Ecology and Epidemiology
The infection efficiency of Plasmopara viticola was determined for the American grape, Vitis lambrusca Catawba. Leaves on potted vines were inoculated with the fungus and exposed to a range of wetness durations (115 hr) at each of six fixed temperatures (530 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 0.070.08 occurred at 1520 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 45 hr; subsequent increase was gradual until a maximum of 0.08 was reached at 15 hr and 15 C. At 10 and 25 C, the initial increase required approximately 810 hr of wetness before leveling off at an efficiency of 0.050.06. |