The most common fungicide used in fruit tree sprays is Captan. Unfortunately, this product is subject to alkaline hydrolysis. This is a process whereby certain pesticides will break down when mixed with high pH water (the pH of the water used to spray various pesticides affects the stability of the solution and how long the pesticide will remain effective while in solution). So let’s say you mix up your spray mixture by adding Captan to 5 gallons of water. If that water has a pH of 7.0, the Captan will break down so that only half of it will still be present in 8 hours. However, if the water you use has a pH of 10.0, half the Captan will break down in 2 minutes. In short, the higher the pH, the faster it breaks down.
Malathion used to be the most common insecticide used for fruit pest control by gardeners but is becoming more difficult to find. It isn’t nearly as sensitive to alkaline hydrolysis as Captan but still will break down under high pH conditions. Fortunately, it is stable at a pH between 5.0 and 7.0. Lambda-cyhalothrin, which is found in Bonide Fruit Tree & Plant Guard, is somewhat of a new product for fruit pest control. It too is stable between a pH of 5.0 to 7.0.
Note that alkaline hydrolysis does not affect all pesticides. Captan is the exception, not the rule. For a listing of common pesticides and their susceptibility to alkaline hydrolysis, see
http://ecommons.library.cornell.edu/bitstream/1813/5149/1/FLS-118.pdf
So, how do you bring down the pH of your spray water if it is high? Commercial farmers use buffering agents, but that may be difficult for homeowners to find. Food grade citric acid can help. If you have a pH of 8.0, add 2 ounces of this citric acid per 100 gallons of water (1 and 1/4 teaspoons per 10 gallons) to bring the pH down to about 5.5.
On a similar note, a practice that will extend the effectiveness of fungicide as well as insecticide sprays when there is the threat of rain, is to add a spreader-sticker to your spray tank. Without a spreader-sticker, a rain would easily render any spay application useless, washing the materials right off. For this reason, spreader-sticker is used in commercial fruit tree sprays to improve the distribution and retention of fungicides and insecticides on fruit and leaves. However, using a spreader-sticker is not a cure all as a rain can reduce the length of time the chemicals are effective. Less than one inch of rain since the last spray will not significantly affect residues. But, one to two inches of rain will reduce the residue by one half. You will need to reduce the number of days until the next spray by one half. More than two inches of rain since the last spray will remove most of the spray residue. Re-spray as soon as possible.

Rip Winkel is the horticulture agent in the Cottonwood District for K-State Research and Extension. Contact him by email at rwinkel@ksu.edu or call 620-793-1910.