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Distribution of Isolates of Sclerotium rolfsii Tolerant to Pentachloronitrobenzene in Texas Peanut Fields

The tolerance to pentachloronitrobenzene (PCNB) of an isolate of Sclerotium rolfsii collected in 1985 was quantified, and a survey of tolerance to PCNB in 377 other isolates of the fungus collected from Texas peanut fields from 1990 through 1994 was conducted. The effective dose (ED)₅₀ of the previously collected PCNB-tolerant isolate was 11.07 μg PCNB/ml and was more than 5-fold greater than the ED₅₀ of PCNB-sensitive isolates. The distribution of tolerance to PCNB among all isolates was slightly skewed, with 18 of the 377 isolates identified as having greater (P ≤ 0.05) tolerance to PCNB than the standard sensitive isolate. No isolate of S. rolfsii collected during the period of 1990 to 1994 had as high an ED₅₀ value as did the 1985 isolate, even among those isolates collected from the same field from which the 1985 isolate was collected. ED₅₀ values of two PCNB-sensitive and five PCNB-tolerant isolates were unchanged after 15 generations on potato dextrose agar amended with 10 μg PCNB/ml or on unamended media. The PCNB-tolerant isolate collected in 1985 was less aggressive than other isolates in greenhouse and microplot tests, but no correlation was observed between ED₅₀ values and disease incidence in these tests for other PCNB-sensitive and tolerant isolates. These data suggest that even though high levels of tolerance to PCNB can be confirmed in some isolates of S. rolfsii, this phenomenon is likely to remain a rare event.