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Factors Affecting Chemical Control of Southern Blight of Peanut in Oklahoma. J P DAMICONE, Assistant Professor (Extension Specialist) ,Department of Plant Pathology, Oklahoma State University, Stillwater 74078-9947. K. E. JACKSON, Assistant Extension Specialist, Department of Plant Pathology, Oklahoma State University, Stillwater 74078-9947. Plant Dis 78:482-486. Accepted for publication 26 January 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0482.

Resistance to pentachloronitrobenzene (PCNB) was not detected among 112 isolates of Sclerotium rolfsii collected from commercial peanut fields in Oklahoma and assayed for inhibition of radial growth in vitro. Values of PCNB concentration that provided 50% inhibition of growth (EC50) were normally distributed about a mean of 2.86 1.45 fig/ml. An isolate of 5. rolfsii from apple, collected where PCNB had never been used, had a comparable EC50 of 3.29 0.18 μg/ml. The recommended PCNB use pattern, one or two applications at 0.56 kg/ha in a wide (30-cm) band over the row, gave variable control of southern blight (23-90%) and peanut yield increases (102-915 kg/ha) in five field trials. In field trials comparing different combinations of PCNB rates and band widths, half rates (2.8 kg/ha) applied in a wide band resulted in no disease control in one trial where full rates (5.6 kg/ha) in a wide band and half rates in a narrow (10-cm) band were effective. None of the PCNB treatments were effective in a second trial. Increasing the concentration of PCNB at the plant crown with full rates in a narrow band did not improve disease control compared to full rates in a wide band. One (1.12 kg/ha) or two (0.84 kg/ha) banded sprays with flutolanil or three foliar sprays of tebuconazole (0.14-0.16 kg/ha) provided more consistent disease control than did PCNB. However, yield increases from the experimental fungicides were not always greater than those from PCNB nor were they as large as those reported in areas where yields are higher and both southern blight and Rhizoctonia limb rot are problems. EC50 values for isolates of S. rolfsii were determined for flutolanil (0.08 0.04 μg/ml) and tebuconazole (0.06 0.02 μg/ ml) for references in resistance monitoring if these fungicides are registered in the future.