Phytotoxicity: When an application appears to do more harm than good

Growers and gardeners can prevent phytotoxicity with these precautions.

Far too often we receive samples in the MSU Diagnostic Services lab where damage is attributed to an application of a product intended to protect or promote plant health. These products might include fungicides, insecticides, herbicides, foliar fertilizers, surfactants and plant growth promoters. These products may have been applied by a commercial grower or a home gardener – phytotoxicity does not discriminate. Organic growers, keep reading; you are not off the hook yet. Consider the fact that in some cases, organic, non-toxic or “safe for use on plants” products are involved – phytotoxicity is not limited to “traditional chemistry products.”

Over a typical week in the diagnostic lab, the staff talks to a wide range of clientele with all sorts of problems. (You can’t make up some of the stuff we run into.) Below are a few of my take-home messages gleaned from experiences in the lab; hopefully sharing them will prevent a few folks from making some of the same mistakes.


The labeled rate and application interval stated on the label must be heeded by the user. Applicators can be tempted to inch the rate up “a tad” to get better control, greener plants, quicker kill, etc. I recently had the opportunity to work with a grower that was fertilizing vegetable transplants with an organic fertilizer. We talked at length about the health of his plants including a problem that was being attributing to root rot. The grower subsequently submitted some of the transplants to the lab for evaluation. Upon examination, we readily observed that the plants were stunted, had extremely limited root growth and constricted stems.

No pathogens were found associated with the tissue; instead, pH and EC (electrical conductivity) measurements of the media revealed the cause (see the Michigan State University Extension article “pH and EC issues still a problem in some greenhouses”). The salt level in the soil was excessively high, causing root damage, stem lesions and eventually plant death. The grower had inadvertently over-fertilized the trays of germinating seedlings – when they weren’t performing as expected, additional fertilizer was added to promote growth, unknowingly increasing the damage.

Murphy’s Law: Watch calculations and injectors

Murphy’s Law says anything that can go wrong, will go wrong. Two common areas for Murphy to strike involve calculations and injectors. Commercial applicators regularly calculate dilution rates to mix chemicals prior to application. Home gardeners do the same with products that are not sold in ready to use formulations. Take the time to logically write out calculations and keep your notes in case the need arises to return to them. Miscalculated dilution rates are a possible source of error leading to phytotoxicity.

Commercial growers use injectors to add fertilizer, acid, pesticides and disinfestants to the irrigation water. Home gardeners use hose end applicators that perform similar function. From time to time, both of these systems malfunction, creating an inappropriate dilution. This is difficult to guard against, but a factor to consider if something does go wrong.

Chemical cocktails

For efficiency, as well as other reasons, growers and homeowners alike often prefer to mix multiple products in the spray tank, creating what you might call a chemical cocktail. Check the label of the product that you applied to a plant most recently. Chances are it says something like, “Product X is compatible with many commonly used pesticides, fertilizers, etc.” Chemical manufactures do their best to trial and evaluate the risks of mixing multiple products for application. When injury results from a known combination or on specific crops, a warning is included on the label. But the fact remains that a chemical producer is unable to evaluate their product in every possible mix on every possible plant.

When using products that are new to the user or new to the crop, apply to a small, sample size. This is especially important when multiple products are mixed. Document the products used in the mix as well as the rate of each product. Wait and observe the treated plant material to be sure that the application did not injure the crop before treating an entire crop with the tank mix.

Science-based information

There are so many sources of information when it comes to determining the preferred products for a particular crop or pest. It is easy to become overwhelmed or persuaded by word of mouth or great marketing. Use available sources wisely and evaluate reports with good, science-based criteria. Data from unbiased trials is available for many commercially available products; if you have trouble finding or interpreting this information, contact your local MSU Extension county educator to assist in this area.

Just because you read something on the Internet does not make it true! It would seem that this is common knowledge, but it bears repeating. For example, last week I spoke to a home gardener that was concerned about damage to trees being caused by blue lasers. I had never heard of such a thing, but she had read about it on the Internet and believed this was entirely plausible.

When you do treat plants with a new product or new combination of product, it is helpful to leave a few plants untreated. This provides an “untreated control” against that you can readily evaluate the performance of the new product applied.

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