Plant protection by the activation of latent mechanisms for resistance

Abstract

The interactions, green bean-Colletotrichum lindemuthianum and cucurbits-Colletotrichum lagenarium, have been used in our laboratory to study the presence and activation of latent mechanisms for disease resistance in plants. Green bean hypocotyls inoculated with cultivar-non-pathogenic races of C. lindemuthianum, the heat-attenuated fungus, or some non-pathogens of green bean were locally and systemically protected from disease caused by subsequent inoculations with cultivar-pathogenic races of the fungus. Cultivars of bean susceptible to all known races of the pathogen were protected against the pathogen by prior inoculation with some non-pathogens. Recently we observed that inoculation of a cotyledon or first true leaf of cucumber, watermelon or muskmelon with Colletotrichum lagenarium systemically protected plants against the pathogen. Protection was evident as a reduction in the number and size of lesions. Inoculation of the first true leaf, when the second true leaf was partially expanded, systemically protected cucumber plants for 4–5 weeks. Plants at this time had 8–12 leaves. A second or booster inoculation 3 weeks after the first inoculation extended protection, and protection was effective with resistant and susceptible cucumber cultivars. A direct relationship existed between the number of spores used for protection and the extent and duration of protection. A single lesion elicited significant protection. Inoculated leaf one was not necessary to maintain protection of leaf two or to initiate and maintain protection in developing leaves once protection was initiated in leaf two. Inoculation of leaf one with the pathogen protected cucumber and watermelon plants in the field against the pathogen. Results from our experiments support the contention that all plants have efficient mechanisms for disease resistance though resistance may be latent in the presence of some pathogens under certain conditions. Activation of mechanisms for resistance may provide a practical means for disease control by plant immunization and suggests a new approach to breeding for disease resistance in plants.