Cultural, pathological, and environmental factors influencing treatment of soybean seeds with fungicides

Abstract

<p>Field emergence and yield tests were conducted in Iowa in 1980 and 1981 on fungicide treated soybean seeds that differed in quality because of small size, mechanical damage, age, or Phomopsis spp. infection. Cultivars used were 'Cumberland', 'Wells', and 'Williams'. Fungicides tested were captan, carboxin-thiram, metalaxyl. Treated and untreated seeds were planted in well-drained loam and poorly drained silty clay loam soils at early (April 28 and 10), normal (May 23 and 15), and late (June 11) sowing times, respectively. Regardless of the cultivar, planting time, or soil type, both captan and carboxin-thiram consistently increased emergence of seedlots with more than 15% Phomopsis spp. infection, but metalaxyl did not. Emergence and yield of the damaged, small-sized, aged, and good-quality seeds were not improved with treatment. Captan treatment did increase the yield of the seedlot with 50% Phomopsis spp. infection;In laboratory studies in which the poorly drained soil was adjusted to different temperature and moisture levels, Pythium and Fusarium colonization of soybean seedlings grown from damaged seeds was measured. Colonization by both fungi was also determined in the field in 1982. Undamaged and damaged seeds were planted on April 30 and June 1 in the poorly drained and well-drained soils. For both laboratory and field tests, fungal colonization of seedlings grown from undamaged seeds was significantly lower than that for damaged seeds. The rate of emergence of these seedlings grown from damaged seeds was well correlated with seedling colonization by Pythium and Fusarium in both laboratory and field tests. Seed treatment with captan, carboxin-thiram, and metalaxyl was effective in reducing Pythium and Fusarium colonization in unsterilized soil, amended with these fungi, but only in Pythium-amended soil did seed treatment increase the emergence rate. Under field conditions, captan reduced Pythium and Fusarium colonization in the well-drained soil, but had no such effect in the poorly drained soil. Treatment had no effect on the emergence rate in either soil type. It was concluded that seed treatment may be beneficial when seeds of poor quality emerge in soils with minimum temperatures of 10-12 C.</p>