In the pest management business, we apply pesticides in numerous ways for our clients. Spraying is extremely popular, although there are also granular applications, dusts, injections, fumigants, and other styles of applying pesticides in order to achieve control of the target pest. Most of the time, this works fine. However, there are occasions where pesticide applications can cause plant injury, which can be used as a general definition for the term phytotoxicity. I will discuss here only plant related injury, without referring to potential injury to people, pets or structures.
This is simply a situation where a pesticide and a plant variety do not get along. Most pesticides are generally considered safe to use on a wide variety of plants. However, experience shows that sometimes the pesticide may be simply be fundamentally injurious to a particular type of plant, even when applied correctly and at label rates. In the old days, such examples might include Malathion on hibicsus, Agrimycin on ivy, Chipco 26019 on peace lily, and copper on bromeliads. I once saw an entire greenhouse full of gardenias defoliated by a proper application of a mancozeb formulation whose label strictly prohibited use on gardenia.
Modern pesticides are usually tested more thoroughly than products from decades ago. Often, during such product testing, investigators will discover that certain plant varieties simply don’t like certain chemicals. For example, Heritage is a terrific strobin fungicide which I and others recommend frequently. However, experience has shown that Heritage is not safe on some apple and cherry varieties, nor on leatherleaf fern. The label therefore strictly prohibits use of this product on these varieties. It is fairly common today for pesticide labels to permit use of a product on a wide variety of turf and ornamentals, yet specifically prohibit use on others. Such counter recommendations are often listed toward the bottom of the pesticide label, after the permitted varieties, so check your pesticide labels carefully.
We all know that some people think if a little is good, more is better. That generally does not apply in the realm of pesticide applications, and it may be downright illegal. You all know that applying the correct dose to dosage of a pesticide is critical. Apply too much, and you can risk plant injury, as well as waste money and product. Apply too little, and you may invite pest resistance. Pay attention not only to dilution rates, but to formulation rates per unit area, such as ounces per thousand square feet. Applicators can not only sometimes mix a chemical too strongly, but they can also apply too much of a properly – mixed formulation per unit area. The pesticide label can be your best friend in such situations. Also, don’t ignore sprayer calibrations, and remember that nozzles do wear over time.
Most of the time, pesticide applications can be properly made according to the label, without incident. Occasionally, if multiple applications are made to the same plants or turf with the same product, phytotoxicity can develop over time. One application may be fine, but multiple applications of the same product can lead to larger accumulations in the plant, resulting in injury. In my experience, this is especially possible with systemic products, both fungicides and insecticides. Watch your rates and rotate your chemistries to help avoid this. Many pesticides today also limit how much product can be applied to a specific area per year.
I am referring here not to potential injury caused to the environment from pesticide applications, but to plant injury that can happen when pesticides are applied during unfavorable environmental conditions. Applying atrazine during summer is a classic example. We Floridians know that chemical injury risk to plants increases during hot weather, or when plants are drought stressed. Any kind of plant stress can potentially increase the risk of spray injury, so keep those factors in mind. When I was a spray man for large ornamental nurseries in the 70s, we tried to never spray when the temperature was above 85°. Landscape plants and turf can generally be sprayed when temperatures are somewhat higher than that, but drought stress greatly increases susceptibility to injury.
Pesticides may often be applied safely when applied to certain parts of a plant, but not to others. For example, granular herbicides such as pre- emergents may be fine when applied to soil at the base of plants. However, if the granules land in a whorl or cup, or on open flowers, injury can occur when it normally wouldn’t. Placement phytotoxicity is more common with granular products, though formulations diluted with water can also cause potential injury when applied to certain plant parts, such as open flowers. Applying a copper fungicide to a fruit tree may be a perfectly appropriate application, but flower injury when applied at the wrong time can substantially reduce fruit yield.
Sometimes chemical X may be safe to apply in a certain situation, as can chemical Y. However, if the two are mixed together, potential injury can result. This can happen with herbicides as well as fungicides and insecticides. Nutritionals can enter the picture as well. I have lectured extensively on tank mixing in the past. Tank mixing is generally safe, but it is a fine and delicate art. It is very dependent on experience and on application details such as weather, dosages, and plant or turf varieties. There are literally quadrillions of potential tank mix combinations, so pesticide labels are often of limited help in this area. Use your experience and be cautious.
Another type of combination phytotoxicity can occur when a singular product is applied which reacts with residue from a previously applied product. Again, either by itself may be fine, but the two together can cause potential problems. A good example of this is injury from applying acidic products too soon before or after metallic products. I know of one fungicide whose label restricts its use within two weeks before or after applications of copper fungicides.
Many times, I have seen pesticides applied properly in terms of application rate and all of the normal label precautions. However, injury can still occur if the pesticide is applied with too much spray pressure, or if the nozzles are utilized to close to tender foliage. Sometimes just the spray pressure alone can cause injury to sensitive plant varieties. Other times, it seems to be that the combination of pesticide and pressure causes injury.
Many of us have experienced situations where we might have applied a pesticide in a normal situation 99 times without incident. Then, on a proverbial 100th application just like all the others, something goes wrong, and injury occurs. It is often quite difficult to figure out just what happened in the case of episodic phytotoxicity. Sometimes it can be environmental conditions, tank or hose residue from previous applications, mixing order, agitation problems, or numerous other possibilities. Such spray injuries are not that uncommon. Both managers and applicators should be honest and straightforward when discussing these things when they occasionally happen. Phytotoxicity is a risk many of us take every day, but better understanding can help us minimize potential problems.