How to Use Milk Spray to Control Powdery Mildew

Powdery mildew on rose foliage

Mark Turner / Getty Images

You can do everything right in your garden and there will still be problems—including powdery mildew. Some things are simply beyond your control. When weather conditions are right, powdery mildew and other fungal diseases move in. Keeping your plants healthy and giving them plenty of air circulation will help minimize damage, but sometimes you need the aid of a fungicide. However, that doesn't mean you have to use chemicals. For powdery mildew, try milk.

What Is Powdery Mildew?

Powdery mildew refers to a group of fungal diseases that all show up as a powdery white coating on leaves, stems, and sometimes even flowers. It does not usually kill plants, but it can weaken them and diminish photosynthesis, leading to poor yield and plants that don't last through the season.

Powdery mildew is spread by spores that are carried by the wind or splashed onto leaves. The fungus favors conditions that are humid but not wet; the spores form when the humidity is high and disperse when the humidity diminishes.

Warning

Spores can overwinter on plants or plant debris and start the process all over again. End-of-season garden cleanup and planting disease-resistant varieties are good regular practices that help to reduce the incidence of powdery mildew. But when it arrives on plants, powdery mildew is a fast-spreading problem, and you need to take action quickly if you want to control it.

How Milk Works as a Fungicide

Milk has become a popular secret weapon in fighting powdery mildew. Actually, it's not so secret, as milk has been used in treating diseases for decades. In addition to its use as an additive to assist spreading other pesticides and helping them adhere, milk has been tested in the fight against the transmission of tobacco mosaic and other viruses, albeit with mixed results. More recently, though, milk has been getting a lot of good press as an anti-fungal spray, specifically against powdery mildew on cucumbers and squash.

In recent years, researchers have been experimenting with spraying a diluted solution of regular milk on a variety of plants, mostly cucurbits (such as squash and cucumbers). The results have been favorable enough to warrant further study. Home gardeners are now getting in on the research, but because such studies often are missing a control group, their results are not definitive.

As to exactly how milk works against the fungus, no one is certain. It appears that the proteins in milk offer an antiseptic-like effect when exposed to sunlight. That's why you should apply the solution in bright sun. (Of course, milk left out in the heat will spoil and smell a bit, but this dissipates pretty quickly.) The protein is in the milk fat, and both whole and skim milk have been used in tests. Researchers have also used whey, a milk by-product because it's cheaper than milk. You can try your own experiments with whatever you have on hand. For accurate testing, spray the solution on only some plants while leaving others untreated.

Author and horticulture professor Dr. Jeff Gillman has also recommended using the milk solution for black spot on roses. There hasn't been a lot of institutional research on this, but home remedies are not profitable and often get short shrift in research. Interestingly, Dr. Gillman also recommends simply spraying plants prone to mildew with water. Since powdery mildew doesn't like getting wet, spraying the plants daily seems to help thwart it.

When to Use Milk to Control Powdery Mildew

Milk works best as a preventative measure rather than as a cure, which means that it should be applied before powdery mildew has appeared on your plants. Through experience, you may already know when the telltale white powdery substance is likely to appear—warm, humid weather is the prime season for powdery mildew on many species. Spray milk solution on these susceptible plants when the prime mildew conditions are setting in, or when the first leaves begin to show signs of mildew.

This can make it hard to determine if the milk solution actually works since you don't know if your plants would have become infected without the milk treatment. On the other hand, if you dutifully spray your leaves with the milk solution and the plants become infected, you know the treatment was less than totally effective on this particular plant.

Project Metrics

  • Working Time: 15 minutes or less for each plant
  • Total Time: Requires repeated treatment every 10 to 14 days for 1 to 2 months
  • Material Cost: None, unless you need to buy a pump spray bottle (less than $5)

What You'll Need

Equipment/Tools

  • Spray bottle or garden sprayer

Materials

  • Milk
  • Water

Instructions

  1. Fill a Spray Bottle

    Most experts advise using a mixture with a 40/60 ratio of milk to water. You may want to experiment with different solution ratios to judge their effectiveness. Either way, simply milk or a milk solution into an ordinary pump spray bottle, mixing the ingredients thoroughly. For large applications, you can put the milk into a garden sprayer.

  2. Spray the Plants

    Spray the milk solution onto the plants, lightly coating all surfaces of the leaves. This treatment is often most successful as a preventive measure, by spraying it on unaffected plants to keep them from developing powdery mildew. Some experts advise spraying the plants in bright sunlight since it is believed that interaction with sunlight is what gives the solution its anti-fungal properties.

  3. Repeat

    Reapply the milk treatment every 10 to 14 days over the normal time period when powdery mildew is likely.

  4. With regular treatment, your plants should remain free of the unattractive white powder of the various fungi that cause powdery mildew.

Tips for Controlling Powdery Mildew With Milk

Powdery mildew is caused by a large arrange of different fungi, and the milk solution is more effective on some than on others. But some of the plants that have successfully responded to the milk/water solution include:

  • Apples
  • Grapes
  • Cucumbers
  • Squash
  • Tomatoes
  • Barley
  • Pumpkins
  • Zinnias

Some users believe that the milk solution can develop an unpleasant odor as it sours on the leaves of plants, but this effect generally vanishes very quickly. The odor of the milk solution is really no more unpleasant than that of commercial fungicides.