Air volume not spray concentration determines in vivo efficacy of volatile organic compounds against Plasmopara viticola

Volatile organic compounds (VOCs) are produced by plants in response to abiotic and biotic stress stimuli. Two possible modes of action against plant pathogens have been reported for VOCs, such as the induction of plant resistance and direct inhibition of pathogen growth. The volatiles 2-phenylethanol, β-cyclocitral, and linalool exhibited inhibitory activity against downy mildew on grapevine leaf disks; however, their efficacy on whole plants remains unknown. The efficacy of the three VOCs against downy mildew was evaluated on potted grapevines under greenhouse conditions. Fumigation with VOCs in a limited air volume showed high efficacy for linalool and, to a lesser extent, for 2-phenylethanol and β-cyclocitral against Plasmopara viticola infections. However, VOCs showed no effect against P. viticola when applied as a liquid spray. In contrast, a standardized VOC dosage in a limited air volume led to a reduction in downy mildew severity, suggesting that improvements in application and formulation methods are required to allow long-lasting release and persistence of VOCs after application.