Occurrence of a Carboxin-Resistant Strain of Ustilago nuda in Italy

Abstract

Loose smut of barley, caused by Ustilago nuda (Jens.) Rostr., is commonly controlled through the use of systemic seed-treatment fungicides. The most widely used active ingredient in these fungicides has been carboxin. However, isolates of U. nuda resistant to carboxin have been reported in France. In 1996 and 1997, unsatisfactory levels of loose smut of barley were observed near Perugia (central Italy), despite the treatment of the barley seed with a carboxin-based fungicide. An isolate of U. nuda (97-255) was collected from this field to determine if the pathogen had developed resistance to carboxin. Germination tests on carboxin-amended agar media indicated that isolate 97- 255 was more resistant to carboxin than a wild type isolate of U. nuda collected in Canada. In tests in which isolate 97-255 was inoculated onto barley cv. Regal, the percentage of smutted plants arising from the inoculated seed was not reduced by a carboxin seed treatment. This is the first report of resistance to carboxin in populations of U. nuda from Italy.