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Effect of Irrigation, Soil Matric Potential, and Seed Priming on Sugar Beet Seed Germination and Damping-Off Caused by Aphanomyces cochlioides. C. M. Rush,Plant Pathology Department, The Texas A&M University System, Texas Agricultural Experiment Station, P.O. Drawer 10, Bushland 79012; K. M. Vaughn, Plant Pathology Department, The Texas A&M University System, Texas Agricultural Experiment Station, P.O. Drawer 10, Bushland 79012. Phytopathology 83:202-206. Accepted for publication 12 November 1992. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/Phyto-83-202.

Laboratory studies were conducted to evaluate how seed priming and soil matric potential affect sugar beet seed germination. Seed treatments included solid matrix priming (SMP), SMP + hydroxyethyl cellulose (HEC) hydrated to form a viscous fluid, and an untreated control. Seed were planted in soils with matric potentials ranging from 100 to 700 J kg1. Evaluations of seed germination and radicle length were made at 2, 3, 4, 5, and 7 days after planting. Soil matric potential had only minimal effect on seed germination and radicle elongation as determined by regression analysis for individual treatments and times after planting. Conversely, seed treatments greatly influenced germination. There was no significant seed treatment matric potential interaction. At 2 days after planting, seed germination, averaged across all matric potential treatments, was 72, 34, and 1% for the SMP + HEC, SMP, and control treatments, respectively. Mean radicle lengths for the same treatments were 3.3, 1.2, and 0.1 mm. By day 7, differences in germination among seed treatments were minimal, but significant differences in radicle length were found. These same seed treatments were used in a greenhouse study to determine how irrigation affected Aphanomyces seedling disease. Seed of each treatment were planted in soil, contained in 10 wooden boxes (1- 2-m), that was artificially infested with oospores of Aphanomyces cochlioides. All boxes were flood-irrigated approximately 10 days before planting; after planting, five of the boxes were irrigated again. Soil water content, seedling emergence, and seedling damping-off were monitored for 3 wk. Seedlings from SMP and SMP + HEC seed treatments emerged significantly earlier than untreated seed, but no seed treatment affected seedling disease. However, irrigation treatment greatly affected disease incidence. In the five boxes that received postplant irrigation, mean seedling disease, averaged across all seed treatments, was 41%, but in the five boxes that received only a preplant irrigation, mean disease incidence was significantly lower, at 7%. Analysis of soil dry-down curves from the two irrigation treatments revealed that soil that received only a preplant irrigation was too dry 5 days after planting for zoospore movement. These results indicate Aphanomyces seedling disease can be significantly reduced by planting into soil wet enough for seed germination but too dry for zoospore movement.