Injury by Application

Most herbicide injury is first observed as a general stunting of the canopy and/or distorted growth of the leaves and stems. Potato plants can be injured even by herbicide to which they are tolerant if they are under stress such as from temperature extremes or damaged by hail. Stress decreases the plant's ability to reduce uptake or deactivate the herbicide.

Applied Glyphosate
Applied Metolaclor
Applied Dinotroanilines
Applied Metribuzin & Linuron
Applied Rimsulfuron
Applied EPTC
Applied Sethoxydim

GLYPHOSATE (ROUNDUP)

Mode of action: amino acid synthesis inhibition
Specific mode: inhibits EPSP synthetase (stops production of phenylalanine, tryptophan and tyrosine)
Chemical family: amino acid derivative
Uptake: foliar only
Movement: systemic throughout plant
Common application: pre-plant
Weed control: non-selective, "TVC" total vegetation control
Weed stage: emerged

Injury by Glyphosate Potato injury: Growing cells (meristems) are affected and plant growth stops due to lack of protein. Injury appears in less than a week after exposure and starts with new, young leaves. Symptoms start with leaf yellowing (chlorosis) and progresses to browning (necrosis) until plants wilt and die. The process takes 10 to 14 days from exposure. There is no recovery. Sub-lethal doses cause parallel veining on leaves and an over-production of vegetative buds (similar to symptoms caused by phenoxy compounds such as 2,4D).

METOLACLOR (DUAL)

Mode of action: seedling growth inhibition
Specific mode: shoot inhibitor
Chemical family: chloroacetamide (acetanilide)
Uptake: stem primarily and root
Movement: systemic throughout plant
Common application: pre-emergence
Weed control: annual grasses and some annual broadleaves [For examples, grasses include foxtails, sandbur, crabgrass, fall panicum, and broadleaves include Eastern black nightshade and pigweeds, and partial control of hairy nightshade.]
Weed stage: pre-emerged (germinating seed)
Injury by Metolaclor -- draw string Injury by Metolaclor -- small leaves

Injjry by Metolaclor Potato injury: Protein synthesis at the shoot and root tips (meristem cells) is inhibited resulting in poor growth. The first symptom of severe exposure is no emergence of potato sprouts due to stunting of underground shoot. If severely affected plants emerge, the stem will not recover and new stem growth must occur from below ground. With mild exposure, affected leaves have a shortened mid-vein resulting in a "heart-shaped" appearance from the "draw-string" effect. Leaf "crinkling" is usually associated with the draw-string effect. Leaves are often very small and may be downwardly cupped before forming the heart shape. Injury results from post-emergence application or application at cracking followed by rain or irrigation which pushes Dual down the soil crack to the sprout.

DINITROANILINES: PENDIMETHALIN (PROWL) and TRIFLURALIN (TREFLAN)

Mode of action: seedling growth inhibition
Specific mode: inhibits root growth
Chemical family: dinitroaniline
Uptake: root primarily and shoot
Movement: remain at site of absorption
Common application: pre-emergence
Weed control: annual grasses [For examples, grasses include foxtails, sandbur, crabgrass and fall panicum, and broadleaves include kochia, lambsquarter and pigweed.]
Weed stage: pre-emerged (germinating seed)

From left: Leaf injury by Pendimethalin, root injury by Pendimethalin, severe injury by Pendimethalin, root injury by Trifluralin

Potato injury: Dinitroanilines inhibit tubule formation thereby stop cell division at the root tips. With no cell division, cells enlarge. Lateral root development is halted causing short, stubby roots with enlarged tips. Root hairs don't form along affected portions of root. Note that root must grow into soil area containing the dinitroaniline. Short thick underground shoots may be seen if they were in direct contact with herbicide. Plants become stunted. Injury appears within a week of exposure. Injury may result from an early post-emergence application. Mildly injured plants show dark-green areas on leaves with some crinkling. Mature plants with leaf symptoms often have brittle stems which can be snapped by wind. Seldom is injury severe enough to cause death since un-affected roots can make up for loss. Some varieties seem more prone to injury than others.

METRIBUZIN (LEXONE and SENCOR), LINURON (LOROX)

Mode of action: Photosynthesis inhibition
Specific mode: C1 class (metribuzin), C2 class (linuron)
Chemical family: triazinone (metribuzin), phenylurea (linuron)
Uptake: root and foliar
Movement: move from roots to vine, no movement from leaves
Common application: pre-emergence
Weed Control: annual grasses, annual broadleaves, perennials
[for Nebraska, grasses as foxtails, crabgrass and fall panicum, and broadleaves as kochia, lambsquarter, pigweed, common ragweed and wild mustard, and thistles]
Weed Stage: pre-emerged and early post-emerged broadleaves

Injjry by Metribuzin Potato Injury: Photosynthesis inhibitors result in the production of free radicals which disrupt cell membranes. Because of compounds upwardly movement in plant's xylem, symptoms appear in the leaves. These compounds do not prevent emergence but become effective when the plants are exposed to sunlight, form leaves and begin photosynthesis. Initial symptoms are a yellowing (chlorosis) of leaf margins and tips especially of older leaves. Yellowing first occurs between the veins and moves inward to the mid-vein. As injury progresses, leaves turn brown (necrotic) and die. Younger leaves are more affected as they enlarge. Plant death is not common but loss of yield and quality is. Symptoms are more pronounced in soils with pHs above 7.2 (alkaline). Varieties vary considerably in their sensitivity to injury. As a general rule with many exceptions, white and red-skinned varieties are more sensitive than russet-skinned ones. Russet Norkotah is one of the least sensitive and metribuzin has been successfully used early post-emergence on this variety.

RIMSULFURON (MATRIX)

Mode of Action: amino acid synthesis inhibition
Specific Mode: inhibits ALS-AHAS enzyme
(stops production of isoleucine, leucine and valine)
Chemical Family: sulfonylurea
Uptake: foliar and root
Movement: systemic throughout plant
Common Application: early post-emergence
Weed Control: grasses and broadleaves (Major use is control of triazine-resistant weeds as pigweeds.) [For examples, grasses include foxtails, volunteer cereals and corn, crabgrass and fall panicum, and broadleaves include kochia, mustards, pigweeds, and partial control of hairy nightshade, common ragweed and common lambsquarter.)
Weed Stage: pre-emerged and early post-emerged

Injury by Rimsulfuron Potato Injury: Growing cells (meristems) slowly stop growing as amino acid reservoir depletes and proteins aren't synthesized. Symptoms are similar to other sulfonylureas and to imidazolinones. Plants may become stunted, and both leaves and stems may turn yellow (chlorosis) and/or purple. Severe injury may result in death of terminal growing points. Growth stops shortly after application but discolorations appear gradually, one to two weeks.

EPTC (EPTAM/GENEP)

Mode of Action: seedling growth inhibition
Specific Mode: lipid synthesis (non-ACCase) inhibitor
Chemical Family: thiocarbamate
Uptake: stem and root
Movement: systemic throughout plant
Common Application: post-emergence and pre-emergence
Weed Control: grasses and broadleaves
[For example, grasses include foxtails, sandbur, crabgrass, fall panicum, cereals and broadleaves include black, hairy and cutleaf nightshades, pigweeds, and common lambsquarter.]
Weed Stage: pre-emerged and newly emerged

Potato Injury by EPTC Potato Injury: Method of inhibiting lipid synthesis is not well understood. Compound causes crinkling or puckering of leaves which also appear thick and dark-green. Leaf buds may not open. Emergence is delayed and plants may be stunted. Symptoms appear one to several weeks after exposure and are similar to those by dinitroanilines except for stem brittleness. Sensitive varieties are uncommon.

SETHOXYDIM (POAST)

Mode of Action: lipid synthesis inhibition
Specific Mode: ACCase inhibition
Chemical Family: cyclohexanediones
Uptake: foliar only
Movement: throughout plant
Common Application: post-emergence
Weed Control: nearly all annual and perennial grasses
Weed Stage: post-emerged

Potato Injury: None, but may get some burn of leaf tips ("fire blight") due to the crop oil additive when applied on a warm day.