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Root Diseases, Populations of Soil Fungi, and Yield Decline in Continuous Double-Crop Corn. Donald R. Sumner, Department of Plant Pathology, University of Georgia Coastal Plain Experiment Station, Tifton 31793-0748. Gary J. Gascho, A. W. Johnson, James E. Hook, and E. Dale Threadgill. Department of Agronomy, Nematologist (USDA-ARS), Department of Agronomy, University of Georgia Coastal Plain Experiment Station, Tifton 31793-0748; and Department of Agricultural Engineering, University of Georgia, Athens 30602. Plant Dis. 74:704-710. Accepted for publication 28 January 1990. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1990. DOI: 10.1094/PD-74-0704.

Grain yield in the spring crop of continuous double-cropped irrigated corn (Zea mays) declined from 11.3 to 7.2 t/ha during 19781983. From 1985 to 1987, tillage, soil, and fertility treatments were applied to determine the causes of yield decline. Soil fumigation with DD-MENCS in February reduced root disease, eliminated symptoms of decline, increased yield, and reduced populations of basidiomycetes, Phoma spp., Fusarium spp., and total fungi in soil. Fertility and tillage practices, a winter crop of rye (Secale cereale), and soil treatments with fenamiphos and metalaxyl did not prevent decline. Phoma terrestris, Pythium arrhenomanes, and Pythium spp. were isolated most frequently from lesions on roots of 11- to 15-wk-old corn. In greenhouse experiments with soil from the field, heat and benomyl treatments reduced chlorosis and increased plant weight in corn. In pathogenicity experiments with corn, Pythium arrhenomanes, P. aphanidermatum, and P. irregulare were moderately virulent and Phoma terrestris and P. americana were slightly virulent.