pH and Pesticides
By Robert B. Martin, Jr., Master Rosarian

The effectiveness of the pesticides used on roses is, in many cases, greatly affected by the pH of the water in which it is mixed and applied. The term "pesticide" here is used in its broadest sense to include insecticides, fungicides and miticides that are used to control the pests and disease which afflict roses.

Water pH

The degree of acidity or alkalinity of water is marked by the symbol "pH" which technically refers to the number of hydrogen (H+) and hydroxyl (OH¯) ions in a solution. The scale for measuring pH runs from O to 14. The lower the pH, the more acidic is the solution, while higher pH indicates that the solution is more alkaline. Water at pH 7 is neutral meaning that there are an equal number of hydrogen and hydroxyl ions in the solution.

Southern California Water

The pH of the water in Southern California is generally quite alkaline. The degree of alkalinity depends on its source. For example, river water, such as from the Colorado River is generally much more alkaline than water pumped from wells. Some water districts draw water from several sources and draw different proportions at different times of the year. Thus, the alkalinity of the water in such areas actually varies throughout the year.

For example, the Pasadena Water and Power Department uses water from wells, surface deposits such as in reservoirs, and also water purchased from the Metropolitan Water District which draws generally on the Colorado River. The pH of the well water varies from 7.1 to 8.2 with an average of 7.7. The pH of the surface water from reservoirs varies from 7.8 to 8.2 with an average of 8.0. The water from the Metropolitan Water District varies in pH from 8.0 to 8.4 with an average of 8.1.

Pasadena also follows the practice of using its own water resources during the summer when the price of MWD water is the highest. Then, during the winter it buys cheaper water from the MWD and pumps it into its wells for storage. As a result, the pH of the water in Pasadena is higher in the winter than it is in the summer.

Water with a high pH also contains a higher number of suspended solids and dissolved minerals. It also typically has high salt concentrations. These substances also affect the performance of pesticides.

The water districts in Southern California follow the practice of mailing a report each year to its customers on the water quality. You can learn a great deal about the pH of your water and the dissolved solids by examining these reports. This information in usually set forth at the bottom of the report under the caption "Secondary Standards," "Additional Parameters."  There is contained information on pH and the presence of quantities of sodium, calcium, potassium and magnesium.

Effect of pH on Pesticides

The effect of pH on pesticides varies from product to product and is also affected by buffering solutions contained in the pesticide. Several of the pesticides commonly used on roses are affected by the pH of the spray water.

The effectiveness of a pesticide is usually measured in terms of half-life. As an example, if a product is 100% effective when first added to a spray solution and has a half-life of 30 minutes, the effectiveness is cut in half to 50% in 30 minutes. During the next 30 minutes it is cut in half again until it becomes useless.

Orthene, a commonly used insecticide, has a half-life of 3 days at a pH of 9.0, 17 days at a pH of 7.0 and 55 days at a pH of 5.0. Thus, Orthene used in an acidified solution lasts much longer. Similarly, Cygon has a half-life of 12 hours at pH 6.0 but 40 minutes at pH 9.0. Malathion is stable at pH 5.0 but rapidly disintegrates at a high pH.

Some fungicides are also affected by pH. The ideal pH for Funginex is 5.0. Benomyl has a half-life of 30 hours at a pH of 5.0 and a half-life of 9 minutes at pH of 9.0. Other fungicides such as Systhane (Rally/Eagle) are not affected by pH.

The pH of water also affects certain miticides. Avid should have a pH of between 4 and 7.5. And Mavrik, a powerful insecticide/miticide, has a half-life of 30 days at a pH of 6.0 and of only 1-2 days at a pH of 9.0.

Although the effect of pH varies, it can be seen from the above that, as a general rule, pesticides are most effective when the spray solution is at a pH of about 5.0. And since Southern California water is considerably more alkaline than this, it is usually desirable to take measures to reduce the pH of your spray solution to 5.0.

A WARNING!!

There are important exceptions to the principle that the pH of spray solutions should be modified to increase its acidity. These primarily arise in the case of metallic fungicides. Aliette, an aluminum based fungicide, effective on downy mildew has a natural pH of 5.0. So the addition of any acidifying agent to the water will lower the pH to dangerous acidic levels with the result being severe leaf burn and defoliation. Also, copper based fungicides are typically very acidic and no steps should be taken to increase the acidity of the solution.

Reducing Acidity

An obvious means of increasing acidity is to add an acid to the spray tank. Some rosarians use vinegar which contains acetic acrid for this purpose. But acids produce inconsistent results and are difficult to handle. To assure proper pH with acids requires use of a properly calibrated pH meter or litmus paper to check the acidity level.

The better approach is to adjust the pH by using a commercially available buffering agent. Buffering agents will stabilize a spray solution at a predetermined pH and keep it at this level. There are reportedly more than 30 products commercially available for such purposes, most of which are unfamiliar to me.

My product of choice is the spray adjuvant Indicate 5 which I have used successfully in my garden since the beginning of the 1994 season. This four plus years of experience have been most satisfactory and the difference in my foliage with the use of Indicate 5 is noticeable.

Indicate 5 contains a red indicator dye that turns the water pink at pH 5.0. I have found that the average application is about 1 teaspoon per gallon. Indicate 5 also contains an excellent spreader sticker which spreads the solution on the foliage and keeps it there.

Another product used by a number of local rosarians is Leaf Act 80B Buffer Spreader. This product also reduces the pH of the solution to around 5.0 and contains a spreader sticker. I have heard good reports on its effectiveness. Note that there are other Leaf Act 80 products that are distinguished by the letter after the "80."  These do not contain the buffering agent in Leaf Act 80B.

Indicate 5 is available from www.rosemania.com. Leaf Act 80B is made by PureGro Company and is available from agricultural chemical suppliers.

Conclusion

Pesticides are expensive, their application is time consuming, and their use is not without risk to ourselves and the environment. By applying them at their optimum pH we can increase their effectiveness and reduce the need for their use. Buffering agents are generally inexpensive and this expense can be quickly recovered through less pesticide use. Getting the most from the pesticides we choose to use is therefore something worthy of "high fives" all around.

Reprinted with permission.

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© Copyright 1998-2013 Robert B. Martin, Jr.. All rights reserved.
Updated January 21, 2013