Get rid of white fuzzy spots on zucchini plant leaves with homemade fungicide

Large leaf vegetable plants, like zucchini (shown above) and yellow squash, may develop powdery mildew, a fungus infection that looks like white fuzzy spots. Left untreated, the spots will spread, killing the leaf and eventually killing the plant. It was the third week of June, while harvesting some zucchini from my own garden, that I first saw the fuzzy spots. There were two to three dozen small white spots on eight leaves of one zucchini plant. The spots were all less than one-quart inch across. The largest leaves, which are also the oldest leaves, are often the first to display powdery mildew.

Causes and cures of powdery mildew

Powdery mildew is the result of several factors such as warm temperatures and high humidity or lack of full sun. The University of California's Agriculture and Natural Resources Department says, "Powdery mildews generally do not require moist conditions to establish and grow, and normally do well under warm conditions." When caught early, there is a good possibility of saving the zucchini through diligent use of a fungicide. I do not use harsh chemicals in my organic garden so I went in search of homemade fungicides.

Homemade fungicide recipes

I tried two different homemade recipes as a fungicidal treatment for powdery mildew. One recipe from Doug Green at Simple Gifts Farm used one part skim milk to nine parts water. The second recipe from John Dromgoole, the Natural Gardener, uses four teaspoons baking soda and one teaspoon liquid dish soap (he suggests Dawn or Ivory) mixed into one gallon of water. The frequency of application varied from daily to weekly; however, daily for an existing infection makes more sense. A weekly spritzing seems more reasonable as a preventive measure to keep powdery mildew from (re)appearing.

Setting up the homemade fungicide experiment

I prepared both mixtures and poured some into separate spray bottles (the cheap plastic spray bottles from the local dollar-type store). I chose a fence post as a marker to visually divide the infected leaves for an experiment to see which homemade product worked better. The plan was to spray the mixtures on the appropriate set of leaves each morning.
  • The milk and water solution would only be applied to affected leaves in Set A.
  • The baking soda, dish soap, and water solution would be applied to affected leaves  to Set B.

The daily results

Day 1 and Day 2, there was no measurable change in the fungus condition of the test leaves.

By Day 3, I noticed a difference in the powdery mildew on both sets of leaves--one good, one bad. On Set A with the milk mixture, there were more mildew spots and older spots were now larger. On Set B with the baking soda/detergent mixture, the spots were still present, but appeared slightly gray like they were less dense.

Day 4, Set A looked even worse. Large areas of the leaves were covered with mildew. Set B's spots were definitely less obvious. Not willing to sacrifice the plant by continuing the milk fungicide experiment, I cut off the leaves with the most fungus and switched to using the baking soda/detergent mixture on the remaining leaves in Set A.

Day 5, Set B's spots are barely visible.

Day 6, the mildew is almost gone.

Day 7, the application of baking soda and dish detergent in water completely eradicated the powdery mildew.

Which worked better?

For my experiment, a combination of baking soda and mild dish detergent mixed in water proved to be more successful at killing powdery mildew than the milk mixture. If you only have a few plants, mix the fungicide in a half-gallon milk container, cutting the recipe in half. For larger vegetable gardens, use a pressure pump sprayer.

Prevention

The best way to prevent powdery mildew on vegetable plants is to purchase disease resistant varieties, plant in full sun, and set plants the designated distance apart for adequate air circulation. I plan a weekly repeat of the homemade fungicide this season to prevent the return of powdery mildew