. Corn smut caused by Ustilago maydis. Corn; Smut diseases. Fig-. 6. Young corn plants severely injured by Ustilago maydis and prostrate on the ground, hence subject to rapid disintegration (Stakman and Christensen, 311). the same whether inoculated with local or nonlocal smut collections; hence, physiologic races do not appear responsible (57). There is some evidence that certain morphological characters, , lack of ligules and tight- ness of husk, may be involved in some cases (157, 193 I. Isolation of U. maydis.—Cultures can readily be obtained by streaking chlamydospores on 1-2% potato-
. Corn smut caused by Ustilago maydis. Corn; Smut diseases. Fig-. 6. Young corn plants severely injured by Ustilago maydis and prostrate on the ground, hence subject to rapid disintegration (Stakman and Christensen, 311). the same whether inoculated with local or nonlocal smut collections; hence, physiologic races do not appear responsible (57). There is some evidence that certain morphological characters, , lack of ligules and tight- ness of husk, may be involved in some cases (157, 193 I. Isolation of U. maydis.—Cultures can readily be obtained by streaking chlamydospores on 1-2% potato- dextrose agar or any other solid nutrient agar favor- able to the growth of the fungus. If the smut gall is infested with other microorganisms, it is desirable to soak the chlamydospores for 20-50 hr in 1% copper sulfate before plating them on nutrient agar. The method of isolating single chlamydospores or sets of sporidia is simple and relatively easy with a micromanipulator (55, 79, 122). A good method is as follows: a thin fiat film of clear nutrient medium con- taining 2('f agar is placed on the surface of a cover- glass. With a dry glass needle attached to a micro- manipulator, a chlamydospore is picked up from another dry cover-glass previously dusted with dry chlamydospores. Then, the spore is transferred to a marked spot on the agar film. This cover-glass is in- verted over a Van Tiegham cell in a moist chamber such as a petri plate. After the chlamydospore germinates, the sporidia can be isolated directly from the promycelium or an indi- vidual sporidium can be drawn away from the promy- celium, along the surface of the agar, until it is some distance from the promycelium and other sporidia. The needle then is pushed into the agar a couple of times to create a slight depression in which the sporidium might bud; and, also, to mark the location of the sporidium (55). The isolated sporidium is allowed to bud for 24-36 hr, depending on the kind of medium: the temperatur
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