CONTROL OF PATHOGENIC MYCORRHIZAL FUNGI IN MAINTENANCE OF SOIL PRODUCTIVITY BY CROP-ROTATION

Pathogenic mycorrhizal fungi may be a major reason crops must be rotated to maintain soil productivity. We studied the role such fungi may have in the maintenance of productivity of soil for tobacco (Nicotiana tabacum L.) by rotation with tall fescue (Festuca arundinacea Schreb). Tall fescue 'Kentucky 31' and continuous tobacco plots were established on a central Kentucky field previously shown to be infested with the mycorrhizal fungus Glomus macrocarpum Tul and Tul., the cause of a stunt disease of tobacco. Fumigation with 67% methyl bromide-33% chloropicrin (MBC, trichloronitromethane) covered with plastic was used as a control. Rotation with fescue for 2 yr and fumigation both reduced disease incidence when tobacco was planted on all plots the third year. Effects of rotation and fumigation were not additive, indicating that both control the same disease agent. Only G. macrocarpum, of three mycorrhizal fungi present in high populations, was associated with the disease. Rotation and fumigation controlled mycorrhizal colonization of roots and the build-up of populations of G. macrocarpum in the root zones of plants. Mycorrhizal fungi should be considered in research on the effects of crop rotation on productivity.