How to Prevent White Powder Mildew

Powdery mildew can afflict vegetable plants.

Powdery mildew can afflict vegetable plants.

The white or gray powder-like substance that afflicts plant foliage, stems and fruits is powdery mildew. Powdery mildew is a fungus that is an issue with a wide range of plants, including desert annuals, fruit trees and vegetables. The fungus spreads by spores and thrives in humid and low-light environments. Your first defense against powdery mildew is to select plant varieties that state they are resistant to this fungus. However, if you decide to grow plants that are not resistant, several cultural habits can help you prevent powdery mildew from becoming a serious problem in your landscape.


Select the sunniest spot that susceptible plants can tolerate. For example, if a plant can be grown in full or partial sun, choose an area that is exposed to at least six hours of direct sunlight a day. The sunlight helps lower humidity, which in turn reduces the likelihood of powdery mildew.


Space plants far enough apart so air can easily flow between plants, even at full maturity. Look at the spacing recommendations for the plants you are planting and use the widest distance listed, at a minimum. Good air flow helps lower the humidity in the middle and base of the plants.


Water plants susceptible to powdery mildew with a garden hose, spraying the plants from overhead. Keeping the foliage wet helps decrease the chances of powdery mildew spores from germinating. It also washes spores away. Because pests can become an issue with overhead watering, the University of California does not recommend this type of irrigation when growing vegetables where pest damage can ruin your crop. Also, if powdery mildew actually becomes a problem, stop overhead watering because it increases the spread.


Clean up plant debris around susceptible plants on a regular basis. This task helps remove spores that might be on the debris. Debris also stays wet longer than the soil, thereby raising humidity around the plants if left in place.


Adopt a regular pruning regimen that includes thinning. This cultural habit keeps the air flowing and it helps sunlight reach the center and bottom of susceptible plants to reduce powdery mildew. Also, prune and discard plant parts that are infested with powdery mildew to prevent spread.


Apply a ready-to-use fungicide labeled to prevent powdery mildew and listed as safe for your plants. Wait until winds are calm and rain is not forecasted for 24 hours. Spray the fungicide until all plant parts are thoroughly covered. Reapply as instructed on the label, often seven to 14 days during the growing season. If the product contains sulfur, do not use when temperatures exceed 90 degrees Fahrenheit.

Things You Will Need

  • Pruning shears
  • Fungicide
  • Horticultural oil
  • Baking soda
  • Spray bottle

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