Why is Puccinia graminis f. sp. tritici, but not P. striiformis f. sp. tritici able to infect an alternate host in the U.S. Pacific Northwest

Authors: WANG, MEINAN - Washington State University; Chen, Xianming

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/7/2015
Publication Date: 11/6/2015
Citation: Wang, M., Chen, X. 2015. Why is Puccinia graminis f. sp. tritici, but not P. striiformis f. sp. tritici able to infect an alternate host in the U.S. Pacific Northwest. Plant Disease. 99(11):1500-1506.

Interpretive Summary: In our previous study, we showed that the stem rust fungus infects barberry while the stripe rust does not under natural conditions in the U. S. Pacific Northwest. To determine why, the viabilities of teliospores of both rust fungal species were investigated from 2010 to 2013 by determining the germination rates using telial samples collected periodically from wheat fields. Stripe rust teliospores usually produced in July became physically degraded during winter, and their germination rate decreased from 50-90% in August to less than 1% in the following March and no germination after May. In contrast, stem rust teliospores usually produced in July and August remained physically intact and physiologically dormant, and could not germinate until February. Their germination rate after winter gradually increased to 90% in May, at which time young leaves of barberry were susceptible to infection. In addition, a time-series experiment was conducted for inoculation of barberry plants with teliospores of the stripe rust fungus. Stripe rust teliospores need a minimum of 40 hours of the dew-forming condition to infect barberry, and infection reaches a peak after incubation of inoculated plants for 88 hours. The lack of protracted moist conditions during the season of telial maturity effectively negates the stripe rust fungus infection of barberry plants in the Pacific Northwest. The information is useful for understanding the pathogen biology and control of the diseases.

Technical Abstract: Sexual reproduction of the stem rust pathogen, Puccinia graminis f. sp. tritici (Pgt), on barberry (Berberis vulgaris) has been shown to provide initial inoculum for the development of the disease on wheat and barley and also generate diverse races of the pathogen. However, in our previous study, the stripe rust pathogen, P. striiformis f. sp. tritici (Pgt), was not found on barberry in the U. S. Pacific Northwest. To determine why Pgt is able to infect the alternate host, while Pst cannot under the natural conditions, the viabilities of teliospores of both Pgt and Pst were investigated from 2010 to 2013 by determining the germination rates using telial samples collected periodically from wheat fields. Teliospores of Pst usually produced in July were physically degraded during winter, and their germination rate decreased from 50-90% in August to less than 1% in the following March and no germination after May. In contrast, Pgt teliospores usually produced in July and August remained physically intact and physiologically dormant, and could not germinate until February. Their germination rate gradually increased to 90% in May, at which time young leaves of barberry were susceptible to infection. In addition, a time-series experiment was conducted for inoculation of barberry plants with Pst teliospores. The results showed that Pst teliospores need a minimum of 40 h dew-forming condition to infect barberry, and infection reaches a peak after incubation of inoculated plants for 88 h. The lack of protracted moist conditions during the season of telial maturity effectively negates Pst infection of barberry plants in the Pacific Northwest.