Herbicide mode of action and sugarbeet injury symptoms

A herbicide's mode of action describes how it kills or controls a plant, from the time the herbicide is absorbed until the plant dies. Herbicides with the same mode of action move through the plant in similar ways and cause similar injury symptoms.

A herbicide's mode of action also helps determine the best time and method to apply it. For example, herbicides such as clopyralid (Stinger) and glyphosate (Roundup PowerMax) have little activity in the soil, so they must be applied after weeds have emerged and must contact the leaves to be effective.

Seedling growth inhibitors such as S-metolachlor (Dual Magnum) or cycloate (Ro-Neet SB) must be applied to the soil to effectively control newly germinating seedlings.

Herbicide selectivity is the ability of an herbicide to control some plants without harming others. Some crops can quickly break down or deactivate an herbicide, allowing them to avoid injury.

For example, corn rapidly deactivates atrazine, and soybeans can deactivate metribuzin (Sencor/Dimetric). Sugarbeet also tolerates desmedipham plus phenmedipham (Betamix) because it quickly breaks down the herbicide after it is absorbed.

Environmental stress can reduce crop tolerance

Sometimes, an herbicide that is normally safe for a crop can cause injury. This is more likely when plants are under environmental stress, which reduces their ability to limit herbicide uptake or break down the herbicide.

Common stress factors include:

• Hot or cold temperatures
• High humidity
• Hail damage

For example, sugarbeet is more likely to be injured by postemergence Betamix applications during hot, wet weather.

Applying too much herbicide can also injure a normally tolerant crop because the plant cannot break down or deactivate the excess herbicide fast enough.

Most weed seeds are small and germinate within the top 0.5 to 1 inch of soil. For the best weed control, soil-applied herbicides should remain in the top 1 to 2 inches of soil, where most weed seeds sprout.

Herbicides can be moved into the soil by:

• Tillage (mechanical incorporation)
• Rainfall
• Irrigation

Herbicide uptake

For a herbicide to work, it must come into close contact with the plant so it can be absorbed by the roots or shoots.

Roots continue to absorb herbicide as long as the growing root tips remain in treated soil. If the roots grow below the treated soil before enough herbicide is absorbed, the plant may survive.

Many soil-applied herbicides are absorbed by shoots before the plant emerges from the soil. This can injure or kill weeds before they appear above the soil surface.

Some herbicides are volatile, meaning they can move through the soil as gases or liquids and enter plant shoots. Examples include:

• Cycloate (Ro-Neet SB)
• Trifluralin (Treflan)

Other herbicides are less volatile and are absorbed only in liquid form. Examples include:

• S-metolachlor
• Dimethenamid-P (Outlook)
• Acetochlor (Warrant)

Warm temperatures, good soil moisture, and other conditions that help crops emerge quickly reduce the time seedlings are exposed to soil-applied herbicides, lowering the risk of crop injury.

Herbicide movement inside the plant

Herbicides differ in how they move after they are absorbed.

Some herbicides, such as trifluralin, do not move within the plant. They cause injury mainly where they enter the plant.

Other herbicides move to different plant parts. For example, atrazine is absorbed by the roots and carried upward with water to the leaves, where injury symptoms usually appear.

Postemergence herbicides are applied after weeds have emerged. For good weed control, the spray must reach and stick to the weed's leaves and stems. Choosing the right spray nozzle, pressure, spray volume, and travel speed helps provide good coverage while reducing spray drift.

Droplet size

Spray droplet size affects how well a herbicide covers the plant.

• Small droplets provide better leaf coverage and are more effective for contact herbicides, which only kill the plant tissue they touch.
• Large droplets are less likely to drift and can penetrate dense plant canopies more effectively.

Droplet size can be increased by:

• Lowering spray pressure
• Using a larger nozzle
• Using drift-reduction nozzles
• Adding certain spray additives (adjuvants)
• Pointing aircraft nozzles toward the rear

Factors that affect herbicide uptake

Several factors influence how much herbicide a plant absorbs, including:

• Plant size and age
• Soil moisture and plant water stress
• Air temperature
• Relative humidity
• Spray additives (adjuvants)

Adjuvants, such as crop oils, methylated seed oils, nonionic surfactants, and some liquid fertilizers, can improve herbicide uptake.

Hot, dry weather, drought-stressed weeds, and older weeds usually absorb herbicides more slowly, reducing weed control.

Fast herbicide absorption improves weed control and reduces the chance that rain or sunlight will remove or break down the herbicide before it enters the plant.

Herbicide movement within the plant

Postemergence herbicides differ in how they move after they are absorbed.

Contact herbicides do not move within the plant. They only control the plant parts they touch, so thorough spray coverage is essential.

Systemic herbicides move through the plant after absorption. For example, growth regulator herbicides such as 2,4-D and dicamba move to the growing points in the shoots and roots, where they cause the greatest injury.

Herbicide resistance is the inherited ability of a weed to survive and reproduce after being treated with a herbicide that would normally kill it.

Some plants are naturally tolerant, meaning they were never controlled by a particular herbicide. These plants are not considered herbicide-resistant because they did not develop resistance over time.

How herbicide resistance develops

Herbicide resistance develops when the same herbicide, or herbicides with the same site of action, are used repeatedly.

Within any weed population, a few plants may naturally have genes that allow them to survive a herbicide application. These resistant plants produce seed, while susceptible plants are killed. Over time, resistant plants become a larger part of the population until the herbicide no longer provides effective control.

How weeds become resistant

Weeds can become resistant in several ways, including by:

• Absorbing less herbicide.
• Moving less herbicide within the plant.
• Breaking down the herbicide more quickly.
• Changing the herbicide's target site so it no longer works.
• Storing the herbicide where it cannot cause damage.
• Producing extra amounts of the herbicide's target protein.

Herbicide resistance can result from:

• A single gene change, which often causes a high level of resistance that spreads quickly through a population.
• Multiple gene changes, which usually cause lower levels of resistance that develop more gradually and can be harder to detect.

Callus tissue – a mass of plant cells that forms at a wounded surface.

Chimera – tissue that is a mixture of two or more genetically diverse types of cells. Chimeras also may arise by a mutation in cells of a growing region. The new kind of tissue may be conspicuously different from the old (as when it is bleached instead of green).

Chloroplast – a membrane-enclosed structure that contains the green pigment molecule (chlorophyll) essential for photosynthesis (food production).

Contact herbicides – a general classification for herbicides that are unable to move within a plant. A contact herbicide’s effectiveness is highly dependent upon uniform coverage of treated soil or plant tissue.

Epinasty -a bending of plant parts (for example, stems or leaf petioles) downward due to increased growth on the upper side of an affected plant part; often associated with the plant growth regulator herbicides.

Herbicide mode of action – the sequence of events from absorption of the herbicide into the plant through plant death; refers to all plant-herbicide interactions.

Herbicide site of action – the primary biochemical site that is affected by the herbicide, ultimately resulting in the death of the plant; also referred to as herbicide mechanism of action.

Necrosis – the death of specific plant tissue while the rest of the plant is still alive. Necrotic areas generally are dark brown.

Phloem – plant tissue that functions as a conduit for the movement (translocation) of sugars and other plant nutrients.

Postemergence application – a time of herbicide application occurring after the crop and weeds emerge from the soil; also referred to as a foliar application.

Preemergence application – a time of herbicide application occurring after the crop is planted but before the crop or weeds emerge from the soil.

Preplanting application – a time of herbicide application occurring before the crop is planted; often followed by an incorporation (mechanical mixing) into the top 1 to 2 inches of soil; often referred to as preplant incorporation treatment.

Systemic herbicide – a general classification for herbicides that are able to move away from the site of absorption to other parts of the plant.

Translocation – the movement of water, plant sugars and nutrients, herbicide and other soluble materials from one plant part to another.

Translucent – an absence of leaf tissue pigments that results in the diffusion of light, making the plant appear off-white.

Xylem – plant tissue that functions as a conduit for the upward movement (translocation) of water from the roots to above-ground plant parts

Sugarbeet is sensitive to temperatures of 28 degrees F or less until true leaves have developed. Plants develop a water-soaked appearance as they thaw. Frosted tissues later turn brown and desiccate. Frost injury is erratic, and a plant may be killed next to another plant that appears uninjured.

Evidence indicates nurse crops planted with sugarbeet may provide some protection against frost. Sugarbeet canopies serve as a short-term insulating barrier to help minimize freeze damage to roots in the fall.

Close contact between insecticide and sugarbeet root can blacken or constrict root growth. Insect damage, including stand loss, can mimic stand loss caused by herbicides such as amino acid synthesis inhibitors or seedling growth inhibitors.

Seepage of blackened exudate from Lygus bug feeding on the petiole also mimics damage caused by an amino acid synthesis inhibitor. Yellowing and discoloration of older leaves and leaf tips mimic photosynthesis inhibitors.

Saturated soil can cause sugarbeet to become bright yellow with leaves that are more erect than normal.

Root rots may occur due to excessive wet conditions and the lack of oxygen movement into root tips when soils are saturated for several days, especially when soil temperatures are high.

Root rots and the odor of fermentation can be confused with the effect of other root-rotting pathogens such as Rhizoctonia solani, Aphanoymces cochlioides, or Pythium spp.

Water damage can cause sugarbeet to become more susceptible to postemergence herbicides.

Water stress plus herbicide causes more sugarbeet injury than water stress or herbicide alone.

Excessive water causes fangy roots at harvest. Fangy roots indirectly increase tare due to the amount of soil lodged between roots.

Water stress causes plant leaves to wilt, especially during afternoon hours when temperatures are high. Leaves in contact with hot soil surfaces can become scorched and eventually dry.

Water stress is relieved, and leaves return to an upright position following precipitation or cooler overnight temperatures.

Leaf scorch may be confused with foliar diseases, including bacterial leaf spot. Leaf wilting may also be a symptom associated with root pathogens such as Aphanomyces or rhizomania. These problems can be distinguished by making evaluations during morning hours.

Hailstorms may occur at any time during the growing season. Hail reduces tonnage and sugar quality, but it depends on timing and intensity.

In general, damage to foliage later in the season has a greater impact on tonnage than damage earlier in the season.

The intensity of defoliation also impacts sugar quality.

The greatest potential for damage from wind occurs in the early stages of growth. Damage is often associated with soil particles blown across the soil surface.

Portions of the root system may be exposed as soil is removed, or small plants may be buried by soil deposits in extreme cases.

Many diseases and insects affect sugarbeet. The Compendium of Beet Diseases and Pests, Second Edition, published by the American Phytopathological Society, provides extensive descriptions and pictures of disease symptoms and insect damage as well as nutritional disorders, drought, hail, lightning, crusting, salt injury and others.