Influence of Conidia Dispersal and Environment on Infection of Grape by Guignardia bidwellii

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Abstract
Artificial field inoculations with suspensions of conidia (5 .times. 104/ml) of G. bidwellii were conducted in a black-rot-free Vitis labrusca ''Concord'' vineyard at East Lansing, Michigan [USA], 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 runoff from leaf lesions bearing pycnidia occurred during the weeks of July 10th, 1974 (4.9 .times. 105 conidia/ml of rainwater collected) and July 2nd, 1975 (6.1 .times. 105 conidia/ml of rainwater collected). Conidia also were trapped in rainwater runoff from newly rotted berries, with a maximum catch of 2.2 .times. 105 conidia/ml of rainwater collected the week of Sept. 3rd, 1975. Conidia were trapped from overwintered rotted berries in 1976, with a maximum catch of 4.4 .times. 105 conidia/ml of rainwater, which occurred the week of July 23rd. 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 3 factors) and amount of rain (cm) was positively correlated with infection (P = 0.075).

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