Abstract
Seedlings of 40 Australian oat cultivars and 154 elite oat lines were tested with various pathotypes of P. graminis avenae. Fourteen cultivars carried Pg-2 and/or Pg-4. One cultivar carried Pg-13 and 4 carried Pg-a. Several cultivars possessed different combinations of Pg-1, Pg-2, Pg-3, and/or Pg-4, but none possessed Pg-8 or Pg-9. Since most elite lines were resistant to all cultures collected from the pathogenicity survey in 1993 and gave similar low infection types, it was postulated that they all carried a common gene, Pg-a. All isolates from the 1993 survey, except one from northern New South Wales, were avirulent for Pg-a. Studies of the effect of temperature on reaction to stem rust showed that resistances in lines possessing Pg-8 and Pg-16 became ineffective at 21.5°C, but were effective at 17°C. Similarly, resistances in lines possessing Pg-4, Pg-12, and Pg-a were effective at 21.5°C, but became ineffective at or above 26°C. Resistances conferred by Pg-1, Pg-2, Pg-13, and Pg-Sa were not affected by temperature. Although resistance conferred by Pg-a was temperature-sensitive, the retardation of fungal growth with decrease in temperature suggested that increasing day temperatures would not cause the breakdown of this resistance provided night temperatures remain cool.
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