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Influence of Environment and Culture Media on Spore Morphology of Alternaria alternata. I. J. Misaghi, Department of Plant Pathology, University of California, Davis, CA 95616; R. G. Grogan(2), J. M. Duniway(3), and K. A. Kimble(4). (2)(3)(4)Department of Plant Pathology, University of California, Davis, CA 95616. Phytopathology 68:29-34. Accepted for publication 22 June 1977. Copyright © 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-29.

Conidia of Alternaria alternata formed in natural habitats usually are larger, have longer beaks, and are more uniform in size than those produced in vitro on common agar media. The size and morphology of conidia produced in vitro were influenced both by the composition of the substrates (e.g., various agar media or pieces of tomato stems) and environmental conditions. Lowering the relative humidity during incubation by removal of the lids from petri-plate cultures for at least 6 hr daily increased the size of matured conidia. Conidia as large as those collected from stem cankers on tomato in the field, but with somewhat shorter beaks, were formed on synthetic media when 2-day-old cultures were exposed to continuous drying by removal of the petri dish lids. Reductions in the water potential of agar media by vapor equilibration or by additions of osmotica before inoculation also increased the size of conidia formed in culture, as did the exposure of cultures to lower temperatures during formation and maturation of conidia. Size and morphology of conidia were not significantly affected by exposure of agar cultures to either white fluorescent or ultraviolet light, to various levels of O₂ and CO₂ or to various pH values between 4 and 8. The number of spores produced was not affected by light but was depressed by CO₂ concentrations greater than 1.3%. These results indicate that only the conidia formed in nature are fully typical of A. alternata, but near-typical conidia can be produced in vitro by choice of media and culture conditions.