Plant Pathology Infobytes
September 26, 1996
Water pH Affects Fungicide and Insecticide Performance
Many home gardeners have found that application of fungicides and insecticides may be needed to keep diseases and insect problems in check over the gardening season.
Provided gardeners use recommended fungicide and insecticide products, mix and apply according to label directions, and otherwise follow good pest control practices, these materials usually provide good results. Occasionally however, fungicides and insecticides don't always perform up to expectations, and as a result, disease and insect problems may get out of hand.
Although several factors affect how well fungicides and insecticides control disease and injurious insects, poor results could be due to the alkalinity of the water used to prepare the spray mixture. Alkaline water has a high pH, and such water sometimes causes some fungicides and insecticides to undergo a chemical breakdown (technically referred to as "alkaline hydrolysis"). The breakdown reaction begins as soon as the insecticide or fungicide is added to alkaline water, and generally means the spray mixture which results isn't going to be very effective in controlling disease or insect problems.
It's important to realize that not all fungicides and insecticides are affected by alkaline hydrolysis. However, several commonly used products are susceptible, and examples include Benlate and captan. These are fungicides which are used by many homeowners to help control diseases of fruits and ornamentals. Sevin, a common insecticide, is also susceptible to alkaline hydrolysis. The rate of breakdown of these materials can be quite rapid. For example, captan loses 50% of its effectiveness within 30 minutes when alkaline water (in the pH 8.5 to 9.0 range) is used to prepare a spray mixture.
What steps should be taken by home gardeners to avoid problems with alkaline hydrolysis when applying insecticides and fungicides? Following these mixing and application guidelines will help:
- Apply spray mixtures soon after mixing to avoid loss of fungicide or insecticide activity. Most should be applied within the same day they're mixed. With some materials, such as captan and Benlate, apply within the same hour if alkaline water is used to prepare the spray mixture.
- Prepare only the needed amount of spray material. This insures little carryover and reduces the temptation to save leftover for the next scheduled spray application.
- It would be a good idea check the pH of your water supply to determine if the potential for a alkaline hydrolysis problem exists. Test kits are often available through garden supply catalogs, or use litmus paper to get an idea of water pH (check with your local druggist as a possible source). Or, your local water association may be able to provide this information.
Infobytes newsletter was written by the late Dr. Frank Killebrew, Extension Specialist.