Photoreduction of Chlorothalonil Fungicide on Plant Leaf Models

Abstract

Photodegradation is seldom considered at the surface of vegetation after crop spraying. Chlorothalonil, a broad-spectrum foliar fungicide with a very widespread use worldwide, was considered. To represent the waxy upper layer of leaves, tests were performed within thin paraffin wax films or in n-heptane. Laser flash photolysis together with steady-state irradiation in n-heptane allowed the determination of the photodegradation mechanisms Chlorothalonil ability to produce singlet oxygen was measured; noteworthy its efficiency is close to 100%. Additionally, chlorothalonil photodegradation mainly proceeds through reductive dechlorination. In these hydrophobic media, a radical mechanism was evidenced. Photochemical tests on wax films under simulated solar light show that formulated chlorothalonil is more reactive than pure chlorothalonil. The field-extrapolated half-life of photolysis on vegetation was estimated to 5.3 days. This value was compared to the half-lives of penetration and volatilization available in the literature. It appears that chlorothalonil dissipation from crops is ruled by both photodegradation and penetration. The relative importance of the two paths probably depends on meteorological factors and on physicochemical characteristics of the crop leaf cuticle.

Direct photolysis mechanism of pesticides in water

“…142) The LFP study with quenching by 3 O 2 showed T* as a key intermediate in the photolysis of chlorothalonil, and reductive dechlorination was the main reaction in water. 27,28) Photosubstitution seems more favorable in the aqueous photolysis of pesticides that have a chlorinated aryloxy moiety. Phenoxyacetic herbicides, such as 2,4-D, 143) 2,4,5-T, 113) and triclopyr, 144) generally produced the corresponding 2-OH derivatives; however, reductive dechlorination at the 4-position was more favorable for dichlorprop.…”

Effect of the spray volume adjustment model on the efficiency of fungicides and residues in processing tomato

“…The field-extrapolated half-life of chlorothalonil due to photolysis was estimated at 5.3 days (Monadjemi et al, 2011). Lichiheb et al (2014) estimated photodegradation of chlorothalonil at 31.1% after 5 days.…”

“…However in field trial on potato the fate of chlorothalonil due to volatilization was only 5% after 7.6 days (Leistra & Van Den Berg, 2007). Furthermore, Monadjemi et al (2011) estimated half-life of chlorothalonil due to photodegradation at 5.3 days. There are studies indicating high azoxystrobin stability on tomato leaves within 1-4 days after spraying (Szpyrka & Sadło, 2008) and even after 7-10 days on fruits (Garau et al, 2002).…”

Environmental Fate and Toxicology of Chlorothalonil

“…Chlorothalonil, exposed directly to light (300-400nm) photolytically degraded more rapidly in natural waters (DT 50 = 0.21-0.76 d) than in a buffered aqueous system (pH 7; DT 50 = 1.1d; Wallace et al 2010). Monadjemi et al (2011) investigated the photodegradation of chlorothalonil on a simulated plant surface, specifically using paraffin wax (irradiated at wavelengths between 300-800nm). A field-extrapolated half-life of 5.3 d resulted, and suggested that chlorothalonil is susceptible to direct photolysis, in addition to surface penetration.…”

“…A field-extrapolated half-life of 5.3 d resulted, and suggested that chlorothalonil is susceptible to direct photolysis, in addition to surface penetration. In addition, these authors found the main degradation route was via reductive dechlorination (Monadjemi et al 2011). Waltz et al (2002) studied the photodegradation of hydroxychlorothalonil (HC), chlorothalonil's major hydrolytic metabolite.…”