Effect of Light and Moisture on Severity of Stemphylium Leaf Spot of Alfalfa. W. A. Cowling, Former Graduate Research Assistant, Department of Plant Pathology, University of California, Davis 95616. D. G. Gilchrist, Assistant Professor, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. 66:291-294. Accepted for publication 8 June 1981. Copyright 1982 American Phytopathological Society. DOI: 10.1094/PD-66-291.

The effect of various light sequences and moisture periods on severity of Stemphylium leaf spot was tested in a controlled environment chamber on a susceptible alfalfa clone inoculated with the cool-temperature biotype of Stemphylium botryosum. Plants inoculated after a 12-hr light period exhibited more disease than those inoculated after a 12-hr dark period, regardless of the postinoculation light sequence. Plants inoculated before a 12-hr light period exhibited more disease than those inoculated before a 12-hr dark period, regardless of the preinoculation light treatment. No leaf spot symptoms appeared if plants were exposed to continuous light after inoculation. Thus, high disease severity was achieved only when plants were exposed to light before and after inoculation, followed by alternating dark/light periods until leaf spot symptoms developed. Disease severity increased when the period of free moisture on the leaves was extended from 1 to 4 days. A 2-day moist period produced symptoms most like those observed in the field in California. Variation in disease severity among treatments was caused by differences in the number, not average size, of lesions. This result illustrates a major distinction in disease expression by the cool-temperature biotype compared with the warm-temperature biotype of S. botryosum.

Keyword(s): environment, lucerne, Medicago sativa, Pleospora herbarum.