FACTORS AFFECTING CHEMICAL CONTROL OF SOUTHERN BLIGHT OF PEANUT IN OKLAHOMA

Resistance to pentachloronitrobenzene (PCNB) was not detected among 11 2 isolates of Sclerotium rolfsii collected from commercial peanut fiel ds in Oklahoma and assayed for inhibition of radial growth in vitro. V alues of PCNB concentration that provided 50% inhibition of growth (EC 50) were normally distributed about a mean of 2.86 +/- 1.45 mug/ml. An isolate of S. rolfsii from apple, collected where PCNB had never been used, had a comparable EC50 of 3.29 +/- 0.18 mug/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 d isease control in one trial where full rates (5.6 kg/ha) in a wide ban d and half rates in a narrow (10-cm) band were effective. None of the PCNB treatments were effective in a second trial. Increasing the conce ntration of PCNB at the plant crown with full rates in a narrow band d id 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 co nsistent disease control than did PCNB. However, yield increases from the experimental fungicides were not always greater than those from PC NB 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 mug/ml) and tebuconazole (0.06 +/- 0.02 mug/ml) for re ferences in resistance monitoring if these fungicides are registered i n the future.