Abstract
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 forVOCs, such as the induction of plant resistance and direct inhibition of pathogen growth. The volatiles 2-phenylethanol, B-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 threeVOCs 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 p-cyclocitral against Plasmoparaviticola infections. However, VOCs showed no effect again st P. viticolawhen applied as a liquid spray. ln contrast, a standardized VOC dosage in a limited air volume led to a reduction in downy mildew severitn suggesting that imProvements in application and formulation methods are required to allow long-lasting release and persistence of VOCs after application. In recent decades, many studies have been conducted to discover new active molecules for plant protectionr'2, including the screeningofbiological control agents such as beneficial fungi or bacteria3 and natural substances such as plant and microbial extractsr. In particular, volatile organic compounds (VOCs) of plant and microbial origin are gaining interest as promising alternatives to synthetic fungicides, as their cfficacy against diverse pathogens (e.g., Rhizoctonia solani, Alternaria alternata, Fusarium oxysporum, Fusarium graminearum) has been demonstrated-8. Additionally, VOCs are associated with sustainable benefits, such as a reduced risk ofresidue persistence on plants and in the environmente, making them a promising new class of active substances for plant protection. However, the effect of VOCs against microbial pathogens was mainly tested in vitro and under iaboritory conditions, while less information is available on their efficacy and application strategies on whole plantsro.-VOCs comprise chemically diverse organic compounds characterized by high vapor pressure under ambient conditionsrr. They can be produced by bacteria, fungi, and plantse and their biological activity has been investigated in numerous studiesr2-r'r. For example, plants emit VOCs in response to abiotic and biotic stimuli, suctias attack by microbial pathogens, herbivoiy insects, mechanical damage, salinity, and droughtr5, as secondary metabolites through different metabolic pathways, including the plastidic methylerythritol phosphate pathway, cytosolic mevalonic acid pathway, shikimate pathway, phenylalanine pathway, and lipoxygenase pathwaylr,lk. Plant VOCs play imporlant roies in intra-and inter-kingdom communicationrT'r8. In partict'lar, VOCs produced by plants can inhibit the growth ofphytopathogenic fungi, and two possible modes ofaction have been reported, nu-.ly th. ind,r.tion ofpl-ant resistincettand the direct inhibition ofpathogen growthl0'20'2r. Foi example, plant VOCs such as 2-phenyleihanol, carvacrol, farnesene, and nonanal can directly inhibit the growth ofplant pathogens22-25. Other VOCs, including p-cyclocitral, ionone, camphene, hexenal, isoprene, and pinene llnstitute for