Squelching Powdery Mildew with Milk

Powdery mildew on rose foliage.

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You can do everything right in your garden and there will still be problems—like 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 humid, rather than wet, conditions. The spores form when the humidity is high and disperse when the humidity diminishes. 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 help to reduce the incidence of powdery mildew. But when it arrives on plants, it is a fast-spreading problem and you need to take action quickly.

Using Milk as a Fungicide

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

To use milk as an anti-fungal spray:

  1. Mix 1 part milk to 2 to 3 parts water in a spray bottle.
  2. Spray the solution onto the leaves of plants, preferably unaffected plants, when the plant is in bright sun. Soak both sides of the leaves until the solution is dripping.
  3. Repeat the spray application every 10 to 14 days.

Milk works best as a preventative, rather than a cure, which makes it hard to determine whether it actually works since you don't know if your plants would have become infected with powdery mildew 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.

How Does Milk Work as a Fungicide?

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 very often a control group is missing, so 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.