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What To Do About Powdery Mildew

By Harley Smith | Last updated: June 8, 2021
Key Takeaways

The key to beating a powdery mildew outbreak in your growroom is staying two steps ahead of the problem. This means growing plants that are resistant to outbreaks in the first place, removing any infected plant parts as you see them, and maintaining optimal growing environments. Read on for more ways to win the battle against powdery mildew, a fungal disease that affects a wide variety of plants.

Caption: Fungal disease powdery mildew on zucchini foliage Source: matunka/Shutterstock

Beating Powdery Mildew with Amino Acids

This discussion on beating powdery mildew in the growroom begins with a trip to the Netherlands. Traveling to the Netherlands is always an eye-opening experience.

The Dutch grow nearly all of their vegetables in zero-runoff hydroponic systems in sophisticated, computer-controlled greenhouses, and they are famous for high yields of gourmet-quality produce. Impressive. They are also strict about the use of pesticides and fungicides.

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In the year 2000, the Dutch government made the use of all synthetic fungicides illegal on food crops. Many of the strawberry growers began losing 40% of their crops overnight due to powdery mildew, gray mold and similar diseases. The hydroponic growers had to do something, so they began experimenting with organic biostimulants in tiny dosages. Amazingly, they worked.

In 2001, I was invited to the Netherlands to interview the strawberry growers who were doing the first biostimulant trials for the Dutch government. They reduced their losses from 40% down to zero, enjoyed a 10-20% increase in yields over any past year, and experienced top-of-the-line sugar content in the fruit—all through the use of a completely organic product with no harmful chemicals.

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Unfortunately, the scientists I interviewed in Holland told me their formula was proprietary and wouldn’t tell me how it worked. But that only made me more determined to solve the mystery.

Powdery mildew on strawberry leaves and ripening strawberryPowdery mildew on strawberry leaves and ripening strawberry. Source: Shutterstock

At the time, strawberry growers in the United States were fumigating the soil with methyl bromide, a chemical that kills life in the soil and destroys the ozone in the atmosphere at the same time. I was inspired and motivated, and knew there had to be a better way. After all, I had seen it with my own eyes.

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After years of research and experimentation in plant physiology, microbiology and nutrition, the pieces finally came together. Amino acids were the key. My well water is loaded with calcium and magnesium carbonate, and lime scale used to turn my reservoirs into rock gardens.

(Read also: Building up the Brix for Healthier Nutrient Dense Crops)

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But when I started experimenting with an amino acid blend I received from a vitamin manufacturer, there was no more lime scale. So where did all of that extra calcium go? Into the plants. I learned that certain amino acids stimulate root cells to open up calcium ion channels, allowing the plants to take up calcium thousands of times faster than simple osmosis.

Calcium is taken up through the roots and transported with the water throughout a plant. Some of the calcium reacts with pectic acid to form pectin, the glue that binds the cell walls together.

Downy mildew under the Microscope living in plantDowny mildew living in plant under the Microscope. Source: Rattiya Thongdumhyu/Shutterstock

Instead of water between the cells, the plants had extra pectin between the cells. The plants became stronger and healthier with thicker cell walls, and I never had to spray a single fungicide or insecticide on them the entire season.

So how does it work? When a mold spore lands on a plant leaf, it wants to send down a feeding tube to get to the water between the cells to germinate and spread. But when the cell walls are thicker and there is an increased amount of calcium-pectate between the cells, the mold spores just sit there.

By the time the germination tube penetrates the cells, it dries up and dies. The pectin doesn’t kill the powdery mildew, it just prevents it from becoming systemic in a plant and spreading.

(Read also: 10 Common Marijuana Leaf Problems and How to Fix Them)

Increased calcium uptake also provides a reserve of natural protection against powdery mildew. Any extra calcium a plant doesn’t use to strengthen the cell walls is pumped into a storage vacuole inside the cell. If a mold spore does happen to germinate, sensors on the surface of the leaf detect the chitin in the cell wall of the fungus. Plants don’t contain chitin—they contain cellulose.

So when chitin is detected, the plants send a signal molecule from the leaf surface down to the vacuole, opening up calcium ion channels inside the cell. The calcium ions released start a chain reaction that causes an oxidative burst—a plant’s first line of defense against powdery mildew.

This all means the judicious use of biostimulants containing amino acids will help improve the plant’s natural resistance to pests and diseases, without harming human beings or the environment.

Spraying/washing leaves that have powdery mildew.Leaf washes can be effective at removing powdery mildew and/or spraying potassium bicarbonate additives can help kill powdery mildew spores. Source: FrankHH/Shutterstock

Additional Prevention and Treatment Options for Powdery Mildew in the Garden

In addition to supplementing amino acids with a full plant nutrition program, there are plenty of other ways to prevent or eradicate powdery mildew from your garden.

Don’t Over-fertilize with Nitrates
Too much nitrate nitrogen encourages soft growth, which means plants will develop large cells with thin cell walls, making them more susceptible to powdery mildew.

Avoid High Humidity
High humidity sets up the perfect environment for molds and mildews, and also interferes with calcium uptake.

Avoid Large Swings in Relative Humidity
It’s best to maintain relative humidity between 40-60%, with 50% being a good target. If powdery mildew is born, a lower humidity stimulates the fungus to make more spores and spread.

Provide Good Air Movement
Provide plenty of space for your plants and use oscillating fans to produce a gentle breeze in your garden. Stagnant air allows moisture to build up on the undersides of the leaves, creating a vapor barrier. If transpiration is disrupted, calcium uptake will be disrupted as well. Adding filters on all intake vents is also a good idea.

Cutting old zucchini leaf with Powdery mildew fungal diseaseCutting zucchini leaf with Powdery mildew fungal disease. Source: KanphotoSS/Shutterstock

Remove Infected Leaves
At the first sign of powdery mildew, carefully remove infected leaves and discard them from the growroom. Be careful not to spread the spores in the process.

Rinse Spores from Leaves
Leaf washes can be effective at killing and removing powdery mildew spores. Powdery mildew spores don’t germinate from surface water on the leaves, so spores can be washed off all the way to the day of harvest. Allow adequate time for the leaves to dry so other fungi aren’t encouraged to form.

Spray Yucca Extracts
Yucca, a natural surfactant that can be safely added to any foliar spray, is said to have natural fungicidal properties. Instead of the water beading up on the waxy surface of the leaves, the spray will spread out in a thin film for better coverage.

Add Silica
When plants are under attack from powdery mildew and other fungi, they mobilize silica to the point of infection, which forms a callus around the surrounding cells. The silica will help prevent the disease from spreading. (Read also: Why Your Plants Need More Silica)

Spray with Potassium Bicarbonate
Potassium bicarbonate additives can help kill powdery mildew spores on contact, while also increasing the pH level on the surface of the leaves. A pH level above 8.3 on the surface of the leaves discourages fungi.

Use Botanical Oils
Some horticultural oils and essential oils improve the effectiveness of fungicidal sprays, just be careful not to combine oils with sulfur treatments.

(Read also: Dealing with Downy Mildew)

Treat with Biologicals
Some micro-organisms produce natural plant-protection agents against pathogenic fungi. Inoculate your soil with a good “maintenance” blend of plant growth-promoting rhizobacteria, and occasionally spray your plants with products containing streptomyces lidicus or other beneficial microbes.

Apply Systemic Fungicides
Most organic controls for powdery mildew only help slow down or contain the fungi, but systemic fungicides kill powdery mildew at the source. Make sure the chemical fungicide is rated for consumable crops and carefully follow all application instructions.

Alternate Treatments
If you must use fungicides, it’s always best to follow up with treatments that have different modes of action. If you use the same active ingredient too many times, the treatment may start to lose its effectiveness over time.

Source Quality Plant Stock
Choose seeds and clones from well-respected breeders. The best breeders choose stock plants that are naturally resistant to powdery mildew and other pathogens, and do their best to maintain a disease-free environment.

Once a crop is infected with powdery mildew, it is always a battle to knock it down and keep it under control. The best defense is growing strong, healthy, disease-resistant crops.

If you provide your plants with a healthy environment and feed them a balanced blend of nutrients and organic biostimulants, you can help your plants reach their true genetic potential. It’s also a lot more fun.

(Read next: Pest and Pathogen Prevention in an Indoor Garden)

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Written by Harley Smith

Profile Picture of Harley Smith
Harley Smith is the director of research for NPK Industries. A veteran in the hydroponics industry, Harley has more than 18 years of consulting and educating experience. He is regarded as an expert on plant nutrition and organic bio-stimulants, performing research and new product development in the US and Europe.

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