Influence of post-inoculation temperature on Puccinia striiformis f. sp. tritici development in wheat

Localized outbreaks of stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst) race 142E30A+ in the northern irrigated areas of South Africa (SA), raised concerns about the possible adaptation of the pathogen to warmer environmental conditions. In this study, we compared the lesion lengths produced by isolates of Pst races 22 days post-inoculation on Avocet S and JIC871, incubated at mean lower- and elevated-temperature regimes. Analysis of variance revealed that temperature, wheat variety and Pst race, as well as their two- and three-way interactions, significantly (p < 0.001) influenced lesion development in flag leaves. In the first trial, mean lesion lengths under the lower-temperature regime (68.4 mm, day/night means of 20.45°C/15.71°C) differed significantly (p < 0.05) compared to the elevated-temperature regime (27.7 mm, 24.5°C/20.96°C) across Pst races and the two wheat varieties. JIC871 was more susceptible than Avocet S, especially at elevated temperatures, with mean lesion lengths that were 9.2 mm and 17.5 mm (p < 0.05) longer in the two trials, respectively. Differences in aggressiveness were observed with Pst race 7E22A- producing the shortest mean lesions (51.9 mm and 48.0 mm, p < 0.05) at the lower- and elevated-temperature regimes, respectively, in the second trial. However, no conclusive evidence was found that isolates of Pst race 142E30A+ were better adapted to elevated temperatures. Overall, the results suggest that the recent high incidence and localized outbreaks caused by isolates of race 142E30A+ in SA and Zimbabwe are more likely the result of wider virulence and increased susceptibility of commercial wheat.