SMALL ARROWS SHOW HERBICIDEMOBILE IN SYSTEM FROM SITE OF

UPTAKE

LARGE ARROWS INDICATEPLACEMENT OF HERBICIDE

DOTS INDICATE A SITE OFUPTAKE WITHOUT HERBICIDE

TRANSLOCATION

Herbicide knowledge is a precious com-

modity when you farm in the environmental age. It’sno longer enough to simply apply herbicides to yourfields; you need to know how they work.

This publication provides detailed explanationsand illustrations on how various herbicides kill or pre-vent weeds from growing. Pictures of crop injuryare included as a means of showing the effects ofherbicidal activity on plants in general and to help

The following diagrams will be usedthroughout this pamphlet to explain theplacement and movement of herbicides.

A Short Course On How HerbicidesKill Weeds and Injure Crops

HerbicidesWork

By David R. Pike,Aaron Hager, andMarshal McGlamery

you diagnose problems from misapplication, drift, orcarryover. The inclusion of trade names does not con-stitute an endorsement of any product or manufacturer.

We urge you to take this guide with you to thefield when evaluating herbicide performance and look-ing for signs of crop injury. If you need additional help,your local extension adviser or chemical company rep-resentative may be able to answer further questions.

HowUniversity of Illinois ExtensionPesticide Impact Assessment Program 1998-05Department of Crop Sciences

Soil Applied HerbicidesCell mitosis inhibitors

Examples - dinitroanilines: trifluralin (Treflan, others), pendimethalin

(Prowl, Pentagon), ethalfluralin (Sonalan)

Symptoms of injury:• Root development is inhibited (rootpruning) which leads to plant stunting andswollen root ends.• Stems and leaves turn purple due tophosphorus deficiency.• A high concentration of herbicide nearthe soil surface may induce the formationof callus tissue on soybean stems, lead-ing to stem breakage or lodging late inthe growing season.

Comments: Carryover of pendimethalin or ethalfluralin is rare, while triflu-

ralin carryover is more likely to injure corn, especially early plantedhybrids which are slow to emerge. Dissipation of these herbicides isenhanced by anaerobic conditions in the soil. Drift injury is uncom-mon. A surface application of pendimethalin may induce callus for-mation on soybean stems. Due to volatility and low water solubility,these herbicides (except pendimethalin) are usually incorporated intothe soil.

Soil applied herbicides reach theroots but are not translocated.Pruned roots

Swollen hypocotyl

Purple corn

Mode of Action: Blocks cell division.

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Corky-stem soybeans

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Soil applied acetamide herbicides moveto the roots and shoot but are nottranslocated.

Leaves failing to unfurl

Examples - acetamides: alachlor (Lasso, Micro-Tech), metolachlor (DualII), acetochlor (Surpass, TopNotch, Harness), dimethenamid (Frontier), ,fluthiamide + metribuzin (Axiom), thiocarbamates: EPTC + acetochlor(DoublePlay), EPTC (Eradicane), butylate (Sutan +)

Mode of Action: Affects cell growth and division, though thespecific sites of action are unknown.

Symptoms of injury:• Stunted plant and leaf crinkling.• Corn seedlings may leaf out underground, making emergence dif-

ficult.• Grass seedling leaves may not unfurl correctly.• "Drawstring" effect on soybeans (ends puckered).

Comments:Carryover injury is not likely, nor is injury from tank

contamination. Direct injury to the crop can occur when thecrop is under additional stress or under conditions favorablefor rapid herbicide uptake from the soil (i.e., cool, wet spellfollowed by warm, windy weather).

Shoot inhibitors

Acetamide injury-drawstring effect

Pigment inhibitors

Examples: clomazone (Command), norflurazon (Zorial), isoxaflutole[experimental]

Mode of Action: Inhibitsproduction of carotenoids andprevents chlorophyll production.

Symptoms of injury:• Affected plant tissue is white orvery light green in color; new andolder leaves may be affected, de-pending on when uptake occurs.

Comments:Carryover of clomazone to susceptible crops is possible where

spray overlap or misapplication has occurred. Volatility injury is alsopossible if certain formulations of clomazone are not incorporated intothe soil immediately after application.

Herbicide, when applied to soil,moves from the roots to newest plantgrowth.

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Pigment inhibition

Isoxaflutole injury

Clomazone injury to ornamental plantsvia vapor drift

Clomazone injury

Photosynthetic inhibitors(mobile within the plant)

Examples: atrazine, cyanazine, simazine (Princep), metribuzin

(Sencor, Lexone)

Mode of Action: Interferes with photosynthesis by blockingelectron transfer, resulting in damaged plant membranes and celldeath.

Symptoms of injury:• Herbicides applied to the soil penetrate the root and translocate

throughout the plant.• Oldest leaves turn chlorotic (yellow) first, with veins remaining

green longest.• Plant becomes stunted and may die if enough leaf tissue is af-

fected.

Herbicide, when applied to soil, movesupwards, with effects seen on oldestleaves first.

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Triazine carryover injury to wheat

Triazine carryover injury to soybean leaf

Comments: Carryover injury to soybeans is common,

particularly in soils with a pH above 7.0. Greatvariation exists among soybean varieties regardingsensitivity to direct applications of metribuzin, aswell as atrazine carryover injury. Cyanazine appli-cations may result in corn injury when applied tocorn growing on sandy soils.

Cyanazine injury

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When applied to soil, herbicidemoves to newest stem, leaf androot growth regions.

Examples: 2,4-D, 2,4-DB, dicamba (Banvel, Clarity),picloram (Tordon), triclopyr (Garlon), triclopyr + 2,4-D(Crossbow), clopyralid (Stinger), MCPA

Mode of Action: Affects growth in the neweststems and leaves by affecting protein synthesis andnormal cell division.

Soil and Foliar Applied HerbicidesHormone (auxin)-type herbicides

Elbowed corn

Fasciated brace roots

Comments:Drift, tank contamination, and direct injury are

common with most hormone-type herbicides. Someof these products are quite volatile and may move fromthe application site many hours after actual spraying.Carryover injury is possible with picloram but un-likely with the others listed, except in extreme cases ofmisapplication.

Leaning corn

When applied to leaves,herbicide moves to newestgrowth, and movesdownward to the roots.

Symptoms of injury:• Stunted, malformed seedlings.• On older plants, new growth is twisted or malformed.• Calluses may form on the stem, and plant may lean

and become brittle.• Grass plants exhibit leaf rolling (“onion leafing”).

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When applied to soil, herbicideworks throughout roots andmoves to newest portion of stem.

Darkened veins on underside ofsoybean leaf

Imazaquin carryover

ALS enzyme inhibitors

Examples: imazethapyr (Pursuit), chlorimuron (Classic,Skirmish), imazaquin (Scepter), nicosulfuron (Accent),thifensulfuron (Pinnacle), primisulfuron (Beacon),chlorsulfuron (Glean), flumetsulam (Python), cloransulam(FirstRate), halosulfuron (Permit), imazamox (Raptor),primisulfuron + prosulfuron (Spirit), rimsulfuron +thifensulfuron (Basis)

Mode of Action: Blocks normal function of theacetolactate synthase (ALS) enzyme, inhibiting plantmetabolism.

When applied to leaves,herbicide movesthroughout the plant.

Comments:Carryover injury is possible and depends on

soil pH and the environmental conditions governingrate of breakdown in the soil. Overlaps and misap-plication can also cause carryover injury. Some cornhybrids and soybean varieties are more sensitive thanothers and may show stunting from direct applica-tions. Drift injury and tank contamination is possible.

Chlorotic soybean leaves

Shortened internodes on soybeans

Symptoms of injury:• When applied to the soil, roots often exhibit bottle

brush growth (unbranched, stubby), resulting inpurple stems and leaves, along with overall plantstunting.

• A soil or foliar application may stunt or slow plantgrowth.

• Leaf tissue becomes light yellow from the outer edgeof the leaf toward the veins, while veins turn lightyellow to dark purple.

When applied to leaves, theherbicide moves down the plantto the growing point of the leavesand to roots.

Foliar Applied HerbicidesACC-ase inhibitors

Examples: sethoxydim (Poast, Prestige), fluazifop (Fusilade DX), quizalofop(Assure II, Matador), fenoxaprop (Option), diclofop (Hoelon), fluazifop +fenoxaprop (Fusion), clethodim (Select)

Mode of Action: Blocks formation of lipids in the shoot (meristem)and roots of grass plants.

Symptoms of injury:• Symptoms are slow to develop.• Plants become stunted when newplant growth is stopped; the grow-ing point of the plant disintegratesand dies first.

Comments:Carryover is not likely at normal application rates. Tank con-

tamination and drift injury can affect sensitive species. Direct injuryto a crop is most likely to be caused by oils and adjuvants used incombination with these herbicides.

Necrotic meristem

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Meristem necrosis on a grass plant

Membrane disrupters

Examples: acifluorfen (Blazer, Status), lactofen (Cobra),fomesafen (Reflex, Flexstar) flumiclorac-pentyl (Resource),sulfentrazone (Authority), carfentrazone (Aim),[experimental](Action ).

Mode of Action: Inhibition of chlorophyll biosynthesisand disruption of cell membranes.

The herbicide, when applied to theleaves, has little translocation.When applied to roots has limitedtranslocation.

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Close-up of leaf contact injury

Sulfentrazone injury to soybeans(chlorotic/necrotic spots on leaf edges)

Comments:Carryover injury is not likely, but drift in-

jury can occur from applications made under windyconditions. Direct injury can occasionally be se-rious if misapplication takes place or the crop isunder stress.

Carfentrazone injury

Sulfentrazone injury

Symptoms of injury:• Water spots develop where herbicide contacts leaf

tissue.• Spots lose color and tissue dies; if sufficient plant

tissue is affected, the plant wilts and dies.

When applied to the leaves, herbicidemoves very little from point of placement.

Photosynthetic inhibitors(nonmobile within plant)

Examples: bentazon (Basagran), bromoxynil (Buctril), paraquat(Gramoxone Extra), pyridate (Tough)

Mode of Action: Blocks the photosynthetic reaction, and disruptscell membranes so that captured light cannot be converted to chemicalenergy.

Symptoms of injury:• Foliar applied herbicides penetrate leaves, then move a little within

the leaf so that the tissue surrounding spray droplets loses colorand dies.

• Plant becomes stunted and may die if sufficient leaf tissue is af-fected.

Necrotic leaf burn

Bromoxynil injury

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Comments:Carryover is unlikely and drift injury is limited to

spots resulting from droplets from nearby applications.Crops show good tolerance to these herbicides (exceptparaquat), but injury may be increased by the additionof spray additives.

Glyphosate Amino Acid InhibitionExamples: glyphosate-IPA(Roundup, Roundup Ultra),glyphosate-TMS (Touchdown)

Mode of Action: inhibits aminoacid synthesis and results incessation of protein development.

Symptoms of Injury:• Overall plant coloration turns lightgreen then relapses to yellow.• Plant dies slowly within 7 to 14days of application.

Comments:Carryover injury does not occur. Drift injury from small spray droplets

is very likely under moderate to windy conditions. Vapor drift injury is not typicallya problem.

Applied to leaves, herbicidemoves throughout the plant.

Examples: glufosinate (Liberty)

Mode of Action: Inhibits amino acid synthesis with a resultant buildup of ammonia in cells, poisoning thephotosynthetic system.

Symptoms of Injury:• Similar to contact action herbicides.

Glyphosate injury on corn

Glyphosate injury on corn

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Glyphosate injury on soybeans

Glufosinate Amino Acid Inhibition

Comments:Carryover injury is very unlikely. Vapor drift injury is also unlikely. Droplet drift injury is likely. Limited

translocation occurs primarily in the dark.

Produced by the University of Illinois with funding provided by the National Agricultural Pesticide ImpactAssessment Program•Photos in this publication include contributions from Dr. William Curran at Pennsylvania

State University and staff at the University of Illinois and Purdue University•Editing and layout byCamille McConkie•To obtain more information contact David Pike•U of I, Cooperative Extension•e-mail

d-pike@uiuc.eduThe Illinois Cooperative Extension Service provides equal opportunities in programs and employment.

The use of trade names in this report does not constitute an endorsement of any product or manufacturer.