Best Fungicide for Powdery Mildew: Trial Results by Crop

The best fungicide for powdery mildew is not one product. It is a rotation. In Cornell University’s pumpkin trials, Vivando alternated with DMI fungicides such as Proline and Procure gave 90% to 96% control on lower leaf surfaces from 2021 to 2023, and Vivando alone gave 86% in 2023 (Cornell Vegetables, cucurbit powdery mildew guide). Sulfur, potassium bicarbonate and oils are the main options for organic growers.

This guide orders the options using trial results and Cornell’s recommendations, explains why a product that worked for years can suddenly fail, and sets out a spray plan for cucurbits and grapes. It also has a calculator to test whether a spray program pays on your own crop. Product names are US examples. Registrations differ by state and country, so check what is approved where you farm.

The best fungicide for powdery mildew, by situation

The right choice depends on the crop, on whether you farm organically, and on what has already been sprayed on that field. The table sums up what extension specialists at Cornell, Clemson and the University of California say. Each row is explained below.

Treat this table as a starting point for a talk with your adviser, not as a recommendation for your field. Always read and follow the label and local rules. The label is the legal limit on rate, crop and number of sprays.

Chart

Rotation against single products, 2023 trial

Percent control on lower leaf surfaces of pumpkin, Long Island, one season

How we chose and what the numbers mean

We used public results from university extension programs rather than manufacturer claims. Most percentages here come from one long-running source: the yearly evaluations on pumpkin at Cornell’s Long Island research center, reported on its cucurbit powdery mildew page (updated April 2024).

Three limits apply to those numbers:

• Control means disease reduction on the lower leaf surface. Cornell scores that surface because the fungus develops best there.
• Products at risk of resistance were often tested alone. Cornell notes that this is neither a labeled nor a recommended commercial pattern. It is done to reveal whether resistance is reducing control.
• The results come from one location. The fungus population changes from year to year and from place to place, so a result on Long Island may not repeat on your farm.

We list no prices, because fungicide prices vary by supplier, pack size and country. Get a current quote and enter it in the calculator further down.

Trial results for conventional powdery mildew fungicides

Cornell’s 2019 results show the gap between products in a hard year. Vivando was the strongest single product at 75% control, close to the alternation program at 92% in statistical terms. Gatten was statistically as effective as Vivando at 65%, while Luna Sensation and Quintec fell behind.

Chart

Control of cucurbit powdery mildew, 2019 trial

Percent control on lower leaf surfaces of pumpkin, Long Island, one season

1. Vivando (metrafenone, FRAC 50)

Best for: the backbone of a rotation on cucurbits. Vivando was the most effective single product in Cornell’s 2017, 2018 and 2019 evaluations, and it reached 86% in 2023. Cornell recommends using it with a silicon adjuvant and not mixing it with horticultural oils. Its activity is limited to powdery mildew. The guide lists three applications a year, with no more than two in a row.

2. DMI fungicides: Proline, Procure and others (FRAC 3)

Best for: rotation partners with strong activity. Proline gave 99.5% control in 2020, the highest figure on Cornell’s page for that year. Procure varied more: 91% in 2016 and 54% in 2017. Resistance to this group is quantitative, meaning the fungus has become gradually less sensitive, so Cornell advises the highest label rate. DMI products are not interchangeable. Cevya, also a FRAC 3 product, gave 44% to 56% control from 2020 to 2022, and Cornell does not recommend it.

3. Gatten (flutianil, FRAC U13)

Best for: an extra mode of action where it is allowed. Gatten was as effective as Vivando in 2019, though not in 2018. Cornell says FRAC 50 and U13 are the two chemistries where resistance had not yet been detected at the time of its update. Clemson’s 2026 watermelon guide names Gatten or Vivando as the best fungicides for powdery mildew on that crop in South Carolina. Gatten is restricted in New York: two applications, and not on Long Island.

4. SDHI mixes: Luna Experience, Aprovia Top, Miravis Prime (FRAC 7 plus a partner)

Best for: sparing use when you also need other diseases covered. Cornell lists these as recommended but to be limited. Luna Experience also contains a Code 3 ingredient, so count that when you plan your rotation. Older SDHI products fare worse: resistance to boscalid, the active in Pristine and Endura, has been found routinely on Long Island since 2009, and Cornell no longer recommends them in New York.

5. Quintec and Torino (FRAC 13 and U6)

Best for: one application at most, and only where the other has not been used. The next section explains why. Note that UC IPM lists these two at the top of its California cucurbit table, which ranks products by IPM value, but that table was last updated in May 2016. Advice can differ by region and by date, because resistance does.

Why a good fungicide stops working

Powdery mildew has a long history of beating single-site fungicides. Cornell calls it the first occurrence of fungicide resistance in the USA. The fungus makes huge numbers of wind-blown spores, so resistant strains can spread fast and travel far.

Quintec shows how sharp the change can be. It gave 96% to 98% control in several early trials. In 2018 and 2019 it scored 42% and 40%, though 2018 was a low year overall (Vivando reached only 54%), and Cornell dates Quintec’s first clear drop below Vivando to 2019. It rebounded to the 89% to 95% band in 2020, which is why Cornell says resistant strains appear to have been uncommon in some seasons and why you cannot predict control without testing isolates from the planting.

Resistance can also cross groups. All the isolates Cornell collected at the end of 2020 that resisted Quintec also resisted Endura and Torino, even though those two had mostly not been applied. So one spray can select for resistance to fungicides in other groups. Cornell no longer recommends Topsin M (FRAC 1), or QoI products such as Quadris, Cabrio and Flint (FRAC 11), in New York. UC IPM’s guideline (text updated 2016) reports strobilurin-resistant strains throughout California.

How to rotate fungicide groups

The FRAC code on the label tells you the mode of action. The Fungicide Resistance Action Committee publishes recommendations for each group. Use these rules together:

• Alternate groups, not brands. Two products with the same code count as the same tool.
• Tank-mix a mobile fungicide with a contact protectant. Cornell recommends this for all targeted products.
• Check premixes. A two-way product uses up two groups in your plan.
• Respect the consecutive limit. UC IPM (table updated 2016) says that in California you should make no more than one application of groups 1, 4, 9, 11 or 17 before rotating, and no more than two in a row for other groups.
• Use an effective label rate. Cornell advises the highest labeled rate for mobile fungicides, within the label. In the UK, FRAG-UK at AHDB gives similar general advice: mix or alternate modes of action, avoid repeating a product, treat before infection is established, and never exceed the maximum dose or number of applications.

Chart

Quintec control over time on Long Island

Percent control on lower leaf surfaces of pumpkin, selected years with a single reported figure

Organic fungicides for powdery mildew

Under the US National Organic Program list (7 CFR 205.601), the allowed plant-disease materials include elemental sulfur, potassium bicarbonate, horticultural oils and polyoxin D zinc salt, and fixed coppers under conditions. Cornell’s guide lists dozens of labeled biopesticides for cucurbit powdery mildew, most of them OMRI-listed. Check with your certifier before you buy any product, and check that it is registered in your state or country.

Cornell says most of these are contact materials, so coverage decides the result. It also says products tested in university trials ranged widely in efficacy, with some as effective as conventional contact fungicides. They do not move to the underside of a leaf you did not hit.

Sulfur

Best for: early, preventive sprays. Cornell calls sulfur one of the most effective and least expensive products. It recommends microencapsulated formulations. The risk is plant injury: UC IPM warns that sulfur can injure plants, especially at 95°F, and that it should not go on sulfur-sensitive varieties. Melons are a known sensitive crop, though tolerant varieties exist.

Potassium bicarbonate

Best for: organic programs that need a short re-entry time. UC IPM’s 2016 table lists Kaligreen at 2.5 to 5 lb per acre, with a 4-hour restricted entry interval and a 1-day pre-harvest interval. Direct contact with the fungus is required, and it suggests the higher rate when pressure is severe.

Oils, including neem

Best for: mild to moderate infections. UC IPM’s home-garden vegetable pest note says horticultural oil or plant-based oils such as neem can control these infections. Keep oil at least 2 weeks away from any sulfur spray, and do not use it above 90°F or on drought-stressed plants. On grapes, UC IPM asks for at least 2 weeks between sulfur and a summer oil, in either direction.

Biological products

Best for: rotation and tank mixes rather than rescue. Cornell’s list includes products based on Bacillus strains, such as Serenade and Sonata, and on plant extracts such as Regalia. On grapes, UC IPM advises stopping sulfurs, biologicals and other soft products as a stand-alone program when pressure is high, because by themselves they will not give adequate control. UC IPM’s home-garden note also says Serenade has not proven as effective as oils or sulfur.

Baking soda and home garden use

The University of Wisconsin clinic reports that 1½ tablespoons of baking soda with 3 tablespoons of lightweight horticultural oil in a gallon of water has been shown to work on ornamentals. It suggests testing on a few leaves first, especially in warm weather. This is garden advice, not a field program. Commercial growers should use a registered product.

▶ A discussion of organic fungicides and treatments for powdery mildew in cucurbit crops. · 24:10

Powdery mildew spray plan for cucurbits

Cucurbits have the deepest public data. The steps below follow Cornell’s guidance. Adjust them to your label and adviser.

1. Choose resistant varieties. Cornell says resistance is standard in modern cucumbers. In other cucurbits it is not enough alone, and Cornell has seen it decline as the fungus adapts.
2. Scout weekly from the start of fruit set. Check both surfaces of five older leaves at each of 10 sites.
3. Act at the threshold. Cornell’s threshold is one leaf with symptoms out of 50 older leaves. Starting later compromises control and promotes resistance.
4. Spray weekly after detection. Rotate mobile fungicides and add a protectant. Cornell’s general range is every 7 to 10 days.
5. Improve coverage. Cornell suggests about 75 gallons per acre at 80 to 120 psi, nozzles 10 inches apart and angled tips. Use water-sensitive paper to check results.
6. Finish with contact products. Cornell says contact fungicides alone can suffice late in the season.

Separate successive plantings too, or plant up-wind of older ones, because older plants supply spores. A spring planting of summer squash can act as an early warning for the area.

How to confirm it is powdery mildew

Look for white, powdery growth on both leaf surfaces, stems and leaf stalks, starting on older and shaded leaves. Yellow spots may form on the upper surface opposite the colonies. Clemson’s scouting advice gives a quick test for yellow spots: powdery mildew spores on the underside are white, while downy mildew spores are purple. The two need different products, so see our guide to preventing downy mildew if your spores are purple.

Peppers and tomatoes can differ. Cornell notes that the pepper fungus, Leveillula taurica, grows inside the leaf and sporulates on the underside only, so symptoms are easy to miss. UC IPM says this group produces yellow patches with little powdery growth.

▶ Organic and chemical strategies for protecting cucurbit crops from viruses, a related threat in the same fields. · 14:25

Grapes: sulfur first, then a risk index

Vineyards work differently because the fungus overwinters in buds or as chasmothecia on the vine. UC IPM’s grape guidelines say a micronized sprayable sulfur or oil should go on before other fungicides, with spray intervals set by the UC Davis powdery mildew risk index.

• Protect early. UC IPM says all powdery mildew fungicides except oil are best used as protectants.
• Watch temperature. Ideal growth is between 70°F and 85°F. Temperatures above 95°F for 12 hours or more slow the fungus.
• Open the canopy. Basal leaf removal improves spray coverage on clusters, and UC IPM says leaf removal alone gives 50% disease control.
• Know when to stop. Treatment may end at 12 Brix for wine and traditionally trellised raisin grapes, but it continues to harvest for table grapes.

The stakes are high. Australia’s wine research institute, the AWRI, says minor disease levels of 3% to 5% can lead wineries to reject grapes, and that diseased berries can cause off flavors in wine. Its note dates from 2010, so check current winery specifications.

Does a spray program pay on your crop?

Powdery mildew costs money in several ways. Cornell lists smaller or fewer fruit, a shorter harvest, sunburned fruit, low sugar in melons, poor rind color and shriveled pumpkin handles. It also says infection predisposes plants to gummy stem blight. Published loss figures vary by crop and year, so we do not quote a single one. Use your own records in the calculator below.

Try the numbers for your own farm below.

Timing, weather and field conditions

Powdery mildew does not behave like most leaf diseases. UC IPM says the spores can infect without liquid water on the leaf, and that moderate temperatures of 60°F to 80°F and shade favor it most. Heat above 90°F and direct sunlight work against it. Its cucurbit guide says high humidity of 50% to 90% favors infection. That is why dry spells give no safety: Clemson says morning dew gives enough moisture for rapid spread on watermelon.

Dense, vigorous canopies are a risk too, because they create shade and block spray. Overhead water will not replace fungicide on susceptible crops, although UC IPM notes that long periods of water on the leaf can kill the spores of some species. Powdery mildew is one of many crop diseases that a tight canopy can worsen. Sound crop management helps here: good spacing, weed control and balanced growth. UC IPM says the fungi generally overwinter on weeds in California.

Farmonaut satellite vegetation maps cannot name powdery mildew, but they can show which parts of a field are losing vigor, so a scout knows where to walk first. Leaf checks still confirm the cause.

FAQ

What is the most effective fungicide against powdery mildew?

No single product wins every year. In Cornell’s trials, the highest single result in 2020 was Proline at 99.5% on lower leaf surfaces, and Vivando was the top single product in 2017 to 2019. Alternating Vivando with DMI products reached 90% to 96% from 2021 to 2023.

What is the best organic fungicide for powdery mildew?

Among protectants, Cornell calls sulfur one of the most effective and least expensive products, but it can injure sensitive crops in heat. Potassium bicarbonate and oils are alternatives. All need thorough coverage. Ask your certifier before use.

Can you cure powdery mildew once it has spread?

Results drop once the disease is established. UC IPM says Quintec is not effective if disease is established. Oils have some eradicant effect, and Clemson advises systemic fungicides for best control after disease starts. Starting at the first symptoms gives better results.

How often should I spray for powdery mildew?

Cornell advises weekly sprays after detection on cucurbits, within a general 7 to 10 day range. On grapes, UC IPM ties intervals to the product and the risk index. The label sets the seasonal limit.

Is chlorothalonil or mancozeb good for powdery mildew?

Chlorothalonil is a contact protectant with activity on upper leaf surfaces, and Cornell uses it as a tank-mix partner. It does not control mildew well on leaf undersides. Cornell notes that mancozeb is an exception among protectants, as it lacks contact activity for powdery mildew.

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