Downy Mildew Fungicide: What Works by Crop and FRAC Group

The best downy mildew fungicide depends on the crop and on which products still work where you farm. For cucumbers, melons, squash and pumpkins, NC State Extension lists cyazofamid (Ranman), oxathiapiprolin mixes (Orondis Opti or Ultra), propamocarb, ethaboxam, fluazinam and zoxamide mixes, each tank-mixed or alternated with mancozeb or chlorothalonil. Clemson Extension reports that in most parts of the United States the pathogen already resists mefenoxam (Ridomil), dimethomorph (Forum) and FRAC Group 11 strobilurins, so those are poor choices on cucurbits.

Below you will find a product table with FRAC groups and pre-harvest intervals, spray intervals before and after an outbreak, notes for grapes, lettuce and basil, a cost calculator for a season of sprays, and the seven non-chemical and chemical methods that make any fungicide work better. Pesticide rules change by country and state: always read the product label, follow local rules and ask your extension service or a licensed adviser before you spray.

Which downy mildew fungicide works best?

No single product is best everywhere. Downy mildew is caused by oomycetes (water molds), not true fungi, so many general fungicides do little. The products that work target oomycete biology. On cucurbits, the NC State Extension cucurbit downy mildew factsheet gives these examples for watermelon, squash and pumpkin, with FRAC group and pre-harvest interval (PHI, the days between the last spray and harvest):

For cucumber and cantaloupe, NC State lists a shorter set: Ranman, Omega, Orondis Opti, Orondis Ultra, Previcur Flex, Elumin, Gavel, Zing! and the two protectants. Clemson goes further and says Orondis Ultra should not be used on cucumber, cantaloupe or honeydew, because the cucumber strain resists its mandipropamid partner. Products, rates and limits differ by state and crop, so treat this list as a starting point for a talk with your adviser, not a spray order.

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Pre-harvest interval of example downy mildew products

Days from last spray to harvest on watermelon, squash and pumpkin, as listed by NC State Extension

Downy mildew fungicide groups and resistance

A FRAC group number tells you how a product works. Two products with the same number share a mode of action, so rotating between them does not slow resistance. The FRAC Code List rates the phenylamides (Group 4, such as metalaxyl-M, also called mefenoxam) as high risk, with resistance known in many oomycetes. It also groups CAA fungicides (Group 40, such as mandipropamid and dimethomorph) together, with cross-resistance between all members.

• Multi-site protectants (M03 mancozeb, M05 chlorothalonil, M01 copper): low resistance risk, no curative action. They are the base of every program and the usual tank-mix partner.
• Group 21 (cyazofamid): a single-site product with strong activity on oomycetes; used in rotation.
• Group 49 (oxathiapiprolin): the active in the Orondis range. Clemson limits it to one-third of all sprays on a crop.
• Groups 40, 43, 45, 28, 22 and 29: mandipropamid, fluopicolide, ametoctradin, propamocarb, ethaboxam or zoxamide, and fluazinam. Each is a separate rotation partner, except that two products in the same group are not.
• Group P07 (phosphorous acid, or phosphite): works partly by boosting the plant’s own defences. Cornell suggests it as a tank-mix partner on basil.
• Groups 4 and 11: mefenoxam and strobilurins such as azoxystrobin. Useful in some crops and regions, but resistance is widespread on cucurbits in the US.

Where resistance has already broken products

The Clemson cucurbit downy mildew guide (February 2025) reports that strains 1 and 2 resist Ridomil, Forum and Group 11 fungicides such as Cabrio, Quadris, Flint, Pristine and Reason in most parts of the United States. In South Carolina, the squash strain also resists Curzate, and resistance to Previcur Flex and Tanos is present in some places, so Clemson does not recommend those three there. Strains collected in Charleston in 2020 and 2021 were also less sensitive to Orondis than in 2015.

NC State explains why cucumber growers have fewer choices: clade 2 isolates, which favour cucumber and cantaloupe, can quickly become resistant to fungicides. That is why the cucumber list is shorter and why protectant partners matter so much.

Growers in the UK can find crop-specific resistance guidance from the Fungicide Resistance Action Group (FRAG-UK) hosted by AHDB.

Five resistance rules that apply to every crop

1. Rotate FRAC groups, not brand names. Gavel and Elumin are both Group 22, so Clemson says not to rotate them with each other.
2. Tank-mix single-site products with mancozeb or chlorothalonil, as NC State advises.
3. Keep Orondis products to one-third of total sprays and do not rotate Orondis Opti with Orondis Ultra (Clemson).
4. On lettuce, UC IPM advises no more than one application of a Group 1, 4, 9, 11 or 17 product before switching groups, and no more than two in a row for other groups.
5. Use the highest labelled rate once the disease is reported in your state; NC State says this gives better control and lowers resistance risk.

When to spray: timing and intervals

Downy mildew fungicides work best before infection. UC IPM says the preventive products for grape downy mildew must be on the leaf before an infection period begins, and new growth after a spray is not protected. NC State gives these spray intervals for cucurbits:

• Before the disease is reported in your area: every 7 days on cucumber and every 10 days on other cucurbits.
• After it is reported nearby: every 5 days on cucumber and every 7 days on other cucurbits, using the shorter interval in cool, wet weather.

Clemson adds three practical points. Spray before a forecast rain, not after, because the product must dry on the leaf first. Once vines touch, use at least 50 psi and 50 gallons per acre to get into the dense canopy. And once downy mildew is in your field, stay on a 7-day schedule.

Timing depends on spore arrival. In the eastern US, cucurbit downy mildew overwinters on crops in southern Florida and Texas and blows north each spring. Clemson notes spores can travel over 600 miles in 48 hours. The Cucurbit Downy Mildew ipmPIPE, run by NC State, maps confirmed outbreaks and offers text or email alerts when new ones are confirmed.

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Cucurbit downy mildew spray intervals

Days between fungicide sprays before and after the disease is reported in your area

What a season of sprays costs

Downy mildew sprays add up fast: a 7-day schedule over a two-month harvest window means about eight trips across the field. The calculator below sets the cost of a program against the crop value it protects. Use your own dealer quotes and yield records; the defaults are round examples set close to break-even, not benchmarks.

The break-even line shows the share of crop value the program must save to pay for itself. If that number is above the loss you expect without sprays, look again at the interval, the product choice or whether a tolerant variety would do more of the work.

Try the numbers for your own farm below.

Downy mildew fungicide choices by crop

Each crop has its own downy mildew species, so a product that works on one crop may not be labelled or effective on another.

Cucumber, melon, squash and pumpkin

The pathogen is Pseudoperonospora cubensis. Clemson recommends a preventive program of chlorothalonil or mancozeb (or copper for organic growers), then adding downy-mildew-specific products when the disease is found in or near your state. The Clemson watermelon fungicide guide says that if downy mildew is seen, spray Ranman or Orondis Ultra at once, followed by the other product the next week. Clemson also notes summer squash, zucchini and acorn squash tolerate some downy mildew and may need fewer sprays.

Grapes

Grape downy mildew (Plasmopara viticola) infects fast. UC IPM says infection can take less than 90 minutes, the incubation period is about 4 days, and the best temperature for the disease is 68 to 77°F. UC IPM suggests early-season copper sprays in vineyards with a history of the disease, especially in wet springs, and adding a spreader-sticker so rain does not wash the spray off.

The Ohio State grape program says most infection happens from early bloom to 3 to 4 weeks after bloom, and advises spraying as soon as possible after rain when temperatures are between 50 and 85°F. In Australia, the Australian Wine Research Institute puts the riskiest window at 3 to 6 weeks after budburst until berries are pea-sized.

Lettuce

Lettuce downy mildew (Bremia lactucae) has many races, and new ones keep appearing. UC IPM calls resistant cultivars the most effective control, but warns they do not stay resistant forever. Where no resistant cultivar fits, apply fungicides before the disease develops; the available products are mostly protectants. Drip irrigation can reduce severity but will not stop an epidemic in conducive weather.

Basil

Cornell Vegetables calls downy mildew the most damaging basil disease in the US since it was first found in south Florida in October 2007. The pathogen, Peronospora belbahrii, is wind-dispersed and seed-borne. Cornell lists Orondis Ultra, Revus, Ranman, Presidio and phosphorous acid products for conventional basil, and copper, Stargus (Bacillus amyloliquefaciens) and other OMRI-listed products for organic growers. Orondis Ultra is limited to one-third of applications and no more than four in total.

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Temperature ranges for grape downy mildew

Degrees Fahrenheit, from two US extension programs

Seven downy mildew treatment methods that work together

Fungicides work far better inside a wider program. These are the seven methods, with what extension sources say about each.

1. Cultural practices for plant disease management

Wet leaves drive infection. NC State lists 6 to 12 hours of leaf moisture and temperatures near 60°F as ideal for cucurbit downy mildew. Space and prune for airflow, water at the soil with drip rather than overhead sprinklers, and water early so leaves dry before night. In grapes, UC IPM advises good soil drainage, removing sources of overwintering spores, and stretching the gap between sprinkler irrigations. Clemson notes that early planting helps: spring cucurbits planted soon after the last frost may need no downy mildew sprays, while summer and fall crops face more pressure.

2. Chemical fungicides for downy mildew

This is the product table and resistance plan above. Start protectants before the disease arrives, add oomycete-specific products when it is reported nearby, rotate FRAC groups, and keep a spray record so you can stay within each label’s limits on number of applications.

3. Downy mildew resistant crops and varieties

NC State says most commercial cucumber varieties resist clade 1 isolates, but no full resistance is available for clade 2. It names the pickling cucumbers Citadel and Peacemaker and the slicers SV3462CS and SV4142CL as tolerant to clade 2. Clemson names Chaperone and DMR-NY401 as cucumbers with partial resistance. For basil, Cornell reports resistant varieties have been sold since 2018. Resistance slows the disease; it does not replace scouting.

4. Biological control and organic options

Organic choices are narrower. NC State says fixed copper products are the only OMRI-labelled active ingredients with some efficacy against cucurbit downy mildew, and Clemson adds that copper only helps if sprayed before infection. On basil, Cornell lists copper products, Stargus (Bacillus amyloliquefaciens) and several other OMRI-listed materials, while noting its trials show how hard downy mildew is to control organically.

5. Induced resistance methods

Phosphorous acid (phosphite, FRAC P07) products act partly by switching on the plant’s own defences. Cornell recommends them as tank-mix partners with other labelled fungicides on basil, not as a stand-alone program. Check that a phosphite product is labelled for your crop before use.

6. Monitoring and forecasting

Scout often, especially early in the morning, when NC State says the dark downy spore growth under leaves is easiest to see with a 20x hand lens. Sign up for the ipmPIPE outbreak alerts for cucurbits, and use weather-based infection models for grapes. Satellite crop maps can help you choose which blocks to walk first, because they show where plant vigour is falling; Farmonaut is one tool that maps this from Sentinel-2 imagery, though a field visit is still needed to confirm the cause.

7. Integrated downy mildew management

Put the six methods together: a tolerant variety, an early planting date where possible, dry foliage, protectant sprays before spores arrive, a rotated set of oomycete products after an outbreak is reported, and steady scouting. Each layer reduces the pressure on the next, which also slows fungicide resistance.

▶ Organic fungicides and monitoring for mildew on cucurbit crops. · 24:10

Symptoms to confirm before you spray

Confirm downy mildew before buying specific products, because look-alike diseases need different treatment. On cucumber, NC State describes angular yellow to brown lesions on the upper leaf that are bounded by leaf veins, with dark downy spores underneath. Clemson notes symptoms on watermelon are more variable, and that the spores underneath are brownish-purple, while powdery mildew spores are white and sit on top of the leaf. NC State lists Alternaria leaf blight, angular leaf spot, anthracnose, powdery mildew and Phytophthora blight as common look-alikes. Our guide to the signs and symptoms of downy mildew has more photos and crop-by-crop clues.

If you are unsure, send a sample to your state or provincial plant diagnostic lab. A correct diagnosis can save a whole season of the wrong sprays.

▶ Angular leaf spot on cucumbers and strawberries, a look-alike to check for before treating downy mildew. · 16:00

How we checked the products

We took product names, FRAC groups and pre-harvest intervals from NC State Extension’s factsheet (reviewed January 2023) and Clemson’s 2025 and 2026 guides, crop notes from UC IPM, Ohio State, AWRI and Cornell, and mode-of-action groups from the FRAC Code List. We did not rank products by percent control, because efficacy changes with local resistance and spray timing. Brand names are examples only and not an endorsement; check that a product is registered for your crop in your state or country.

▶ How to monitor crop health on your fields with satellite maps in the Farmonaut web app. · 8:43

FAQ

What is the best fungicide for downy mildew on cucumbers?

NC State’s example program for cucumber after the disease is reported rotates Ranman plus Bravo, Previcur Flex plus Bravo, and Orondis Opti every 5 to 7 days. Clemson advises against Previcur Flex in South Carolina because of resistance, and against Orondis Ultra on cucumber. Ask your local extension service which products still work in your area.

Is downy mildew a fungus?

No. It is caused by oomycetes, often called water molds. That is why fungicides aimed at true fungi often fail, and why products in groups such as 21, 40, 43 and 49 are used instead.

Can you cure downy mildew once you see it?

You cannot heal leaves that already have lesions, but you can slow spread. Clemson advises spraying a downy-mildew-specific product at once on watermelon and keeping to a 7-day schedule after the disease appears in a field.

What organic products work on downy mildew?

Fixed copper is the main organic option on cucurbits, according to NC State, and it must go on before infection. On basil, Cornell lists copper, Stargus and other OMRI-listed products but reports that organic control is hard.

Why did my downy mildew fungicide stop working?

The local strain may resist it. Clemson reports resistance to mefenoxam, dimethomorph and Group 11 strobilurins across most of the US on cucurbits. Rotating FRAC groups and tank-mixing with a protectant slows this.

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