A Quantitative Model for Describing the Sporulation ofPlasmopara viticolaon Grape Leaves

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Abstract
Leaves from potted grape vines of Vitis labrusca ''Catawba'' infected with downy mildew were placed in a dark moist chamber maintained at constant temperatures of 10, 15, 20, 25, and 30 C. A humidifier, located within the chamber, maintained atmospheric moisture at or near saturation. Plants were removed for observation after 6, 7, 5, 9, 10.5, and 12 hr. The largest number of spores, approximately 300,000 sporangia per square centimeter of lesion area, were produced after 12 hr at 20 C, while no sporulation was observed at any time at 10 or 30 C. When the asymptote, rate, and intercept parameters of the Richards model were represented as quadratic functions of temperature, the model fitted to the pooled data described 90% of the variation in number of sporangia. The model produced that approximately 50% of the sporangia were formed at 20 C after 8 hr of incubation. Although sporangia were produced after only 7.5 hr of incubation, they failed to germinate, possibly indicating a lack of maturity. However, germination increased as the duration of the sporulation period was increased, until a maximum of 70% was observed for sporangia harvested after 12 hr at 25 C.

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