By Jim Kamas (April 2017)
What are Synthetic Auxin Herbicides
Synthetic auxin herbicides are a broad group of related compounds originally designed to eliminate broadleaf weeds in monocotyledonous crops such as wheat, rice and other cereal grains. They are also commonly used to manage broadleaf weeds in grassy pasture lands and lawns, for pre-plant burn down applications and as brush killers along fence lines and industrial right of ways. More recently crops such as cotton, corn and soybeans have been genetically engineered to be resistant to auxin-like herbicides which has resulted in increased risk to growers of sensitive crops in areas where these new resistant crops are being grown.
In general, most dicotyledonous (dicot) plants from ornamentals, to garden crops and broadleaf weeds are susceptible to injury and damage when exposed to these compounds. Grapevines are however several orders of magnitude more sensitive to injury from these products than other crops considered to be very sensitive such as tomatoes. In Washington state, drift from grain growing regions to the east has been documented to have caused damage to vineyards along the Columbia River, 50 miles away from the point of application. While this injury resulted from highly volatile formulations of 2,4- D, it accentuates how sensitive grapevines are to these products in even very low rates of exposure.
There are other groups of auxin-like herbicides that can also cause damage similar to 2,4-D but the symptoms and physical characteristics of these compounds vary. It may be difficult to tell which exact herbicide is the most likely culprit and obtaining the assistance of an experi- enced consultant/ investigator may be required if it is not possible to get accurate information on what herbicides were used in the vicinity of your vineyard. The first group known as the Phenoxycarboxylic acids includes 2,4-dichlorophenoxyacetic acid (2,4-D), mecoprop and MCPA. 2,4-D is the member of this family most commonly encountered as a risk to grape growers because of the immediate and severe effects on vines and due to its ability to volatilize and drift as a gas. These products are used in pasturelands and cereal crops and pose a threat to vineyards throughout the state and the region. Under conditions favoring temperature inversions, volatile vapor can rise several feet, encounter an inversion layer, then travel at a mile or more where vapor exposure to sensitive plants can cause injury. The ester formulations are the most volatile, however while less volatile, amine and salt formulations can still be problematic. Newer “low volatility formulations” of 2,4-D have reportedly been developed for use with the new GM crops, but actual “safety” for surrounding sensitive crops is still not fully documented.
Benzoic acids like dicamba are also commonly encountered by vineyards near pasturelands. Dicamba, sold under the trade names Banvel®, Marksman®, Celebrity®, and many others, is labeled for grain crops and grasslands. It is commonly used to control thistles and other weeds not readily managed by other phenoxy herbicides. Dicamba is commonly package-mixed with 2,4-D for broader spectrum weed control. While not as volatile as 2,4-D, dicamba injury on grapevines can be equally severe and long lasting. It is now being marketed as part of the RoundUp Ready Xtend crop system for cotton and soybeans as Xtendimax® and in combination with glyphosate as RoundUp Xtend® (pending EPA approval) with VaporGrip® technology.
A third important family of auxin herbicides are the pyradine carboxylic acids. Compounds within this group include picloram and triclopyr which are used for brush control on right of ways or CRP land, or as additions to 2,4-D for enhanced broadleaf control in pastures. Products include Grazon®, Crossbow® and Garlon®; and while not known for volatile movement, much like dicamba, even very limited exposure to these chemicals can cause extensive injury to grapevines.
How Injury Occurs
While injury caused by these hormone-like herbicides is the same in all broadleaf plants, the extent of injury or damage varies considerably from family to family with grapevines being among the very most sensitive. Immediately after exposure, grapevines begin showing toxic effects on new growth. The plant response is referred to as epinasty. Epinastic growth is the result of the upper portion of the tissue growing at a faster rate than the lower portion, resulting in a downward curving or “cupping” of the tissue. In grapes, it is characterized as a twisting, curling and cupping of foliage that has not fully expanded. The primary response these herbicides induce is unregulated cell expansion. To further exacerbate the problem, these herbicides are readily translocated to the phloem which is responsible for the transport of sugars and other products of photosynthesis. This unregulated cell expansion in phloem tissue causes a collapse of the nutrient transport system within grapevines. A few days after exposure, shoot tips may abort, plants can develop leaf chlorosis and many shoots can begin to die back. New growth may exhibit severe shoot and petiole twisting, leaf cupping, stunting, curling, roughness, crinkling of the leaf surface, vein discoloration, and fingering of the leaf margins. Because photosynthate transport is shut down in portions of the plant, grapevines exposed to hormone-like herbicides cannot adequately ripen fruit, cannot move carbohydrate to storage vessels, and are consequently severely predisposed to winter injury. High exposure to these herbicides has been known to express itself for at least three seasons after the exposure event. Arguably, if vines are still expressing exposure symptoms, the fruit remains unusable because of the possible contamination of fruit with these herbicides. Because none of these herbicides are labeled for use in vineyards, EPA does not have food tolerances for these herbicides in grapes. Few wineries will take the risk of processing fruit from vineyards showing herbicide damage. Therefore, even reduced crops may be considered as complete crop losses due to contamination of the fruit.
Classic dicamba injury results in a halo of extreme chlorosis on severly cupped leaves
While it is the case that from a physiological perspective all synthetic auxin herbicides likely work in a similar way to kill plants, the exact means by which they kill is not well understood, and likely varies somewhat from species to species. There is some variability in the extent to which various auxin herbicides effect the different chemical/hormonal pathways within the plant. This is why while symptoms from injury by these various herbicides may vary somewhat, ultimately they result in broadly the same outcome. This does not mean however that understanding these symptomatic differences holds no importance. Being able to recognize the difference in visual symptoms of various synthetic auxin herbicides is a valuable tool in knowing which herbicide is responsible for an observed injury. If you need to have your vines tested for exposure to a synthetic auxin, knowing which family(ies) are the most likely culprit(s) will help in choosing a laboratory for testing and in choosing which tests to order. UC Davis has an extensive online gallery of herbicide injury.