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Effect of temperature on the expression of seedling resistance to Puccinia graminis f. sp. avenae in oat

The expression of resistance to stem rust caused by Puccinia graminis f. sp. avenae in infected oat seedlings has previously been reported to be influenced by temperature. The present study evaluated the effect of incubation temperatures (15, 18, 21, 24, 25, 26, 27, and 30 °C) on the expression of resistance in seedlings of 14 oat differential lines with a single Pg resistance gene, of 'Omega' (Avena sativa 'Kyto'/Avena sterilis S-66AB667), and of the Pg-a complex of two recessive genes (A. sativa 'Rodney 0'*2/'Omega'), using races NA1, NA8, NA28, and NA58 of P. graminis f. sp. avenae. Genes Pg1, Pg2, and Pg6 were stable in expression of resistance from 15 to 30 °C. Gene Pg10 conferred an intermediate reaction to infection (infection type (IT) = 23+n) at temperatures between 15 and 24 °C, but a higher level of resistance (IT = 0;1N to 12N) at temperatures > 25 °C. Gene Pg3 was the most unstable and lost effectiveness at temperatures ≥ 21 °C. Genes Pg9, Pg12, and Pg16 lost effectiveness at 25 °C. Genes Pg4 and Pg15, and the Pg-a complex lost effectiveness at 26 °C, while genes Pg8 and Pg13 and 'Omega' tolerated slightly higher temperatures before expressing a compatible (susceptible) response. Genes Pg14 and Pg17 and A. sativa 'Makuru' control were susceptible to all races tested. Incubation at 15 °C generally produced lower ITs and necrosis on many lines after inoculation with virulent races; it did not give a true representation of infection phenotypes. According to the results from the present study, it is critical to use an incubation temperature near 18 °C to achieve a consistent and stable expression of all known Pg genes, when evaluating seedling reactions to stem rust.