Powdery Mildew How To Guide: Containment, Control & Prevention

“How do I get rid of powdery mildew?” is one of the most common questions we get at GrowersHouse. The good news: powdery mildew control is absolutely doable—but it takes a combination of strategies and a little consistency. The most reliable approach is a simple 3-step system:

1. Identify
2. Prevent
3. Treat

Step 1: Identify Powdery Mildew

Do I have powdery mildew? What does it look like?

Powdery mildew is one of the most common diseases in greenhouse and indoor agriculture, recognized by its powdery white spots. This crop-damaging fungus can infect many plant species and spread quickly through indoor and outdoor environments—especially where plants share airflow, tools, or handling. It can also be transmitted by pests like aphids, or by contamination from clothing, boots, hands, and shared ventilation between “dirty” and “clean” spaces.

Early symptoms typically appear as dusty gray or white spots on the upper or lower leaf surfaces. Left untreated, powdery mildew produces spores and spreads into stems, fruits, and flowers. While plant death is rare (usually only in extreme cases), powdery mildew can significantly reduce yield, flower/fruit quality, appearance, and overall crop value—especially in intensive hydroponic production.

*IMPORTANT*
If powdery mildew is detected, bag and remove affected plants from the environment immediately. Increase manual monitoring frequency to reduce spore spread. Clean and sanitize tools or equipment that contact multiple plants to prevent cross-contamination.

Once you research the specific fungal species affecting your crop, you can tighten your environmental targets and prevention plan. No matter which method you choose, the best results come from combining a systemic strategy with a surface (contact) strategy. This combo helps:

• Kill pathogens on contact
• Suppress spore production and germination
• Limit systemic spread through plant tissue
• Improve ongoing plant protection

It’s also smart to rotate methods/products over time. As commercial growers know, fungal strains can develop resistance if the same treatment is used repeatedly.

Step 2: Prevent Powdery Mildew

Cleaning, sanitation, and environmental stability

Powdery mildew and many molds thrive when foliage stays dry while the surrounding environment remains humid. That’s why greenhouses and indoor hydroponic setups are especially susceptible. In general, aim to keep relative humidity (RH) low and stable—typically 40% to 55%. Fluctuating conditions can make infection more likely.

Prevention also improves when you:

• Provide adequate spacing between plants to reduce canopy humidity and improve spray coverage
• Trim lower growth when appropriate to reduce the most vulnerable, crowded leaf zones
• Use liquid water strategically on high-risk surfaces (it may discourage spore germination), while avoiding conditions that promote other water-favoring diseases

Active sanitation and disinfection practices minimize contamination and reduce the odds of recurring outbreaks.

Sanitize tools, equipment, surfaces—and people

• Boot baths or sprays for walk-in greenhouse areas help prevent tracking in spores and pests
• Hand washing, hand sanitizers, and antimicrobial soap between plant handling; alcohol-based sanitizers often perform best for quick resets
• Tool dip disinfectant for scissors and trimmers—immersing tools for ~2 minutes can reduce cross-contamination (example: Physan 20 at 4 tsp/1 gal water)
• Hard surface sanitation for pots, trays, walkways, reusable equipment, evaporative cooler water, hydroponic reservoirs/tubing, and replacing or washing HEPA filters
• Area disinfection between crop cycles using rated hard-surface disinfectants, plus cleaning transport vehicles or truck beds to prevent reintroducing spores

Here are the same product references with non-relative links (so they work consistently across platforms): Physan 20 Fungicide Virucide, ProKure V Disinfectant, BioSafe SaniDate.

These best practices are easy to implement in both new and established hydroponic facilities. Once your operation is running, inspect plants on a regular schedule—paying extra attention to mid and lower leaves, where infections often begin. Between production cycles, thoroughly sanitize all surfaces and remove leftover debris and foliage.

If your region or facility is highly susceptible, consider alternating diluted preventatives between cycles (example products: Serenade Garden Disease Control or PM Wash). A common conservative approach is using approximately ¼ strength as a preventative.

Step 3: Treat Powdery Mildew Safely

UV (ultraviolet) tools

Because spores spread readily through airflow, UV tools can help reduce transmission risk. In limited exposure, a UV device designed for horticultural use may be directed at leaf surfaces to deactivate or kill powdery mildew, fungus, and bacteria already present in the grow room. Use care: overexposure can damage plant tissue. For best results and safety, use UV tools specifically designed and tested for powdery mildew suppression when used as directed.

Chemical treatments

Inorganic options: Manufactured chemical treatments can be effective, efficient, and economical—often while still being safe for use around live plants when applied correctly. For example, potassium salts and phosphorous acid can help protect and treat fungal and bacterial infections by strengthening crops and reducing disease advantage.

Stronger chemistries are sometimes used for prevention and sanitation before disease ever enters the room. Peroxide solutions are commonly used as an eco-friendly way to clean hard surfaces. For higher-grade sanitation, chlorine dioxide solutions can rapidly kill microorganisms and pathogens on hard surfaces.

Organic treatments

Organic fungicides come from plant-derived compounds and natural mechanisms. Common organic tools for powdery mildew control include neem and other oils, insecticidal soaps, sulfur, citric acid, bicarbonates, milk/whey sprays, and more.

• Neem oil and other plant oils contain broad pest-control compounds and can help suppress mildew pressure
• Garlic naturally produces antimicrobial compounds that can be extracted for spray use
• Milk (or dry milk powder) has shown effectiveness; some farms use whey-based foliar sprays to inhibit multiple powdery mildew species
• Insecticidal soaps (potassium fatty acids) have very low mammal toxicity and have been approved for powdery mildew use in the U.S. since 2011
• Sulfur helps prevent germination by lowering leaf-surface pH; it can be vaporized or sprayed, but it’s an eye/lung irritant—use protective measures and ventilate away from living spaces
• Potassium bicarbonate makes leaf surfaces less hospitable and is commonly applied as a spray solution
• Sodium bicarbonate (baking soda) can be used similarly in solution
• Beneficial microorganisms can help compete with disease; Bacillus subtilis is one example often referenced for biological protection

Climate control

Powdery mildew depends on specific environmental cues, so consistent climate control is one of the biggest “set-and-forget” advantages you can build into a hydroponic operation. Stable humidity is the goal: if spores don’t get the RH signal to germinate, outbreaks are far less likely. Because fluctuating humidity can trigger problems, pairing humidification and dehumidifiers may help maintain optimal RH even as temperature changes between the room and the outside environment.

Many climate controllers integrate thermostats and hygrometers, and some offer Wi-Fi alerts—useful for catching swings early before powdery mildew gains traction.

Key Takeaways for Hydroponic Growers

Powdery mildew spreads fast, but it’s also very manageable when you treat it like a system problem—not a single-product problem. The strongest outcomes come from:

• Fast removal of infected material to reduce spores
• Consistent sanitation (tools, surfaces, and traffic control)
• Stable RH targets (commonly 40%–55%) and reduced environmental swings
• Layered control using both systemic and surface approaches
• Rotation of methods to reduce resistance risk

When these are in place, growers typically see fewer outbreaks, stronger crop quality, and better protection of yield and plant aesthetics.