Plant-mediated interactions between insects and phytopathogens in cucumber

Plants are generally attacked by communities of pathogenic microbes and herbivores. Preliminary work in the field with hybrid zucchini-gourd plants demonstrated effects of plant disease on insect occurrence. I hypothesized that prior infection of cucumber by fungal necrosis and bacterial vascular wilt pathogens would induce plant chemical responses and would alter resistance to further pathogen infection and feeding and reproduction by insects.; Infection by cucurbit fungal scab in cucumber induced plant-wide increases in peroxidase enzyme activity and phenolics, and simultaneously generated systemic acquired resistance to cucurbit anthracnose fungus. I hypothesized that these chemical changes would decrease the suitability of cucumber for herbivores. Cucumber beetles fed more upon partially necrotic leaves in choice and no-choice tests but avoided infected plants in the field, suggesting compensatory feeding caused by reduced host quality. Melon aphids produced more offspring on symptomatic plants. These trends were not evident on symptomless leaves, suggesting that nutrients and not defenses may have determined cross-effects in partially necrotic leaves. Inoculation with vascular wilt bacteria did not induce plant-wide peroxidases or systemic acquired resistance. I hypothesized that cross-effects here would be mediated by cucurbitacins, toxic triterpenoids which increased in wilted leaves of young cucumber plants in the field. Cucumber beetles and melon aphids did not show altered feeding or reproduction in detached leaf assays, but beetles were present in higher numbers on wilted leaves of young and mature field plants, suggesting age-dependent mediation by cucurbitacins.; I hypothesized that peroxidase induction would occur in similar ways after insect herbivory and abiotic mechanical stress as after pathogen infection. Neither restricted cucumber beetle herbivory nor simulated herbivory with or without the addition of beetle saliva led to induction. A response was evident after a long period of wind-generated mechanical stress. No acquired resistance resulted from wind stress, but melon aphid reproduction was reduced. None of these results indicate specific mediation of cross-effects by peroxidases.; Pathogen infection specifically alters host suitability in cucumber for other pathogens and for insects, but chemical mechanisms vary. Pathogens alter the spatial and temporal distributions of insects within and between host plants, potentially changing the nature of entire communities.