Agriculture and Agri-Food Canada, Morden Research Centre, Morden, Manitoba, Canada R6M 1Y5

Background and objectives
Powdery mildew, caused by Erysiphe pisi Syd., has been a major disease problem of field pea production in western Canada for the last two years. Although two resistance genes (er-1, er-2) which provide complete control of this disease have been identified in pea [1], they have not been widely incorporated into field pea varieties to date. Of a total of nearly 100 pea varieties currently registered in Canada, only AC Tamor, Tara and Highlight are resistant to the disease. Since most commercial varieties are susceptible to powdery mildew and there is a potential for resistance to be overcome if the appropriate virulence races are selected in the pathogen population, it is important to identify and introduce new sources of resistance to this disease. The objective of this study was to identify sources of resistance to powdery mildew in the local and exotic pea germplasm.

Materials and methods
335 pea cultivars and accessions were evaluated in naturally infested fields at Morden, Manitoba in 1995 and 1996. Of this pea germplasm, 14 were Canadian varities; 306 originated from 34 other countries including Afghanistan (19), Australia (2), Austria (1), Bulgaria (2), China (14), Czech Republic (4), Denmark (4), Ecuador (1), Ethiopia (30), Finland (4), Former USSR (14), Former Yugoslavia (3), France (5), Galicia (5), Germany (2), Greece (6), Guatemala (1), Hungry (2), India (22), Latvia (1), Mongolia (1), Nepal (1), Netherlands (12), New Zealand (2), Peru (3), Poland (26), Spain (3), Sweden (12), Syria (21), Tunisia (1), Turkey (27), USA (52), UK (3); and 15 were from unknown origins. Peas were planted in two-row plots in the first week of June in 1995 and the last week of May in 1996. The seeding dates were two weeks later than normal seeding time, to allow severe disease development. A uniform development of powdery mildew for all the plots was observed in each year. Disease severity was visually estimated using a scale of 0 (no disease) to 9 (all of the foliage area infected) for each plot at pod-fill to dry seed stages which occurred in the third week of August in 1995 and the second week of August in 1996.

Results and conclusions
Disease reaction among genotypes varied from highly resistant to highly susceptible, with most genotypes in the extremes in both 1995 and 1996. Of the 335 genotypes, eight were virtually immune to the disease in the two years of field evaluations, and were considered resistant; and six had mean disease reaction of 1-4 on the 0-9 scale used, and were considered moderately resistant. The remaining genotypes has disease scores greater than 6 in both years, and were considered susceptible to the disease. Among the eight resistant peas, Highlight, AC Tamor and Tara are currently registered cultivars in western Canada and carry er-1 gene for resistance [2]. The genetic basis of resistance of accessions 267-PS210713, 89-2910, JI1543, PI273605, and PI311112 and all of six moderately resistant varieties (JI 87, JI95, JI96, Carneval, JI100, and PI164523) are not known. The resistance of these genotypes has not been previously reported. Genetic studies are being undertaken to determine whether these accessions carry unique genes for resistance. If so, incorporation of the newly identified resistance into commercially adapted cultivars will aid in stabilizing yield and reducing costs of field pea production in western Canada.

1. Reiling TP, 1984. In DJ Hagedorn, ed. Compendium of pea diseases. American Phytopathological Society, St. Paul, MN, pp 21-22.
2. Tivari, KR, Penner, GA, Warkentin, TD, 1997. Can. J. Plant Sci. 77, 307-310.