Abstract
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.
Highlights
Plants protect themselves against a diversity of attackers through constitutive and inducible defense strategies
Germination rates of inoculated seeds immersed in methyl jasmonate (MeJA), inoculated seeds sprayed in MeJA and inoculated untreated seeds were 64, 51 and 41%, respectively, the first and third rates differing significantly (P < 0.05)
Five weeks post inoculation, conditioning treatments with MeJA did not protect seeds against challenging inoculations with F. oxysporum, and mortality of seedlings was significantly higher if seeds were immersed in MeJA than if seeds were not treated (P < 0.05) (Table 1)
Summary
Plants protect themselves against a diversity of attackers through constitutive and inducible defense strategies. Foliar sprays of SA or of benzothiadiazole to Pinus radiata significantly decreased plant infections by Diplodia pinea or by Phytophthora cinnamomi, respectively (Reglinski et al, 1998; Ali et al, 2000) Within forestry, both elm and pine trees are appropriate hosts for testing active elicitor molecules, since previous research on these species reported several types of induced resistance to be operative (Solla and Gil, 2003; Bonello et al, 2006; Gordon et al, 2010; Martín et al., 2010; Kim et al, 2010). The study of disease control methods based on the direct application of natural molecules on trees is gaining interest by researchers and foresters (Holopainen et al, 2009)
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