The significance of stilbene phytoalexins in the Plasmopara viticola-grapevine interaction

Abstract

Among 17 Vitis spp. tested for Plasmopara viticola resistance, cultivars were selected that allowed no sporulation, limited sporulation, or heavy sporulation of the parasite. With the exception of one group of Vitis spp. which exhibited resistance despite an inability to synthesize high concentrations of stilbene phytoalexins, resistance was positively correlated with the capacity for the synthesis of resveratrol and ε-viniferin following induction by UV irradiation. The necrotic reaction of plant tissue after inoculation was correlated with phytoalexin accumulation rather than with reproduction of Plasmopara viticola.

The phytoalexin response of the host was dependent on inoculum density. In three selected Vitis species where the resistance was correlated with phytoalexin production, increased inoculum densities resulted in characteristic stilbene synthesis for each cultivar. The resistant Vitis species, i.e. cultivar, cv. Castor accumulated five times the maximum resveratrol concentration of the susceptible “Riesling” and maintained these high levels during pathogenesis whereas the resveratrol concentration in cv. Riesling decreased. The intermediate Vitis rupestris 6544 accumulated resveratrol concentrations comparable to those of cv. Castor in the first few hours following inoculation, but then the levels declined considerably. Associated with the time of fungal sporulation was an increase in resveratrol concentration. ε-Viniferin was not detected during pathogenesis of Plasmopara viticola in the three Vitis spp.

Reproduction of Plasmopara viticola was directly related to inoculum density. Increasing the number of applied sporangia resulted in more rapid colonization of plant tissue, as well as in earlier sporulation in cv. Riesling, and caused a loss of resistance in V. rupestris 6544 and cv. Castor.

Resveratrol was stable in water and sporangia suspensions where it inhibited germination by 75% at 60 mg ml⁻¹. ε-Viniferin at the same concentration degraded in water but more rapidly in sporangia suspensions and was half as fungitoxic as resveratol. The results suggest that reservatrol is more important for the regulation of the Plasmopara viticola-grapevine interaction than ε-viniferin.