Weather Variables in Relation to an Epidemic of Onion Downy Mildew

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Abstract
Development and spread of downy mildew (P. destructor (Berk.) Casp.) from a point source of inoculum was observed in relation to weather factors in a field plot. Disease developed in a succession of 2 minor steps and 2 major steps of spread and intensification. Production of spores (sporangia) in the predawn hours required temperatures below 23-24.degree. C on the previous day and continuous high relative humidity (.gtoreq. 95%) at favorable temperatures from 0200 h until dawn. No sporulation was observed on portions of leaves with moisture films of rain or dew. Initial dispersal of spores at 0700 hours occurred 1.5 h after sunrise but generally did not coincide with initial decline in relative humidity, with initial rise in temperature, with initial wind, or with the end of wet periods. Peak dispersal of spores at 0800-0900 h coincided with declining relative humidity, drying of the leaves, and increasing wind speeds. Spores were trapped mainly when wind speeds in the onion canopy were 0.3-1.0 m/s. Spores apparently survived and infected the leaves on days with persistent moisture and cloud cover but not on dry and sunny days. The steplike pattern of the epidemic appeared related to a sequence of short periods (< 1-2 days) required for sporulation and infection alternating with long latent periods (10-16 days).

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