Over-summering of wheat powdery mildew in Sichuan Province, China

Wheat powdery mildew, caused by Blumeria graminis f. sp. tritici, is a serious disease worldwide, and a problem in China, especially in Sichuan Province, where it causes partial or total loss of grain yield on susceptible varieties when climatic conditions are favorable. In this study, we clarified ways and areas of the over-summering of this fungus, and provided information for control of the disease in Sichuan Province. We observed cleistothecial survival under a variety of different storage conditions both indoors and outdoors. Cleistothecia in humid conditions completely died by mid-July; those in dry conditions maintained a 40% survival rate into the autumn. Cleistothecia were not only able to survive until fall, but also to produce ascospores as initial inoculums for autumn-sown wheat. Field inoculation experiments using ascospores and conidia at different altitudes in Ya'an, Yibin, and Bazhong demonstrated that B. graminis f. sp. tritici is able to over-summer at an altitude of 595 m and above in Sichuan Province. Volunteer wheat plants are abundant in these areas. Through systematic monitoring, volunteer wheat plants were found to be heavily infected during the summer months, providing an inoculation bridge to the fall planting season. The type of crops following wheat also had an important influence on the over-summering of the fungus; high-shade corn was more favorable to volunteer wheat survival and the occurrence of powdery mildew than short-shade soybeans. The most effective strategy for reducing inoculation would be to eliminate the remaining wheat straws and the volunteer wheat before fall seeding.