Herbicide Damage

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.

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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.

Physical spray-particle drift

Physical spray-particle drift is the off-target movement of fine droplets generated during herbicide application. Small droplets are produced when herbicides are applied with small nozzle tips at high pressure and low spray volume. The distance that droplets may travel depends on droplet size, with smaller droplets traveling farther than larger droplets. High wind speed, low relative humidity, high temperatures, and height above the ground where the herbicide is released also may increase herbicide drift. Spray droplets may travel a few feet to several miles from the targeted area, depending on weather conditions and spray application; but the potential for drift damage decreases with distance because droplets are deposited or become diluted in the atmosphere. In addition, a special consideration should be given to evaporation of water from the spray droplets when a spray droplet of any given size moves off target. In a long distance drift, it isn't uncommon for water droplets to evaporate completely; and herbicide will be airborne in a dry form. Research showed that small herbicide droplets that dry to particle forms before contact with leaves are biologically inactive. If the dry particles, however, fall on a leaf that is already wet, or if water falls on the leaf in a larger amount but not in a large enough amount to wash it off,  some of the dry particles could be dissolved and  pass into the leaf.

Vapor drift

Vapor drift, or volatility, refers to the ability of an herbicide to vaporize and mix freely with air. The amount of vapor drift varies depending on herbicide, formulation, and weather and soil conditions. Some herbicides are more volatile than others. Volatile herbicides may produce vapors that can be carried great distances from the target area to other crop sites. Vapor drift also may depend on the volatility of formulation. For example, the synthetic auxin herbicide 2,4-D is available in formulations that differ in volatility. The order of 2,4-D volatility is 2,4-D ester (short chain) >2,4-D ester (long chain) >2,4-D amine. MCPA, clopyralid, and triclopyr are other synthetic auxin herbicides (besides 2,4-D) that are produced in ester forms. Dicamba is another hormonal-type herbicide that may drift in vapor form even though it is formulated as a salt. In general, herbicide injury symptoms and damage are more severe and more often from physical spray-particle than from vapor drift.

Herbicide-contaminated soil drift

Herbicide may drift from a treated site by adhering to soil particles and traveling as herbicide-contaminated soil. Herbicide may contaminate soil in several ways: when it is applied directly to the soil, when foliar applications are not intercepted by the foliage, or when herbicide is washed off foliage by rain or overhead irrigation. Subsequent soil disturbance by wind or cultivation may cause soil-adsorbed herbicide to become airborne and to be deposited downwind on plant foliage. However, research shows that herbicides might adsorb tightly to soil particles and not easily release to be absorbed by established plant foliage. The amount of herbicide-contaminated soil deposited on plants would have to be extremely large to cause symptoms or injury, so it is unlikely this would occur under field conditions.

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Preventing herbicide drift injury

Awareness is the key to preventing herbicide drift. Once applicators are aware of the hazards and possible consequences of misuse, they can take several steps to prevent problems:

1. Learn the locations of sensitive crops in the area. Avoid herbicide application near sensitive plants or select herbicides that do not cause injury to nearby plants. Be a good neighbor and do not trespass with herbicides. You will be held liable for damage even if it is unintentional.
2. Herbicide labels warn applicators to avoid using herbicides in the vicinity of susceptible crops. Therefore, it is important to be aware of any sensitive crops grown close to herbicide application. Although there is no legal obligation for herbicide applicators to consult and cooperate with neighbors in matters of herbicide use, it is advisable to do so.
3. Leave a buffer zone between treated fields and sensitive plants. Herbicide labels may specify the width of the buffer zone. The buffer zone will allow larger droplets to settle before reaching sensitive plants. The buffer zone may not be effective in settling small droplets.
4. Avoid the use of highly volatile formulations of herbicides in any area near sensitive crops.
5. Do not apply herbicides when wind is blowing toward sensitive plants. Apply herbicides when a light breeze is blowing away from sensitive crops. Drift is minimal when wind velocity is between 2 mph and 10 mph. Do not spray when temperature inversions are likely or when wind is high or blowing toward sensitive crops, gardens, dwellings, livestock, or water sources. High wind, and no wind situations, may result in serious herbicide drift.
6. Spray when temperatures remain below label temperature restrictions to minimize vaporization and droplet evaporation.
7. Use sprayer application techniques that minimize the production of fine droplets. Selecting proper spray tips, lower spray pressures, and using drift reducing agents will decrease the number of fine droplets. Use drift-reduction nozzles such as drift-guard or air induction types that operate at a low pressure. When using venturi nozzles, higher pressures will be required to maintain an effective pattern. As the pressure is increased with these nozzles, the drift potential will increase; but it less than from other nozzle types.
8. Use wide-angle nozzles, keep the nozzles close to the soil and keep the boom stable.
9. It is also important to use lower application speeds. It is likely that higher speeds may increase herbicide drift.
10. Use shielded booms to minimize herbicide off-target movement.
11. Use spray additives within label guidelines to reduce production of small spray droplets. This will result in less potential for drift. Avoid tank mix ammonium sulfate with volatile herbicides as ammonium sulfate increases volatility.
12. In the case of trees or vines exposed to herbicide drift, consider pruning off the affected leaves or branches to prevent the spread of the herbicide into the plant.
13. Read and follow the directions on product labels. Instructions on the product label are given to ensure the safe and effective use of herbicide to minimize risk to people and the environment. Many drift complaints involve application procedures in violation of the label.

Rights of injured parties: Those who grow sensitive plants that may be injured as a result of herbicide misuse have rights protected by law. Through civil proceedings injured parties may attempt to regain financial losses or secure punitive judgments.

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Preliminary Herbicide Drift Diagnosis

Investigating herbicide drift cases should start when a grower observes unusual symptoms on their crops or observes nearby spraying during weather conditions that may cause drift. The following information should be collected to document herbicide drift incidents.

1. Look for symptom patterns in the field and document the severity of symptoms. Is there a symptom-intensity gradient across the field? Patterns of injury may help identify the source of the problem. The direction of herbicide drift can sometimes be determined by finding "drift shadows" by trees, buildings, or elevated roads. Anything that intercepts or deflects spray droplets can cause an area of undamaged plants on the downwind side of the object.
2. Check to see if other species, especially weeds, develop symptoms similar to symptoms on the species at issue.
3. If there is open ground or a crop between the damaged field and the sprayed field, check for herbicide symptoms on plants in that area. Draw a map or use GPS to locate injured plants in the field. It will be helpful if you record the date when injury symptoms were first observed in the field.
4. Report the description of injury symptoms and photograph typical symptoms of foliage, roots, and bio-indicator plants such as weeds. Continue to report and photograph symptoms through the growing season. Take a large number of quality photos including close-up photos. Record the date and location of each photo. Aerial photos may help to show the pattern and severity of herbicide damage.
5. Plant tissue and soil can be analyzed for herbicide residue. However, growers need to take several precautions when analyzing tissue or soil:
   • Select a reputable laboratory that is certified to conduct GLP (good laboratory practices) analysis. In addition, check the detection level for the procedure used to analyze herbicide residue; and select the laboratory with the most sensitive procedure. The detection level should be at a level below the concentration that causes biological effect. If you select a laboratory that has low detection levels, they may not detect any residue even though you may see injury symptoms.
   • Sample plant tissue or soil from areas where symptoms are intense. The depth of soil sample is important for herbicide detection. Try not to sample too deep because it may dilute the herbicide residue. Plant tissue or soil samples should be packed in dry ice and sent to the lab immediately after sampling. Laboratories should analyze samples immediately.
   • Chemical analysis is costly and may not provide a positive identification of some of the herbicides that damage plants because detection levels are not high enough. Some herbicides rapidly degrade in plants and soils and may be gone before the sample is taken and analyzed. Analytical procedures are specific to each herbicide and must be specified. Chemical analysis may determine the presence of herbicide residue but cannot determine the source of drift or any yield loss caused.
6. Try to create a timeline of the drift incident by investigating all events in the surrounding area. Drift is most likely from adjacent areas but also may occur from farther away. Try to determine the date and time of herbicide application, herbicide name and formulation, wind speed and direction, temperature during application, name of applicator, boom height, nozzle type, spray pressure, and gallons per acre.
7. Collect and record the crop and herbicide history of damaged fields to prove that damage is not due to your own spray.
8. Contact the County Agricultural Commissioners Office (California) or Department of Agriculture immediately after observing herbicide injury symptoms to file an official complaint and arrange for their visit to your field. If you intend to litigate, try to obtain legal advice at an early stage of the litigation.
9. Try to estimate the extent of yield loss. EARLY SYMPTOMS ARE NOT A GOOD INDICATOR OF YIELD LOSS. Crops frequently recover from slight to moderate symptoms and may yield similar to unaffected fields. Actual yield loss generally is less than expected from early season observed herbicide symptoms. The best method to estimate yield loss is to compare the yield from damaged areas to the yield of plants that do not show any herbicide injury symptoms. Comparison of yields between years is not reliable because yields fluctuate between years. But, historical yield data will help substantiate normal production levels.

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Herbicide Misapplication

Misapplication is when an herbicide is applied to soil or a crop that it was not intended to be applied on, such as glyphosate applications to nontolerant varieties of corn. These mistakes rarely happen and can be easily avoided if special attention is given when tank mixes are prepared or when fields are sprayed to ensure that the correct field is treated and the correct herbicide applied. The symptoms and level of injury will depend on the type of herbicide that contacted the plant, herbicide rate, plant species, stage of growth, and weather conditions. While crop injury can happen from all types of herbicide application, most injuries may occur from postemergent herbicides.

If soil-applied herbicides are misapplied preplanting or preemergence, symptoms may appear right after new plants begin to germinate and often are more severe at high rates of application or when seeds are shallowly planted. If incorrect and postemergent herbicides are applied by mistake, symptoms may appear within hours to a few days after application depending on herbicide, rate, stage of growth, and weather conditions. In general, symptoms are more severe on young plants or when plants are metabolically active.

When misapplication occurs, symptoms of treated plants are usually uniform throughout the treated area. It is likely that plants in the treated area need to be destroyed due to significant injury and illegal herbicide residue on the plants.

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Herbicide-Contaminated Tank

Herbicide symptoms may occur when the sprayer is not properly cleaned after a previous herbicide application. Sprayer contamination is problematic in highly diversified cropping systems. This problem can be easily avoided by ensuring that sprayers are properly cleaned between different herbicide tank loads. Herbicide symptoms from sprayer contamination can occur up to several months after using the uncleaned sprayer, since dry herbicide particles can be redissolved causing symptoms. Just spraying until the sprayer is empty does not mean the sprayer is clean. There are herbicide residues that can be on the side of the spray tank, in the spray lines, sumps, pump, filters, and nozzles. All of these parts can be a potential source of contamination. Small amounts of herbicide residue in the spray lines or filters can cause significant damage to the next crop to be sprayed.

In general, postemergent herbicides sprayed directly on the crop foliage have greater potential injury than soil applications, especially when surfactant or adjuvants are included to enhance pesticide spread or uptake. Injury from sprayer contamination can affect crop growth and development for several weeks after application and in severe cases can reduce crop yields.

The field pattern can provide clues to the sprayer filling routine in the field where the crop damage occurred. Crop injury that is associated with one or two sprayer tank loads would suggest sprayer contamination. Symptoms from contaminated tanks are usually worse at the beginning of the spray with damage diminishing with spraying and tank reloading.

Always follow the herbicide label for directions and recommendations for the best method and cleaning agent to use when cleaning out the spray equipment. Consult labels for the products that were previously in the tank and for the products that will be used for the next application. Rinsing with just water may not remove the residue and the herbicide may remain tightly adsorbed in the sprayer through several loads. Further loads that contain other herbicides, oils, fertilizers, or basic pH blend may cause the herbicide to desorb, disperse into the spray solution, and damage susceptible crops.

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Herbicide Carryover

Herbicide residues may persist in the soil and affect susceptible crops for one or more years following application. Crop sensitivity depends on the crop, soil properties, soil moisture and temperature and herbicide.

Crop injury from herbicide residue in the soil, however, is not restricted to persistent residual herbicides applied the previous year. It may happen from herbicide applied to burndown weeds before planting. For example, dicamba and 2,4-D applied to burndown weeds before cotton or soybean planting may severely injure these crops. Herbicide labels often provide guidelines on intervals between herbicide application and the planting of sensitive crops.

Herbicide injury symptoms on sensitive plants can occur from exposure to low soil concentrations. Herbicide carryover can cause crop injury ranging from minimal to complete crop loss or plant kill. Injury problems have typically arisen where normal breakdown of herbicides has been inhibited by factors such as drought and pH.

Herbicide carryover can have considerable field variation in acreage affected and severity of plant injury. Injury can occur anywhere in the field and may be patchy. Uneven plant stands can affect crop maturity. Areas of low organic matter, headlands, corners, or overspray may have more symptoms.

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