Effectiveness of beneficial bacteria to promote systemic resistance of grapevine to gray mold as related to phytoalexin production in vineyards

The rhizospheric Bacillus subtilis PTA-271 (271) and endophytic Pseudomonas fluorescens PTA-CT2 (CT2) and Pantoea agglomerans PTA-AF2 (AF2) bacteria are able to induce systemic resistance (ISR) in grapevine against B. cinerea, but ISR markers and their costs remained unknown in vineyards. In this study, we investigated the relationship between the effectiveness of single and binary combinations of selected bacteria to induce ISR and their ability to trigger phytoalexin accumulation, as a potential marker for disease resistance, in leaves and berries, as well as their impact on grape yield in vineyards. Grapevine plants were treated during 2006 in two vineyards by drenching soil with single or binary mixtures of bacteria. Induced resistance against B. cinerea was evaluated and stilbenic phytoalexins were analyzed by HPLC in both leaves and berries. Grape yield was also assessed as number and weight of clusters at ripening. Both single and mixtures of bacteria were effective in reducing gray mold severity in the leaves and berries in vineyards. Disease control was accompanied by a significant accumulation of stilbenic phytoalexins, trans-resveratrol and e-viniferin, in both leaves and berries in the bacterized plants. δ-Viniferin also accumulated, but only in berries of the treated plants. Reduction of disease symptoms and accumulation of resveratrol and viniferins were higher in the plants treated with single CT2 compared to AF2 and 271. Treatment of grapevine plants with binary mixtures of these isolates resulted in a significant performance of CT2+AF2 in leaves and CT2+271 in berries. On the other hand, bacterial treatments did not show any negative effect on grape yield. These results revealed the efficacy of CT2 alone or in combination with AF2 or with 271 in triggering grapevine resistance against B. cinerea and enhancing systemic accumulation of resveratrol and viniferins, without compromising grape yield.