Pathogen-activated induced resistance of cucumber: response of arthropod herbivores to systemically protected leaves

Restricted (non-systemic) inoculation of cucurbits, green bean, tobacco, and other plants with certain viruses, bacteria, or fungi has been shown to induce persistent, systemic resistance to a wide range of diseases caused by diverse pathogens. The non-specificity of this response has fueled speculation that it may also affect plant suitability for arthropod herbivores, and there is limited evidence, mainly from work with tobacco, which suggests that this may indeed occur. Young cucumber plants were immunized by restricted infection of a lower leaf with tobacco necrosis virus (TNV), and upper leaves were later challenged with anthracnose fungus, Colletotrichum lagenarium, to confirm induction of systemic resistance to a different pathogen. The response of arthropod herbivores was simultaneously measured on non-infected, systemically protected leaves of the same plants. As has been reported before, immunization with TNV gave a high degree of protection from C. lagenarium, reducing the number of lesions and the area of fungal necrosis by 65-93%. However, there was no systemic effect on population growth of twospotted spider mites, Tetranychus urticae Koch, on upper leaves, nor did restricted TNV infection of leaf tissue on one side of the mid-vein systemically affect mite performance on the opposite, virus-free side of the leaf. Similarly, there were no effects on growth rate, pupal weight, or survival when fall armyworm larvae were reared on systemically protected leaves from induced plants. In free-choice tests, greenhouse whiteflies oviposited indiscriminately on induced and control plants. Feeding preference of fall armyworms was variable, but striped cucumber beetles consistently fed more on induced than on control plants. There was no increase in levels of cucurbitacins, however, in systemically-protected foliage of induced plants. These findings indicate that pathogen-activated induced resistance of cucumber is unlikely to provide significant protection from herbivory. The mechanisms and specificity of induced resistance in cucurbits apparently differ in response to induction by pathogens or herbivores.