THE INFLUENCE OF ROTATION AND TILLAGE ON LATE SEASON SOYBEAN DISEASES
Abstract
The influence of three tillage systems: conventional, chisel, and no tillage; three rotations: soybean-soybean (S-S), corn-soybean (C-S), and wheat-corn-soybean (W-C-S); and three levels of weed management were evaluated for their effect on soybean diseases in the Eastern corn belt. Disease pressures were manipulated by fumigation with sodium methyl dithiocarbamate, inoculations with Cercospora or Phomopsis and application of foliar fungicide. Diseases caused by Cercospora and Diaporthe/Phomopsis were the major factors associated with yield losses. Incidence and severity of Diaporthe/Phomopsis were highest in the S-S and lowest in the W-C-S rotation suggesting that a two year rotation better limits disease development in stems, pods, and seed. Diaporthe/Phomopsis prevailed in all tillage systems. Cercospora incidence was greatest with the S-S no tillage. Weed management did not directly influence disease development. Cercospora inoculation resulted in extensive foliar and seed disease development which were inversely related to Diaporthe/Phomopsis disease levels in the rotation and tillage systems. Fumigation limited Diaporthe/Phomopsis and increased yield similarly to one year of crop rotation. At the R6 growth stage 5% of the soybean plants in these plots died in 1985 and 1986. In 1985 symptoms of affected plants were similar to those reported for 'Sudden Death Syndrome' (SDS). Diaporthe/Phomopsis was consistently associated with affected plants. These plants were shorter, lighter, and had fewer nodes suggesting the plants had been diseased or under stress and their sudden death was the result of a sustained process. In 1986 symptoms were predominantly those of Brown Stem Rot caused by Phialophora gregata which was isolated from affected plants. More plants were affected in the S-S (11%) than in W-C-S (2%) rotation and fumigation reduced early dying from five to three percent. Weather conditions influenced which pathogen developed and manifested SDS symptoms more than crop rotation or fumigation.
Degree
Ph.D.
Subject Area
Plant pathology
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