Studies on winter survival strategies of Erysiphe pisi in Manitoba

Erysiphe pisi, the causal agent of powdery mildew, is an important disease of field pea in western Canada, but very little information is available on whether or how it survives the winter in the prairie provinces. Therefore, studies were conducted of the role of cleistothecia on pea stubble, the po ssibility of seed transmission, and survival on other plant species. In 199 6 and 1997 cleistothecia were abundantly formed on heavily infected plants in late August to September under field conditions. Microscopic observation during the winter indicated that by May more than 95% of the cleistothecia had degenerated under field conditions, whereas in samples stored at room temperature, 50% of the cleistothecia contained intact ascospores. When see ds from plants heavily infected with powdery mildew were grown in a greenho use, none of the 4200 plants examined developed powdery mildew symptoms, su ggesting that the possibility of transmission of E. pisi through infected s eed is remote. When isolates of powdery mildew fungi originating on plant s pecies in the vicinity of pea fields were inoculated onto pea, no infection occurred. Furthermore, none of four legume crop species (chickpea, lentil, field bean and faba bt an) inoculated with E. pisi from pea became infecte d.