Characteristics of Pecan Scab Lesions on Mockernut Hickory and Pecan Cultivars of Differing Susceptibility. S. V. DIEHL, Department of Entomology and Plant Pathology, Mississippi State University, P.O. Box 9655, Mississippi State, MS 39762. C. H. GRAVES, JR., Department of Entomology and Plant Pathology, Mississippi State University, P.O. Box 9655, Mississippi State, MS 39762. Plant Dis. 78:512-516. Accepted for publication 14 February 1994. Copyright 1994 The American Phytopathological Society 512. DOI: 10.1094/PD-78-0512.

Leaves of pecan (Carya illinoensis) cultivars Schley, Stuart, Success, and Stevens and of mockernut hickory (C. tomentosa) plus nut husks from Schley and Stuart were compared by scanning electron microscopy for differences in surface morphology. Internal tissue structure was also compared by freeze-fracture of some selected samples. Scab lesions on leaves of susceptible Schley pecan were made up of dense, compact mats of mycelium with prolific sporulation. There was complete breakdown of host spongy and palisade parenchyma cell structure. Leaf lesions on resistant mockernut hickory had sparse mycelial growth, no sporulation, and minimal disruption of surface and internal tissue structure. The reactions of other cultivars varied between these extremes. Both Stuart and Schley nut husks appeared to support fungal development and sporulation and had similar internal tissue breakdown. Differences seemed to be in lesion size and were not related to the extent of fungal development. Mycelium appeared to be closely associated with noncollapsed plant trichomes, whereas conidiophores and mycelium often appeared to emerge from the trichome base. Few trichomes were collapsed. Trichome density does not seem to be a factor in the resistance of hickories other than pecan.

Keyword(s): Cladosporium caryigenum, phenolic compounds