Powdery mildew is unavoidable in some geographic locations. Keeping it in check is key to a healthy crop. Keep reading for our science-backed, organic, low-tox approach that has been shown to cut powdery mildew by up to, and even over, 90 percent.

If you are here because you are staring at chalky leaves, do not fret. In many places, powdery mildew is inevitable. It rarely kills a plant, but it does reduce photosynthesis, which can weaken plants if left unchecked. The good news? There are absolutely organic and low-tox ways to keep it in check. The solutions are simple and natural, but this article goes deep (ya know, like we do). Let’s goooo!

This post will cover:

• What powdery mildew is and how it spreads
• The conditions that favor outbreaks
• High-risk regions and settings
• Our prevention strategy for low-tox, organic systems
• Our eradication plan when powder shows up
• Exact spray mix and application tips
• No-spray cultural practices that help

Powdery Mildew: What Is It?

Powdery mildew is one of the most common plant diseases. It is caused by several fungi that thrive with warm days, cool nights, and high humidity. As the name suggests, infected leaves and stems look dusted with white powder. Unlike many fungal diseases, it does not require free water on leaves to spread. A humid microclimate is enough.

Powdery Mildew: Favorable Conditions

• Temperature
  • Optimal between 60–80°F (15–27°C)
  • Growth slows above about 90°F (32°C) and below about 50°F (10°C)
• Humidity
  • High relative humidity, roughly 60 percent or greater, encourages spore germination
  • Free water, like prolonged rain, can actually suppress some powdery mildews
• Daily cycles
  • Warm, humid days followed by cooler nights are a classic trigger
  • Morning dew or nighttime condensation provides just enough surface moisture
• Airflow
  • Poor ventilation, stagnant air, or dense canopies increase risk
  • Shade and lower leaves are usually the first infection sites

Powdery Mildew: High-Risk Regions

Powdery mildew shows up in many climates. In our Midwestern fields, humid summers can create issues if we are not proactive. A few notable hot spots and settings:

Region / Setting	Climate Conditions	Why It’s High-Risk
California (vineyards, Central Valley, coast)	Warm to hot summers, little rainfall, coastal fog or irrigation humidity at night	Dry days mean spores aren’t washed off, while humid nights favor germination
Oregon & Washington (Willamette Valley, Puget Sound, greenhouse crops)	Mild, relatively dry summers (July–Sept) with cool, damp nights and morning dew	Days are not rainy but humidity stays high, night cooling drives sporulation
Northeastern U.S.	Warm, humid days and cool nights in summer	Fluctuating humidity + dense canopies create ideal mildew cycles
Midwestern U.S. (incl. WI)	Hot summer days, humid air, cool nights with dew	Classic “humid day + cool night” pattern in mid-to-late summer
Europe (Mediterranean & Central)	Mediterranean: dry summers, humid nights from coastal influence; Central: mild, moist cycles	Both regions have long histories of endemic mildew in vineyards and orchards
Greenhouses & High Tunnels (worldwide)	Warm temps, restricted airflow, high relative humidity	Artificial environments trap humidity and prevent natural spore wash-off
Shaded / Dense Plantings (anywhere)	Lower canopy stays humid and cooler than ambient	First infection often starts on shaded lower leaves and moves upward

Some plants are especially prone to powdery mildew: cucurbits like cucumbers, squash, pumpkins, and melons; ornamentals such as zinnias, dahlias, roses, phlox, and delphiniums; and edibles including peas, grapes, apples, and strawberries.

Susceptibility varies by crop and variety, and some years bring far heavier pressure than others. In 2025, for example, we saw more powdery mildew than usual, and it appears we were not alone. I’ve received countless messages from growers across the country, some who have never experienced powdery mildew before.

So I thought: let’s taco ’bout it!

Powdery Mildew: Prevention & Eradication

When it comes powdery mildew, prevention is key, but oftentimes, infection is unavoidable.  Below, I’ve outlined our science-backed, organic (and low-tox) approach to powdery mildew, both from the lens of prevention, but also what to do when powdery mildew strikes.

I share prevention first for those of us currently experiencing powdery mildew, so we can implement an earlier solution next time🙂 But honestly, this list is actually backwards of my lived experience. Because truly, I don’t introduce any controls until after I’m aware there is a problem. If a plant has never displayed powdery mildew, why introduce a preventive? However, once I experience a persistent issue, I’m equipped to prevent and treat early.

Powdery Mildew: Prevention

As the common saying goes, “An ounce of prevention is worth a pound of cure,” and this is especially true for powdery mildew. Why? Because the best time to act is before the white powder appears. Visible powder means mature colonies that are harder to knock back.

The words “spray” and “fungicide” can sound scary, but we’re not talking conventional chemicals here. These words are simply to indicate their mode of action. In our context, these are low-tox, natural, and organic-compatible tools.

Do not want to spray? No problem! Skip down to No-Spray Cultural Practices. They help more than you might think.

Powdery Mildew Biological Fungicide: Actinovate®

For crops with a history of mildew, we like to start early with a biological preventative and then alternate with a contact-kill mineral. Our preferred biological is Actinovate® (larger version we get here), an OMRI-listed product for growers and home gardeners. It contains a beneficial soil bacterium that colonizes roots and leaves, competes with pathogens, and produces antifungal metabolites. It suppresses a range of soilborne and foliar diseases.

Actinovate® Key Features

• Shelf-stable for 24 months
• Can be stored under normal conditions despite being live bacteria
• Effective on a range of soil and foliar pathogens
• Non-phytotoxic
• Can be applied as a foliar spray and/or soil drench
• Compatible with IPM programs and traditional fungicides
• Low Re-Entry Interval (4 hours) and no Pre-Harvest Interval
• The biological active ingredient in Actinovate poses no threat of resistance formation

Pathogens Controlled

• Soil: Damping Off Fungus, Root Rot, Turf Brown Patch, Pythium, Rhizoctonia, Phytophthora, Fusarium, Verticillium, Sclerotinia, and many others.
• Foliar: Downy Mildew, Gray Mold, Powdery Mildew, Botrytis, Phytomatotricum, Sclerotinia, Alternaria, and more.

Source: mycorrhizae.com

Actinovate®: Regulatory & Certification Status (2025)

• OMRI (Organic Materials Review Institute, U.S.)
  • Actinovate® AG and Actinovate® SP are OMRI-listed for use in organic production.
  • Approved under USDA National Organic Program (NOP) standards as a biological fungicide.
• EPA (United States)
  • Registered as a biological fungicide (EPA Reg. No. 73314-1).
  • Recognized for suppression of soilborne pathogens (Pythium, Phytophthora, Rhizoctonia, Fusarium) and foliar diseases (powdery mildew, Botrytis).
  • Classified as a low-risk pesticide—exempt from residue tolerance requirements.
• EU Regulations
  • Streptomyces lydicus WYEC 108 (the active ingredient in Actinovate) is registered as a microbial plant protection product in the EU.
  • Approved for use in organic farming under EU organic regulation (EC No. 834/2007 and subsequent).
• Canada
  • Registered with the Pest Management Regulatory Agency (PMRA) as a microbial fungicide.
  • Listed in the Canadian Organic Standards as an allowed microbial biocontrol product.
• Other international approvals
  • Authorized in several countries (e.g., Australia, New Zealand) as a microbial inoculant/biological fungicide.
  • Typically classified under “beneficial microbes” or “biological control agents.”

Bottom line: Actinovate is widely accepted across the U.S., EU, and Canada as an organic-compliant biological fungicide. It has the advantage of being a living microbe, which means it colonizes plant surfaces and roots, offering both disease suppression and potential growth promotion.

Why we use it early It works by colonization and competition, not by contact kill. Early introduction helps it establish before mildew does. One study on cantaloupe reported about 72 percent reduction in powdery mildew severity after a series of applications.

The downside? It’s pricey. However, we’ve seen so many benefits, we almost always have it on hand as a beneficial biological ally (we use it on a number of crops). 

Our 2025 field note As fate would have it, we were conducting an experiment with Actinovate® on our dahlia crop in 2025. For simplicity at planting, we sprinkled a small amount of Actinovate® in dahlia planting holes rather than mixing a drench. Surprisingly (and not the intention of the original experiment), we found that rows treated this way showed no or minimal powdery mildew compared to untreated rows. There are variety effects to consider, but the difference was strong enough for us to keep using it at planting in future seasons.

Please Note: Actinovate® is a water-soluble powder that is traditionally pre-mixed into a spray or a water drench. I  chose to use it as a dusting for simplicity, as our soil has plenty of moisture in spring. Always follow the label for rates and precautions.

Actinovate®: Further Considerations

Interestingly, Actinovate® has also been shown to significantly increase the root mass in certain crops. Anecdotally, and admittedly this could be quite a reach, but I wonder if it has the potential to increase root production in dahlias (i.e., tubers)?

Powdery Mildew: Alternate Preventative

Our alternate spray application works as both a preventative and eradicant, so if you’re currently dealing with powdery mildew, keep reading, as we’ll cover this in the eradication section. As a preventive, this has been shown to cut powdery mildew by over 90%, with curative treatments still suppressing infections by 60–80% Bonus: This control is low cost and can easily be made at home.

Powdery Mildew: Eradication

Once powdery mildew is visible, you’ll want something that can knock it back on contact. Our go-to? Potassium bicarbonate.

Potassium Bicarbonate

Potassium bicarbonate is a naturally occurring mineral salt that raises the leaf surface pH, destabilizing and rupturing fungal cells on contact. Because of this mode of action, it works both preventively and curatively when applied with good coverage and a wetting agent.

In research trials, potassium bicarbonate has consistently shown strong activity against powdery mildew. Preventive sprays often reduced disease incidence by over 90%, while curative applications on already infected plants achieved 60–80% suppression. Efficacy depends heavily on timing, frequency, and spray coverage, and results improve when potassium bicarbonate is used with a surfactant.

Potassium bicarbonate is classified as a “low risk, multi-site contact fungicide.” Because it breaks down into potassium (K⁺) and bicarbonate (HCO₃⁻) ions—both naturally occurring—it’s considered acceptable in organic systems.

Potassium Bicarbonate: Regulatory & Certification Status (2025)

• OMRI (Organic Materials Review Institute, U.S.)
  • Listed by OMRI for use as a fungicide in organic crop production.
  • Allowed under USDA National Organic Program (NOP) standards.
  • Commercial formulations (e.g., MilStop®, Kaligreen®, Armicarb®) are OMRI-listed.
• EPA (United States)
  • Registered as a biochemical pesticide (low-risk category).
  • Recognized as a contact fungicide for powdery mildew and similar diseases.
  • Exempt from residue tolerance requirements (safe for food crops).
• European Union
  • Approved as a basic substance under EU plant protection rules (EC No. 1107/2009).
  • Permitted in organic farming across the EU.
  • Widely used on grapes, cucurbits, and ornamentals in European organic systems.
• Canada
  • Listed in the Canadian Organic Standards as an allowed substance for disease management.
  • Registered for use on food and ornamental crops as a low-risk fungicide.
• Other International Approvals
  • Australia & New Zealand: Registered as a fungicide for powdery mildew in horticultural crops; approved for use in organic production systems.
  • Japan: Approved as a plant protection agent; permitted in Japanese organic standards.
  • FAO/WHO Codex: Recognized as a food additive and safe for agricultural use, supporting broad international acceptance.

Bottom line: Potassium bicarbonate is one of the most widely accepted organic-compatible contact fungicides in the world. It’s registered as a low-toxicity, minimal-residue tool in the U.S., EU, Canada, Australia, New Zealand, and Japan, and it is generally permitted wherever organic standards are enforced. Its global approval reflects both its safety profile and its effectiveness against powdery mildew.

OMRI Product or DIY? OMRI-listed products include MilStop®, Armicarb 85®, and Kaligreen®. We currently use MilStop and follow the label. Before that, we mixed our own with food-grade potassium bicarbonate (recipe below).

A DIY mix can perform very well, because the active ingredient is molecularly the same. The difference is that commercial products are finely milled for easy mixing, tested for crop safety, and come with an EPA label and organic certification.

If you’re not required to meet certification standards and don’t mind adding your own surfactant, the DIY version works just as well in practice, at least in my experience (oh, and it is cheaper). If you are certified organic, use labeled, OMRI-listed products and check with your certifier prior to introduction.

Either way: Avoid spraying in hot conditions and always spot test a few plants before treating a block.

Powdery Mildew Spray Mix (DIY)

Per 1 gallon of finished spray

Core ingredients

• One Gallon Sprayer
• Food-grade potassium bicarbonate
  • Preventive or early pressure: 1 tablespoon per gallon
  • Established mildew: 2 to 3 tablespoons per gallon
  • Follow product labels when using commercial formulations
• Wetting agent
  • We use ThermX-70 (yucca extract) 1 teaspoon per gallon, or
  • Liquid castile soap 1 teaspoon per gallon as a simple substitute
• Water: bring total volume to 1 gallon. Mix fresh and use entire contents within 24 hours.

Mixing

1. Fill the sprayer halfway with water.
2. Add potassium bicarbonate, agitate until fully dissolved.
3. Add the wetting agent, stir gently.
4. Top up with water to 1 gallon.
5. Spray immediately. Coat upper and lower leaf surfaces to just before runoff.

Application notes

• Interval: every 7 to 10 days preventively. Every 5 to 7 days when conditions are favorable or mildew is present.
• Timing: early morning or evening on cool leaves.
• Coverage: thorough coverage is critical. Hit the undersides.
• Heat caution: avoid spraying when temperatures exceed about 85°F (29°C) to reduce phytotoxicity risk.
• Spot test: treat a few plants and wait 24 to 48 hours before spraying the block.

Powdery Mildew Extra: Horticultural Oil

If powdery mildew is persistent, consider alternating potassium bicarbonate with a high-quality horticultural oil such as JMS Stylet-Oil®.

How it works: The oil forms a thin film over the fungus and spores. This physical coating can continue to suppress spores for a short period after the spray has dried, so it sometimes provides a bit more residual effect than a simple contact knockdown.

Why this oil? JMS Stylet-Oil® has a relatively low phytotoxicity risk compared to heavier oils. Studies report less disruption of beneficials than some alternatives, especially when applied as a direct foliar spray rather than broadcasted into the air. It does not carry the same soil microbe impacts that some broad-spectrum products do.

Bonus: Horticultural oils can reduce aphid transmission of certain nonpersistent viruses. That includes CMV and DMV in the dahlia world. That is a rabbit hole for another day. If you want a follow-up on this, tell me in the comments.

Side Note: Never mix oil and sulfur. Give a safe interval if sulfur has been applied recently. Always read labels for compatibility and temperature limits.

No-Spray Cultural Practices

Not into sprays? No problem! There are still things that you can (and should) do:

• Increase spacing beyond the minimum when varieties are mildew-prone
• Remove lower leaves where airflow is poor, if the crop tolerates it
• Scout early and often and remove infected leaves promptly
• Water at the base when possible to avoid humid leaf canopies in protected culture
• Improve airflow with pruning, trellising, and vent management in tunnels

Conclusion

Powdery mildew happens. You can still keep plants productive and beautiful with a low-tox plan. Prevent on crops with a history of mildew using a biological like Actinovate®. If powder appears, knock it back with potassium bicarbonate and consider rotating with a quality horticultural oil if pressure is unusually high. Pair these tools with smart spacing, airflow, and sanitation. Start simple, be consistent, and adjust based on what you see in your rows.

What about you? Do you have a powdery mildew hack that could help the collective?! Share in the comments below!