Backyard Aquaponics
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Understanding Phytotoxicity and Plants

The term phytotoxicity is roughly equivalent to spray injury. We have probably all applied sprays at one time or another that inadvertently resulted in plant injury in contrast to a positive response and many times we don’t know exactly why it occurred, and therefore do not know what we can do to prevent it in the future.

Often, a grower will apply a particular spray mixture on a regular basis without incident; then suddenly the same mixture results in injury. There are several different types of phytotoxicity. The names of these types of spray injury are my own, as I have not seen this subject formally referred to in the published literature.

Phototoxicity
· Fundamental
· Overload
· Cumulative
· Combination
· Placement
· Episodic

Fundamental Phototoxicity
Is simply when a plant variety is sensitive to a particular chemical. Examples would be the sensitivity of Aralia to Vydate or Hibiscus to Malathion. There are simply situations where a plant and a chemical just don’t get along. The activity of selective herbicides can also fall under the category of Fundamental Phytotoxicity.

Overload Phototoxicity
A second type of phytotoxicity I have identified, where an excessive rate of a chemical that is otherwise safe, is applied, and therefore causes injury.
You may also cause Overload Phytotoxicity by mixing too many elements in your spray tank. I have seen growers mix six or eight different chemicals in a tank, all at proper and safe rates. By themselves, these materials should not cause phytotoxicity. Bear in mind however, anytime you mix three or four different materials in a spray tank, the potential for Overload Phytotoxicity increases.

Cumulative Phototoxicity
When individual applications are not the problem, but that phytotoxicity occurs via build-up from regular applications of the same type. I have seen Spathiphyllum sprayed regularly with iron to the point of inducing iron toxicity. And while individual applications of Subdue fungicide may not cause a problem, applied too many times in succession, and at too close an interval, phytotoxicity can occur.

Combination Phototoxicity
This occurs when a chemical or set of chemicals may be applied without injury, but when mixed with incompatible material, results in crop injury.
For example, Daconil and Vendex are safe by themselves on numerous crops, but, when you mix them together, which you should not do, the risk of spray injury is great. Aliette mixed with copper fungicides also presents great risks, whereas individually the materials are quite safe.

Placement Phototoxicity
A somewhat rare type of phytotoxicity, which occurs when a material applied in the correct fashion is perfectly safe, but is placed where it shouldn’t. A good example would be applying Ronstar to a soil for preemergent weed control. That in itself is normally very safe, but if the Ronstar granules end up in the whorl of a sensitive plant phytotoxicity can damage that plant.

Episodic Phototoxicity
This refers to an episode where a common spray, for some unknown reason, and where it has never occurred before, suddenly causes plant injury. Usually in this type of situation weather conditions are a factor. Some sprays are safe in cooler weather whereas they can become very dangerous in high heat conditions.
Water-stressed plants can be very sensitive to otherwise safe spray applications. Improper cleaning of the spray tank from a previous application can cause Episodic Phytotoxicity. Sometimes, the causes of Episodic Phytotoxicity remain unknown.

Preventing Phototoxicity
What can a grower do to prevent all these potential problems?
First, it is important to note that phytotoxicity is a relatively rare event, occurring perhaps only once in every 500 applications on average.
To reduce those odds even more, the rules are simple:
· Clean your spray tank thoroughly between each application
· Use a separate, and marked accordingly, sprayer for herbicides only
· Watch your application rates carefully, and try not to mix more than three or four items in the tank
· Do not apply a tank mix unless your experience or chemical labels indicate a mixture is safe
· Read the chemical labels
· Don’t spray in excessive heat, or when plants are stressed or wilted
· Finally, when you are unsure about a spray mixture, there are a number of sources for useful information you can tap into, such as consultants, extension agents, other growers, chemical companies, ag sales people or the Internet
If you pay attention to what you are doing regarding the application of agricultural sprays and are reasonably diligent, phytotoxicity can be avoided almost all of the time.