In the humid south we can usually expect plant diseases to start showing up in June and July. Some of them are difficult to manage at all, but powdery mildew (PM) has a surprisingly effective organic solution… milk!

Photo Caption: A simple sprayer and one part milk to nine parts water is around 90% effective at preventing powdery mildew.

I know, milk… it seems like one of these too-good-to-be-true crank organic remedies, right?

The September 1999 issue of Crop Protection reported about scientist Wagner Bettiol’s study on using diluted milk as a control for powdery mildew on cucurbits. Backed by Embrapa, the Brazilian Agricultural Research Corporation, the study reported that a solution of 10% – 20% raw milk in water sprayed weekly was equally effective or better than the most trusted chemical fungicides on the market (around 90% effective, depending on the plant variety’s natural disease resistance). Continued studies at the University of Adelaide in Australia seek to understand why milk works so well against powdery mildew (and also botrytis blight, which you may have seen on your squash flowers or strawberries — it’s that fuzzy mold that appears in wet conditions).

The University of Adelaide has also found that the milk solution works on crops other than cucurbits, such as grapes and roses. The full potential of this remedy is still being explored. Scientist Peter Crisp has talked about his findings with ScienceNews Online and the Australian Journal of Grape and Wine Research. The ScienceNews article also quotes professor and winemaker David Bruer saying that raw milk is not required for success. Skim milk and rehydrated powdered milk both offer the same results.

Not only is this toxin-free solution effective, it is very easy! After raiding the fridge (how do you explain this one to your family when there is no milk for cereal?), simply mix 1 part milk to 9 parts water and thoroughly coat susceptible plants once a week. (Bruer says he only sprays once every 2 weeks).

Milk sprays work best in full sun so try to do applications in the morning hours and on non-cloudy days. More isn’t better! A different mold has been found to grow on plants if the mixture contains 30% or more milk.

Photo Caption: Spa treatment! It is important to start spraying your plants when they still look healthy to get the best protection. This is how my cukes looked yesterday just prior to their milk bath.

There may be one small catch: at the Organic Growers School early this spring I learned that milk sprays in our region typically work best as a preventative rather than as a cure. They suggested keeping an indicator crop to let you know when powdery mildew has shown up in the area. PM susceptible crops that give an early heads-up are European grapes (not muscadines), zinnias, and peas. If those plants start to get sick, start spraying!

You can also keep an eye on your neighbor’s gardens. Yards with less air circulation, more shade, poorer soil nutrients, or uneven watering may come down with diseases before you see them at your house. If you see any signs of powdery white spots on the upper or lower sides of leaves, it’s time to spray.

Photo Caption: I became alerted to the presence of disease in my neighborhood when a fellow gardener down the street sent me this photo of cucumbers. Once a plant is this distressed it may not respond to milk treatment. (Note: this plant also has cucurbit downy mildew).

Once your plants get sick, it may be too late, but you can try removing the sickest leaves and then spray. Be careful, if you remove more than 1/3rd of the foliage then your green friend will also struggle to survive from a lack of energy. You may want to give your plant a dose of liquid fertilizer in addition to the milk spray to encourage replacement growth.

Photo Caption: This winter squash leaf is showing early spots from powdery mildew. If left alone the disease would soon coat the leaf, giving it a silvery appearance. Heavily infected leaves rapidly begin to curl, turn brown, and fall off the plant. Throw diseased foliage in the garbage instead of the compost pile.

If you don’t have an indicator crop on hand you can watch for the earliest signs of small, powdery white spots on your plants leaves. Or, if you’ve kept good garden records, you can start spraying a week or two prior to when your plants became ill in previous seasons.

Photo Caption: A thorough coating of milk once weekly can make normally susceptible crops all but immune to powdery mildew. Leaves should look shiny on both sides with one or two drips falling from them after you spray.

When you spray you will find that the garden suddenly has the smell of a dairy’s milk room. That smell is lactic acid being dispersed into the air. Unless you have a pronounced milk allergy there should be no need to wear the protective gear recommended for harsher fungicides. You can even feel comfortable using your bare hands to lift plant parts in order to spray the underside of the leaves. (If you’re using a new, clean sprayer and would like to give the neighbors something to talk about, try drinking some “fungicide” while they’re watching). 😉

Milk may also work as a foliar fertilizer and can feed the soil when it drips off of plants. The Columbia Daily Tribune has reported that dairy farmers are using leftover skim milk from cheese processing to fertilize their fields with tremendous results! Some of us would probably choose dentist visits and toilet cleaning over spraying the garden, but at least this chore offers a spectrum of benefits.

Also, though saving a buck on garden care is always tempting, please consider buying high quality, organic milk for your spraying needs. Inferior milk may work as well but your organic purchase votes for the integrity of farmers with well-treated, healthy cows and land.

Photo Caption: We decided to spray our zinnias for powdery mildew along with our vegetables.

Definitive studies only exist on a handful of crops but you can safely try milk on most plants that are in danger of powdery mildew or botrytis blight. Milk might damage plants with delicate tissue (perhaps African violets, for instance) but shouldn’t harm sturdy foliage.

Try milk to prevent PM on cucumbers, summer/winter squash, pumpkins, melons, okra, peas, beans, zinnias, crape myrtle, roses, dogwood, phlox, or any other plant that contracts this disease. Get milk!