by Kassim Al-Khatib
Plant Sciences, University of California–Davis
Although the intent in using herbicides is to kill unwanted plants in order to enable food crops or ornamentals to thrive, sometimes the use of herbicides has the unintended consequence, when applied inappropriately, of injuring nontarget plants.
Herbicide damage on nontarget plants may cause slight to serious injury symptoms and can occasionally cause economic damage as well.
Herbicide chemistry and physical properties usually determine how herbicides interact with the biological and physical systems of the plant. Factors determining herbicide efficacy and crop safety are complex and include plant species, plant size, stage of growth, soil chemical and physical properties, soil moisture, temperature, and relative humidity. Postemergence herbicide uptake and efficacy can be affected by spray additives that enhance the performance of the herbicide but may also increase the risk of crop injury.
Herbicide symptoms vary depending on the herbicide, the rate of application, stage of growth, type of exposure, and the plant species receptor involved. In general, herbicides with the same mode of action produce similar injury symptoms, because the outward appearance of injury is a function of herbicide effect on the plant at the cellular level. Therefore, it is much easier to diagnose symptoms belonging to different herbicide modes of action than herbicides within the same modes of action. In addition, diagnosing herbicide symptoms can be difficult because herbicide symptoms may look very similar to symptoms caused by diseases, nutrient deficiencies, environmental stress, and soil compaction.
While sometimes it is not possible, by visual observation alone, to determine what particular herbicide from the same mode of action may have caused plant damage, it is possible to do so with some other modes of action. For example, there are five types of herbicide chemistry that inhibit acetolactate synthase. Herbicide chemistries, and the individual herbicides within them, may have different physicochemical properties, biological activities, weed control spectrums, soil activities and half-lives but all generally produce similar injury symptoms on nontargeted plants. On the other hand, there are 11 types of herbicide chemistries that inhibit photosynthesis; however, some of these herbicides may cause specific symptoms that can be identified. Furthermore, herbicides from the same mode of action or chemistry may cause different symptoms and injury on the same species. For example, pyridine carboxylic acid herbicide picloram causes different symptoms on cotton compared to other pyridine carboxylic acids such as clopyralid and triclopyr.
In general, annual plants that rapidly translocate herbicide are more susceptible to herbicide damage and may show more injury symptoms. Conversely, perennial plants tend to translocate herbicide slower than annual plants and are also able to dilute herbicide in larger biomass systems, resulting in less injury. In addition, perennial plants may have more ability to breakdown herbicide and recover from injury symptoms. It is not uncommon for plants affected by herbicide to recover from symptoms, even with the occurrence of considerable dieback. This is particularly true with trees and other woody plants that have the ability to store carbohydrates and also have protected meristems in dormant buds. Trees have a remarkable ability to survive and recover from herbicide injury.
Herbicides can injure foliage, shoots, flowers, and fruits. If injury is severe enough, either from one incident or repeated exposure, it may reduce yield, produce poor fruit quality, distort ornamental or nursery plants, and occasionally cause plant death. Herbicide symptoms may be visible for a few days to several years depending on the herbicide involved, plant species, stage and rate of growth, environmental and soil conditions, and cultural practices. In addition, herbicides may reduce nontarget plant vigor, increase susceptibility to disease, and shorten the life cycle of a plant. Herbicide injury to nontarget plants also may result in illegal residues on the exposed crop. In ornamental nursery plants even slight herbicide symptoms may affect the marketability of damaged plants.
Several herbicide injury symptoms, such as general and interveinal chlorosis, mottled chlorosis, yellow spotting, purpling of the leaves, necrosis, and stem dieback, may result from causes other than herbicide exposure. If herbicide damage is suspected, the progression of symptoms and the study of herbicide symptomology in its entirety are critical. Research at several universities, including the University of California, shows that many symptoms from biotic and abiotic stresses mimic some herbicide symptoms and can be difficult to distinguish for the untrained observer.
Accurately diagnosing plants that show herbicide injury symptoms is difficult. In many cases, other biotic and abiotic causes may be involved or it may be unclear what herbicides were applied. Trained researchers, however, may be able to confirm or discount the possibility of herbicide injury by examining plant symptoms, injury progression, and studying other information such as type of herbicides used and history, herbicide rates and application timing, injury patterns, plant species affected, weather data, and soil conditions. However, positive confirmation of herbicide symptoms requires lab testing of the live plant tissue and/or the soil while the chemical is still present at detectable levels. In cases investigating herbicide symptoms, it is easier to accurately diagnose these symptoms from contaminated tanks, soil carryover, misapplication, or sprayer overlapping than from herbicide drift.
Herbicide Drift
Drift is defined as physical movement of an herbicide through air, at the time of application or soon thereafter, to any site other than that intended. The three ways herbicides may move to nontarget areas are physical spray-particle drift, vapor drift, and herbicide-contaminated soil.
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