Infection of Wheat Embryos byPythiumSpecies During Seed Germination and the Influence of Seed Age and Soil Matric Potential

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Abstract
Pythium spp. (mainly P. irregulare and P. ultimum var. sporangiiferum) were isolated from the scutellum region of 30-55% of random winter wheat [Triticum aestivum] seedlings dug 3 wk after sowing (just energed) from five sites, yet stand establishment was not a problem at any of the sites. The presence of Pythium spp. in the scutellum region was traced to embryo infections that occurred during the first 2 days after sowing. In pot experiments, infection of wheat embryos by Pythium spp. at 2 days after planting (20 C) occurred maximally (45%) at -0.1 bar matric potential and was essentially prevented at -0.4 to -1.2 bar. Seedlings produced from seeds pregerminated for 48 hr on moist filter paper in petri dishes then transplanted into Pythium-infested soil showed no evidence of leaf distortion or stunting and were identical to those produced from seeds sown directly into Pythium-free (pasteurized) soil. The older the seed (e.g., 3-7 yr old), the greater the incidence of seed decay caused by Phythiim spp. An exception was a 7-yr-old seedlot of cultivar Daws stored at a constant 5 C, which emerged 87.5% in Pythium-infested soil, in contrast to a portion of the same seedlot stored at the normal fluctuating summer/winter temperatures, which emerged 35% in Pythium-infested soil. Both seedlots emerged 100% in Pythium-free soil. Treatments of the old seed with carboxin and thiram, captan, or metalaxyl improved emergence, but the seedlings were still smaller than those produced from new seed.