Diagnosing Suspected Herbicide Damage in Tomatoes

(For more information, please refer to Extension publication Preventing Off-target Herbicide Problems in Tomato Fields, W295-A)

Agricultural chemicals, particularly pasture and right-of-way herbicides, have the potential to cause off-target damage to tomatoes. Although these herbicides control many troublesome weeds, off-target damage to tomatoes often results in expensive fines and/or lawsuits, lost productivity for growers, and even crop rejection. Several management practices can be adopted to avoid these problems. 

  • Herbicide Selection — Although highly effective on several broadleaf weeds in pastures and rights-of-way, the auxin or growth regulator herbicides can damage sensitive crops if not used properly. The characteristics of these herbicides determine which product to use in different situations.
    • Volatile herbicides — change from a liquid to gas or vapor and move away from the target. Typically, dicamba and 2,4-D are more volatile than aminopyralid or picloram.
    • Persistence of herbicides — The persistence of herbicides can affect future plans for a field. While dicamba and 2,4-D are highly active on cotton even in minute doses, these materials are relatively non-persistent in soil and in treated pasture grasses and hay. This is not the case with aminopyralid or picloram. Both of these herbicides can stay active in soil, pasture grass and hay for a year or longer
    • Water Solubility — Picloram is more soluble than aminopyralid and therefore more likely to be moved off-site by runoff.
  • Prevention of Drift — Several factors can contribute to herbicide drift to sensitive areas. physical and vapor, can occur. (Refer to the section “Spray Today?” in this website)
    • Physical Drift — can occur when the herbicide is blown away from the target area on windy days and/or in runoff after a hard rain. It is important to monitor weather conditions several days prior to and after spraying to prevent physical drift.
    • Vapor Drift — is the movement of spray vapor away from the target after the herbicide has been deposited on the target. It is mainly influenced by air temperature, but also by relative humidity (RH) and herbicide formulation.
  • Field Selection — The location, characteristics and history of a field influence future management strategies. A proper risk assessment should be performed before spraying a pas- ture with some of these herbicides. Rains can wash certain herbicides downhill to sensitive areas.
  • Sprayer Contamination — A common way for pasture herbicides to make it into cotton fields is through sprayer contamination. Pasture herbicidesare notoriously difficult to rinse from sprayers. Rinsing withwater does not remove all herbicide residues from a sprayer and hoses
  • Monitoring — Producers are encouraged to assess the performance of herbicides in pastures and hay fields. Tracking results will guide future decisions for weed control. It is important to keep a log of all applications with dates, products, field locations and weather conditions.

(For more information, please refer to Extension publication Diagnosing Suspected Off-target Herbicide Damage to Tomato, W295-B)

Many herbicides used in pastures and hay fields are growth regulators that mimic the plant hormone auxin. When observing herbicide damage in tomato, it is often difficult to distinguish between these herbicides. The symptoms below can help to identify certain characteristics of each of these herbicides in tomato. The following are descriptions of commonly observed symptoms resulting from tomato exposure to synthetic auxin herbicides:

  • Curling – folding of edge of leaf margins.
  • Epinasty – twisting, bending and/or elongation of stems and leaf petioles.
  • Blistering – appearance of raised surfaces on leaf tissue.
  • Chlorosis – yellowing or whitening of leaves resulting from loss of chlorophyll.
  • Necrosis – browning of tissue resulting from cell death.
Herbicide Damage to Tomato
Tomato plant exhibits curling, epinasty and chlorosis.

Common Name – Chemical Family – Trade Names

  • aminocyclopyrachlor* – Pyrimidine-carboxylic acid – Not registered for use in pastures and hay fields
  • aminopyralid – Pyridine-carboxylic acid – Milestone, ForeFront R&P, ForeFront HL, GrazonNext
  • picloram – Pyridine-carboxylic acid – Tordon, Surmount, GrazonP+D
  • 2,4-D** – Phenoxyacetic acid – Various names and mixtures
  • Dicamba – Benzoic acid – Banvel, Clarity, Oracle, Rifle, Brash, Rangestar, Weedmaster

*Products containing aminocyclopyrachlor (MAT28) are registered for non-cropland use, but are not yet registered for use in pastures.
**Picloram, aminopyralid, and dicamba are often sprayed in combination with 2,4-D.

Tomato plants exposed to picloram typically exhibit symptoms relatively soon.

  • Leaf petioles begin drooping within one day after treatment. New leaves are cupped and often curled around the margins.
  • Leaf petioles then become epinastic and start to curl over themselves (Figure 1).
  • Within three days, the upper stem is twisting, and later the main stem often bends horizontally at the base (Figure 2).
  • Approximately one week after exposure, older leaves show signs of yellowing and all leaves begin to appear withered (Figure 3). The main stem is swollen and is marked with several bumps and lesions (Figure 4).
  • As the main stem continues to elongate and bend, petioles become more epinastic, even near the base (Figure 5).
  • At three weeks, cracks are apparent on the main stem and most of the plant is yellowed (Figure 6).

The following plant images were photographed over time to illustrate the development of symptoms after plants were exposed to picloram.

Exposure to Picloram Injury-Figure1
Exposure to Picloram Injury-Figure2
Exposure to Picloram Injury-Figure3
Exposure to Picloram Injury-Figure4
Exposure to Picloram Injury-Figure5
Exposure to Picloram Injury-Figure6

  • Approximately two days after treatment, older petioles are drooping; however, the drooping is not as severe as with picloram. The main stem is bending and new leaves have started to curl around themselves. Young leaflet margins are often curled underneath. (Figure 7).
  • Within three or four days, leaves have curled so they are bunched around the main stem (Figure 8).
  • Within one week the main stem is bent over and small brown lesions begin to appear (Figure 9).
  • Bumps form on the main stem as it continues to elongate and bend and the epinasty is more pronounced in younger leaves (Figure 10).
  • Within two weeks, the stem is cracked and has large brown lesions, and leaves are withered and yellow (Figure 11).
  • At lower rates of exposure to aminocyclopyrachlor, new petioles appear stringy and have underdeveloped leaflets at three weeks after exposure (Figure 12).

The following plant images were photographed over time to illustrate the development of symptoms after plants were exposed to aminocyclopyachlor.

Exposure to Aminocyclopyrachlor Injury-Figure7
Exposure to Aminocyclopyrachlor Injury-Figure8
Exposure to Aminocyclopyrachlor Injury-Figure9
Exposure to Aminocyclopyrachlor Injury-Figure10
Exposure to Aminocyclopyrachlor Injury-Figure11
Exposure to Aminocyclopyrachlor Injury-Figure12

Symptoms from exposure to aminopyralid are similar to aminocyclopyrachlor and picloram.

  • (Figure 13).
  • Approximately three days later, the petioles continue twisting and the main stem can become bent horizontally (Figure 14).
  • Later, cracks start to appear in the main stem and petioles continue to twist and bend (Figure 15). As the main stem elongates, it often has sharp curves (Figure 16).
  • Within two weeks, bumps and brown lesions appear on the stem (Figure 17).
  • At lower rates of exposure to aminopyralid, the stem and petioles are long and stringy, with small, curled leaflets (Figure 18).

The following plant images were photographed over time to illustrate the development of symptoms after plants were exposed to aminopyralid.

Exposure to Aminopyralid Injury-Figure13
Exposure to Aminopyralid Injury-Figure15
Exposure to Aminopyralid Injury-Figure16
Exposure to Aminopyralid Injury-Figure17
Exposure to Aminopyralid Injury-Figure18

Symptoms begin to appear slower with 2,4-D than with picloram or dicamba.

  • Within two days after exposure, the upper stem is bent and leaf petioles are drooping (Figure 19).
  • Within four days, all petioles are twisting and new leaves are often cupped upward (Figure 20).
  • Approximately one week after exposure, the main stem is often bent over and cracks begin to appear (Figure 21). Large, red to dark brown patches will appear on the main stem .
  • Within two weeks after exposure, bumps are apparent on the main stem (Figure 23).
  • At three weeks, new leaves have parallel venation and margins may appear toothed (Figure 24).

The following plant images were photographed over time to illustrate the development of symptoms after plants were exposed to 2,4-D.

Exposure to 2,4-D Injury-Figure19
Exposure to 2,4-D Injury-Figure20
Exposure to 2,4-D Injury-Figure21
Exposure to 2,4-D Injury-Figure22
Exposure to Dicamba Injury - Figure 25
Exposure to 2,4-D Injury-Figure24

Overall, symptoms develop quickly in plants exposed to dicamba.

  • All petioles are drooping by two days after exposure (Figure 25).
  • (Figure 26).
  • Approximately one week after treatment, leaves are yellowed and new growth is limited (Figure 27), and bumps appear along the base of the main stem (Figure 28).
  • Within two weeks, leaves are withered and brown necrotic lesions appear on the stem (Figure 29).
  • At lower rates of exposure to dicamba, new leaflets can have parallel venation and are stunted (Figure 30).

The following plant images were photographed over time to illustrate the development of symptoms after plants were exposed to dicamba.

Exposure to Dicamba Injury - Figure 25
Exposure to Dicamba Injury - Figure 26
Exposure to Dicamba Injury - Figure 27
Exposure to Dicamba Injury - Figure 28
Exposure to Dicamba Injury - Figure 29
Exposure to Dicamba Injury - Figure 30

Although diagnosing herbicide injury in the field is difficult, the following steps can be taken to determine possible causes:

  • Always record the date, time, location and description of observed symptoms.
  • Photographs of injury can help document symptom development, especially since the appearance of plants can change over a short period of time.
  • Try to rule out other causes of plant stress, such as weather, soils, insects or misapplied fertilizer.
  • Off-target movement of herbicides will cause multiple plants over a large area to exhibit similar symptoms.
  • Pay particular attention to leaf margins, new growth, and the main stem, as these areas can offer several clues for herbicide damage.
  • If herbicide injury is suspected, it can be difficult to determine if the herbicide was placed there by:
    • tank-contamination,
    • drift,
    • moved well after application due to volatility,
    • possibly placed there by manure from livestock who fed on treated forage.
  • Research is important to narrow down the source of contamination.
    • determine when symptoms first appeared
    • whether livestock were given access to the field in the off-season,
    • what the previous crop was and what herbicides were applied in the previous three seasons,
    • whether manure was used,
    • if there was an application of pesticides soon before the symptoms appeared.
  • Looking for patterns in fields can also narrow down the source of contamination.
    • Scattered patches of herbicide damage may indicate carryover in manure and urine.
    • If the majority of plants are injured, then a change in the intensity of symptoms in the field may indicate from which direction the herbicide came.
    • Vapor drift can travel several miles, though, making the direction of origin difficult to determine.
  • Herbicide residue testing is expensive, especially if the herbicide or family of herbicides is unknown. Being able to narrow the list of possible herbicides can significantly lower the cost of residue testing.
  • One important thing to remember is that picloram, aminopyralid and dicamba are often sprayed in combination with 2,4-D. Even though pasture herbicides damage tobacco in similar ways, the descriptions listed on this webpage can help to verify the source of injury