Cereal rust control in Canada

The major cereal crops grown in Canada are wheat (11 Mha), barley (4 Mha), and oat (1.5 Mha). Over 90% of the total cereal production area is in the western provinces of Manitoba, Saskatchewan,and Alberta. Historically, the disease of major concern in wheat was stem rust, caused by Puccinia graminis f. sp. tritici. The first significant stem rust resistant cultivar in Canada was Thatcher, grown extensively from 1939 until the early 1970s. The stem rust resistance in Thatcher was relatively effective, with the exception of susceptibility to race 15B epidemic in the 1950s. Thatcher, however, was very susceptible to leaf rust, caused by Puccinia triticina. Over time, improved resistance to both stem and leaf rust was achieved with the release of cultivars with additional genes for resistance, primarily Sr2, Sr6, Sr7a, Sr9b, Lr13, Lr14a, Lr16, and Lr34. Over the years genetic resistance has adequately controlled stem rust but leaf rust continues to cause significant losses, partially due to changes in the P. triticina population which reduced the effectiveness of resistance genes such as Lr13 and Lr16. Stripe rust on wheat, caused by Puccinia striiformis f. sp. tritici, was historically a problem under irrigation in southern Alberta, but since 2000 it has been found annually in the central Canadian prairies and southern Ontario. The genetic basis of resistance to stripe rust in most Canadian wheat cultivars has not been determined, although Yr18 provides partial resistance in many cultivars. In contrast to wheat, rust diseases have generally not caused concern for barley producers. Stem rust, caused by P. graminis f. sp. tritici, is the primary concern for barley growers, and has been controlled through use of gene Rpg1 since 1947. In 1988 race QCCJ with virulence on Rpg1 was detected in the prairie region but to date has not caused significant economic losses in barley. The resistance gene rpg4 is effective against QCCJ, but no commercial varieties have yet been produced with rpg4. In oat, both stem rust, caused by Puccinia graminis f. sp. avenae, and crown rust, caused by Puccinia coronata f. sp. avenae, have caused significant yield losses. Both rusts have been controlled mainly through host resistance and early planting. Stem rust resistance genes Pg2 and Pg13 have been the most effective and occur in many current oat cultivars. However, in 1998, 2 races, NA67 and NA76, with virulence on both Pg2 and Pg13 were detected in the prairie region. Currently, race NA67 is predominant in the prairie region and thus all Canadian cultivars are susceptible to stem rust. Since the 1980s, improved resistance to crown rust has been achieved through use of resistance derived from Avena sterilis. Pc39 was the first of the genes derived from this wild relative to be deployed in a new cultivar, followed by the release of cultivars possessing both Pc38 and Pc39. These 2 genes remained effective until the early 1990s. From 1994 onward, a series of cultivars with the highly effective Pc68 gene introgressed from A. sterilis were released. Virulence to Pc68 appeared in 2001, and in 2005 cultivars with this gene were severely rusted. The cultivar Leggett with Pc68 plus the highly effective Pc94 gene from the diploid A. strigosa was released in 2004. Rhamnus cathartica, the alternate host of P. coronata, is widespread in Canada and removal of these woody shrubs in the vicinity of oat fields is important to reduce the severity of crown rust. The increased use of fungicides on all cereals in the past 10 years has been fairly effective in rust control but represents an added input cost for producers.