Temperatures, Pesticides and Phytotoxicity

Posted: August 8, 2011

It is important to pay attention to the weather for a variety of reasons. Most growers pay attention to rain events to better time pesticide sprays. Some herbicides need a rain event to activate the product in the soil. At other times, an upcoming rain event may delay a spray as fungicides or insecticides can wash off. An equally important factor to take into consideration is temperature.

As always, the label is chock full of information and will give a warning to the applicator on temperature issues.  For example, sulfur products are often utilized to control powdery mildew on a variety of vine crops such as pumpkins and cucumbers.  Pesticide products that contain sulfur usually include a statement that alerts the applicator that the product should not be applied when temperatures are over 90 degrees F.  As of the time of writing this article (July 29, 2011), there have been 14 days since June 1st where temperatures reached 90 degrees F or above in central Pennsylvania.

Certain insecticides are also at the mercy of hot temperatures.  Insecticidal soaps also have warnings about hot temperatures and the potential injury with use of product.

Some herbicides are also affected by hot temperatures.  Probably the most common phytotoxicity problem seen in vegetable fields/high tunnels is 2,4-D damage.  Not that growers are using 2,4-D directly on the vegetable crop, but that the grower or grower's neighbor may have used this product nearby.  Certain formulations of 2,4-D can volatilize (go from a liquid to vapor state) and move off site.

2,4-D labels will mention the importance of temperature.  For example, the Riverdale 2,4-D L.V. 4 Ester label has the phrase, "Although this product is a low volatile formulation, at temperatures above 90°F vapors may damage susceptible crops growing nearby."
There are times when cool temperatures are just as bad as hot temperatures.  Copper is used to control a variety of organisms in both fruit and vegetable production.  In Penn State's Pennsylvania Tree Fruit Production Guide 2010-2011, it states that, "Because copper has the ability to kill all types of plant tissue, the use of copper fungicides carries with it the risk of causing injury to fruit plants."  It goes on to say, "Factors that can promote injury include:  failure to use enough lime; cold, wet weather conditions that apparently increase copper's solubility, allowing more into the plant and resulting in toxicity…"

Although not an exciting read, take the time to become familiar with all aspects of a pesticide label.  Not only can it prevent human harm, but also plant death.