Relationship Between Juvenile-Leaf Resistance to Anthracnose and the Presence of Juglone and Hydrojuglone Glucoside in Black Walnut

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Abstract
The effects of juglone (5-hydroxy-1,4-naphthoquinone) and hydrojuglone glucoside (HJG) on anthracnose (Gnomonia leptostyla) development in black walnut (J. 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 3 leaf spot pathogens of black walnut and 3 nonpathogens was monitored. Two of 3 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 .mu.g/ml with a reduction in growth observed at 1000 .mu.g/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 has the highest juglone levels over the growing season. The pathogenic relationship between the anthracnose fungus and black walnut may be modified by juglone and its precursors in juvenile leaves.