Methyl jasmonate spray for the protection of broad-leaf trees against oomycete and fungal pathogens

Methyl jasmonate (MeJA) is a volatile hormone produced by plants in response to stress. Exogenous application of MeJA enhances resistance to pathogens in conifers, but little is known if resistance is also enhanced in broad-leaf trees. This article reports on five independent experiments carried out to determine the preventive and curative effects of MeJA spray in broad-leaf trees in response to relevant pathogens. In Castanea sativa seedlings, preventive MeJA spray at 1 mM was the most effective dose against Phytophthora cinnamomi, and protection lasted one year. For Quercus ilex and Q. suber seedlings, double spray of MeJA at 0.2 mM, before and after inoculation, and single spray of MeJA at 1 mM before inoculation were the most effective treatments against P. cinnamomi, respectively. MeJA spray had no effect on the mycorrhization of plants. Quercus robur and Fraxinus excelsior plants were sprayed with 0, 0.2, 1, 5 and 10 mM MeJA, before and after Phytophthora plurivora infection, but no protection was observed. Finally, Ulmus minor trees were sprayed at 1 and 10 mM MeJA, and protection against Ophiostoma novo-ulmi was dose- and genotype-dependent. It is concluded that MeJA spray can induce resistance in broad-leaf trees against widespread and highly virulent pathogens, but the effect may vary depending on the tree species and pathogen combination. Protection of broad-leaf trees could be obtained only if the appropriate dose and timing is used.