Accumulation and decay of chlorothalonil and selected metabolites in surface soil following foliar application to peanuts

One of the principal uses of the fungicide, chlorothalonil, is control of f oliar peanut diseases. Recent assessments indicate its runoff from treated fields in southeastern states presents risks to aquatic life. Two factors t hat control its runoff are how much reaches soil surfaces and degradation r ates. To address these questions and to evaluate accumulation and decay of key metabolites, soil samples (0-2 cm)were collected after seven chlorothal onil applications on experimental peanut plots in south central Georgia dur ing the 1999 growing season. At the start of and during laboratory incubati ons, samples were analyzed for the parent and degradates by HPLC-PDA-APCI-M S. The maximum observed residue levels were after the second application, a fter which canopy closure reduced soil deposition from later applications t o 5-10% of applied amounts. After the last spray, <5% of the cumulative chl orothalonil applied was detected in the soil. Foliar interception and dissi pation and rapid soil degradation contributed to low residue levels. Soil h alf-lives were <1-3.5 days for chlorothalonil and 10-22 days for its princi pal degradate, 4-hydroxychlorothalonil. Other daughter and granddaughter pr oducts were detected, some of which accumulated during the growing season. Results emphasize the plant canopy role in controlling the amount of fungic ide sprays that reach soil surfaces and suggest concentration-dependent chl orothalonil degradation with degradation rates increasing as soil loading d ecreases. The study indicates that the 30-day field half-life often used fo r risk assessments of this pesticide is too long for one of its most import ant agronomic uses, i.e., in southeastern peanut production. It also indica tes that the principal metabolites are more persistent than the parent, and more study is needed to identify and quantify their fate pathways.