A new model for secondary infection of grapevine downy mildew

The life cycle of Plasmopara viticola (Berk. & M. A. Curtis) Berl. & De Toni, the causal agent of grapevine downy mildew, consists of sexual and asexual cycles that occur throughout the grape-growing season. Within 5 to 10 days after the initial infection, sporangiophores and sporangia are produced and form a dense, raised, white-cottony mildew on the underside of lesions. Sporangia become airborne and release clonal zoospores as secondary inoculum that are able to infect all green tissues of the vines following the same process as in primary infections. Secondary disease cycles occur under suitable conditions for sporulation, dispersal and infection. Different models have been developed to simulate the progress of downy mildew epidemics due to infections caused by asexual sporangia of P. viticola in Switzerland, France, Austria, Germany, and Italy. All these model have been developed using common, old literature (mainly referring to experiments carried out under laboratory conditions or with host plants different from Vitis vinifera) and work in a similar way; they provide a prediction of disease severity or a risk index that guides the scheduling of fungicide applications. Recent studies were performed on P. viticola sporulation, dispersal and infection that provided new knowledge that can be incorporated in the disease models. Therefore, a new, weather-driven, mechanistic model for secondary infection of P. viticola was developed based on the new knowledge following the systems analysis. The process leading to secondary infections was separated into different stages: from the appearance of oil spots on leaves, to repeated sporulation on lesions, sporangial dispersal to new leaves, survival of sporangia, and finally infection. The system changes from one stage to another at different rates, according to mathematical equations which depend on the environmental conditions. Based on a recent study carried out under vineyard\u2019s conditions, 3 consecutive wet hours at nighttime are used into the model to predict sporulation of P. viticola. Once sporulation has initiated, sporangia are produced at high rate for the first 4 days and then sporulation slows down until it stops; the production of sporangia increases with humidity of the sporulation period. The sporangia are released over a wide range of weather conditions, including dry periods. Based on another study carried out under environmental controlled conditions, zoospores are released from sporangia after 6 to 9h at 10 to 25\ub0C and 5\ub0C, respectively. On sensitive varieties of V. vinifera, sporangia cause infection, even if with low efficiency, after 1h of wetness when incubated between 10 to 25\ub0C, and infection efficiency increases with increasing duration of the wet period. Incubation and latency periods are both influenced by temperature; below 15\ub0C, P.viticola causes infection but sporulation never occurred. The model was used for scheduling fungicide applications for a 2-year period in 20 organic vineyards across Italy providing the same disease control of the calendar-based grower\u2019s applications, with 38% less copper