Reaction of field populations of sunflower downy mildew (Plasmopara halstedii) to metalaxyl and mefenoxam

Abstract

Seed treatment with phenylamide fungicides (metalaxyl and mefenoxam) provides an effective chemical control of sunflower downy mildew caused by the oomycete Plasmopara halstedii. Resistance of the fungus to metalaxyl has been reported in Spain and differences of disease incidence (DI) caused by field populations (FPs) have been observed. The sensitivity of several FPs to seed treatment with metalaxyl was compared. Different groups of FPs were discriminated on the basis of DI caused on sunflower plants. None of the FPs was completely controlled by the fungicide at the dose used. In addition, the efficacy of metalaxyl and mefenoxam against FPs resistant to metalaxyl were evaluated. Three genotypes of sunflower were treated with the fungicides at two different doses. Neither metalaxyl or mefenoxam nor doses had a significant effect on the DI caused by the FPs, suggesting the absence of effect of the double quantity of active enantiomer provided by mefenoxam as compared with metalaxyl. Nevertheless, significant differences on DI were observed among genotypes. The different DIs could be related to: a) different germination rates and, consequently, size of the radicle in the moment of inoculation, b) higher amount of fungicide provided to the confectionery genotype, which has a larger size of seed.