Welcome to the National Alfalfa & Forage Alliance · Welcome to the National Alfalfa & Forage Alliance

Dan UndersanderFred A. GrayKeith KellingMarlin E. Rice

Third edition

Alfalfa Analyst

alfalfa analyst

Alfalfa is a vigorous and productive

crop. Like all crops, however, alfalfa

is subject to stand and yield loss from

disease, insect injury, nutrient deficien-

cies, and other environmental stresses.

Prompt and accurate diagnosis of a

problem can allow early treatment to

modify or correct the situation before

yields are seriously affected or stands

are lost.

The purpose of the Alfalfa Analyst is to

provide an identification guide to some

of the more important alfalfa yield and

stand limiters. Please use it to help

identify any alfalfa problems you may

have. Then secure specific up-to-date

management recommendations from

your local Extension service.

Aphanomyces root rot Aphanomyces stunts and kills seedlingsand causes a chronic root disease in estab-lished plants. On seedlings, roots andhypocotyls are first gray and water-soakedthen become brown while remaining rigid.Cotyledons and foliage appear yellowishgreen. On established plants, aphanomycesreduces root mass and nodules may beabsent. Lateral roots may have a browndecay and lesions with mucus. Overallstunting and yellowing is common.Symptoms are similar to nitrogen defi-ciency. Infected plants are often slow torecover following harvest or winterdormancy. Aphanomyces is an importantdisease of wet soils. There are two majorraces of this disease. Plants with symptomsthat have resistance are likely resistant torace 1 but not race 2.

Anthracnose Diamond-shaped tan lesions with darkborders appear near the base of the stems.Centers of the lesions contain small blackbodies that produce spores.The fungusmay girdle and kill stems, crown buds, andeventually the crown. Crown rot due toanthracnose is characteristically bluish-black (gunmetal).The “shepherd’s hook”(stem top curved over) is often observed inyoung, dead shoots, especially on firstgrowth of the year. Dead, straw-colored,erect stems scattered throughout the fieldmay indicate anthracnose infection.Thedisease is favored by hot, most weather.Diseased plants are susceptible to winterkill.

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severe moderate mild


Bacterial wilt Early symptoms appear in the taproot with ayellowish-brown discoloration just belowthe surface. In cross sections of the root, dis-coloration initially rings the outer edge; itspreads inward as the disease progresses. Inthe field, scattered plants become stunted,yellow-green in color, with cupped leaves.The bacteria are in the soil and usually enterthrough wounds. Bacterial wilt typicallyappears in the second or third year.

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Brown root rot Dead or stunted plants occur throughoutalfalfa stands of 2- to 3-year-old fields. Dugtaproots show various stages of rot. Rootinfection occurs during dormancy in latefall and early spring. Early root symptomsconsist of circular and slightly sunkenlesions with black borders. Lesions spreadand coalesce, girdling and rotting off theroot. Small black fungal structures(pycnidia) are embedded in the center ofdead tissue in the spring. Some plantdeaths blamed on winterkill may be due tobrown root rot. Seedlings are not attackedby brown root rot. This disease has beenconfirmed in Idaho, Minnesota, Montana,New York, Wisconsin and Wyoming. It isexpected to affect the entire northern tierof states extending southward in thehigher elevation mountain regions.

Crown rot Crown rot may be caused by a complex ofas many as four types of fungi (Rhizoctonia,Stagnospora, Colletotrichum, and/orFusarium species). Symptoms often beginas a small cone-shaped discolorationbelow the base of a cut stem. The rottedarea enlarges and may merge with rotfrom other infection sites until the centralportion of the crown is destroyed, oftenleaving a rim of living crown tissue andstems. Some plants may partially recoverand be productive for several years, whileothers progressively worsen until theentire crown is destroyed and the plantdies. Plant death is often blamed on win-terkill. The crown and buds usually rotduring the second and third year, withsymptoms becoming progressively worseover time. Crown rot is usually more severewhere machine traffic is heavy or in fieldsthat are heavily grazed. Parasitic stemnematodes create entry points on theroots for the fungi that cause crown rot.

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Downy mildew Light green to yellow blotches appear onupper leaves. Shoot tips are often dwarfedand the leaves twisted or rolled. A grayishcottony growth, the mycelium and fruitingstructures of the fungus, is often visible onthe underside of the leaflets. Plants do notlose their leaves. New spring seedings maybe severely injured or destroyed. Cool,moist weather and sprinkler irrigationfavor disease development.


Fusarium wilt This disease may cause stems on only oneside of the plant to wilt and die or it mayaffect the entire plant. Stems and leavesappear bleached. Diseased plants are scat-tered throughout the field. In a crosssection of the taproot, brown to brick redstreaks initially discolor parts of the outerring. As the disease progresses, the discol-oration rings the root and the plant dies.The fungus lives in the soil and entersthrough wounds or fine roots. Fusariumwilt is more severe in association withroot-knot nematodes.

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Leaf spot, common Small, circular chocolate brown spotsappear on leaves. Eventually, smaller raiseddiscs, usually lighter in color, erupt throughthe leaf surface in the center of the spots.Infected leaves turn yellow and drop off.Severe defoliation may occur. This diseaseoccurs in most alfalfa stands and is favoredby warm, moist conditions.

Leaf spot, leptoSmall black spots on the leaflets are sur-rounded by a halo. The spots enlarge andacquire a tan center with an irregularbrown border. As the disease progresses,lesions often grow together. Infectedleaves die and cling to the stem for a time.Only young leaves become infected; thegreatest damage is seen on new growthwhen moist weather follows clipping. Inolder growth, only the young upper leavesbecome infected. Disease is worse in cool,wet summers. Resistant cultivars are notavailable.

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Phytophthora root rot Seedlings grown in cool, wet soils may bekilled just before or after emergence. Thisfungal root disease is favored by cool con-ditions and wet soils. Seedlings thatsurvive the initial infection have rottedroots and yellowish-red leaves; they typi-cally die within several months. On matureplants, the disease causes lesions ontaproots. In severe cases the roots rot offcompletely. Shoots are stunted but other-wise appear normal until the plant col-lapses and dies or is killed over winter.Infected, unproductive plants may survivefor several years before dying.


Rhizoctonia stem blight,web blight, and root canker Stem blight girdles stems at their base, even-tually killing them. Dead, straw-colored, erectstems may resemble symptoms caused byanthracnose. However, the characteristicshepherd’s hook is absent.Web blight, char-acterized by dead leaves sticking to eachother and to stems by strands of the fungus.This disease occurs in warm, humid areas ofthe Unites States, particularly after extendedrainy periods. Root canker is common in thesummer in the irrigated valleys of Arizonaand California.These large cankers are oftendarker at the margins and turn black withage.The disease is most severe under hightemperatures and moist conditions.

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Root-lesion nematodePlants appear unhealthy and stunted,usually in spotty areas within an otherwisehealthy stand. The parasitic nematodes aremicroscopic worms that feed on root hairs,feeder roots, and nitrogen-fixing nodulesof alfalfa. They reduce the plant’s ability totake up soil nutrients and fix nitrogen.Root-lesion nematodes reduce yield andthin stands. Nematode populations can bereduced by rotating to row crops or fallow-ing for 2 months following incorporationof forage crop residue. Moderate resistanceis available in some varieties.

Summer black stem Large, usually circular, light gray to blackspots usually appear on lower leavesduring the summer and early fall. Youngspots on the leaves are often surroundedby a halo. The disease causes leaf drop,starting with the lower leaves and pro-gressing up the stem. Reddish brown tochocolate brown oval lesions form on thestem and merge to discolor most of it. Thedisease is favored by warm, moist weather.Symptoms are worse on second and thirdcuttings.

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Spring black stem and leaf spot Dark spots with irregular borders appearon the older, lower leaves. They enlargeand merge until much of the leaf iscovered. Leaves turn yellow and drop. Onstems, lesions are dark green at first, laterturning black. Lesions spread to covermost or all of the lower portion of thestem. Young shoots are often girdled andkilled. The plant dies when infectionspreads to the crown and roots. Thedisease is most severe on spring growthand is favored by cool, moist weather. Canbe devastating on first growth in thecooler regions of the western states, espe-cially under sprinkler irrigation.


Stem nematode Germinating seedlings may be attackedand killed. Seedlings are swollen anddeformed. Stem buds of older plants arestunted, swollen, brittle and easily brokenoff, providing open wounds for crown-rotting organisms. Stems are usuallyswollen at the base, with short internodes,resulting in compacted crowns andstunted plants. Bases of the stems becomebrown to black. White leaves and stems(“white flagging”) develop on some plants.Infected plants are predisposed to win-terkill. Stem nematodes are worse infurrow-irrigated fields with heavy soils. Thechrysanthemum foliar nematode producessimilar symptoms and is frequently foundin diseased plants along with the stemnematode in the western states.

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severe

California biotype

moderate mild

Stemphylium leaf spot Two biotypes, eastern and California,produce somewhat different symptoms.The eastern biotype causes oval, slightlysunken, light brown spots bordered withdark brown and often surrounded by alight yellow halo. With age, lesions becomeconcentrically ringed and may affect alarge portion of the leaflet. When severe,lesions may cause general leaf yellowingand early defoliations. Stems may also bediseased and turn black. The Californiabiotype produces somewhat irregularlesions that are light tan (almost white),with a sharply defined light brown border.Spots vary in size, but are seldom longerthan 3⁄16 inch. Spots do not change withage. Forage quality may be reduced, butearly defoliation is rare. The disease showsup in the spring in the interior valleys andthroughout the growing season in thecooler coastal areas. Stems are notaffected.

Sclerotinia crown and stem rot The first symptoms occur in the fall assmall brown spots on leaves and stemsthat eventually wilt and die. The fungusthen spreads to the crown. In early springthe crown and basal part of the stembecome soft and discolored. As infectedparts die, a white fluffy fungal myceliumgrows over the area, forming hard blackspores (sclerotia) which may adhere to thesurface of or be imbedded in the stem andcrown. Plants less than a year old are mostsusceptible. Disease is favored by cool,moist conditions.

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Verticillium wilt Symptoms first appear during the prebudto floral stage of first cutting. On warmdays, the upper leaves temporarily wilt onscattered plants. Leaflets turn yellow topinkish brown and curl along the midrib.Stems remain erect and green or they maytake on a bleached appearance (chlorotic)for a while after the leaves die. Within thetaproot, a yellowish to brown discolorationis usually present. Regrowth appearsnormal in most infected plants, butsymptoms reappear as top growthapproaches the prebud stage. Plantsbecome progressively weaker and usuallydie later in the season when plants arestressed for moisture. Disease is favored bycool, wet conditions. It is most prevalenton spring and fall growth.


Alfalfa blotch leafminer The adult leafminer is a tiny dull-blackhumpbacked fly which emerges in lateMay from overwintering pupae on theground. The fly’s presence is best recog-nized by pinholes in leaflets caused by egglaying. The female deposits one to threeeggs per leaflet and she may lay over 3,000eggs during her brief lifetime. Small yellowmaggots hatch within the leaf and beginfeeding between the upper and lower leafsurfaces. A tunnel is formed as feedingprogresses, usually beginning at the baseof the leaf, widening as it approaches theleaf tip, and ending with an enlargedcomma-shaped blotch. The mature larvaeleave their mines and drop to the groundto pupate. There can be up to four genera-tions a year of this fly in alfalfa.

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Alfalfa weevil Alfalfa weevil damage starts in early springwhen the larvae hatch from eggs laid inalfalfa stems. Larvae have black heads anda white stripe down the back. Newlyhatched larvae are yellow or light green;older larvae are darker green. Larvae feedfirst in the growing tips and shred thefoliage, which gives heavily infested fieldsa grayish cast. Adult beetles are about1⁄4 inch long, light brown in color, with adarker brown stripe down the back.Damage to first-cutting alfalfa is mainlyfrom larvae, but both larvae and adults canbe present after the first cutting, feedingon the regrowth. Larvae mature to adultsin early summer and leave the fields butmay return in the fall and start depositingeggs. Biological control agents (parasiticwasps and a fungus) have greatly reducedthe impact of the alfalfa weevil in theeastern regions of the country.



Pea aphid This large green aphid is common onalfalfa. It builds up large populations whichcover the stems and terminal buds duringcool, wet springs. It causes damage bysucking sap from the terminal leaves andstem, causing the plants to wilt. Feedingcan wilt plants or even cause death if infes-tations are large. Plants that survive heavyinfestations are stunted with more lightcolored tops than healthy plants. Injury issimilar to that caused by blue alfalfaaphids. Large infestations also produceexcessive honeydew which may interferewith harvesting. Usually, as drier andwarmer weather develops, natural enemieshelp reduce infestations.

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Blue alfalfa aphid The blue alfalfa aphid was first found inCalifornia in 1974 and now occurs inseveral western and midwestern states. It issimilar to the pea aphid in appearance, butcan be distinguished by its bluish-greencoloration in contrast to the yellowish orlight green color of the pea aphid. Theantennal segments of the blue alfalfaaphid are uniform brown in contrast to thepea aphid which have narrow brownbands at the tip of each segment. Severeaphid infestations slow alfalfa growth,reduce yields, and may kill plants. Alfalfa’sfeed value and palatability can be reducedby a black fungus that grows on thehoneydew excreted by the aphids.



Cowpea aphid This aphid is the only black aphid foundinfesting alfalfa. It is relatively small andthe adult is usually shiny black, whereasthe nymph is slate gray. The legs andantennae are usually whitish with blackishtips. Large populations will stunt alfalfagrowth and it produces a considerableamount of honeydew upon which sootymold grows. The honeydew also makes thealfalfa sticky, which can cause problemswith harvest. Damage has been greatest inCalifornia, but it also occurs in theMidwest.

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Spotted alfalfa aphid This tiny aphid is light yellowish green orstraw colored, with rows of dark spots onits back. Unlike the pea aphid, it developsunder hot, dry conditions. It causes severestunting and yellowing of plants and willkill seedling stands. It secretes an abun-dance of sticky honeydew on which asooty black fungus may develop. Thisaphid is most severe in the arid areas ofthe western and southwestern UnitedStates.



Blister beetlesSeveral species of blister beetles arecommon in alfalfa fields. Among them, theorange and brown striped, black, and gray-colored blister beetles are common in theMidwest. Although these insects causelittle injury to alfalfa, they are significant aspests when they are killed by cuttingequipment and incorporated into thebaled hay. Later, when horses ingest thedead beetles along with the hay, a toxinknown as cantharidin in the beetles maycause serious illness and possible mortal-ity. As few as 25 beetles consumed by asmall horse may kill it, while larger horsesmay suffer from diarrhea, colic, and internalbleeding. The first cutting of alfalfa oftenhas the fewest blister beetles while mid-and late summer cuttings typically havelarger populations of these insects.

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Clover leaf weevil Larvae look similar to alfalfa weevil larvaeexcept they are larger (1⁄2 inch long), havebrown heads, not black, and the whitestripe is frequently tinged along the edgewith pink. They feed on alfalfa at night oron overcast days, very early in the spring.Clover leaf weevil populations are reducedby a fungal disease before they causeextensive damage to alfalfa. Larvaeattacked by this fungal disease often curlaround a stem before dying.



Clover root curculio The adult clover root curculio is a small,slender, dark gray snout beetle about 3⁄16

inch long. It feeds on the foliage, but thisseldom causes economic damage.However, larvae can extensively damagealfalfa taproots, particularly in older stands.Damage is characterized by extensivescarring of the outer layers of the roots.These lesions may become avenues ofentrance for various disease pathogens,causing wilt and root rot, and reducing thelife of the alfalfa plant.

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Grasshoppers The differential and redlegged grasshop-pers are two of several species ofgrasshoppers that attack alfalfa and causeserious damage. Grasshoppers becomemost numerous in uncultivated areas.Consequently, heaviest infestations areusually found in field margins, fence rows,pastures, grass waterways, etc. Their popu-lations are often the largest during hot, dryyears. Young alfalfa is a favorite food ofgrasshoppers and they will move fromfield margins into these new seedlings,causing extensive damage.



Meadow spittlebug The meadow spittlebug is an early springpest. A characteristic feeding symptom is afrothy spittle secreted by the yellowishgreen nymphs as they feed. The spittleaccumulates on the alfalfa stem andprovides protection for the nymph.Feeding by numerous nymphs on a plantmay cause stunting with a shortening ofinternodes so that the leaves are bunchedtogether in a rosette appearance. Thebrown-banded adults emerge in early Juneand cause little economic damage toalfalfa.

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Plant bugThere are several species of plant bugscommon in alfalfa fields. The tarnishedplant bug, and the alfalfa plant bug, arethe most common in the Midwest and inthe East. Plant bugs cause serious damageto seed but are not usually consideredforage pests. However, they suck the juicesfrom the foliage, resulting in crinkled andpuckered plant tips and which occasion-ally causes forage loss.



Potato leafhopper This tiny (1⁄8 inch) light green insect is acommon cause of alfalfa yellowing, knownas “hopperburn.”The nymph or immaturestage is light yellowish green and charac-teristically walks sideways. The adults arerapid jumpers and flyers. Feeding causessevere stunting of the plants and yellow-ing of the foliage. Leafhopper damagestarts as a V-shaped pattern at the tips ofthe leaves. The first cutting of alfalfa is notusually affected, but subsequent cuttingsmay be severely damaged. Extensive hop-perburn can reduce long-term yield andplant stand health. New seedings are espe-cially susceptible to damage.

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Variegated cutworm This cutworm prefers non-grass crops andcan cause extensive damage to alfalfaduring warm, wet springs. When fullygrown, it is about 2 inches long and mayrange in color from almost black to lightgreenish yellow or tan. It has a distinctiverow of light yellow diamond-shaped spotsaligned down the middle of the back. Theworms feed mainly at night and hideunder soil clods or in soil debris during theday, or rest on the shaded side of alfalfastems. Larvae are most damaging on theregrowth after the first alfalfa cutting.Other species of cutworms occasionallydamage alfalfa.



Acid soilsAlfalfa grown on low pH, acid soils are lightgreen and stunted. Stands are difficult toestablish and thin quickly. Yields aregreatly reduced. Aluminum and man-ganese may be toxic to alfalfa plants onhighly acid soils. Adding agricultural lime-stone increases soil pH and reduces theavailability of these toxic elements.Increasing the soil pH on many soilsincreases the availability of soil molybde-num, stimulating nitrogen-fixing bacteriain root nodules. It also may increase theavailability of soil phosphorus and someother nutrients. Lime provides varyingamounts of calcium and magnesiumdepending on the type of limestone.

PotassiumPotassium deficiencies are much morecommon in areas that receive 25 or moreinches of precipitation per year. The dis-tinctive symptom of small white spots orflecks around the outer edges of theleaflets is very characteristic. Although thespots are usually most obvious on theupper leaflets, marginal yellowing isgreatest on the lower leaves. Under severeconditions the size and number of spotsincrease and the leaves turn yellow anddry, and may drop off the plant. Alfalfastand failure, winterkill, and encroachmentby grasses may also be signs of low potas-sium levels. Soil tests and plant analysismay be used for predicting potassiumneeds.

PhosphorusPhosphorus-deficient alfalfa often doesnot exhibit distinct symptoms other thanstunted growth. Roots may be light brownand tops stunted, stiff and erect. Leaves,especially on acid soils, may be small andabnormally dark or bluish-green. Well-calibrated soil tests may be particularlyuseful for predicting phosphorus needs.

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SulfurSulfur deficiency is most common onexcessively drained, sandy, low organicmatter soils located away from urban orindustrial areas. However symptoms willoccur on silty and loamy soils and distancefrom urban areas is becoming less of afactor as air pollution is being reduced.Manure contains sulfur and may meetplant needs if applied in adequatequantity. Deficient plants tend to beentirely pale green or yellow, stunted, andunthrifty. Soil tests typically do not accountfor all sulfur inputs. They usually correctlyidentify the available sulfur status onlywhen adequate amounts of sulfate arepresent in the plow layer. Plant tissueanalysis may be helpful in diagnosingsulfur deficiencies,

BoronBoron-deficient alfalfa is sometimes called“yellow top” and is often misdiagnosed asleafhopper damage. The top leavesbecome yellow or reddish yellow. Planttops may become bunched with short-ened internodes and the growing tip maydie while the lower leaves and branchesremain green. Deficient plants do notbloom normally and produce poor seedyields. Symptoms are most prevalentduring dry periods, particularly on coarse-textured soils with low organic matter.When moisture conditions improve, sidebranches may resume growth and extendbeyond the main stem.

MolybdenumMolybdenum-deficient alfalfa tends to bepale or light green because molybdenumis essential for nitrogen fixation. Deficiencyoccurs on very acid soils. Increasing the soilpH increases solubility and availability ofsoil molybdenum.

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HeavingHeaving usually occurs on soils with signif-icant clay content, high in moisture andsubject to freeze/thaw cycles. Freezingcauses the wet soil to expand and exertupward pressure on the crown. Whenfreezing alternates with thawing and soilshrinkage, the pressure can be greatenough to lift the root and crown. Thetaproot will often break when the base ofthe root is frozen solid. Heaving is mostcommon in the spring but may be severein late summer or fall seedings because ofless-developed root systems on newseedings.

Traffic damage/compactionWhen alfalfa is subjected to excessivetraffic, or, especially, traffic when the soil iswet, loss of yield and stand can result. Thesoils become compacted and less aeratedwhich results in poor nutrient utilization.Traffic can also break stems and increaseroot disease problems and crown damage.Stand loss in high-traffic areas is common.

damage

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no traffic traffic

Winter injuryAlfalfa plants form buds in the fall forgrowth the following spring. When any ofthese buds are killed over winter, plantsgreen up with fewer shoots than optimum.Severe winters, lack of snow cover, insuffi-cient winterhardiness, and/or poor man-agement may contribute towards makingplants vulnerable to winter injury. Injuredplants often form new buds in the spring,however, shoots arising from these budswill be significantly behind in develop-ment. Thus, when the tallest stems are 6inches tall, shoots starting from spring-formed buds will be only 2 to 3 inches tall.To evaluate the extent of winter injury,examine fields when the tallest shoots areabout 6 inches tall. Look for high shootdensity per plant and for uniform height ofshoots.

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Winter kill When alfalfa crowns are exposed to tem-peratures below 15°F, the entire alfalfaplant may be killed. Contributing factorsinclude lack of snow cover (an excellentinsulator), lack of crop residue, lack ofgenetic winterhardiness, low soil fertility,and/or low pH. Symptoms will be visibleas the ground thaws. Crowns and taprootsturn soft and fibrous, and a distinct brownline is often visible across the taproot 2 to3 inches below the soil surface.

Dan Undersander,Extension Forage Agronomist, University of Wisconsin

Fred A. Gray,Professor of Plant Pathology, University of Wyoming

Keith Kelling,Extension Soil Scientist, University of Wisconsin

Marlin E. Rice,Extension Entomologist, Iowa State University

National Alfalfa Alliance100 N. Fruitland, Suite BKennewick, WA 99336509-585-6798www.alfalfa.org

Documents

Welcome to the National Alfalfa & Forage Alliance · Welcome to the National Alfalfa & Forage Alliance