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Effect of Light on the Behavior of Alternaria tagetica In Vitro and In Vivo. P. J. Cotty, Department of Plant Pathology, University of Arizona, Tucson 85721; I. J. Misaghi, department of Plant Pathology, University of Arizona, Tucson 85721. Phytopathology 75:366-370. Accepted for publication 3 October 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-366.

Light was found to affect growth, sporulation, and zinniol production by Alternaria tagetica, as well as the number of lesions formed on marigold inoculated with the fungus. Fungus growth was inhibited by both continuous and alternating light. The fungus sporulated only on one of the three test media and only under alternating light. The patterns of zinniol production in the dark and in the light were not the same. Zinniol production in the light occurred only during the active growth of the fungus; the bulk of the toxin was produced in the dark after cessation of growth. In both the light and the dark, the quantity of zinniol in filtrates rapidly declined once zinniol production ceased. More lesions were produced on inoculated plants kept in the dark than on those kept in the light. Low illuminance also was more conducive to lesion development than high illuminance and more lesions developed on plants exposed to low illuminance for 48 hr prior to inoculation than on those exposed to high illuminance. The limitations of studies on the behavior of Alternaria spp. in vitro and in vivo in which the effect of light has been overlooked are discussed.