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 amndinacea 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.
Peronospora tabacina is an obligate plant pathogen that causes downy mildew disease on several species of Nicotiana, including N. tabacum (tobacco). The primary objective of this study was to use gnotobiotic associations to describe interactions between the pathogen and roots of either N. tabacum (cv. KY14) or N. repanda. We found that the pathogen was capable of moving systemically from shoots to roots of both host species and emerged from the root tissues as hyphae. We also demonstrated that root-associated hyphae were infectious on roots of nearby plants and resulted in new systemic infections. Following overnight darkness, sporulation of the pathogen was observed on infected roots exposed to air on both host species. We also found that within 2 months in culture, structures resembling resting stages of Peronospora tabacina were produced on hyphae emerging from roots of N. repanda but not N. tabacum. These findings appear relevant to both the epidemiology of the disease and to future studies of this and other downy mildew pathosystems.
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