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Resistance

Relationship Between Juvenile-Leaf Resistance to Anthracnose and the Presence of Juglone and Hydrojuglone Glucoside in Black Walnut. Steven Cline, Research assistant, Department of Plant Pathology, University of Illinois, Urbana 61801; Dan Neely, plant pathologist, Illinois Natural History Survey, 172 Natural Resources Building, 607 East Peabody, Champaign 61820. Phytopathology 74:185-188. Accepted for publication 9 August 1983. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-185.

The effects of juglone (5-hydroxy-1,4-naphthoquinone) and hydrojuglone glucoside (HJG) on anthracnose (Gnomonia leptostyla) development in black walnut (Juglans nigra) seedlings were correlated with resistance expressed in juvenile leaves. Anthracnose lesions on ontogenetically immature leaves were smaller and had fewer acervuli in contrast to fully expanded leaves. In shake culture, the effect of juglone on the growth of three leaf spot pathogens of black walnut and three nonpathogens was monitored. Two of three pathogens were more tolerant of juglone than were the nonpathogens. Hydrojuglone glucoside, isolated from walnut husks, stimulated the growth of the anthracnose fungus in shake culture at concentrations up to 750 μg/ml with a reduction in growth observed at 1,000 μg/ml. A high-pressure liquid chromatographic method was developed to determine the juglone and HJG content in walnut leaves. Higher levels of both compounds were observed in juvenile seedling leaves than in mature leaves. The concentration of juglone decreased while the HJG level increased in anthracnose-infected leaves. Seasonal levels of juglone and HJG varied considerably between trees and over time in field-grown trees. Immature leaves had the highest juglone levels over the growing season. We conclude that the pathogenic relationship between the anthracnose fungus and black walnut may be modified by juglone and its precursors in juvenile leaves.