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.
Root production of four cultivars of flue-cured tobacco was quantified in the field, greenhouse and phytotron. The cultivars ranged in level of partial resistance to the black shank pathogen, Phytophthora parasitica var. nicotianae, from susceptible to highly resistant. In the field, root-observation plates were installed approximately 10 cm from plants, and in greenhouse and phytotron studies, plants were grown in 4-liter containers with one sloping transparent side for root observation. Root growth was determined weekly for four weeks after transplanting in the field and daily up to 14 days after transplanting in the greenhouse and phytotron. Root tracings were made on acetate sheets placed against the sloping transparent side of the containers or against the transparent observation plates in the field following removal of soil from the outside of the observation plate. Root growth was quantified by retracing the root pattern on the acetate sheets over a digitizing tablet attached to a personal computer. Numbers of roots, root length, and mean and maximum rate of root growth were determined. Cultivars Hicks (susceptible) and K-326 (low level of resistance) had significantly larger root systems than moderately resistant G-28 or highly resistant NC 82. Differences in total root length were due to increased branching that resulted in development of significantly greater numbers of roots in Hicks and K-326. For example, between day 21 and 28, Hicks produced more than three times the number of new roots as NC 82 in the field. The mean rate of root extension observed (2.17 mm hr-l) was similar in all four cultivars. Infection efficiency on the different cultivars was determined in the field by inoculating roots with zoospores of P. p. nicotianae. Lesions were visible as water soaked areas within 24 hr of inoculation. At 48 hr after inoculation, percentages of inoculations that resulted in lesion formation were 57, 46, 23, and 16% for Hicks, K-326, G-28 and NC 82, respectively. The possible role of rooting intensity as a mechanism of avoidance to P p. nicotianae in tobacco cultivars is discussed.
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