Seed dressing to control downy mildew of basil.

Four experimental trials were carried out in order to evaluate the efficacy of different products (chemicals, resistance inducers and natural products) as well as heat air when applied to basil seeds naturally infested by Peronospora belbahrii, the causal agent of downy mildew of basil, a pathogen which is seed transmitted. Seed quality, as vigour index, was also evaluated. In general, seed treatment had a positive effect on the reduction of disease incidence and on plant biomass at the end of each the trial, with very satisfactory results when disease incidence was lower than 10% and still satisfactory results with a higher disease incidence. Although many of the fungicides, resistance inducers and thyme oil treatments tested showed a significant disease reduction compared with the untreated control, the protection offered was only partial. Moreover, the effectiveness of the tested seed treatments varied considerably between the trials. The seeds treatment with heat air (65°C for 10 min) significantly reduced the number of infected plants per container. The interaction effect between the fungicides (at the lowest dosage tested, with the exception of acibenzolar-S-methyl, and dimethomorph), thyme oil and the heat air treatment was also significant. In terms of biomass, most treatments not significantly improved the fresh weight in comparison with the untreated control. The highest biomass was observed in the plots where seeds were dressed with mancozeb combined with heat air. Interestingly, thiram is also effective against other pathogens affecting basil and the thermal treatment and potassium phosphite are fully compatible with the rules of organic farming. Seed dressing represents the starting point of a full integrated approach for downy mildew management.