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 with the same problem. Drought conditions can cause an awful lot of stress on plants and when rain and heavy, humid air appear later, conditions became 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 took place 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 results of the study 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.” Which 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.”Â
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)
I have heard of the milk cure, never heard of mixing with compost tea, but I like the sounds of that; very organic! Another method of combating PM that I learned about almost two years ago is using a solution of colloidal silver, which can be made for pennies a gallon. I haven’t made it myself because I just buy it pre-mixed from my organic Guru in Morro Bay. From what I understand, once you find a place to buy the concentrated silver colloid, you simply add water in the correct proportion (again, I don’t know what the mix is) and put that in a spray bottle (ideally once that doesn’t let in light because light degrades the strength of the solution) and apply directly to the affected area. Colloidal silver can kill many types of fungus and pathogens, and is completely safe and organic (you can even drink it to wipe out disease-causing bacteria, I’ve heard it can even cure bladder infections!).
Kyle: I absolutely would not recommend using colloidal silver. According to the ENVIRONMENTAL CONTAMINANTS ENCYCLOPEDIA issued by the National Park Service, “In fish and amphibian toxicity tests with 22 metals and metalloids, silver was the most toxic tested element as judged by acute LC50 values. In solution, ionic silver is extremely toxic to aquatic plants and animals.”
Todd: Colloidal silver and ionic silver are two different things. Colloidal silver does not have the toxic properties that you cited in the NPS’s description of ionic silver. This page (http://www.frequencyrising.com/colloidal_silver.htm) talks about the difference between these two types of silver, and this article (http://www.cashforsilverusa.com/medical-uses-make-silver-truly-shine) gives supplemental information on why colloidal silver has medically accepted properties. Regardless, we’re not talking about spraying this in an aquatic environment. I posted this solution because from personal experience it will eliminate powdery mildew in an organic gardening environment without causing harm to the plant, soil or humans and animals coming in contact with the plant.
Kyle: I fact checked myself and yes indeed, colloidal and ionic silver are two different solutions. However, I can find no reliable, authoritative research on using colloidal silver in one’s garden, so I can’t recommend it. I found a few folks who reported good personal experience with it in their gardens, but without knowing the full ramifications of using an electrically charged silver solution on a living organism, I’d find an alternative, proven method. And by the way, a “cash for silver” link isn’t exactly an authoritative source.
Hey Todd: Fair enough! You have a point that not a lot of scientific study has been published about it’s use in gardening. It’s been used for a really long time for medical purposes, but there certainly is some debate over the extent of the benefits of ingestion, but most of the concern is based on the quantities that can be consumed by drinking it daily.
The EPA has set safety standards for the amount of silver that may be ingested daily (https://articles.mercola.com/sites/articles/archive/2009/02/07/new-guidelines-released-for-safe-usage-of-colloidal-silver-supplements.aspx) without causing discoloration to the skin. The linked article also clarifies the differences between different silver solutions and provides links to further research about silver’s effectiveness.
Also, in terms of finding research on it’s use in gardening it is difficult to find any writing that is not from a commercial site, but the author of this article (https://www.lifedevice.com/General%20Info.htm) seems to know what they are talking about, and even gives information on how to prepare the solution (there is a paragraph at the end about gardening applications, but i wish it were longer and more detailed)
I hope that all makes for interesting reading! If you are interested, I may be able to do more research and come up with something more definitive on the subject. Let me know. Best Regards ~Kyle
A number of alternative materials for powdery mildew control are available to grape growers overseas or have shown promise in Australian trials (e.g. Crisp et al. 2003). These include fungal and bacterial preparations, dairy by-products, sodium salts, silicate compounds, compost teas and vegetable and mineral oils. However, because these materials are not registered for use on grapes in Australia, they cannot be detailed in this Agriculture Note.