Vinews

No. 14 — July 1, 2019

Phytotoxicity: Be Careful With the Spray Cocktails

This week has been the week of phytotoxcity on grapes that resulted from tank mixing two or more pesticides together. I cannot say for certain that the symptoms growers are observing are the result of the pesticide cocktails. However, when new products come on the market and damage occurs to the crop, then I believe that growers should be informed of some potential problems.

There is also another issue that growers need to understand. The pesticide label often states some of the potential damage that can occur if the label is not adhered to fully. Obviously, pesticide companies cannot test every tank mix combination that growers may concoct. Therefore, if you are going to tank mix pesticides that have not been applied before then consider spraying a few vines and then observe them over a period time to determine if phytotoxicity develops.

The environment also plays a role in phytotoxicity. Weather conditions that delay the drying of the pesticide can result in phytotoxicity. These weather conditions can be described as wet and cool. Heavy dew and cool temperatures in the early am hours result in the pesticide(s) going back into solution on the leaf surface. The cool temperatures delay the drying of the pesticide(s) and prolong the period of time in which the pesticides may be absorbed by plant tissue. A common phytotoxicity that occurs during cool conditions is from copper. On the other end of the temperature spectrum, hot temperatures (> 85 degrees F) can induce sulfur phytotoxicity. Similarly, applying foliar iron sprays during hot temperatures can cause phytotoxicity.

close-up of grape cluster showing phytotoxicity spots and symptoms from sprays
Figure 1. Phytotoxicity from a tank mix of Satori, Rally, and Assail. Satori contains the active ingredient azoxystrobin and an adjuvant Leci-Tech. Photo credit: submitted.

Adjuvants can also play a role in phytotoxicity. If the label does not state that an adjuvant needs to be added then don’t add an adjuvant. A newly introduced fungicide product has the trade name Satori and has the active ingredient azoxystrobin which is the same active ingredient as Abound. In addition Satori also contains an adjuvant (Leci-Tech) which is derived from soybean. A grower recently tank mixed Satori, Rally and Assail and this resulted in phytotoxicity (Figure 1).

The take home message on phytotoxicity is the following:

  • Read and follow the label.
  • If applying a tank mix of pesticides that has not been applied before, then mix a small amount in a hand-pump sprayer and treat a few vines. Observe the vines for two to three days after treating to determine if phytotoxicity develops.
  • Environment can play a role in the development of phytotoxicity. Environmental conditions that promote rapid drying of the pesticide on the plant surface is often beneficial.
  • If the label of the pesticide being applied does not state that an adjuvant needs to be added, do not add an adjuvant to the spray solution.
  • Plant growth that occurs during cool cloudy conditions results in thinner cuticles and this plant tissue is more prone to phytotoxicity.

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