INFLUENCE OF SOYBEAN GENOTYPE ON RATE OF SEED MATURATION AND ITS IMPACT ON SEEDBORNE FUNGI
Abstract
Maturation rate, measured as length of late season growth stage intervals and/or rate of moisture lost, was studied during a three-year period on a diverse group of soybean genotypes (plant introductions and adapted cultivars) to determine the role of plant and seed drydown on Diaporthe / Phomopsis spp. and Cercospora kikuchii seed infection. A modified scale for late season growth stages was developed to better assess moisture loss and seed infection that occur after physiological maturity. The length of the R7$\sb1$-R8 period, i.e. when major carpel and seed drydown occurs (55% to 16% moisture content), was consistently associated in all field tests with the incidence of internal colonization by seedborne fungi. Among genotypes with near-identical maturities, those classified as resistant to seedborne diseases has shorter r7$\sb1$-R8 intervals than those classified as susceptible. Although pod infection levels were similar for both resistant and susceptible entries under natural field inoculum conditions or following conidial inoculation with Phomopsis sp. or C. kikuchii, only susceptible entries showed a sharp increase in seed infection between R7$\sb1$ and R8. Growth regulators and bean pod mottle virus (BPMV) altered the rate of maturation and were used to study genotype reaction. Morphactin CF 125 and BPMV delayed maturity, extended the length of late season growth stage intervals, and increased seed infection in entries susceptible or moderately resistant to seed diseases. Ethrel tended to hasten maturity, shorten late season growth stage intervals, and decrease the percentage of internally borne fungi in the seed in all but the highly resistant genotypes. The entries PI 417274, PI 417460, Gnome, PI 80837, PI 417303, and PI 417017 showed the highest levels of resistance to seed diseases. Entries PI 361093, Miama, Amsoy 71, PI 417520, PI 361065B, PI 361095, Keller, and PI 398371 were susceptible; and PI 404169A, PI 416946, PI 417046, and PI 416921 were moderately resistant to seedborne fungi in these tests. Evaluation of maturation and drydown rates, as described in this study, holds promise as a practical screening procedure for the identification of soybean genotypes with improved seed quality.
Degree
Ph.D.
Subject Area
Plant pathology
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