Chlorothalonil (2,4,5,6-tetrachloroisophthalonitrile) is an organic compound mainly used as a broad spectrum, nonsystemic fungicide, with other uses as a wood protectant, pesticide, acaricide, and to control mold, mildew, bacteria, algae.[2] Chlorothalonil-containing products are sold under the names Bravo, Echo, and Daconil. It was first registered for use in the US in 1966. In 1997, it was the third most used fungicide in the US, behind only sulfur and copper, with 12million pounds (5.4million kilograms) used in agriculture that year.[3] Including nonagricultural uses, the United States Environmental Protection Agency (EPA) estimates, on average, almost 15millionlb (6.8millionkg) were used annually from 1990 to 1996.[2]
Uses
Chlorothalonil use in the US in pounds per square mile in 2002 (USGS data)
In the US, chlorothalonil is used predominantly on peanuts (about 34% of usage), potatoes (about 12%), and tomatoes (about 7%), although the EPA recognizes its use on many other crops.[2] It is also used on golf courses and lawns (about 10%) and as a preservative additive in some paints (about 13%), resins, emulsions, and coatings.[2]
Chlorothalonil is commercially available in many different formulations and delivery methods. It is applied as a dust, dry or water-soluble grains, a wettable powder, a liquid spray, a fog, and a dip. It may be applied by hand, by ground sprayer, or by aircraft.[2]
Mechanism of action
Chlorothalonil reacts with glutathione giving an glutathione adduct with elimination of HCl. Its mechanism of action is similar to that of trichloromethyl sulfenyl fungicides[4] such as captan and folpet.[5]
Toxicity
Acute
According to the EPS, chlorothalonil is a toxicity category I eye irritant, producing severe eye irritation. It is in toxicity category II, "moderately toxic", if inhaled (inhaled LD50 0.094mg/L in rats.) For skin contact and ingestion, chlorothalonil is rated toxicity category IV, "practically nontoxic", meaning the oral and dermal LD50 is greater than 10,000mg/kg.[2]
Chlorothalonil was found to be an important factor in the decline of the honey bee population, by making the bees more vulnerable to the fungal pathogen Nosema ceranae.[6]
Chlorothalonil is highly toxic to fish and aquatic invertebrates, but not toxic to birds.[7]
At a concentration of 164μg/L, chlorothalonil was found to kill a species of frog within a day.[8]
Contaminants
Common chlorothalonil synthesis procedures frequently result in contamination of it with small amounts of hexachlorobenzene (HCB), which is toxic. US regulations limit HCB in commercial production to 0.05% of chlorothalonil. According to the EPA report, "post-application exposure to HCB from chlorothalonil is not expected to be a concern based on the low level of HCB in chlorothalonil. 2,3,7,8-Tetrachlorodibenzodioxin being one of the most potent carcinogens known is also a known contaminant".[2]
The main breakdown product of chlorothalonil is SDS-3701 (structure shown below). SDS-3701 has been shown to be 30 times more acutely toxic than chlorothalonil and more persistent in the environment.[11][12] Laboratory experiments have shown it can thin the eggshells of birds, but no evidence supports this happening in the environment.[2]
In 2019, a review of the evidence found that "a high risk to amphibians and fish was identified for all representative uses", and that chlorothalonil breakdown products may cause DNA damage.[13][14]Agrochemicals are claimed to be the strongest factor in bumblebee population decline.[13]
Important metabolites of chlorothalonil
SDS-3701
R417888
R471811
Gag order
Syngenta sued to stop communication by Swiss health authorities with the Swiss public regarding the "relevance" of specific metabolites of chlorothalonil that Swiss authorities detected in high concentrations in the groundwater from which hundreds of thousands of Swiss people obtain drinking water. The court banned Swiss health authorities from communicating with the public about the dangers posed by some of the metabolites.[15]
Bans
In March 2019, as a result of the previously mentioned research, the European Union banned the use of chlorothalonil[13] dated to take effect May 20, 2020.[16] Switzerland followed in December 2019.[17]
↑ Tillman, Ronald; Siegel, Malcolm; Long, John (June 1973), "Mechanism of action and fate of the fungicide chlorothalonil (2,4,5,6-tetrachloroisophthalonitrile) in biological systems: I. Reactions with cells and subcellular components of Saccharomyces pastorianus", Pesticide Biochemistry and Physiology, 3 (2): 160–167, doi:10.1016/0048-3575(73)90100-4
↑ Siegel, Malcolm R. (1970). "Reactions of certain trichloromethyl sulfenyl fungicides with low-molecular-weight thiols. In vivo studies with cells of Saccharomyces pastorianus". J. Agric. Food Chem. 18 (5): 823–826. doi:10.1021/jf60171a034. PMID5474238.
↑ Jeffery S. Pettis, Elinor M. Lichtenberg, Michael Andree, Jennie Stitzinger, Robyn Rose, Dennis vanEngelsdorp "Crop Pollination Exposes Honey Bees to Pesticides Which Alters Their Susceptibility to the Gut Pathogen Nosema ceranae" PLOS One, 24 July 2013, Online: 9 April 2014.
↑ Taegan McMahon, Neal Halstead, Steve Johnson, Thomas R. Raffel, John M. Romansic, Patrick W. Crumrine, Raoul K. Boughton, Lynn B. Martin, Jason R. Rohr "The Fungicide Chlorothalonil is Nonlinearly Associated with Corticosterone Levels, Immunity, and Mortality in Amphibians" Environ Health Perspectives, 2011, Online: 4 April. doi:10.1289/ehp.1002956
↑ White LM, Ernst WR, Julien G, Garron C, Leger M (2006). "Ambient air concentrations of pesticides used in potato cultivation in Prince Edward Island, Canada". Pest Manag. Sci. 62 (2): 126–36. doi:10.1002/ps.1130. PMID16358323.
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