Captan is a broad-spectrum fungicide used since the 1950s to control a wide range of fungal diseases on plants. As a general-use pesticide, it is available in various formulations, including powders, dusts, and liquids, for both agricultural and home settings. This substance belongs to the phthalimide chemical class and functions as a non-systemic, contact fungicide, meaning it protects the plant surfaces it directly covers without being absorbed into the plant’s tissues.
How Captan Functions
Captan operates as a multi-site inhibitor, disrupting several metabolic processes within fungal cells simultaneously. It specifically targets and binds to enzymes involved in cellular respiration and energy production, which halts the fungus’s ability to grow and reproduce. This disruption of the electron transport chain leads to a reduction in ATP, the energy currency of the cell, ultimately causing cell death.
The fungicide also interferes with the synthesis of the fungal cell wall. It inhibits the activity of chitin synthase, an enzyme responsible for producing chitin, a component that provides structural integrity to the cell wall. This action weakens the wall, leading to its eventual breakdown. Because captan attacks multiple pathways, it is effective in preventing the development of resistance in fungal populations, as it is more difficult for fungi to develop defenses against numerous simultaneous threats.
Common Agricultural and Horticultural Uses
The broad-spectrum and preventative capabilities of captan make it useful in both commercial agriculture and home gardening. It is commonly applied to fruit crops like apples, strawberries, and grapes to manage diseases like scab, black rot, and brown rot. In apple orchards, for instance, it is used to control secondary scab and bull’s eye rot, often applied in cover sprays throughout the growing season.
Beyond fruit production, captan is applied to vegetable crops, including tomatoes and potatoes, to control blights and leaf spots. Its use extends to ornamental plants and turfgrass in nurseries and greenhouses, where it protects against diseases like black spot on roses and brown patch on lawns. Another application is as a seed treatment for crops like corn and beans, where it protects seeds from soil-borne pathogens that cause damping-off.
Human Health and Safety Considerations
Exposure to captan can lead to irritation of the skin and eyes. Acute dermal contact may cause dermatitis, while eye contact can result in more severe outcomes, including conjunctivitis or irreversible corneal opacity. Inhalation of the fungicide, a primary route of exposure for agricultural workers during mixing and application, poses risks. If ingested in large amounts, captan can cause symptoms such as vomiting and diarrhea.
The U.S. Environmental Protection Agency (EPA) classified captan as a “probable human carcinogen” based on studies where high doses caused intestinal tumors in mice. However, the EPA later clarified this effect is linked to prolonged irritation not expected at the low-level dietary exposures the public might encounter. The agency determined that when used according to the label, captan is not likely to pose a cancer risk to humans. Residue levels on food are monitored, and regulatory agencies establish a reference dose (RfD) for safe daily consumption.
Environmental Impact
Captan poses a hazard to aquatic ecosystems. It is classified as moderately to very highly toxic to fish and aquatic invertebrates like water fleas and oysters. Because of this, runoff from treated agricultural fields into nearby streams, ponds, or rivers is a concern, as it can lead to fish kills and disrupt aquatic food webs. For this reason, its application near water bodies is often restricted.
In contrast to its aquatic toxicity, captan is considered practically non-toxic to birds and mammals. Captan breaks down relatively quickly in both soil and water, which limits its persistence in the environment. Its half-life in soil can range from less than a day to a few weeks, and in water, it hydrolyzes even faster, with a half-life of just a few hours depending on the pH. This rapid degradation means it is unlikely to bioaccumulate, or build up, in the tissues of wildlife.
Safe Handling and Application Practices
Personal Protective Equipment (PPE) is required when applying captan, including:
- Long-sleeved shirts and long pants
- Chemical-resistant gloves
- Shoes with socks
- Protective eyewear such as goggles or a face shield
For tasks involving mixing, loading, or handling the concentrate, a chemical-resistant apron is also necessary. It is important to wash hands thoroughly after handling and before eating or drinking, and to wash PPE separately from other laundry.
Regulations also dictate specific intervals to protect workers and consumers. The Re-Entry Interval (REI) is the time immediately following a pesticide application when entry into the treated area is restricted. For captan, the REI can vary from 24 hours for crops like apples to 48 hours for grapes, and even longer for specific tasks involving high foliage contact. The Pre-Harvest Interval (PHI) is the minimum time that must pass between the last pesticide application and when the crop can be harvested. For many crops treated with captan, such as apples and cherries, the PHI is zero days.