Differential effect of resistance inducers on the susceptibility of lettuce cultivars to Sclerotinia sclerotiorum and Botrytis cinerea

Abstract: The preventive use of resistance inducers has been shown to be an interesting method to reduce dependency on pesticides for plant protection. However, little is known on possible differences in the protective effects of such methods for different varieties of a given crop. In the present study, we assessed the effect of three compounds (acibenzolar-S-methyl, a calcium-based mineral compound and a yeast extract) for the protection of six lettuce cultivars against two major pathogens, Sclerotinia sclerotiorum and Botrytis cinerea. The compounds were sprayed on the plants three days before inoculation. A water spray was used as a control. The protective effect of the compounds was then assessed by comparing the size of lesions developing on inoculated leaves. For both pathogens, none of the compounds fully inhibited disease development. However, reduction in lesion size was observed on some of the leaves. The effect of the three compounds was different for the two pathogens. For tests with B. cinerea, effects of treatment were not statistically significant. In contrast, significant effects were found for cultivars inoculated with S. sclerotiorum. Overall, the yeast extract provided the highest level of protection against that pathogen. However, for all compounds, the extent of the protective effect depended on the cultivar. Furthermore, in some cases the effect the compound was opposite to that desired and disease was more severe on treated plants than on the water control. Possible consequences for field application of such methods will be discussed.