You don't have to garden long to become acquainted with the disease called powdery mildew, which creates whitish patches on the leaves of pumpkin, winter squash, and other members of the cucumber family. A fast-growing fungus, powdery mildew is also among the worst enemies of rosemary, and is a well-known disease of monarda, grape, and zinnia. Each of these unrelated plants hosts a different strain of powdery mildew, but I have found that they all can be controlled with regular sprays of milk and water.
What is Powdery Mildew?
To understand how milk sprays prevent powdery mildew and thus use them most effectively, you must first understand your enemy. Powdery mildew fungi are present in many environments, so that even in the cleanest gardens, outbreaks can begin from spores spread by windblown rain, or on the feet of insects and birds. But when the right strain of powdery mildew finds a suitable host plant, it quickly sinks root-like structures into the cells on the leaf's surface. There is stays, taking nutrition from the leaf while developing a matrix of thread-like structures over the surface. This is when we gardeners notice unusual patches of white or light gray with a powdery or furry texture, usually on the top sides of leaves.
For the infected plants, powdery mildew cripples its ability to conduct photosynthesis by blocking out light, and stops up the leaf's gas exchange system, too. Powdery mildew can quickly spread to nearby leaves, so it's always a good idea to clip out leaves that show early spotting. Also make use of resistant varieties of cucumbers, squash and melon, which can be of tremendous help in preventing powdery mildew. Resistant varieties have special characteristics that make it extremely difficult for powdery mildew fungi to enter leaves, which they do with cell-melting enzymes.
Using Milk for Plant Mildews
More than 50 years ago, researchers in Canada discovered that milk sprays could help prevent powdery mildew on tomato and barley. Then the age of fungicides began, with no further published research on the milk cure until 1999. Since then, numerous small studies from around the world have validated the use of milk sprays on powdery mildew on a wide range of plants. Most recently, a spray made of 40% milk and 60% water was as effective as chemical fungicides in managing powdery mildew of pumpkins and cucumbers grown in mildew-prone Connecticut. In Australia, milk sprays have proven to be as effective as sulphur and synthetic chemicals in preventing powdery mildew on grapes. In New Zealand, milk did a top-rate job of suppressing powdery mildew in apples.
Scientists are not exactly sure how milk sprays work, but most think proteins in the milk interact with sun to create a brief antiseptic effect. Any fungi present are "burned" into oblivion, but there is no residual effect after that. In order to be effective, milk sprays must be used preventively, must be applied in bright light, and should be repeated every 10 days or so.
On the downside, some writers have suggested that milk sprays give off a bad odour after they have been applied, but this has not been my experience. I use a hand-held pump-spray bottle to wet both sides of the leaf until it's dripping, and usually spray in mid to late afternoon on a sunny day. In the days that follow, I never smell a thing.
There is no consensus on which dilution of milk to water is best, with the most concentrated recommended mixture 40% milk and 60% water, and the most dilute 10% milk and 90% water. I fall in between using 30% milk to 70% water, with good results. It does not matter if the milk you use is skim or whole because it is the protein rather than the milkfat that is working on your behalf.
With experience, you will learn which types of powdery mildew are likely to develop in your garden, and this knowledge will take you far in managing this disease. Like other fungicides, milk sprays work best when used preventatively, before the disease can gain a foothold. If you often see powdery mildew on your squash, grapes or zinnias, start milk sprays before the plants show signs of infection. You have nothing to lose beyond a cup of milk.
By Barbara Pleasant
Electron microscope photograph of powdery mildew from the Max Planck Institute for Developmental Biology.