Resumen
Damping off is probably the most common disease affecting seedlings in forest nurseries. In south-western Europe, the pitch canker and the Dutch elm disease cause relevant economic looses in forests, mostly in adult trees. The ability of the chemical plant elicitor methyl jasmonate (MeJA) to induce resistance in Pinus pinaster against Fusarium oxysporum and F. circinatum, and in Ulmus minor against Ophiostoma novo-ulmi was examined. In a first experiment, an aqueous solution of MeJA 5 mM was applied to P. pinaster seeds by immersion or spray, and different concentrations of MeJA (0, 0.1, 0.5, 1, 5 and 10 mM) were tested in seedlings before inoculations with F. oxysporum (105 and 107 spores mL?1). In a second experiment, 6-months-old P. pinaster seedlings were sprayed with 0 and 25 mM of MeJA, and later challenged with mycelium of F. circinatum. Finally, 4-year-old U. minor trees were sprayed with 0, 50 and 100 mM of MeJA and subsequently inoculated with O. novo-ulmi (106 spores mL?1). MeJA did not protect P. pinaster seeds and seedlings against F. oxysporum, probably because plants were too young for the physiological mechanisms responsible for resistance to be induced. Based on the morphological changes observed in the treated 6-months-old P. pinaster seedlings (reduction of growth and increased resin duct density), there is evidence that MeJA could have activated the mechanisms of resistance. However, 25 mM MeJA did not reduce plant mortality, probably because the spread of the virulent F. circinatum strain within the tree tissues was faster than the formation of effective defense responses. Based on the lack of phenological changes observed in the treated elms, there is no evidence that MeJA would cause induction of resistance. These results suggest that the use of MeJA to prevent F. oxysporum and F. circinatum in P. pinaster seedlings in nurseries and O. novo-ulmi in U. minor trees should be discarded.