Not all plant injuries are caused by a biotic factor, like bacteria or fungi; some symptoms are caused by abiotic (non-living or environmental) factors. Chemicals like pesticides, fertilizers, and growth regulators can cause damage when used inappropriately and often look like those caused by biotic factors. Learn more about biotic and abiotic symptoms in this article: Biotic vs. Abiotic - Distinguishing Disease Problems
Herbicide damage is one of the most common chemical injuries observed in the home garden. If herbicide damage is suspected, more information can be found in this article: Herbicide Injury to Garden Plants.
How Plants are Damaged | Symptoms | Diagnosis | Management | More Information
How Plants are Damaged by Off-Target Chemical Applications
Plants may be unintentionally exposed to pesticides and other chemicals through the following ways:
- Drift or accidental applications from pesticides applied to nearby lawns, landscapes, fields, or other areas
- Improper use of a product or using the product for a use not listed on the label
- Accidental spills or applications of chemicals not intended for garden use (such as motor oil or drain cleaner)
Drift and Accidental Applications
Pesticide drift occurs when spray particles or vapors move off-target due to environmental factors like wind, temperature, humidity, or improper sprayer setup. Vapor drift is influenced by chemical formulation, temperature, relative humidity, wind, and inversions. Depending on how the drift happens, it can affect plants only within the immediate area or plants in excess of a mile away.
Accidental applications occur when sprayers aren’t properly rinsed, contaminating subsequent treatments. For example, if a sprayer with herbicide residue is used for fungicide, unintended plants may be damaged. Accidental applications also occur when applicators are careless or distracted and apply chemicals to a plant they did not intend to spray.
Spills
Spills are typically accidental or unintentional applications of any type of chemical that can kill or cause damage to plants. Often, they are chemicals that are not intended for plant use and have toxic properties. Examples include gasoline, motor oil, antifreeze, diesel fuel, drain cleaner, bleach, ammonia, paint, cleaners, and lighter fluid. Often, these chemicals are toxic to animals and humans as well as plants.
Improper Use
When any chemical is not used according to the label directions, unintentional injury can occur. It is unlawful to use any pesticide in a way not outlined on the product's label. Improper use can take many forms.
Use on Inappropriate Species
Always confirm that the plants within or near the pesticide application are listed on the label. Never apply any insecticide, fungicide, or other pesticide to any plant species not listed on the label.
Use on Stressed Pants or During the Wrong Stage of Growth
Plants experiencing stress from drought, heat, insect damage, disease, frost, or other environmental factors are more susceptible to damage from chemicals.
Some products can affect certain plants differently at different stages of growth. The product label will advise what stage of growth or time of year is most appropriate for an application. This could include recommendations to not apply when plants are in bud, have large amounts of new growth, or during germination, for example.
Over Applied
Any pesticide applied too frequently can cause damage to plants. Product labels outline the maximum number of applications that can be made within a certain time period. If this maximum is exceeded, damage can occur.
Improperly Mixed or Prepared
Always mix and apply the pesticide, fertilizer, or other chemicals as outlined on the label. Mixing the product at a higher concentration than what is listed on the label will not make it work better or faster. Instead, it will frequently lead to damage.
Some chemicals can be combined in the same sprayer and applied at the same time to save time. If incompatible products are mixed, it can cause phytotoxicity on plants. Additionally, many products for home gardeners are sold "ready to use." If amendments, like surfactants, are added to these products, they can potentially cause damage.
Use During Unfavorable Weather Conditions
Damage can occur when chemicals are applied in conditions that are windy, hot, humid, sunny, overcast, or other unfavorable weather conditions. Applying chemicals without a good understanding of the weather forecast can also lead to damage if chemicals are applied and shortly after, weather conditions change.
The ideal weather conditions for a chemical application are listed on the label and vary based on the product. For example, some pesticides, especially soaps, oils, and sulfur compounds, are more likely to cause damage when temperatures and humidity levels during application are too high. Conversely, some pesticides, such as copper fungicides, are more likely to cause damage when conditions are too cool and damp.
Assuming the Chemical is "Safe"
Even "non-toxic" or "organic" chemicals, like insecticidal soaps or horticultural oils, can cause damage if they are not applied appropriately. Fertilizers are often considered good for plants and promote growth, but can cause damage when applied too frequently or at higher concentrations.
Poor Post-Application Care
Following post-application instructions is crucial to prevent plant injury. This could be things like not watering for several hours after application, avoiding the use of other chemicals for a period after application, delaying the harvest of fruit, staying out of the area for a period of time, or waiting a certain period of time before reapplication, among other things.
Even if the product is mixed in accordance with the label directions, if it is applied again too soon or applied too many times in one growing season, it can potentially cause damage. Always adhere to the reentry times, waiting intervals, and use guidelines printed on the label.
Symptoms of Chemical Injury
Symptoms of chemical injury can vary widely depending on many factors. The damage that occurs from a chemical application is termed "Phytotoxicity."
Symptoms of phytotoxicity are typically things like leaf burn, leaf speckling, blotches or spots on leaves, bleaching, chlorosis (yellowing) or necrosis (browning) of the leaf edges, leaf cupping or twisting, tip die-back, leaf or tip burn, stunted growth, and/or plant death.
Factors that Determine the Level of Injury
The injury that occurs depends on several factors including (but not limited to) active ingredient, formulation, concentration, mode of action, source, and amount of chemical as well as the species of the plant affected, application method, equipment used, time of year, plant developmental stage, and weather conditions during and after application. Remember, even "safe," "non-toxic," or "organic" products can cause injury if not applied according to label directions.
Easily Confused with Disease and Environmental Stress
Damage from improperly applied chemicals can be easily confused with diseases caused by pathogens (viruses, bacteria, and fungi) as well as problems caused by poor environmental conditions. This can make it difficult to pinpoint chemical damage as the source of injury over things like insect feeding, mottling due to a virus, or die-back due to dry soil conditions.
Diagnosis of Chemical Injury
Use these factors to separate chemical injury from damage caused by abiotic factors (like disease) and environmental stress.
Knowledge of Chemical Applications or Spills
Always keep a record of when and where you applied any chemical (pesticides, fertilizers, et.al.) Note accidental spills and nearby applications of chemicals by other people. Often, it is not known what is being applied to nearby areas, but if symptoms appear after the application, there could be a correlation.
It is important to remember that just because a chemical was sprayed nearby does not mean it is the cause of injury to the plants. The chemical application to your neighbor's lawn could have just as easily been a fertilizer as it could have been a herbicide, and you cannot assume it is the source of phytotoxicity until it is known what was applied and if that particular chemical could cause the damage observed.
Damage Patterns Match Spay Patterns
If phytotoxicity is only observed in areas known to be sprayed, it could be the cause of the injury. Plants located closer to the sprayer or in areas where the spray pattern may have overlapped are more likely to show damage.
If drift is suspected, plants located closer to the suspected source will show more damage than those further away.
Time for Symptoms to Appear
Phytotoxicity may develop within a few hours, a few days, to more than a week after the application, depending on the product used. While the time it takes for injury symptoms to appear could vary, when caused by a misapplied chemical, it will likely appear all at once and have a regular distribution over the entire area treated.
Multiple Species Affected
Many insect pests and most diseases only affect specific plant species and those closely related in the same genus or plant family. When chemical misapplication or drift occurs, it typically affects many different plant species in the area in a similar way.
There may be some variation in the level of injury symptoms, as some plant species are more sensitive to chemical injury than others. Additionally, poor environmental conditions, such as drought, can cause all species in an area to show injury symptoms.
Compare Treated and Untreated Plants
If plants exposed to a chemical application are showing injury symptoms and plants of the same species growing in similar conditions were not exposed and show no injury symptoms, then misapplied chemicals could be the reason for the plant damage.
Sensitivity
Some plant species are more sensitive to certain types or formulations of pesticides than others. Before spraying, check the label for a list of sensitive plants or a precautionary statement stating which species or varieties the product is not recommended for use. In situations of accidental applications and drift, sensitive species will show more extensive damage at distances further from the application than non-sensitive species.
New Growth is Unaffected
Many insecticides, fungicides, miticides, and other pesticides can cause damage that is irreversible. However, as plants grow, the damage will remain on the older leaves, and the new growth will emerge healthy.
Those plants with chemical injury from herbicides or fertilizers do not have the same response. Typically, these chemicals can cause new growth to look damaged as well. More information can be found in this article: Herbicide Injury to Garden Plants
Diagnosing Chemical Injury Can be Difficult
With so many factors and variables to consider, it can be difficult to determine if the injury observed is due to a chemical misapplication or due to poor environmental conditions or biotic factors like disease. Phytotoxicity symptoms are often very similar to other problems, and especially in situations of drift or accidental applications, there is often not enough information available to make a conclusive determination.
It is also possible for drift, accidental applications, or spills to occur and no phytotoxicity is observed. Proper management of any plant exposed to a misapplication of any chemical is important, whether it shows signs or symptoms of injury or not.
Management of Chemical Injury to Plants
If plants are damaged by a pesticide or other chemical, they may or may not recover, depending on the severity of the damage. All that can be done is to wait and see what happens while providing good care for the plant. Preventing other stresses from harming the plant is the best way to minimize chemical injury. This includes watering during hot and dry periods, fertilizing if the soil is deficient, and scouting for other diseases and insect pests.
When phytotoxicity is observed, there are no chemical sprays or nutrient/fertilizer applications that can reverse the damage. New growth may be unaffected and long-term damage may not occur. This depends on the plant species, its overall health, and the product it was exposed to.
Management of Edible Plants with Chemical Injury
Whether a crop is safe to eat after accidental exposure to a chemical depends on many factors, including the product used, active ingredient(s), concentration, its mode of action, and many others. The safest course of action is not to consume any part of a plant that has been exposed to a known or unknown chemical and to remove and replace the plants. For perennial edible plants, like fruit trees, fruit should not be consumed as long as the chemical is present in the soil or the surrounding environment. The length of time before you can consume the crop varies depending on the chemical and growing conditions. In most situations, it is at least one growing season. Check the product label or website for the pre-harvest interval to know how long you have to wait before harvest.
In many cases, the product affecting the edible plant is not known. Even if you know the product used, the label may not provide safety information for use on fruits and vegetables since they may not be intended for use on these plants.
Depending on the time of year and source of contamination, it may be possible to replant the crop. Some vegetables will not be able to be replaced and have enough time to mature and be harvested. However, there may still be some alternative crops you can replant. Count the number of days from the anticipated replanting date to your average first frost date. Locate the days to maturity printed on the label or seed packet. As long as the days to maturity are fewer than the days left until frost, it’s worth attempting to replant. Additionally, many garden centers have large, established vegetables in containers that could be purchased to replace damaged plants and farmer's markets can be a good source of locally grown vegetables and fruits.
Prevention is Important
When chemical damage occurs, there is nothing that can be done other than wait and see and continue normal practices that maintain plant health. Because of this, preventing chemical damage is essential.
Always Follow Label Directions
Always apply pesticides as directed on the label. Pay close attention to the instructions on the label and follow them precisely. Only apply pesticides when the environmental factors, such as wind direction, wind speed, and temperature, are appropriate. Align and follow all other factors outlined on the label such as personal protective equipment (PPE), re-entry interval (REI), and pre-harvest interval (PHI). Equipment used for mixing and spraying must be cleaned thoroughly after the application of a chemical to prevent contamination. More information on reading pesticide labels can be found in these resources:
- What You Need to Know about Reading a Pesticide Label from Penn State Extension
- The Environmental Protection Agency (EPA) pesticide information page
Use New Products Carefully
When applying a pesticide or other chemical you've never used before on a particular plant species, test it out on a few plants to see if phytotoxicity occurs. This precautionary step can help to avoid extensive damage to the entire garden.
Be Aware of Nearby Applications
Scout often and be aware of spraying that occurs in your landscape or in surrounding fields, yards, and gardens, and watch your plants for symptoms of damage in the following days. Knowledge of the pesticides and other chemicals applied in the area can help identify the potential cause of the symptoms.
Prevent Spills
Whenever using potentially toxic chemicals that are not pesticides, such as motor oil, handle them carefully to avoid spills. Store them in the original container and always be sure the lid is secure to prevent leakage.
Testing Plant Material for Chemical Residue
An analysis of plant material for chemical residue can be done, but they do not always provide conclusive results. Additionally, little is known about acceptable thresholds for many pesticides on most vegetables, trees, shrubs, and other garden plants. So even if you conduct a chemical residue test and now know the concentration of pesticide found in the leaves or other parts of the plant, it still does not give any guidance on what to do with the plant since the acceptable threshold of contamination is not known.
If you are still interested in testing the plant material for residue, Iowa State University does not provide this service. The Iowa Department of Agriculture and Land Stewardship (IDALS) provides a list of private labs that can perform a pesticide residue test. IDALS Pesticide Labs List. Information on costs and how to collect the sample is found on the websites of each respective laboratory.
Reporting and Enforcing Pesticide Drift
The IDALS Pesticide Bureau provides enforcement of the Iowa Pesticide Act, which governs pesticide use in Iowa. This would include any pesticide application situations that involve misapplication, drift, or negligence on the part of the applicator. More information on pesticide use investigations and enforcement can be found on their website: IDALS Pesticide Bureau
Information on how to file an "Incident Report" with the IDALS Pesticide Bureau to report any pesticide complaints (drift, misapplication, etc.) can be found here: Pesticide Investigation & Enforcement
More Information
- American Phytopathological Society (chemical injury section)
- Herbicide Injury to Garden Plants
- IDALS Pesticide Bureau
- Home and Garden Pesticide Guidelines
- Toxicity of Common Lawn, Garden, and Ornamental Pesticides in Iowa
- Understanding Pesticide Labels
- Small Sprayer Calibration
- Factors Affecting Pesticide Drift
- Integrated Pest Management for Home Gardens and Landscapes
- Protecting Bees from Pesticides
Photo credit: 1:Tomasz/AdobeStock; 2: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org; 3: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org; 4: dean/AdobeStock; 5: Aaron Steil; 6: kornwipa/AdobeStock; 7: Bruce Watt, University of Maine, Bugwood.org; 8: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org; 9: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org