SOYBEANS IN WISCONSIN

Authors: Smith, Richard; GRAU, CRAIG - UNIV OF WISCONSIN-MADISON; Gray, Lynn

Submitted to: Plant Disease
Publication Type: Research Notes
Publication Acceptance Date: 11/14/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: In recent years a soilborne fungus has caused stand failure in Wisconsin of the perennial forage crop, birdsfoot trefoil, and yield losses in soybeans. The fungus has been identified as Mycoleptodiscus terrestris, often called just Mycoleptodiscus or Myco. Subsequent to detecting this fungus on these two crops, we determined that red clover and alfalfa are equally as susceptible to the fungus. The fungus infects the roots and crowns of the plants and causes root and crown rot in the infected plants, eventually leading to death of the plant. Our research also suggests that very little resistance to the fungus can be found in the currently grown varieties of birdsfoot trefoil, red clover, alfalfa, and soybean. Although recognized in the soil in states south of Wisconsin, this is the first time this specific fungus has been implicated in poor health of these crops in Wisconsin. Warm temperatures and very moist conditions are necessary for the growth of the fungus and the development of the rotting disease on the crop plants. All these legumes are equally infected by the fungus, and they are grown throughout Wisconsin and the Midwest U.S. The significant implication of these results is that a potential exists for severe losses to occur in these crops when the environmental conditions are most favorable for infection.

Technical Abstract: 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 newly established and mature yield trials in 1994 and 1995 in Wisconsin. Dark brown lesions were evident on root and crown tissue which occasionally spread to newly initiated stem tissue. Isolates of the fungus produced dark green mycelium and black sclerotia on potato dextrose agar. Setaed conidia were 2-celled, oval in appearance, and measure 24-30 x 4.5- 7.4 millimicrons. These morphological characteristics of the fungal mycelium and conidia are typical of the fungal pathogen, Mycoleptodiscus terrestris. Similar isolates have been isolated from soybean plants grown in Wisconsin that expressed a premature decline symptom at the R6-7 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. M. terrestris isolates have also been baited from other soils from southern Wisconsin using red clover (Trifolium pratense L.), 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.