Chlorothalonil Fungicide: Uses, Rates and Legal Status

Chlorothalonil fungicide is a broad-spectrum contact protectant in FRAC group M5. It stays on the leaf surface and stops spores from germinating, so it has to go on before infection. Uses include vegetables, potatoes, peanuts and turf. It is legal in the US, with limits, but it has not been approved in the EU since 2019.

This guide covers what it controls, how it works, sample rates from extension guides, US residue limits and the legal position by region. It also has a calculator that shows how much yield a spray programme must save to pay for itself.

What chlorothalonil is and how it works

Chlorothalonil is the common name for 2,4,5,6-tetrachloroisophthalonitrile. The US EPA fact sheet of April 1999 calls it a broad-spectrum, non-systemic pesticide, first registered in the US in 1966 for turfgrass. Well-known US brands include Bravo, Echo, Equus and Initiate, as listed in the University of Florida tomato production chapter. Daconil is a brand name used in turf.

The University of Hertfordshire Pesticide Properties DataBase describes the action as multi-site. The chemical prevents spore germination and stops zoospores from moving. EPA noted in 1999 that the exact mechanism was not known, so treat any very detailed account of its biochemistry with some care.

What contact action means in the field

• It protects, it does not cure. The film works on spores that land on treated tissue. It does not move into the plant to reach an infection that has already started.
• New growth is bare. Leaves that open after a spray carry no film, so fast-growing crops need shorter gaps between sprays.
• Coverage decides the result. Spots the spray misses stay open to infection. Water volume, nozzle choice and canopy density all matter.
• Rain and dew matter. Long wet periods drive infection and also wear down the film, so check the rain-fast guidance on your label.

Why the M5 code matters for resistance

Fungicide Resistance Action Committee codes group products by how they act. Codes that start with M are multi-site products. Chlorothalonil is listed as M05, and extension guides pair it with single-site products to slow resistance. For example, the Florida chapter lists Quadris Opti (azoxystrobin plus chlorothalonil, codes 11 + M5) and Orondis Opti (oxathiapiprolin plus chlorothalonil, codes 49 + M5) as ready-made mixes. Each still carries its own rotation limits. The Clemson guide also builds week-by-week rotation into its programmes for systemic products.

Chlorothalonil fungicide uses by crop

EPA lists field, vegetable and orchard crops plus turf among the registered sites. It also lists non-crop uses such as a mildew additive for paint. The table shows crop uses named by current extension guides. Labels differ between products, so a crop or disease named here may not be on the product in your shed.

Older guides also list wheat and barley. That use has ended in the EU and Great Britain, where the substance is no longer approved. Elsewhere, read the current product label for cereals.

Potatoes and tomatoes

These are the classic blight crops. A chlorothalonil spray goes on before disease arrives and is repeated while the weather favours infection. It will not rescue a canopy where blight is already spreading fast, so growers often move to a mix with a product that has a different code. Our video on Alternaria covers how to tell the signs apart.

Melons and other cucurbits

Clemson’s programme shows how a contact product fits into a season. In the spring programme, the vine-run spray is mancozeb plus copper, and chlorothalonil comes in at the next spray. A note in the guide then says to use mancozeb instead of chlorothalonil starting week 5, to avoid injury to fruit on hot, sunny days. That is a useful warning for any crop with exposed fruit.

Turf, nursery and greenhouse

Turf was the first US use. UC IPM ranks chlorothalonil first in its dollar spot table for IPM value, with a 12-hour restricted entry on the entry it shows. Greenhouse workers need extra care: EPA’s 1999 agreement requires respirators for handlers in enclosed spaces. EPA also lists a smoke generator form as a restricted use product.

▶ Identifying Alternaria blight on potato and tomato and how to manage it · 16:21

Sample rates, harvest intervals and limits

There is no single rate. It changes with the product strength, crop, disease and country. The label is the legal limit. These entries show how guides express the numbers:

• Snap bean rust (Maryland): chlorothalonil 6F at 2.0 to 4.0 pints per acre, on a 7 to 14 day schedule, 14-day pre-harvest interval, 12-hour re-entry.
• Tomato (Florida): rate per the individual label, 0-day pre-harvest interval, half-day re-entry, with higher rates at fruit set and lower rates before it.
• Potato late blight mix (UC IPM): Ridomil Gold Bravo SC, a mefenoxam mix, at 2.5 pints per acre.

The harvest intervals above differ by two weeks across crops, which is why a rate or interval from one crop must never be carried to another. A 6F product holds 6 lb of active ingredient per gallon, so one pint carries 0.75 lb.

Does a spray programme pay? Run your own numbers

A protectant only pays if the yield it saves is worth more than the product and the trip. Enter your own figures below. The result shows the yield saving needed to break even.

Try the numbers for your own farm below.

US residue limits on food crops

EPA sets tolerances, the highest residue allowed on a food. Those for this active ingredient are in 40 CFR 180.275. They cover chlorothalonil and its 4-hydroxy metabolite as the regulation defines them. A limit is a legal ceiling. It does not tell you what residue a sprayed crop carries.

Chart

US chlorothalonil residue limits, selected crops

Tolerance in parts per million; selected entries from the list

The spread is wide. Celery has a limit of 15 ppm and potatoes 0.1 ppm. The regulation lists the limits but does not explain the differences. An export crop has to meet the limit of the buying country too. Since the EU no longer approves the substance, check its residue limits early if you sell into Europe.

Regulatory status: US, EU, UK and elsewhere

United States

EPA completed its reregistration decision in 1999. As part of it, registrants agreed to lower maximum rates for many uses and to cut the hexachlorobenzene impurity to 40 ppm by 2003. They also agreed to untreated buffers next to estuarine and marine water: at least 150 feet for aerial and air-blast application and 25 feet for ground application. Products are barred from home lawns. Chlorothalonil as such was not a restricted use pesticide in that decision. Later rules or label changes can be stricter, so the current label governs.

European Union

The EU decided not to renew approval in Commission Implementing Regulation (EU) 2019/677, adopted on 29 April 2019. Any member-state grace period had to end by 20 May 2020. The regulation gives these reasons:

• Several metabolites were predicted in groundwater above 0.1 μg/L in all pertinent scenarios for all proposed uses.
• A genotoxicity concern could not be ruled out for residues that consumers may be exposed to.
• A high risk to amphibians and fish was found for all the uses evaluated.
• The authority said the carcinogen classification should move from category 2 to category 1B.

United Kingdom and others

The Pesticide Properties DataBase lists the substance as not approved in Great Britain, with no UK approval for use as a plant protection agent. If you farm in Canada, Australia or New Zealand, check the product register of your national regulator: PMRA, the APVMA or the EPA in New Zealand. A label from another country does not make a product legal on your farm.

How much is used in the US

EPA’s 2008-2012 sales and usage report ranked chlorothalonil 10th among conventional active ingredients in the US farm sector for 2012, at 6 to 16 million pounds. It also ranked second in the industry, commercial and government sector that year, at 5 to 7 million pounds. These are ranges, not exact totals, and they are old. USGS county-level use maps run to 2019 for a more local view.

Chart

Estimated US farm use of chlorothalonil

Million pounds of active ingredient, low and high ends of EPA ranges

Health and environmental risks

People

EPA’s 1999 fact sheet places chlorothalonil in Toxicity Category I for eye effects, the most severe group, after severe eye irritation in rabbit tests. Inhalation is Category II. Oral and skin acute toxicity are Category IV, the least toxic group. EPA classed it as a likely human carcinogen. It judged dietary cancer risk from food and surface water at that time to be negligible, but some handler estimates, such as for wettable powders, exceeded levels of concern. That helps explain the handling rules.

The joint FAO/WHO expert meeting records an acceptable daily intake of 0 to 0.02 mg per kg of body weight for the parent substance, and a lower 0 to 0.008 mg per kg for the metabolite SDS-3701, in its 2010 JMPR evaluation. The EPA fact sheet and the EU decision are 20 years apart, and the EU decision turned largely on groundwater metabolites.

Water life and wildlife

EPA described chlorothalonil as very highly toxic to fish and highly to very highly toxic to aquatic invertebrates. In tests, fathead minnow hatching and survival were affected between 3 and 6.5 parts per billion. EPA named peanuts, potatoes and turf among the uses of concern for water life, and marine mollusks as a group at particular risk. Spray drift, runoff and eroding soil are the routes to surface water.

Acute effects on land are milder, with one caution. EPA called it practically non-toxic to birds in acute tests but reported effects on bird reproduction at 5 ppb, and it called the chemical relatively non-toxic to honey bees, while the Pesticide Properties DataBase rates it moderately toxic to bees, birds and earthworms. Follow the bee wording on your label.

What happens in soil

The parent chemical breaks down fairly fast in soil. The database gives a typical half-life near 3.5 days in the lab and about 18 days in the field. The concern lies with what it turns into. EPA said SDS-3701 appears to be more persistent and mobile than the parent, and it found breakdown products in groundwater although the parent rarely appears there. This is why a short soil half-life does not settle the groundwater question.

Chart

How long the parent chemical lasts in soil

Half-life in days, EU regulatory data as held by the database

Chlorothalonil compared with mancozeb and azoxystrobin

Control levels change with crop, disease, weather and timing, so a single efficacy figure for each product would mislead. This comparison uses facts you can check.

Other options include copper for bacterial problems and polyoxin D, which the Florida chapter lists with a four-hour re-entry time. Where both multi-site products are gone, as in the EU, programmes have to lean on other modes of action plus resistant varieties, hygiene and forecasting. Your local extension service or adviser can name what is legal and effective for your crop.

How to use chlorothalonil safely and well

1. Confirm the legal basis. Check that the product is registered in your country for the crop and disease.
2. Read the current label. It sets the rate, seasonal maximum, spray gap, harvest interval, re-entry time, mixing limits and buffers.
3. Identify the disease. Bacterial diseases need a different answer. Send a sample to a clinic if you are unsure.
4. Time it before infection. Scout weekly. Clemson advises walking fields each week and the day before spraying.
5. Use a forecast. UConn IPM explains that Tom-Cast uses temperature and leaf wetness to time sprays for early blight, Septoria leaf spot and anthracnose in tomato, and that Simcast does the same for late blight. UConn adds that Tom-Cast intervals are too long if bacterial disease is present.
6. Wear the right gear. Eye protection deserves special attention. Under EPA’s 1999 terms, handlers of several forms must wear gloves, and workers who re-enter within 7 days must be told about eye hazards and have eye-flushing equipment on site.
7. Protect water. Keep buffers, avoid spraying before heavy rain, and use nozzles that reduce drift.
8. Rotate. Follow FRAC codes and the limits on any partner product.
9. Keep records. Note date, field, product, lot, rate, weather and operator.

Avoiding crop injury

Phytotoxicity is damage caused by the spray itself. Clemson’s warning about melons on hot days is one example, and the Florida note about adjuvants with Quadris Opti is another. Test any new mix on a small area first, and do not add an adjuvant unless the label allows it. Our video on phytotoxicity shows what the symptoms look like.

▶ Phytotoxicity in crops: symptoms and managing chemical applications · 19:28

Where satellite monitoring helps, and where it does not

Satellite vegetation maps show where a crop canopy is weaker than the rest of the field. They do not name the disease. A low-vigour patch could be blight, water stress, nutrient shortage or other crop stress. Their value is in telling you where to walk first, and in checking the same patch after a spray. The Farmonaut Satellite + AI Farm Monitoring System provides maps of this kind on web and mobile. A field visit should still decide whether to spray.

Related pages on our site: agriculture web applications, carbon footprinting, product traceability, large farm management, crop loan and insurance monitoring, satellite and weather API documents, satellite API access, web app and telling aphids from mealybugs.

▶ How to read satellite data for crops in the Farmonaut web app · 10:16

FAQ

What is chlorothalonil fungicide used for?

It protects crops and turf from fungal leaf diseases. Extension guides name it for early and late blight, gummy stem blight, downy and powdery mildew, anthracnose and dollar spot. Check that your own label lists your crop and disease.

Is chlorothalonil systemic or contact?

Contact. EPA describes it as non-systemic. It works on the plant surface and needs to be in place before infection starts.

What is the chlorothalonil mode of action?

It is a multi-site fungicide in FRAC group M5. It prevents spore germination and stops zoospores from moving. Acting at many sites makes it a common rotation partner for single-site products.

Is chlorothalonil banned?

It has not been approved in the EU since the 2019 non-renewal, and it is listed as not approved in Great Britain. It remains registered in the US with limits, and EPA’s 1999 agreement barred its use on home lawns. Check your national register elsewhere, and get local advice if you are unsure.

Is chlorothalonil harmful to people or fish?

It can seriously irritate eyes, and EPA classes it as a likely human carcinogen. It is very highly toxic to fish. Follow the label on protective gear, re-entry times and water buffers.

How long does it last on the crop?

Guides set spray gaps; Maryland’s bean guide uses 7 to 14 days for rust. Rain, new growth and disease pressure shorten protection, so use the gap on your label.