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Influence of Conidia Dispersal and Environment on Infection of Grape by Guignardia bidwellii. Donald M. Ferrin, Graduate Research Assistant, Department of Botany and Plant Pathology, Michigan State University, East Lansing, MI 48824, Present address of senior author: Yoder Bros., Inc., Ft. Myers, FL 33901; Donald C. Ramsdell, associate professor of Plant Pathology, Department of Botany and Plant Pathology, Michigan State University, East Lansing, MI 48824. Phytopathology 68:892-895. Accepted for publication 14 November 1977. Copyright © 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-892.

Artificial field inoculations with suspensions of conidia (5 × 104/ml) of Guignardia bidwellii were carried out in a black-rot-free Vitis labrusca ‘Concord’ vineyard at East Lansing, MI at various times during the 1975 growing season. Maximum leaf and berry infection resulted from inoculations made at mid-bloom and at 1-cm berry-diameter stages. Inoculations made after berry color change were not successful. Maximum conidia catches in rainwater run-off from leaf lesions bearing pycnidia occurred during the weeks of 10 July 1974 (4.9 × 105 conidia/ml of rainwater collected) and 2 July 1975 (6.1 × 105 conidia/ml of rainwater collected). Conidia also were trapped in rainwater run-off from newly rotted berries, with a maximum catch of 2.2 × 105 conidia/ml of rainwater collected the week of 3 September 1975. Conidia were trapped from overwintered rotted berries in 1976, with a maximum catch of 4.4 × 105 conidia/ml of rainwater, which occurred the week of 23 July. Healthy potted Concord grape ‘trap’ plants placed in the field for 1-wk periods, became infected only during weeks with rainfall. Environmental factors that correlated positively with infection were number of hours of leaf wetness following rainfall, number of rain events, duration of rain (P = 0.01 for all three factors) and amount of rain (cm) was positively correlated with infection (P = 0.075).

Additional keywords: epidemiology, grape black rot, Vitis sp.