Herbicide Stewardship | Diagnosing Suspected Herbicide Damage in Tobacco

Tobacco is extremely sensitive to auxin or growth regulator herbicides. Even minute doses can seriously injure the crop. Off-target damage to tobacco often results in expensive fines and/or lawsuits, lost productivity for growers, and bad publicity for the industry. Herbicide damage can lead to poor leaf quality, reduced yield, and possible rejection by buyers. Fortunately, preventive steps can be taken to avoid these problems.

Preventing Herbicide Injury in Tobacco

(For more information, refer to Extension publication Preventing Off-target Herbicide Problems in Tobacco Fields W290-A)

• 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 tobacco 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. Before planting tobacco, research the history of the field to see if it has been treated with any persistent herbicides in the past three years.
  
  
• Movement of Cattle and Handling of Manure -- Cattle that graze on pastures treated with persistent herbicides (aminopyralid and picloram) should be monitored carefully. Aminopyralid and picloram remain intact in treated pasture grasses or hay, and when these forages are consumed by animals, the chemical passes through their digestive and urinary systems without change and into the manure and urine.
  
  
• Handling of Treated Hay -- When purchasing hay, producers should know the origin of the hay and whether it was treated with persistent pasture herbicides.
  
  
• Monitoring Results -- 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.

• Hooding — Downward folding and cupping of entire leaves.
• Curling — Folding of edge of leaf margins.
• Epinasty — Twisting, bending, and/or elongation of stems and leaf petioles.
• Chlorosis — Yellowing or whitening of leaves resulting from loss of chlorophyll.
• Necrosis — Browning of tissue resulting from cell death.

Diagnosing herbicide injury to tobacco in the field can be difficult.

Herbicides Evaluated for Symptomology in Tobacco

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

 

 

 

 

 

 

Aminocyclopyrachlor Injury Symptoms

Plants treated with aminocyclopyrachlor display similar symptoms to to picloram.

• Within three days after exposure, the youngest leaves curl downward along the leaf margin (Figure 7).
  
  
• By 10 days after exposure to aminocyclopyrachlor, those same leaves maintain their hooded shape, but new leaves formed after exposure are often cupped upwards (Figures 8 & 9).
  
  
• More that 10 days after exposure, new leaves have reduced lateral expansion and are often spade-shaped (Figure 10).
  
  
• Interveinal chlorosis is apparent in older leaves three to four weeks after treatment (Figure 11).
  
  
• Abortion of the apical meristem and development of necrotic symptoms, such as stem splitting and brown lesions, appear slower than with picloram.



• Older leaves often have prominent ridges and wavy leaf margins (Figure 12).

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

Aminopyralid Injury Symptoms

Symptoms develop slightly sooner from aminopyralid than aminocyclopyrachlor, but not as rapidly as with picloram.

• Youngest leaves are cupped downwards around three days after treatment (Figure 13).
  
  
• Similar to aminocyclopyrachlor, newly formed leaves curl upwards, while older leaves exhibit downward hooding (Figure 14).
  
  
• Approximately three weeks after treatment, the newest leaves are often heart-shaped (Figure 15).
  
  
• Bud leaves are light green in color and are clustered around the meristem (Figure 16).
  
  
• Younger leaves emerging from the stem appear thick and strap-shaped several weeks after exposure (Figure 17).
  
  
• At higher rates, the meristem is aborted and turns white, and older leaves begin to yellow approximately four weeks after treatment (Figure 18).

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

2,4-D Injury Symptoms

Symptoms begin to appear sooner with 2,4-D than with aminocyclopyrachlor, but not as soon as picloram and dicamba.

• The first symptoms to appear with 2,4-D-treated plants are hooding of middle-aged leaves and an inversion of new leaves (Figure 19).
  
  
• Approximately 10 days after treatment, plants tend to exhibit more stalk elongation and curvature, resembling a zigzag shape (Figure 20).
  
  
• At two weeks after treatment, new leaves may have serrated edges and a thin needlelike projection at the end (Fig. 21).
  
  
• At higher rates, lesions appear on a good portion of the main stem at three weeks after exposure (Figure 22).
  
  
• Within a month, older leaves are wrinkled (Figure 23) and younger leaves from the stem resemble a piece of worn leather (Figure 24).

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

Dicamba Injury Symptoms

Overall, symptoms develop quickly in dicamba-treated plants, similar to picloram.

• Initially, the youngest leaves exhibit hooding and curling from the base (Figure 25).
  
  
• Approximately 10 days after treatment, newer leaves are severely hooded and curled underneath, all the way down the petiole (Figure 26).
  
  
• Approximately two weeks after exposure, stems develop lesions near the base (Figure 27) and leaves develop interveinal chlorosis beginning from margins (Figure 28).
  
  
• Three weeks after exposure, the drooping of older leaves is very apparent.
  
  
• After three weeks, the apical meristem is aborted, bud leaves are nearly white, and necrotic lesions have formed near the top of the main stem (Figures 29 and 30).

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

Guidelines and Recommendations

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