Chlorothalonil (Chlorothalonil)

Chlorothalonil is a multisite contact fungicide, widely used for preventive control of fungal diseases in various agricultural crops.

Belonging to the chemical group of isophthalonitriles (FRAC M5), it has a broad spectrum of action against pathogens such as Alternaria spp., septoria spp., cercospora spp. And Colletotrichum spp.

It is especially valued in crops such as soybeans, corn, cotton, potatoes, tomatoes and fruit trees, being fundamental in resistance management programs due to its multi-site mechanism of action.

Developed by Diamond Shamrock Corporation in the late 1960s, it was first registered in 1969. In Brazil, its use was approved in the 1980s, becoming one of the most widely used fungicides in national agriculture due to its effectiveness and low risk of resistance.

Common name (ISO): Chlorothalonil (Chlorothalonil)

Official chemical name: 2,4,5,6-tetrachloro-1,3-benzene-dicarbonitrile

Gross chemical formula: C₈Cl₄N₂

Number CAS: 1897-45-6

Chemical class: Isophthalonitrile (chloronitriles)

Molecular structure:

•Molecular weight: 265,90 g/mol

•Functional group: Halogenated aromatic nitrile

Mode of action

Chlorothalonil acts as a multisite fungicide, simultaneously interfering with multiple metabolic pathways of fungi. Its main mechanism involves reaction with sulfhydryl groups (-SH) present in essential enzymes, causing protein dysfunction and interrupting vital cellular processes.

Biochemical target: cysteine ​​thiol groups in enzymes critical for cellular respiration, including components of the mitochondrial electron transport chain and glycolytic enzymes.

FRAC group and resistance code: Group M5 (multisite contact fungicide). Due to its multisite mechanism, it presents a very low risk of resistance development.

Physiological consequences in the pathogen: inhibition of cellular respiration, interruption of protein synthesis, disruption of cell membrane integrity and blockage of spore germination and mycelial growth.

Control spectrum

The versatility of chlorothalonil is manifested in its broad spectrum of control over several fungal pathogens in multiple agricultural crops. In soybean farming, the product has proven effective against Asian rust caused by Phakopsora pachyrhizi, the target stain caused by Corynespora cassiicola, anthracnose caused by Colletotrichum truncatum and cercosporiosis caused by Cercospora kikuchii. Recommended doses for soybean crops vary between 1,0 and 1,5 liters of commercial product per hectare, depending on disease pressure and environmental conditions.

In corn cultivation, chlorothalonil has been shown to be effective in controlling turcicum leaf spot (Exserohilum turcicum), cercosporiosis (Cercospora zeae-maydis) and common rust (Puccinia sorghi), with recommended doses between 1,2 and 2,0 liters per hectare.

Cotton farming also benefits significantly from this fungicide, especially for the control of ramularia (Ramularia areola), angle spot (Xanthomonas citri p.v. malvacearum) and alternariasis (Alternaria macrospora), using doses between 1,0 and 1,5 liters per hectare.

In crops such as potatoes and tomatoes, chlorothalonil plays an even more critical role due to its effectiveness against late blight (Phytophthora infestans), one of the most devastating diseases of these Solanaceae. In potatoes, it also efficiently controls early blight (Alternaria Solani), while in tomatoes it is effective against septoria (Septoria lycopersici) and anthracnose (Colletotrichum coccocodes). The doses for these crops are generally higher, ranging from 1,5 to 3,0 liters per hectare, reflecting the greater susceptibility of these crops to fungal diseases.

Resistance and sustainability

Resistance management represents one of the most relevant aspects of the use of chlorothalonil in modern agriculture. Its classification as a multisite fungicide gives it a unique position in integrated disease management programs, acting as an anchor for preventing the development of resistance to other fungicides with specific mechanisms of action.

The strategy of rotating and mixing fungicides with different FRAC groups has chlorothalonil as a fundamental component. Its use in alternation with fungicides from groups G (triazoles), 3 (benzimidazoles), 11 (strobilurins) and 7 (succinate dehydrogenase inhibitors) allows maintaining the effectiveness of the available fungicide arsenal. Furthermore, its application in mixtures with systemic fungicides enhances the effectiveness of the treatment, providing preventive control through chlorothalonil and curative action through the systemic component.

Current technical recommendations always recommend using the maximum recommended dose for each crop, ensuring not only maximum product efficacy but also contributing to the preventive management of resistance to other fungicides used in mixtures. This "high rate" approach contrasts with dose reduction strategies that, although they may seem economically attractive in the short term, may compromise the sustainability of the control system in the long term.

Safety and environmental impact

Chlorothalonil's toxicological classification as Class II (highly toxic, yellow band) requires special care in its handling and application. This classification reflects not only the intrinsic toxicity of the product, but also the need to implement appropriate safety measures to protect applicators, rural workers and the environment.

The environmental behavior of chlorothalonil is characterized by moderate persistence in soil, with a half-life ranging from 30 to 100 days, depending on soil temperature, moisture and pH conditions. The adsorption coefficient (Koc) between 1.380 and 5.000 mL/g indicates low mobility in soil, resulting in low leaching potential and, consequently, low risk of groundwater contamination. This characteristic is particularly important in regions with sandy soils or in areas close to water supply sources.

The risks to non-target organisms deserve special attention when making decisions about the use of chlorothalonil. For bees, the product has moderate toxicity, requiring that applications be avoided during the flowering period of crops or at times of greatest pollinator activity. For fish and aquatic organisms, toxicity is considered high, making it essential to maintain adequate distances from bodies of water during applications. The impact on beneficial soil microorganisms is considered low to moderate, not significantly compromising the biological activity of soils when used in accordance with technical recommendations.

Chronic exposure to the fungicide chlorothalonil negatively affects the development and reproduction of Drosophila melanogaster, according to a recent study (doi.org/10.1098/rsos.250136).

Compatibility and interactions

The versatility of chlorothalonil also extends to its compatibility with other phytosanitary products and foliar fertilizers. This characteristic allows its integration into complex management programs, where multiple products are applied simultaneously to optimize operational costs and maximize efficiency.

Among the permitted and recommended mixtures, combinations with systemic fungicides from different FRAC groups, such as triazoles, strobilurins and succinate dehydrogenase inhibitors, stand out. This compatibility allows the formulation of control strategies that combine the multisite preventive action of chlorothalonil with the systemic and curative action of the other components. Organophosphate insecticides and pyrethroids also present good compatibility, enabling the simultaneous control of diseases and pests.

However, some mixtures should be avoided due to the potential for physical or chemical incompatibility. Highly alkaline products with a pH greater than 8,5 may cause hydrolysis of chlorothalonil, reducing its effectiveness. Mineral oils in high concentrations may alter the release dynamics of the product, while products containing copper at high pH may form precipitates that compromise the effectiveness of the application.

The phytotoxic potential of chlorothalonil is considered low when used at recommended doses, but may manifest itself in extreme environmental conditions. Temperatures above 35°C combined with high relative humidity may predispose plants to symptoms of phytotoxicity, especially in more sensitive crops or in critical phenological stages.

Agronomic efficiency

The agronomic efficacy of chlorothalonil is significantly influenced by environmental conditions during and after application. The occurrence of rain is one of the most critical factors, requiring a minimum interval of 2 to 4 hours after application for the product to properly adhere to the leaf surface. Heavy rains within this period can significantly compromise the efficacy of the treatment, requiring reapplication.

The temperature range between 15 and 30°C provides optimal conditions for the action of chlorothalonil, while extreme temperatures may reduce its effectiveness or increase the risk of phytotoxicity. Conditions of high relative humidity favor the action of the product, since they facilitate adhesion and uniform distribution over the leaf surface.

The strategic positioning of chlorothalonil in different cropping systems reflects its versatility and importance. In soybean farming, its use is concentrated in the first applications of the fungicide program, especially between the V4 and R2 stages, acting as an anchor for the preventive control of late-cycle diseases. This strategy is particularly important in regions where Asian rust pressure is high, allowing a solid base of protection to be established before the application of more specific systemic fungicides.

In corn cultivation, preventive application in the period between V8 and VT has proven to be essential, especially in regions with a history of cercosporiosis and turcicum. Application during this critical period protects the upper leaves of the plant, which are essential for grain filling, ensuring that production potential is preserved.

Cotton farming has particularities that make chlorothalonil especially valuable. The application program starting at the B1 stage, with intervals of 14 to 21 days, always mixed with systemic fungicides, has proven effective in controlling ramularia, one of the main diseases of the crop. The preventive nature of chlorothalonil is essential in this context, since ramularia is a disease that is difficult to control once it is already established.

In sugarcane, aerial application of chlorothalonil for preventive control of rust and other foliar diseases has gained increasing importance, especially in more susceptible varieties and in regions with climatic conditions favorable to the development of diseases.