Seeding date, temperature, and seed treatment affect pythium seedling blight of field pea

Pea root rot and seedling blight caused by Pythium spp. often result in poo r seedling establishment and patchy stands. Laboratory and field trials wer e conducted to determine the role of soil temperature and seeding date in t he severity of these diseases, since weather records indicate a gradual inc rease in mean soil temperature from 4.5 to 17.3 degreesC over the April 15 to June 15 planting season. In a gradient plate test to assess the impact o f temperature on infection of field pea seedlings, the optimum temperature for infection was 15-22.5 degreesC for a strain of Pythium ultimum and 17.5 -27.5 degreesC for Pythium irregulare. Field trials, conducted from 1994 to 1996 at Westlock, Alta., and Saskatoon, Sask. using a mixed inoculum of Py thium irregulare and Pythium ultimum applied in the seed row at seeding, sh owed that inoculation consistently reduced emergence and yield relative to the untreated control. There were few differences in emergence or seed yiel d between the seeding dates in late April to mid-May, but emergence was 10- 15% lower and seed yield was 20-50% lower when the crop was seeded in late May to early June. There were no seeding date x inoculation interactions at any site. We conclude that low soil temperatures do not increase the sever ity of pythium root rot and seedling blight in field peas and that the crop should be seeded early on the northern prairies to maximize yield, even wh ere soils are infected with Pythium spp. Seed treatment fungicides generall y improved seedling emergence and decreased root rot severity relative to t he inoculated control, especially with metalaxyl. Metalaxyl and thiram + ca rbathiin produced 15-50% higher seed yield than the inoculated control.