VIEW ARTICLE

Resistance

Cytochemical Responses of Pecan to Cladosporium caryigenum: In Situ Localization and Quantification of Fungitoxic Phenols. Susan V. Diehl, Department of Plant Pathology and Weed Science, Mississippi State University, Drawer PG, Mississippi State, MS 39762; Clinton H. Graves, Jr.(2), and Paul A. Hedin(3). (2)Department of Plant Pathology and Weed Science, Mississippi State University, Drawer PG, Mississippi State, MS 39762; (3)Crop Science Research Laboratory, USDA-ARS, Mississippi State, MS 39762. Phytopathology 82:1037-1041. Accepted for publication 16 June 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/Phyto-82-1037.

Selective indicators and a microspectrophotometer were used to quantify juglone, isoquercitrin, and condensed tannin in fresh tissue sections of pecan (Carya illinoensis). Some significant differences were observed among seven pecan cultivars and two other hickory species (nutmeg [C. myristicaeformis] and mockernut [C. tomentosa]) for all three compounds in leaves. Concentrations of juglone and isoquercitrin in nut husks were not significant among three pecan cultivars, but Schley pecan contained significantly less condensed tannin. Juglone and isoquercitrin concentrations were significantly different among nutmeg, shagbark hickory (C. ovata), and black walnut (Juglans nigra) nut husks, whereas condensed tannin concentrations were significantly less in black walnut. Pecan leaves and nut husks infected with Cladosporium caryigenum contained significantly more juglone, isoquercitrin, and condensed tannin than uninfected tissues. This increase in phenolic content was confirmed by transmission electron microscopy, suggesting a physiological response by the plant to the presence of C. caryigenum. In addition, hickory species other than pecan consistently contained greater concentrations of all three phenols in infected and uninfected material. These higher phenolic concentrations may be responsible for the greater resistance of hickories to C. caryigenum.