The Root of It All: Powdery mildew and compost ‘rules’

2012-09-02T05:00:00Z 2013-11-30T18:50:05Z The Root of It All: Powdery mildew and compost ‘rules’PATTI NAGAI Horticulture educator for Racine County UW-Extension Journal Times

We have powdery mildew on our squash, cucumbers and even on the collards. I was told you have a recipe for treating it, using oil, water and baking soda. Is there a formula for the solution? — Maggie, Racine.

Powdery mildew is the common name for a disease that primarily affects the foliage of many types of plants. It can show up as spots of talc-like material on the leaves, and eventually results in the browning and death of the leaf tissue. It is especially bad in the cucurbits, which includes squash, cucumber, pumpkin, melon and gourd. The fungus that affect the cucurbits is different from the fungus that causes powdery mildew in collard (or phlox, rose or turfgrass), but if you have the powdery symptoms on many types of plants, it means the environmental conditions are right for the diseases to proliferate.

You can’t control temperature and humidity, but next year consider planting farther apart to allow more air movement. Many of the cucurbits can be grown on trellises, which also helps increase air movement and decrease humidity. In addition, when diseased leaves and plants are removed from the garden, bury them or throw them away to reduce the number of spores carried over for next year.

Fungicides containing dinocap, dithiocarbamates, myclobutanil, triadimefon, triforine, sulfur or thiophanate methyl are registered for use against powdery mildew, but if you are using organic methods for your garden, look for an organic fungicide that contains sulfur. A mix-it-yourself recipe that has been tested and approved by many research institutions is:

1½ tablespoons of baking soda

3 tablespoons of a light weight horticultural (e.g., Sunpray) oil

1 gallon of water

Mix well and continue to shake the mixture as you apply it to the foliage. It is best to apply it in the evening to avoid leaf scorch in the bright sun and high temperatures.

If you choose a labeled product from the garden center, be sure to read and follow all label instructions to ensure that you use the fungicide in the safest and most effective manner possible.

For more information on powdery mildew, please read the Wisconsin Garden Fact Sheet at

Compost ‘rules’

I’ve heard that you cannot put citrus into compost, is that true? Are there things that should not go into the compost piles? — Scott, Waterford.

Compost can be made of a variety of organic materials. Large composting facilities use “recipes,” but that recipe can change depending on availability of starting materials. In general, try to achieve a balance of one part green to three parts brown in your compost pile. Green materials would be fresh yard trimmings, coffee grounds and vegetable scraps while brown materials would include dry yard trimmings, chipped leaves or peanut shells.

Keep the pile moist and aerated to promote decomposition. Having particles of different sizes (chopped, chipped, cut, but not “powdered”) helps with aeration, and turning the pile regularly keeps it aerated also.

Technically, everything organic is compostable. For home composting we generally recommend that no animal-related materials such as cheese or bones are put in the pile to keep animals disinterested. Diseased plant material should not be composted unless it has been “killed” first by baking under the hot sun in a black plastic bag for a few days. Orange or other citrus peels can be put in compost, but it may take longer for them to decompose. This is remedied by chopping them into smaller pieces before putting them in the compost.

For more information on composting, visit the Solid and Hazardous Waste Education Center website at:

More questions?

Master gardener volunteers serving as plant health advisors are able to answer your questions at or by calling the horticulture helpline at (262) 886-8451 (Ives Grove) or (262) 767-2919 (Burlington).

Dr. Patti Nagai is the horticulture educator for Racine County UW-Extension. Submit your questions for the Journal Times Q&A column to Dr. Nagai at and put “Question for RJT” in the subject line.

Copyright 2014 Journal Times. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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