Plant-mediated interactions between insects and a fungal plant pathogen and the role of plant chemical responses to infection.

Diverse organisms simultaneously exploit plants in nature, but most studies do not examine multiple types of exploiters like phytophagous insects and fungal, bacterial, and viral plant pathogens. This study examined patterns of induction of antipathogenic peroxidase enzymes and phenolics after infection by the cucurbit scab fungus, Cladosporium cucumerinum, and then determined if induction mediated ecological effects on Colletotrichum orbiculare, another fungal pathogen, and two insect herbivores, spotted cucumber beetles, and melon aphids. Peroxidase induction occurred in inoculated, `local,' symptom-bearing leaves 3 days after inoculation, and in `systemic,' symptom-free leaves on the same plants 1 day later. Phenolics were elevated in systemic but not in local leaves 3 days after inoculation. Detached systemic leaves from plants inoculated with C. cucumerinum developed significantly fewer and smaller lesions after challenge with C. orbiculare. Spotted cucumber beetles did not show consistently significant preferences for infected versus control leaf disks in comparisons using local or systemic leaves, but trends differed significantly between leaf positions. In no-choice tests, beetles removed more leaf area from local but not from systemic infected leaves compared to control leaves, and melon aphid reproduction was enhanced on local infected leaves. In the field, cucumber beetle and melon aphid densities did not differ between infected and control plants. Antipathogenic plant chemical responses did not predict reduced herbivory by insects. Other changes in metabolism may explain the positive direction and spatially dependent nature of plant-mediated interactions between pathogens and insects in this system.