Herbicide, Planting Date, and Root Disease Interactions in Corn

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Abstract
The interaction of herbicides and root disease of corn caused by Rhizoctonia solani AG-2 type 2 and R. zeae was studied in tests in a greenhouse and environmental chambers. In a greenhouse test with R. solani AG-2 type 2, root disease severity increased and root and shoot development were restricted in soil treated with pendimethalin and metolachlor. Alachlor, cyanazine and cyanazine plus atrazine caused variable effects, and butylate and atrazine did not influence root disease severity or plant growth. Plant height was decreased significantly by X and 2X rates of the herbicides in uninfested soil but only by the X rate in infested soil. In environmental chambers, pendimethalin and metolachlor increased root disease severity and reduced plant height at night-day temperatures of 9-20, 15-25, and 21-32.degree. C in untreated or heat-treated Dothan loamy sand. In soil infested with R. solani or R. zeae, pendimethalin caused more swollen club-shaped root enlargements at temperatures of 12-25.degree. C than at 8-21 and 20-32.degree. C. In field tests in Bonifay sand, root disease severity increased in soil treated with pendimethalin in corn planted on 3 March but not in corn planted on 21 Feb., 17 and 25 March, or 4 April. Grain yield was lower in soil treated with pendimethalin in the 3 March planting, and in soil treated with pendimethalin, metolachlor and butylate in the 25 March planting. The herbicides did not increase root disease severity or decrease yield at any planting date on Tifton loamy sand. There were more enlarged roots in pendimethalin-treated plots in the first 2 plantings than in the last 3 plantings. In early plantings of corn when soil temperatures are 5-18.degree. C, severe root injury and increased root disease severity may occur in sand and loamy sand soils treated with pendimethalin.