Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/14/1995
Publication Date: N/A
Citation: Interpretive Summary: Food safety could be increased by reducing the use of synthetic fungicides to protect crops. Naturally occurring mechanisms in plants that inhibit fungal growth could provide protection against pathogens. However, these mechanisms must first be identified. Growth of the fungus causing scab disease on pecan was studied with light and electron microscopy under controlled environmental conditions on leaves of pecan and other plants that do not develop scab disease. Researchers at the Russell Research Center, the Southeastern Fruit and Tree Nut Lab and the University of Georgia discovered that early stages of fungal growth occurred on all plants whether or not they develop the disease. The fungal growth necessary for developing pecan scab disease occurred within the leaf. Thus, developing naturally occurring control measures for scab disease should be focused on growth that occurs within the pecan leaf and not the leaf surface as was long believed.
Technical Abstract: Germ tube, appressorium, and subcuticular hypha development were analyzed on host and non-host leaves for the fungus causing scab on pecan. Plant features characterized for supporting fungal growth were genotype, adaxial and abaxial leaf surfaces, and leaf maturity. Germ tubes and appressoria developed on all plant leaves, despite genotype, leaf surface or maturity. Germ tube frequency on susceptible host plant, 'Wichita', was not significantly different from the non-host plant tobacco and significantly lower than resistant host cultivar 'Elliott'. Appressoria were of equal frequency on leaves of both pecan cultivars and tobacco. Adaxial and abaxial leaf surfaces were not different within a given genotype for supporting fungal development. 'Elliott' but not 'Wichita' leaf maturity influenced frequency of germ tubes and appressoria. Only immature leaves of susceptible 'Wichita' pecan supported subcuticular hyphae growth, the fungal stage specific for host susceptibility. Hence, resistance occurred at a plant site beneath the cuticle and at the fungal growth stage of hyphae development. Thus, resistance to the fungus causing scab occurs later than germ tube or appressorium development on the leaf surface and may result from mechanisms within, not on the surface, of the leaf cuticle.