Chlorothalonil
| Pesticide Type | Fungicide (Group M05) |
|---|---|
| Chemical Class | organochlorine |
| Common Trade Names* | Bravo®, Daconil®, Echo®, Initiate® |
| Registration Status | EPA: Registered since 1966 MN: Registered |
| Structure |
|
*No endorsement is implied in the referencing of trade names.
Chlorothalonil is a broad-spectrum contact fungicide. It is widely used on potato and numerous other crops in Minnesota to protect against fungal and oomycete diseases such as late blight and downy mildew. It is also commonly used on turf and ornamentals including use on golf course turf and in sod production for control of dollar spot and snow molds. Additionally, chlorothalonil is used in paints and surface treatments as a mildewcide. In 2022, approximately 304,000 pounds of chlorothalonil were sold in Minnesota.1
Mode of Action
Chlorothalonil has been classified by the Fungicide Resistance Action Committee (FRAC)2 as an M05 fungicide (mode-of-action group M, multi-site). It has multiple sites of action and causes inactivation of cell sulfhydryl enzymes that disrupts metabolic processes in fungi cells. Multi-site fungicides in Group M have a low risk of resistance development and are often tank mixed or alternated with single site fungicides that have medium to high risk for resistance development to help mitigate resistance concerns.
Movement and Breakdown in the Environment
Chlorothalonil enters the environment directly through its labelled uses. In the environment, chlorothalonil breaks down or degrades, and it can also move offsite. It is relatively insoluble in water (less than 1 mg/L), binds tightly to soil (average Koc = 3113 mL/goc), and is considered slightly to hardly mobile.3 Chlorothalonil is not considered to be persistent in the environment, and half-lives less than 23 days have been reported in field dissipation studies. Chlorothalonil is stable to hydrolysis under acidic conditions and is moderately susceptible to hydrolysis under neutral to basic conditions (half-life: 4-64 days). It can also degrade via photolysis in clear, shallow water with environmentally relevant half-lives of 0.4 (pH 7) and 23 days (pH 5). Volatilization is not expected to be a major route of dissipation from either soil or water (Henry’s Law Constant = 2.6 x 10-7 atm-m3/mol).3
The main route of dissipation is transformation to the major degradate 4-hydroxychlorothalonil through soil metabolism (aerobic soil half-life: 2-58 days). Half-life may be dependent on application rate with higher rates resulting in longer half-lives.3 4-Hydroxychlorothalonil is more mobile and persistent than chlorothalonil.3
Detection in Minnesota Waters
The Minnesota Department of Agriculture (MDA) monitors groundwater and surface water throughout Minnesota for over 180 pesticide-related chemicals. Since the MDA began monitoring for chlorothalonil in 2001, it has only been detected a few times in groundwater and surface water, and all detections have been well below the Minnesota Department of Health (MDH) health-based guidance value (health risk limit: 1 µg/L).4
In 2020, the MDA began monitoring for 4-hydroxychlorothalonil, a major degradate of chlorothalonil. From 2020 through 2023, 4-hydroxychlorothalonil has been detected in approximately 10% of groundwater samples (n=888), and all detections have been in the central sands region of Minnesota. 4-Hydroxychlorothalonil has been detected at concentrations above the MDH health-based guidance value (risk assessment advice: 2 µg/L) in groundwater.4 In 2023, there were 12 detections of 4-hydroxychlorothalonil above the health-based guidance value in groundwater, with a maximum concentration of 12.7 µg/L.5
Chlorothalonil and Non-target Organisms
The Environmental Protection Agency (EPA) classifies chlorothalonil as practically non-toxic to birds and small mammals on an acute oral basis and practically non-toxic to honey bees on an acute contact exposure basis.6 It is, however, classified as very highly toxic to fish and aquatic invertebrates on an acute exposure basis. Fathead minnow hatching success and survival are affected at 3 and 6.5 µg/L, respectively.
On an acute oral basis, the 4-hydroxychlorothalonil degradate is more toxic to birds and mammals than chlorothalonil, but it is significantly less toxic than chlorothalonil to aquatic taxa.6
Chlorothalonil and Human Health
Based on animal studies, impacts from exposure to chlorothalonil may be seen in the gastrointestinal, liver, and kidney systems in the body, which may increase the risk of certain kinds of cancer. The United States Environmental Protection Agency classifies chlorothalonil as “likely to be carcinogenic to humans.”7 No potential acute dietary risks of concern were identified but chronic dietary risks of concern were identified for chlorothalonil. In all instances where potential risks of concern were identified, drinking water exposure from groundwater contamination was the major source of exposure. It appears that 4-hydroxychlorothalonil is much more acutely toxic than chlorothalonil.7
1Minnesota Department of Agriculture. 2024. Pesticide Sales Database. Accessed June 5, 2024.
2Fungicide Resistance Action Committee. 2024. FRAC Code List 2024: Fungal control agents sorted by cross-resistance pattern and mode of action.
3US Environmental Protection Agency. 2020. Chlorothalonil: Drinking Water Assessment for Registration Review.
4Minnesota Department of Health. 2023. Chlorothalonil and 4-Hydroxychlorothalonil and Water.
5Minnesota Department of Agriculture. 2024. Water Monitoring Reports and Resources. Accessed June 28, 2024.
6US Environmental Protection Agency. 2020. Chlorothalonil: Draft Ecological Risk Assessment for Registration Review.
7US Environmental Protection Agency. 2023. Chlorothalonil Proposed Interim Registration Review Decision Case Number 0097 September 2023.
UPDATE: July 2024
