This study indicates that induced disease resistance might be useful to control branched broomrape. Strains of the rhizosphere bacteria Pseudomonas spp. (Proradix®), salicylic acid derivates (Bion®), and extracts of the algae Ascophyllum nodosum L. (Goemar Fruton Spezial®) can decrease branched broomrape infection to 80%. Results suggest that agents working as elicitors for resistance in other plant–pathogen interactions by induced systemic resistance or by systemic acquired resistance also could reduce branched broomrape infestation. These findings suggest that activation of immune responses before infection of plants could be an innovative control method for parasitic weeds.

β-Aminobutyric acid (BABA) is known to induce resistance to microbial pathogens, nematodes and insects in several host plant/pest systems. The present study was undertaken to determine whether a similar effect of BABA occurred against the Asian citrus psyllid, Diaphorina citri Kuwayama, in citrus. A 25 mM drench application of BABA significantly reduced the number of eggs/plant as compared with a water control, whereas 200 and 100 mM applications of BABA reduced the numbers of nymphs/plant and adults/plants, respectively. A 5 mM foliar application of BABA significantly reduced the number of adults but not eggs or nymphs when compared with a water control treatment. In addition, leaf-dip bioassays using various concentrations (25–500 mM) of BABA indicated no direct toxic effect on 2nd and 5th instar nymphs or adult D. citri. BABA-treated plants were characterized by significantly lower levels of iron, magnesium, phosphorus, sodium, sulfur and zinc as compared with control plants. The expression level of the PR-2 gene (β-1,3-glucanase) in BABA-treated plants that were also damaged by D. citri adult feeding was significantly higher than in plants exposed to BABA, D. citri feeding alone or control plants. Our results indicate the potential for using BABA as a systemic acquired resistance management tool for D. citri.

Chitinase is often produced in higher plants as a general defence response after wounding or pathogenic attack. Since germinating seeds are exposed to soil pathogens, the activity and expression of chitinase in muskmelon (Cucumis melo L.) seeds was investigated. One acidic and three basic chitinase isoforms were detected, beginning 40 d after anthesis in developing and fully mature seeds. Both acidic and basic chitinase isoforms were found in endosperm tissue during imbibition and after radicle emergence. Basic chitinase isoforms, but not acidic isoforms, were detected in the embryonic axes of imbibed seeds and in seeds before germination, indicating that chitinases are developmentally regulated in specific seed tissues. Two complete cDNAs, Cmchi1 and Cmchi2, were cloned from germinated muskmelon seeds and are predicted to encode chitinases that show 95% identity to a class III chitinase from cucumber (Cucumis sativus L.) and 61% identity to a class II chitinase from soybean (Glycine max L.), respectively. Southern blotting indicated that Cmchi2 was present only once in the muskmelon genome, while Cmchi1 may be present in one or two copies. Cmchi1 and Cmchi2 mRNAs were only detected in radicles of germinating seeds and in roots of mature plants, so additional genes other than Cmchi1 and Cmchi2 must be responsible for the chitinase activity in developing seeds. Salicylic acid and benzothiadiazole stimulated the expression of Cmchi1, but not Cmchi2, after radicle emergence. A putative role for chitinase in muskmelon seeds is defence against fungal pathogens.

In previously reported laboratory experiments, infection of Rumex obtusifolius by the rust fungus Uromyces rumicis was decreased on leaves which had prior herbivory by the beetle Gastrophysa viridula. In this paper we investigate whether this interaction is robust for natural infection by a variety of fungi in field experiments carried out in spring and autumn with plants given different levels of nitrogen fertilization. Grazing by G. viridula led to a decrease in lesion density of Ramularia rubella and Venturia rumicis in the spring and V. rumicis and U. rumicis in the autumn experiment. For V. rumicis and U. rumicis significant reductions in lesion density occurred on the undamaged leaves of damaged plants, compared with similar leaves on undamaged plants, suggesting systemic induced resistance. This induced resistance was usually independent of the amount of nitrogen fertilization, although the inhibitory effect of grazing on R. rubella in the spring and V. rumicis in the autumn experiment was enhanced by increasing nitrogen fertilization and was inhibited by increasing nitrogen fertilization for V. rumicis in the spring. In both experiments, the lesion density of V. rumicis was greater on leaves on which R. rubella was also present, and the presence of U. rumicis in the autumn experiment was linked to a similar but greater effect on V. rumicis lesion density. We found no evidence of induced resistance by fungi against fungi in these experiments. We highlight the complex interactions between inhibitory and facilitatory processes acting on leaf fungal infection. These results are compared with the proposed molecular mechanisms of induced resistance(s) and we consider the benefits of closer integration between molecular and ecological investigations of induced resistances that occur in the field.