Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 5, 2010
Publication Date: July 1, 2010
Citation: Mengistu, A., Smith, J.R., Bellaloui, N., Paris, R.L., Wrather, A. 2010. Evaluation of selected soybean genotypes against Phomopsis longicolla under irrigated harvest times. Crop Science. 50:2055-2064. Interpretive Summary: Phomopsis seed decay is a disease of soybean seed caused by a fungus (mold), known as Phomopsis longicolla. Infection by this fungus occurs in hot and humid production environments. Infection of soybean by this mold is known to be influenced by timing of harvest and climatic conditions that occur during and after maturity along with using irrigation. However, evaluation of soybean accessions have not been performed under these conditions. Sixty soybean accessions were evaluated in two irrigation environments and at two harvesting times. Only 13 of the 60 accessions were classified as moderately resistant under delayed harvest across the two irrigations. The high level of seed infection due to P. longicolla from delayed harvest combined with irrigation application allowed the identification of soybean accessions that withstood adverse conditions that generally occur after senescence. This study indicates that researchers and plant breeders should evaluate soybean breeding lines at delayed harvest in irrigated environments to select for resistance to Phomopsis seed decay rather than selecting in non-irrigated or rain fed environments.
Technical Abstract: Many soybean [Glycine max (L.) Merr.] growers in the midsouthern USA have in the past decade shifted to an early-season production system. Incidence of Phomopsis longicolla in seed is frequently high and quality is low in such production system. Cultivars resistant to this disease have not been available. Infection of soybean by Phomopsis longicolla Hobbs is known to be influenced by timing of harvest and climatic conditions that occur during and after physiological maturity. However, evaluation of soybean accessions has not been performed under these conditions. The purpose of this research was to evaluate the reaction of a select set of soybean accessions to P. longicolla when grown under two irrigation environments and at two harvest times. Sixty soybean accessions that included six maturity groups were screened for resistance. The seed infection index when harvest was delayed was significantly higher than the infection index at harvest maturity (first time seed dried to less than 14% moisture) in both irrigated and non-irrigated environments. When soybean accessions were evaluated across the two harvest dates, there were 20 accessions that were classified as resistant in the non-irrigated environment while none was identified as resistant in the irrigated environment. When these accessions were evaluated for seed infection at each harvest, 41 accessions were classified as resistant at harvest maturity, and 20 accessions were classified as resistant at delayed harvest in the non-irrigated environment. Under irrigation, 21 accessions were classified as resistant at harvest maturity while none was classified as resistant under delayed harvest. However, 13 accessions were classified as moderately resistant under delayed harvest across the two irrigations. The correlations between harvest date and seed infection were positive and significant in both non-irrigated (r=0.42, P<0.0001) and irrigated environments (r=0.45, P<0.0001), indicating that as harvesting was prolonged seed infection also increased. The high level of seed infection due to P. longicolla from delayed harvest combined with irrigation application allowed the identification of soybean accessions that may withstand adverse weather conditions that may occur after senescence.