Distribution and temperature sensitivities of genes for stem rust resistance in Australian oat cultivars and selected germplasm

Seedlings of 40 Australian oat cultivars and 154 elite oat lines were teste d 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 c ultivars 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 simil ar low infection types, it was postulated that they all carried a common ge ne, Pg-a. All isolates from the 1993 survey, except one from northern New S outh Wales, were avirulent for Pg-a. Studies of the effect of temperature o n reaction to stem rust showed that resistances in lines possessing Pg-8 an d Pg-16 became ineffective at 21.5 degrees C, but were effective at 17 degr ees C. Similarly, resistances in lines possessing Pg-4, Pg-12, and Pg-a wer e effective at 21.5 degrees C, but became ineffective at or above 26 degree s C. Resistances conferred by Pg-1, Pg-2, Pg-13, and Pg-Sa were not affecte d by temperature. Although resistance conferred by Pg-a was temperature-sen sitive, the retardation of fungal growth with decrease in temperature sugge sted that increasing day temperatures would not cause the breakdown of this resistance provided night temperatures remain cool.