Q: I have several apple and pear trees that I have treated with the fungicide Captan for several years to try to prevent fungal disease problems. It doesn't seem to work. Last year, I specifically didn't treat some branches of the trees to see if I could tell the difference. There was no difference. What can I do to protect my fruit trees?
A: Captan is a fungicide that is used commercially on many fruit crops, and it is used in landscaping for flowering plants, such as azaleas and roses. It is also used on lawns to treat fungal diseases. It is probably the most common fungicide used in the landscape and found in many products. But it has a dark secret that I have not seen mentioned on the product labels: It becomes ineffective if the pH of the water is too high. Many companies have their product labels on the internet; I checked a bunch of them, but I didn't find any reference to the need to check the pH of the water used in mixing the product.
Most pesticides are sold as concentrates and need to be mixed with water before use. Around the country, tap water varies in acidity and dissolved minerals. When water has a high pH, it can break down the pesticide into nontoxic chemicals or the pesticide may become attached to the dissolved minerals in the water and become ineffective.
If the water has a pH value higher than 7, a process known as alkaline hydrolysis can degrade the pesticide. Insecticides are more likely to degrade than fungicides or herbicides. Pesticides can be tested to determine how long it takes for half of the product to degrade.
Captan probably has the worst half-life rating of any pesticide. With water at an acidic pH of 5, it has a half-life of 32 hours. So, if you try to store it for use the next day, it is about half as effective as when it was mixed. At a neutral pH of 7, Captan's half-life is 8 hours. The problem occurs when the water's pH is higher. At a pH of 8, the half-life is just 10 minutes, and at a pH of 10, it is just two minutes! I have tested the water coming out of my hose, and it is always over a pH of 9. So, I can't use Captan.
Since the label doesn't fill you in on this information, how do you find it? There are a few university websites you can look at, and you can check with your local Cooperative Extension Service office. Do a search for Michigan State University or Cornell University and "pH stability of pesticides," and you will find tables that list many pesticides with their half-life ratings.
For instance, Mancozeb, which is another fungicide used to treat many of the same problems as Captan, has a half-life of several days at a wide range of pH levels.
Besides using a different fungicide, you could use a buffer to treat the water to better control the pH. This is often difficult to do and not useful for a homeowner.
It is best to use all of a pesticide when it is first mixed, not letting any sit. Aside from degrading, any unused pesticide has the potential to spill or become contaminated.
The other way to look at the half-life issue is that we don't want the pesticides to last too long in water, as we don't want them contaminating soil and water in the landscape. The faster they degrade, the faster they can't pollute.
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