Gardening Laidback Gardener Tip of the Day Pesticides

Insecticidal Soap Is Also a Fungicide

Insecticidal soap can also be used as a biological fungicide. Source:

Did you know that insecticidal soap, the popular organic insecticide/miticide so many gardeners routinely use in treating mites and insects on their house and garden plants, is also a fungicide? And this is not just a case of wishful thinking, so common in the world of home gardening: this use has actually been carefully studied. In the United States, for one, insecticide soap has been officially approved for use as a fungicide, specifically in controlling powdery mildew on vegetables and roses.

At first, powdery mildew looks harmless, ike a coating of icing sugar on plant leaves, but can quickly kill them. Source:

Apply it at the dilution recommended on the package (it may vary from one manufacturer to another) on cucumbers, squash, tomatoes, lettuce, etc. at the first signs of the disease (an apparent coating of white powder on the foliage) and repeat up to three times at 7- to 10-day intervals. Insecticidal soap stops the progress of the disease and also kills spores present on the leaves, keeping the disease from spreading.

Note too that insecticidal soap can be used up until the day of harvest … but personally, I’d recommend rinsing any treated vegetable thoroughly before eating it. Soap may not be toxic to humans, but it still doesn’t taste good!

3-in-1 Spray


There are also commercial preparations, often called “3-in-1 sprays” (because they treat insects, mites and plant diseases), that combine insecticidal soap with sulfur, a biological fungicide, to increase the fungicidal effectiveness of the soap and extend its use beyond only that of powdery mildew, as 3 in 1 spray is also registered for use in controlling black spot and rust on a wide range of plants.

On the other hand, 3 in 1 sprays are well-known, having been on the market for a long time. What is more surprising is to discover that, in many cases, good ol’ insecticidal soap alone could have done the job.

Insecticidal soap can also be a fungicide? Who knew!

Larry Hodgson is one of Canada’s best-known garden communicators. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He has written for many garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 60 other titles in English and French. He is a past president of the Garden Writers Association (now Garden Communicators International) and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. He resides in Quebec City, Quebec, Canada.

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