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Powdery Mildew

By August 3, 2014 December 8th, 2015 Pests & Troubleshooting, Uncategorized

Powdery mildew looks like a grey powder-like substance on all of your leaves.

Powdery mildew is the common name for a number of species of fungus that negatively affect your plant leaves. It’s spread by wind and often overwinters on plants. Watch for it in the warm days and cool nights of late summer. Your crops usually won’t die from powdery mildew, however it may affect the flavor, yield, and aesthetics of your crop.

Mild Powdery Mildew that will appear on most squash leaves during the mid-summer. Not affecting plant growth, but keep an eye on it at this stage.

Mild Powdery Mildew that will appear on most squash leaves during the mid-summer. Not affecting plant growth, but keep an eye on it at this stage.

Severe Powdery Mildew affecting plant growth

Severe Powdery Mildew affecting plant growth

How Plants Get Powdery Mildew

Powdery mildew is most frequent when you water your squash plants from the top, resulting in wet leaves. Powdery mildew is also bad when relative humidity is high around the plants, even if the leaves aren’t getting wet. Most frequently, relative humidity gets high when plants are spaced too closely together and aren’t thinned properly.


  1. Use drip irrigation instead of hand watering. If hand watering, avoid the leaves!
  2. Plant where there is good air circulation, and thin to the recommended number of crops per square.
  3. Sunny locations help inhibit spore germination.
  4. There are many mildew-resistant or mildew-tolerant strains of plants.


  1. Selectively prune overcrowded plant material to help increase air circulation. Prune off the leaves that are getting hit the worst. This helps reduce relative humidity and infection (don’t compost them!)
  2. To take direct action against powdery mildew, spray affected crops with a mixture of 1/4 cup of milk and 1 quart of water.
  3. Anti-Fungal Baking Soda Spray
    • For plants that already have powdery mildew – hose down all the infected leaves prior to treatment. This helps to dislodge as many of the spores as possibly to help you get better results. Use as prevention or as treatment at first signs of any of the diseases.
    • To make: Mix 1 tablespoon baking soda, 2 1/2 tablespoons vegetable oil with one gallon of water. Shake this up very thoroughly. To this mix add 1/2 teaspoon of dishwashing soap and spray.
    • This won’t get rid of the fungus on leaves that already have it, but it will prevent it from spreading to the rest of the plant.
    • Repeat every 5-7 days as needed

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One Comment

  • Ruth says:

    Thanks for the recipes; I’d like to try them. I have had quite good success using just a half teaspoon of baking soda in a quart of water; it is especially effective preventing the spread of the fungus; I prune leaves that are badly affected. After hearing the suggestion to treat athlete’s foot (a fungal infection) by peeing on one’s feet in the shower, I got the idea of putting human pee to work in the garden to see if that would fight the fungus, and found it to be very effective, especially if catching the fungus in early stages. That was my sole strategy last year, and had an almost completely fungus-free garden all season, the first ever. This year I tried a commercial Neem product and found it to be very effective in the greenhouse.

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