Overwintering of Puccinia striiformis f. tritici on Winter Wheat at Varying Altitudes in Gansu and Qinghai Provinces.

Stripe rust, caused by Puccinia striiformis f. tritici, is an important wheat disease in China. P. striiformis f. sp. tritici overwintering and nonoverwintering regions based on the temperature were described elsewhere ( Shi et al. 2005 ). The temperature limit for P. striiformis f. sp. tritici overwintering is derived from field observations. However, P. striiformis f. sp. tritici has recently been observed to overwinter at sites where overwintering is predicted to be unlikely. We studied P. striiformis f. sp. tritici overwintering across several sites in regions close to or further away from the current P. striiformis f. sp. tritici "overwintering boundary" in China. Plants with P. striiformis f. sp. tritici symptoms and uredinia were tagged in late autumn and moved to the laboratory in early spring the following year for quantification of P. striiformis f. sp. tritici biomass via a quantitative reverse-transcription polymerase chain reaction method and for assessment of P. striiformis f. sp. tritici symptoms and sporulation after incubation in a greenhouse. The molecular method detected P. striiformis f. sp. tritici in leaves and sheath in most samples, much greater than the observed incidence of P. striiformis f. sp. tritici symptoms and sporulation after incubation. Thus, further refinement may been necessary to calibrate this molecular method in order to avoid overestimating P. striiformis f. sp. tritici overwintering potential. Active sporulation (hence, successful overwintering) was observed for all sites except one. Increasing altitude led to decreasing incidence of visible P. striiformis f. sp. tritici symptoms and sporulation; in addition to lower temperatures in high altitudes, wind chill may also explain this negative relationship between P. striiformis f. sp. tritici overwinter potential and altitude. P. striiformis f. sp. tritici sporulation on plants subjected to different treatments (control, two oldest leaves, or all leaves removed) indicated that P. striiformis f. sp. tritici overwinters in young green leaves as latent infection established in late autumn. The present study suggests that using only temperature to predict overwintering potential of P. striiformis f. sp. tritici at a given site is insufficient for mountainous regions.