Effect of light, temperature and leaf wetness on the infection efficiency of Plasmopara viticola, the causal agent of grapevine downy mildew

Although the Plasmopara viticola patogen life's cycle and the disease epidemiology have been studied intensively, quantitative dependency of some infection processes on environmental variables is still unclear. The combined effects of light, temperature, and wetness duration on the release of zoospores from zoosporangia and the infection efficiency of these zoospores were investigated trough environmental controlled and semi-field experiments. Zoospores were released after 6h of incubation between 10 and 25°C, after 9h at 5°C, and at lower percentages at 30°C.When these zoospores were inoculated on Vitis vinifera leaf disks, their infection efficiency were negatively related to the time elapsed between release and inoculation. It seems that swimming in water progressively reduced the infection efficiency of zoospores. Irrespective of the light regime, sporangia inoculated on V.vinifera leaves caused infection after only 1h of wetness when incubated between 10 and 25°C, nonetheless longer wetness duration was needed to reach the maximum infection efficiency: 3h at 20/25°C and 6h at 10/15°C. No infection occurred at 5 and 30°C. Combination of temperature/wetness duration also influenced disease severity. These results were fit to a nonlinear model to predict infection severity for each combination of temperature/wetness duration; this model can be used in infection prediction instead of the simpler criterion of Blaeser and Weltzien (1979) as occurs for V.labrusca (Lalancette et al., 1988). Incubation and latency periods were mainly influenced by temperature; when incubated at 10/15°C the pathogen required 9days for the appearance of symptoms, and only 4days at 20/25°C. Below 15°C, P.viticola caused infection but did not produce sporangia.