Successes in breeding for and managing durable resistance to wheat rusts.

Stripe rust, leaf rust, and stem rustcaused by Puccinia striiforrnis Westend., Puccinia recondita Roberge ex Desmaz., and Puccinia graminis Pers., respectively-have been considered to be the most widely destructive diseases of wheat (Triticum aestivum L.) in the world. Rust epidemics have been common throughout history and have frequently caused severe yield losses. During the first half of the twentieth century, rust resistance was short-lived. Within a few years after the release of a new resistant cultivar, new, virulent races (pathotypes) of the rust pathogens would appear and severely damage the previously resistant cultivar. Within the last 30 to 40 years, major progress has been made in developing cultivars with superior, more sustainable resistance to the rusts, and in the application of improved methods of managing that resistance. Consequently, rust epidemics have been infrequent, and when they have occurred, damage caused by the rusts has been less severe and less extensive. The following is a brief discussion of some of the successes and a few of the failures in controlling the rusts. The information is based on firsthand knowledge of the rusts and corroborative information from many other rust specialists. Emphasis will be on control of the diseases in North America. Wheat production and rust development in North America. The environmental conditions that affect wheat production and rust epidemics have a major impact on the strategies and methods of breeding for rust resistance and the management of that resistance. Wheat is grown in many regions of North America under a wide range of environmental conditions. It is the environment within those regions that determines the wheat types and market classes that are grown, the importance of each of the three rusts, and the effec-