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Seasonal Progress of Brown Stem Rot and Its Impact on Soybean Productivity. Alemu Mengistu, Former graduate research assistant, University of Wisconsin-Madison; C. R. Grau, Associate professor, University of Wisconsin-Madison. Phytopathology 77:1521-1529. Accepted for publication 18 June 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1521.

The seasonal progress of brown stem rot was measured for two growing seasons in plots irrigated at growth stages VCR1, R1R8, VCR8, and in nonirrigated plots. Disease was assessed by disease incidence based on stem symptoms and the isolation frequency of Phialophora gregata, and disease severity based on proportion of internal stem discoloration, severity of foliar symptoms, and loss of symptomless leaf area. Each measurement of disease was related to yield loss, which was estimated as the percentage difference in yield between resistant and susceptible cultivars. The pathogen was recovered from roots and stems in advance of internal stem symptoms. Internal stem symptoms were minimal during vegetative growth but rapidly developed after flowering. Although isolation of the pathogen and progress of internal stem symptoms were less during periods of no irrigation, disease incidence and proportion of internal stem symptoms were similar regardless of irrigation regime by growth stage R7. Foliar symptoms did not develop until growth stage R4 and were most severe on plants in plots that received postflowering irrigation. The loss of symptomless leaf area was correlated with the severity of foliar symptoms as measured by the Horsfall-Barratt system. Yield loss with brown stem rot ranged from 13 to 30%. The extent of yield loss was affected by severity of brown stem rot, which in turn was affected by irrigation at specific stages of growth. Yield loss was greatest if both stem and foliar symptoms were present. The severity of foliar symptoms accounted for more yield loss variation than did stem symptoms, especially if multiple point models or an area under the disease progress curve was used to assess disease. Growth stage R5 was an optimum period for a single-point assessment of stem and foliar symptoms.