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.
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!
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