RELATION OF DATE OF SEEDING WINTER WHEAT IN SOUTHERN ALBERTA TO LOSSES FROM WHEAT STREAK MOSAIC, ROOT ROT, AND RUST

Diseased immature wheat is the most highly infectious reservoir of the streak mosaic virus, Marmor virgatum McK. and its mite vector, Aceria tulipae Keifer, in southern Alberta. Dates of seeding experiments showed that losses from wheat streak mosaic were greatest in winter wheat seeded before nearby winter or spring wheat crops that harboured the disease were mature. Wheat that emerged after the diseased crops had matured was seldom infected. Infection was able to spread from diseased volunteer wheat as long as it was living, or until the weather became cold in late October or November. Wheat sown in August was more severely damaged than that sown later. Immature wheat harbouring mosaic infection was completely destroyed by one operation with a mouldboard plough, but not by one cultivation with the one-way disk or subsurface type of cultivator. Root rot was most severe, and stem rust least severe in wheat sown in August. In the absence of disease, yields decreased in seedings made after mid-September. Early September appears to be the best time to seed winter wheat in southern Alberta.