Penetration Phenomena and Facultative Parasitism in Alternaria, Diplodia, and Other Fungi

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Abstract
Strains (88) of fungi were used in 5,413 somewhat random inoculations of 78 species and varieties of flowering plants. Seedlings growing aseptically in large test tubes were inoculated without mechanical injury. After 3-7 days, strips of epidermis were removed from the inoculated areas, stained, and mounted in glycerine. In the greenhouse, drops of water containing spores or mycelium were placed on normal uninjured leaves and covered with cover glasses, the spores often becoming arranged in rings marking the margin of the glass. Corresponding rings of infection often resulted. Inoculated plants, covered with bell jars lined with wet filter paper and shaded, developed leaf spots 2-7 days after inoculation or not at all. Helminthosporium gramineum sporulated on barley leaf spots 3 days after inoculation. Species of Alternaria, Macro-sporium, Helminthosporium, Acrothecium, Diplodia, Colletotrichum, and Cephalosporium caused the formation of globular hyaline to brown bodies on the cell walls of wheat coleoptiles and the plumules of many other plants; the term "callosities" is proposed for these bodies. Their formation seems to be a typical reaction to infection by these fungi and it is believed that the callosities inhibit further development of the parasite. All species of Alternaria and Macrosporium placed on wheat coleoptiles produced callosities; all except 3 produced internal hyphae. Alternaria induced callosities also in the seedlings of many common cereals and vegetables. Helminthosporium and Cephalosporium induced callosities in field corn. Different combinations of hosts and fungi resulted in the formation of characteristically different callosities. Alternaria spp. produced hyphae within swollen, anticlinal cell walls of wheat, cabbage and muskmelon. When strips of wheat coleoptiles recently infected with Alternaria were stained with Congo red, rings appeared surrounding the points of incipient penetration, indicating differential chemical alteration in the cell walls near the infection points. Purely mechanical injuries of wheat and sorghum stems under aseptic conditions also caused prominent discolorations. Species of Penicillium, Botrytis, Cephalothecium, and Epicoccum produced appressoria on wheat coleoptiles, but caused no infection. The leaves of a number of common greenhouse and other plants were not infected by any of the fungi tested.

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