DEVELOPING THE PECAN SCAB FUNGUS ON SUSCEPTIBLE AND RESISTANT HOST AND NONHOST LEAVES

Germ tube, appressorium, and subcuticular hypha development were analy zed on host and nonhost leaves for Cladosporium caryigenum (Ell, et La ng. Gottwald), the fungus causing scab on pecan [Carya illinoinensis ( Wangenh,) C, Koch], Plant features characterized for supporting fungal growth were genotype, adaxial and abaxial leaf surfaces, and leaf mat urity, Germ tubes and appressoria developed on all plant leaves, despi te genotype, leaf surface, or maturity, Germ tube frequency on the sus ceptible host, 'Wichita', was lower than on the resistant host, 'Ellio tt', but was not significantly different from the nonhost, tobacco (Ni cotiana tabacum L,), Appressoria formed with equal frequency on leave of both pecan cultivars and tobacco, Adaxial and abaxial leaf surfaces were not different within any given genotype for supporting fungal de velopment, Immature leaves of 'Elliott', but not of 'Wichita', had a h igher frequency of germ tubes and appressoria than mature leaves, subc uticular hyphal development occurred only on immature leaves of suscep tible 'Wichita' pecan, Hence, subcuticular hyphal development is a pri me candidate for being the fungal stage specific for host susceptibili ty, Resistance to C. caryigenum infection appears to be expressed at t he plant site beneath the cuticle as fungal hyphae did not develop in a resistant pecan genotype or on nonhost leaves, Thus, resistance to t he fungus causing pecan scab likely is expressed after both germ tube and appressorium development and operates beneath, not on the surface, of the leaf cuticle, Furthermore, technology developed to make these assessments would be adaptable in pecan breeding programs to screen fo r scab resistance.