Virulence of collections of Ustilago avenae and Ustilago kolleri sampled from oat fields in Canada during 1995-1999

Loose and covered smut of oat are effectively controlled through the use of resistant cultivars or carbathiin seed treatment in Canada. The potential for development of carbathiin-insensitive genotypes or of races with new vi rulence genes or new virulence gene combinations makes it desirable to regu larly monitor the pathogen populations to detect changes. This study was co nducted to define the virulence spectrum and carbathiin sensitivity of isol ates of Ustilago avenae (Pers.) Rostr. and Ustilago kolleri Wille in Canada . Smutted oat panicles were collected from 17 farm fields in Manitoba and S askatchewan during annual surveys from 1995 to 1999. Four more collections were obtained from collaborators in Ontario and Alberta. Nineteen of the co llections were identified as U. avenae and two were identified as U. koller i. All the isolates were found to be sensitive to the fungicide carbathiin at 1 mug/mL, using a petri dish test. The virulence of the smut isolates wa s determined using the Canadian oat smut differential set. Eleven races, vi rulent on 3-10 differential host lines, were identified from the 19 isolate s of U. avenae, and 2 races, virulent on 5 and 7 differential host lines, r espectively, were identified from the 2 isolates of U. kolleri. None of the races were virulent on the oat differential lines Fulgham, Camas, or Markt on or the oat cultivars AC Medallion, AC Assiniboia, AC Belmont, AC Preakne ss, or Tibor, but races virulent to cultivars AC Lotta, CDC Boyer, and Derb y were detected. Carbathiin seed treatment and current resistant oat cultiv ars are still effective for the control of loose and covered smut of oats i n Canada.