Evaluation of resistance components to Plasmopara viticola in grapevine genotypes

Downy mildew (DM) caused by the oomycete Plasmopara viticola is one of the most important grapevine diseases. The use of DM resistant grapevine varieties is getting increasingly consideration and importance in Europe, as a possible mitigation measure to reduce the use of fungicides. The goal of the present work is to characterize the resistance components to DM in grapevine genotypes (specifically Regent, Rkatsiteli, Bronner, Johanniter, Solaris, 32-078, 34-111, Villaris, Calandro, Felicia, Reberger, Palava, Royal Merlot, Gf.93-22-6, Early Merlot) bringing different resistance genes (Rpv 3, 4, 10, 11 and 12) from Muscadinia and Vitis American species. The following resistance components were assessed in monocyclic experiments carried out under environment controlled conditions: (i) infection efficiency of sporangia (as a percentage of the inoculation sites showing DM symptoms); (ii) AUDPC (area under the disease progress curve); (iii) incubation length, (iv) latency period, (v) sporangia produced per unit of infected area (as number of sporangia per mm2 of lesion area); and (v) infectiousness of the produced sporangia. Resistance components were assessed after artificial inoculation of leaf discs with a natural population of P. viticola. Leaf discs were excised from fully developed, young leaves collected in 2014 and 2015 at shoot growing and at flowering/fruit set. Resistance components were expressed at different degrees in the different genotypes and at the different sampling times, with significant differences in comparison to the susceptible Vitis vinifera variety used as reference (Merlot). For instance, infection efficiency ranged between 60 and 99%, the latency period lasted between 104 and 132 degree-days, and the production of sporangia varied between 80 and 1.1×103 sporangia per mm2 of DM lesion. The research was partially funded by the European collaborative project “Innovine”, that has received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement n° 311775.