| Summary: | Pea (Pisum sativum L.) suffers significant yield and quality losses because of infection by the parasitic fungus Erysiphe pisi Syd., the causal agent of powdery mildew. Resistant cultiv rs and lines were intercrossed and crossed with susceptible lines to determine the genetic basis of resistance. A high level of resistance in most of the resistant lines, including field pea cultivars grown in Canada (Highlight, AC Tamor and Tara), was conferred by er-1; resistance in JI 2480 was conferred by er-2. Variability in virulence was examined in aturally occurring populations of E. pisi in western Canada and NW USA. Thirty-one single colony isolates were tested on a set of 14 different pea lines, using a detached leaf assay. A low level of variability among the isolates was evident. Ten of the 14 pea lines were evaluated for powdery mildew reaction in Canada, NE USA, SW USA, NW USA, UK and Nepal. Reaction in Nepal differed from that observed in other locations for three of the ten lines. The cultivars/lines Highlight, JI 2480, JI 1559, JI 210, JI 82, Radley and JI 1758 were suggested for use as differential lines for future studies. In a study of winter survival strategies of E. pisi in Manitoba, cleistothecia from infected leaves and stems were examined microscopically on a periodic basis throughout the winter of 1996/97. Most ascospores were degraded by spring under field conditions. In a seed-transmission study, where seeds from severely infected plants were sown in a greenhouse in 1996 and 1997, none of the 4200 plants examined was infected with powdery mildew. Powdery mildew inoculua from other plant species found in the vicinity of pea fields did not infect pea. As molecular markers are useful in gene pyramiding and marker-assisted selection, three random amplified polymorphic DNA (RAPD) markers, OPO-18, OPE-16 and OPL-6 were identified as linked to er-1 by screening progenies of the cross Highlight/Radley (susceptible cultivar), using bulked segregant analysis. Five amplified fragment length polymorphism (AFLP) markers linked to er-2 were identified by screening progenies of the cross JI 2480/Radley using bulked segregant analysis.
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