Can Milk Protect Your Garden Against Powdery Mildew?

The University Of Connecticut conducted an interesting study on how milk spray can be used to prevent powdery mildew disease on squash and pumpkins. The results were very surprising.

After my post on how powdery mildew infected my pumpkins, I heard from many gardeners about the same problem. Drought conditions can cause an awful lot of stress on plants, and when rain and heavy, humid air appear later, conditions become ripe for fungi to grow. Powdery Mildew and other infections suddenly appear. One reader commented that he uses a milk spray to suppress powdery mildew, a technique which I was unaware of (one of the great things about writing this blog is that I learn so much from all of you!).

So I did a little digging and found a thesis paper published by the University Of Connecticut Graduate School. The paper evaluated treatment sprays for suppressing powdery mildew on acorn squash, delicate squash, and pumpkins in fields and greenhouses (plants in the squash family, cucurbits, are notorious for powdery mildew infections). Squash and pumpkins in test plots were treated with various sprays, including chemical fungicides, and control plots were established that received no sprays. All test sprays were applied on a regular schedule before symptoms appeared, and visual inspections were performed on a weekly basis. Some plants were treated with combinations of the compost teas, milk, humic acid, and seaweed.

The sprays used to treat the plants were:

• Aerated Compost Tea
• Non-Aerated Compost Tea
• Compost teas with humic acid and liquid seaweed
• Diluted Whole Milk (40% milk, 60% water)
• Chlorothalonil (used widely in agriculture to prevent powdery mildew)
• Chlorothalonil accompanied by Mycolubutanil

UCONN’s explanation of compost teas: “Compost tea is a liquid extract of compost made by steeping the compost in water for various time periods… Compost teas are thought to suppress disease by promoting the proliferation of beneficial microbes, which then act as a biological control over pathogens… There are two main types of compost teas: Actively aerated compost tea (ACT) is made by the constant induction of air into a water-compost mixture. Non-aerated compost tea (NCT)  is made by simply placing the compost into water and allowing it to steep for a predetermined length of time. The addition of air to the compost-water mixtures creates aerobic conditions for the microbial growth, while the stagnate NCT’s provide conditions primarily for the growth of anaerobic microorganisms.”

Compost tea and milk were nearly as effective as chemical fungicides

The study’s results were surprising. In the field tests, Non-Aerated compost tea used in conjunction with diluted powdered milk was nearly as effective in suppressing powdery mildew as the chemical Chlorothalonil. The compost teas used alone, however, had virtually no effect on the disease. In the greenhouse plots, the milk treatment was just as effective as the chemical control.

Official conclusion of the paper: “Both greenhouse and field results indicate that milk provides control of powdery mildew similar to the control provided by a fungicide. Organic and conventional growers could benefit from using milk in place of the fungicides typically sprayed to control powdery mildew.” This begs the question, why aren’t agricultural growers using milk instead of fungicides? It’s cheaper and environmentally safer.

3 reasons milk spray prevents powdery mildew

The paper suggests three reasons why milk might suppress powdery mildew: “A simple physical barrier may be created reducing the attachment efficiency of the fungus to the leaf surface. The pH of milk, which is slightly basic, could be altering the pH of the leaf surface and slowing the development of this surface living fungus. Milk also contains lactoferrin, which is a natural antimicrobial agent.” 

It was a big surprise that compost teas applied alone had virtually no effect on preventing powdery mildew. That’s not to say that compost teas are useless – they definitely are not – just not effective in this case. The conclusion is obvious: As a preventative measure – before any powdery mildew symptoms appear on your squash family plants – spray them with a diluted milk solution.

Read the entire paper, Compost Tea and Milk to Suppress Powdery Mildew (Podosphaera xanthii) on Pumpkins.(pdf)