Development of an Infection Efficiency Model forPlasmopara viticolaon American Grape Based on Temperature and Duration of Leaf Wetness

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Abstract
The infection efficiency of Plasmopara viticola was determined for the American grape, Vitis labrusca "Catawba''. Leaves on potted vines were inoculated with the fungus and exposed to a range of wetness durations (1-15 hr) at each of six fixed temperatures (5-30 C) in a growth chamber. The Richards function was then used for describing the results. The maximum infection efficiency obtained at each temperature, a measure of the asymptote parameter of the Richards model, was derived as a function of temperature using a second-order polynomial; the models fit to the pooled and averaged data described 84 and 96%, respectively, of the variation in this parameter. An optimum maximum efficiency of 0.07-0.08 occurred at 15-20 C, whereas little or no disease occurred at the 5 and 30 C extremes. Similarly, the rate parameter of the Richards model also could be expressed as a quadratic function of temperature; the models fit to the pooled and averaged data explained 72 and 82%, respectively, of the variation in the rate. Values of this parameter ranged from 0 at 3.8 and 30 C to 0.36 at 16.9 C. After substituting the polynomials for the asymptote and rate parameters in the linearized version of the Richards function, the model described 73% (pooled data) and 84% (averaged data) of the variation in infection efficiency. At 15 and 20 C, the efficiency rapidly increased from approximately 0 after 2 hr wetness to 0.06 after 4-5 hr; subsequent increase was gradual until a maximum of 0.08 was reached at 15 h and 15 C. At 10 and 25 C, the initial increase required approximately 8-10 hr of wetness before leveling off at an efficiency of 0.05-0.06.