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Physiology and Biochemistry

Fungicidal and Fungistatic Effects of Carboxin on Ustilago nuda. G. Newcombe, Research Station, Agriculture Canada, 195 Dafoe Road, Winnipeg, Manitoba, Canada R3T 2M9; P. L. Thomas, Research Station, Agriculture Canada, 195 Dafoe Road, Winnipeg, Manitoba, Canada R3T 2M9 Phytopathology 80:509-512. Accepted for publication 10 December 1989. Copyright 1990 Department of Agriculture, Government of Canada. DOI: 10.1094/Phyto-80-509.

The systemic fungicide carboxin is a respiratory inhibitor of Ustilago nuda, the fungus that causes loose smut of barley. Concentrations of 0.01 to 1,000 ?g/ml of carboxin were applied to 24-hr sporelings of three isolates of U. nuda growing in agitated liquid culture. Sporelings were not affected by 24 hr of exposure to carboxin concentrations below 0.03 ?g/ml. Carboxin concentrations of 1 ?g/ml or higher stopped hyphal growth of sporelings without causing death of apical cells. However, lower concentrations (between 0.03 and 1 ?g/ml) not only reduced growth but also killed apical cells. An agar growth test showed that most sporelings were dead after 5 days at 0.125 ?g/ml, whereas few sporelings were killed by 10 ?g/ml for 5 days. Paradoxically, carboxin has a fungicidal effect on hyphae of U. nuda at concentrations lower than those at which it exhibits only a fungistatic effect. In contrast, using the same methodologies, the respiratory inhibitors antimycin A and carbonyl cyanide m-chlorophenyl-hydrazone were increasingly fungicidal with increased concentration. Respiratory inhibitors probably have fungicidal effects only when a certain minimum of energy for autolysis is available.