Variability in the sensitivity of biotrophic grapevine pathogens (Erysiphe necator and Plasmopara viticola) to acibenzolar-S methyl and two phosphonates

The antifungal properties of two phosphonates (fosetyl-Al and a fertilizer) and acibenzolar-S-methyl (ASM) were evaluated to assess their potential for protecting grapevine leaves against grapevine mildews (Plasmopara viticola and Erysiphe necator), and to determine their effects on the development of various mildew isolates, taking into account the inter- and intra-species variability of the pathogens. The phosphonates directly and significantly inhibited the growth of these pathogens. By contrast, ASM had no direct effect on spore production and growth of P. viticola and of E. necator at 1.9 mM. Applied before inoculation, the mean EC50 of ASM was 0.50 ± 0.04 mM and 1.00 ± 0.07 mM for downy and powdery mildew isolates, respectively. The EC50 of the fosetyl-aluminium (FOS) was 0.50 ± 0.02 mM for downy mildew and the EC50 for powdery mildew varied depending on the genetic group under consideration (0.89 ± 0.32 mM for group B 3.30 ± 0.46 mM for group A, respectively). The EC50 of the potassium phosphonate fertilizer (PK2) was 0.96 ± 0.45 mM for downy and 6.9 ± 0.76 mM for powdery mildew isolates. These compounds showed differences in their efficacy depending on the variability of mildews and could be an alternative or additional method to traditional pest management in the grapevine.