Phytotoxicity is injury to plants caused by chemicals, including air pollutants, fertilizers, and pesticides. Because they are designed specifically to kill plants, herbicides pose the greatest risk of phytotoxicity when desirable plants are exposed these pesticides.
Unless you definitely know desirable plants were exposed to a kind of herbicide that affects plants of that age and type, investigate whether other causes of similar symptoms are responsible. Causes of symptoms resembling herbicide damage include
Herbicide damage is usually most severe on plants nearest to where it was applied. Unlike with most diseases and insects, when an herbicide is the cause, often more than one plant species is damaged.
Necrotic spots on leaves of a similar age may indicate drift of postemergence herbicide. Chlorosis or necrosis caused by a soil-applied, preemergence herbicide is usually most prominent in older foliage, since time has allowed the herbicide to accumulate. Symptoms are often most prominent in new growth when inappropriate soil conditions or poor root health are the cause.
Inspect surrounding plants, including weeds, to observe where herbicide was applied and to determine whether injury resulted from root absorption or aerial drift.
Phytotoxicity is most likely when label directions are not followed, such as applying excessive rates or exposing desirable plants that are sensitive to the herbicide. Landscapes can be injured as a result of herbicide application and movement (e.g., drift) from nearby properties, rights-of-way, or turf, and when tree roots grow in treated soil. Spray equipment contaminated with minute quantities of certain herbicides can damage plants when used later to apply fertilizers, fungicides, or insecticides.
Each type of herbicide or chemical family produces characteristic damage symptoms. Some types of herbicides affect only plants of a certain age (e.g., only seedlings) or type (only broadleaves or grasses, but not both).
Preemergence herbicides must be applied before seeds germinate; they kill germinating seeds for several weeks or months after application. Postemergence herbicides are applied to emerged weeds. Certain products contain multiple herbicides e.g., two or more postemergence herbicides.
Postemergence herbicides. Some herbicides are nonselective (broad-spectrum); they can kill both weeds and desirable plants that are sprayed. Postemergence herbicides that are nonselective, or are selective for (designed to kill) broadleaves, are most likely to damage landscapes; avoid using them in established landscapes, or use them only when you can avoid exposing desirable plants.
Some postemergence herbicides move systemically (translocate) within plants. Treating cut stumps with systemic herbicide to prevent resprouting can damage nearby plants because natural root grafts are common between adjacent trees of the same species; herbicide translocates from treated stumps through grafted roots to nearby trees.
Broadleaf herbicides. Postemergence, broadleaf herbicides include 2,4-D, dicamba, and triclopyr. They can be absorbed through roots and translocate within plants, resulting in twisted shoots and leaf petioles and dwarfed, distorted, and discolored foliage. These herbicides can severely damage or kill broadleaf trees and shrubs if their roots grow under treated lawns, if droplets drift during application, or after application when warm weather causes herbicide to vaporize and move in air. Spray equipment used to apply them becomes contaminated with their difficult-to-remove residues; using the same equipment to apply insecticides or fungicides can result in phytotoxicity.
Dicamba and 2,4-D are contained in some lawn "weed-and-feed" products. Dicamba in particular can injure woody broadleaves exposed to even very low amounts.
Nonselective herbicides. Postemergence, nonselective herbicides include glyphosate, glufosinate, and pelargonic acid. Glyphosate contamination on broadleaves’ basal buds, foliage, or thin or green bark can cause leaves to turn yellow or mottled green and sometimes die. Plants exposed to this nonselective herbicide in the fall or winter may not exhibit symptoms until months later when new growth appears in the spring. Plant growth is then distorted and stunted, and leaves appear needlelike, puckered, and undersized. Roses in the landscape are quite sensitive to glyphosate absorbed through green stems, and the symptoms are often delayed.
Preemergence herbicides. With preemergence herbicide damage, aboveground symptoms first appear on foliage. Often, damage is observed on leaves as brown or yellow spots where herbicide granules landed and were not washed off. In plants with leaves in whorls, herbicide may accumulate in the whorls and damage will not be observed until new leaves emerge.
After root exposure to preemergence herbicide, leaves may develop white or yellow veins, or veins remain green and tissue between them becomes chlorotic or pale. Sometimes chlorosis progresses to necrosis (blotchy, interveinal, or marginal) and foliage dies.
Some preemergence herbicides can persist for months. If roots grow into treated soil, such as nearby fence lines or roadsides, damage can occur long after the application.
Label instructions for preemergence herbicides often require adjusting the application rate according to soil type. Not knowing the specific soil type and failing to properly adjust the rate, increase the likelihood of phytotoxicity.
Many preemergence herbicides are tolerated by established woody plants, especially plants with healthy, well-developed root systems. Damage sometimes occurs if preemergence herbicide is applied at high rates, incorporated too deeply into the root zone, is not labeled for the site, or is applied near poorly rooted young plants or shallow-rooted plants.
Each type of herbicide or chemical family produces characteristic damage symptoms, including
- malformed, distorted leaves and shoots
- stunted shoots and roots
- chlorotic, necrotic, or spotted leaves
In some cases plants will be stunted, growth will be atypically slow. In the worst cases, desirable plants will die.
Avoid phytotoxicity by applying herbicides and other pesticides carefully, as directed on the label. Use nonchemical weed management methods when feasible. If phytotoxicity has occurred, be diligent about providing plants with proper cultural care, especially appropriate irrigation.
Incorporating activated charcoal, compost, manure, or organic mulch into topsoil and keeping soil moist when temperatures are warm can help reduce the concentration of certain preemergence herbicides through microbial degradation or because the herbicides bind to organic particles. It takes time for herbicide residues to completely degrade and new, uninjured growth to appear.
If phytotoxicity is suspected, learn
- what herbicides have been used on-site or nearby
- when they were used
- at what rate they were applied
- conditions at time of application
- what plants are susceptible to them, and
- the type of damage caused by them
You may be able to diagnose phytotoxicity by having a laboratory test soil for preemergence herbicides or test foliage for systemic pesticides or spray residue. Samples should be tested soon after plant exposure. Such tests are expensive, and laboratories require you to identify the specific herbicides for which you want tests. Be aware that some soil-active herbicides can affect plants at concentrations that are below the minimum detection limit of the laboratory.
If preemergence herbicides are suspected, bioassays may be useful as the first step to determine whether herbicide residue is causing the problem. Collect soil from the upper 2 inches and separately from one or more deeper areas in the root zone. Separately collect uncontaminated soil that is otherwise similar. Plant seeds into small containers of these soils. Use species known to be susceptible to the suspect herbicides, such as target weeds listed on the label or sensitive, desirable species the label warns you not to spray. Provide good growing conditions and appropriate cultural care and compare the emergence or growth of plants from the different soils.
Consult Abiotic Disorders of Landscape Plants and the Herbicide Symptoms website for help in diagnosing phytotoxicity.