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Effect of Chloroneb on the Growth and Metabolism of Ustilago maydis. Ronald W. Tillman, Research Assistant, Department of Botany, University of Maryland, College Park 20742. Present address of senior author: Crop Protection Institute, P.O. Drawer “S”, Durham, New Hampshire 03824; Hugh D. Sisler, Professor, Department of Botany, University of Maryland, College Park 20742. Phytopathology 63:219-225. Accepted for publication 28 June 1972. DOI: 10.1094/Phyto-63-219.

Chloroneb (8 µg/ml) prevented multiplication of sporidia of Ustilago maydis, but had little effect on increase in absorbance at 450 nm or dry weight until after 3 hr. Subsequent increases in absorbance and dry weight were strongly curtailed. Colony-forming ability was not affected when sporidia were incubated in phosphate buffer with chloroneb for 12 hr, but was reduced 80% when sporidia were incubated 12 hr in nutrient medium with chloroneb. The greatest uptake of chloroneb by sporidia in nutrient medium or phosphate buffer occurred within the first 15 min of incubation. Additional uptake of the toxicant was proportional to the increase in sporidial dry weight. No detectable metabolism of chloroneb occurred during a 24-hr incubation period. Concentrations of chloroneb (8 or 12 µg/ml) which prevented sporidial multiplication had no effect on the oxidation of glucose or acetate. Protein, RNA, and DNA of treated and untreated cultures increased at approximately the same rate until failure of sporidial division in the treated cultures: Subsequent increase of protein, RNA, and DNA in treated samples was strongly inhibited. Synchronous sporidia exposed to chloroneb did not form buds, but underwent mitosis and formed binucleated sporidia with a cross wall between the two nuclei. Thus, inhibition of protein, RNA, and DNA syntheses appears to result from the failure of sporidia to form buds and divide normally, rather than from a direct action of the toxicant on these syntheses. A mutant of U. maydis resistant to chloroneb was isolated from medium containing 8 µg/ml of the toxicant. There was no appreciable difference in the uptake of chloroneb by the mutant or wild type sporidia. Neither mutant nor wild type metabolized chloroneb. The chloroneb-resistant mutant was also resistant to 2,6-dichloro-4-nitroaniline, diphenyl, hexachlorobenzene, naphthalene, p-dichlorobenzene, pentachloronitrobenzene, and sodium-o-phenylphenate. Tolerance to chloroneb in progeny from a cross of the resistant mutant to a sensitive wild type indicated single gene control of the resistance.