HOW TO IDENTIFYRHODODENDRON

ANDAZALEA PROBLEMS

Washington State University Extension Bulletin1229

COOPERATIVE EXTENSIONWashington State

University

HOW TO IDENTIFY RHODODENDRON AND

AZALEA PROBLEMSBy A. L. Antonelli, Extension Entomologist; R. S. Byther, Extension PlantPathologist; R. R. Maleike, Extension Horticulturist, WSU Western WashingtonResearch and Extension Center, Puyallup; S. J. Collman, Extension Agent,Snohomish County; and A. D. Davison, Superintendent, WWREC, Puyallup.

TABLE OF CONTENTS

AZALEA and RHODODENDRONCULTURE ..............................................................1

SYMPTOMS ................................................. 3-24

Missing Portions of Leaves 2-4..............................Root Weevils 2..............................................................Caterpillars 3................................................................Sawflies 4.......................................................................

Spotting, Discoloration, orDead Areas on Plant Parts 5-14...............................Physiological Leaf Spot 5............................................Fungus Leaf Spot 5.......................................................Azalea Leafminer 6........................................................Rhododendron Leafminer 6..........................................Marginal Leaf Necrosis 7..............................................Iron or Manganese Deficiency 8..................................Heat Damage (sunscald) 9...........................................Root Problems 9............................................................Nitrogen Deficiency/Wet Feet 9...................................Virus Diseases 10..........................................................Bud Blight 10................................................................Cold Injury/Buds 11.....................................................Frost Injury/New Growth 11.......................................Leaf Senescence 12.......................................................Normal Bark Development 12......................................Lacebug 13.....................................................................Chemical Injury 14......................................................

Curling, Distortion, orMisshapen Plant Parts 15-17..................................Leafroller 15...................................................................Cold Response/Leaves 15...........................................Azalea Leaf Gall 16......................................................Aphids 17......................................................................

Powdery, Granular, or OtherUnusual Material on Leavesand/or Stems 18-22...................................................Rust 18..........................................................................Powdery Mildew 18-19...............................................Sooty Mold/Bark Scale 20...........................................Lecanium Scale 20.......................................................Indumentum 21...........................................................Lichens 22.....................................................................Algae 22.........................................................................

Dieback, Total Decline, orPoor Performance 23-24.........................................Crown Girdling 23......................................................Root Rots 24..................................................................Stem Diseases 24........................................................Broken Branches 24.....................................................Mountain Beaver 24....................................................Small Leaves 24............................................................Planted Too Deep 24...................................................

SELECTED REFERENCES ...........................25

GLOSSARY ...................................................... 26

APPENDIX ARhododendron Species Resistantto Weevils ..........................................................27

APPENDIX BRhododendron Hybrids Resistantto Weevils ..........................................................28

FOREWORDAzalea and Rhododendron Culture

Both azaleas and rhododendronsbelong to the genusRhododendron, which is in theEricaceae or heath family. Othermembers of the heath family in-clude heaths, heathers, blueberries,mountain laurel, and andromeda (Pieris). Cultural needs of plants inthe heath family are all somewhatsimilar.

Because of the diversity in thegenus Rhododendron, it is impos-sible to give exact cultural require-ments for each variety. There aresome varieties which will toleratetemperatures to - 25'F, whileothers do not tolerate frost. Somevarieties almost demand full sun,while others require full or partial

shade. It is important to select thecorrect variety for a given set ofenvironmental conditions. If theconditions do not suit a given va-riety, for example, if too wet, toodry, too cold, and/or too sunny,the plant will become stressed.Stressed plants are much moresusceptible to damage from insectsand diseases.

Another extremely importantconsideration is the soil in whichthe rhododendron is growing. Itmust be an acid soil with a pHrange of about 4.5-6.0. If the pH ofthe soil is much above or belowthese figures, it should be ad-justed. Secondly, the soil must bewell drained. Insufficient drainageleads to root problems. Thirdly,the plants need ample moisture.Irrigation will be required in all

areas of Washington in the sum-mer. Finally, an organic mulch,such as sawdust, bark chips, orground bark, is very beneficial tokeep roots cool and moist.

The hardiness of a plant may bedefined as the ability to with-stand either cold or warm temper-atures. If the plant can withstandrather cold temperatures it is saidto be cold hardy, and if the plantcan tolerate warm temperatures itis said to be heat hardy.

Hardiness ratings with mini-mum temperatures and a few ex-amples of each hardiness groupare listed below. There are manymore within each group. (Checkwith a local grower or gardencenter for availability of varietiessuited to your area.)

H-1-Hardy to -25°F (Probablyhardy in many areas of easternWashington)

AmericaCatawbiense AlbaIgnatius Sargent

R. mucronulatum (deciduous)Nova ZemblaPioneer (dwarf)PJM (dwarf)Ramapo (dwarf)Roseum ElegansSome of Exbury and Knap Hilldeciduous azalea group

H-2-Hardy to - 15 ° F

Anah KruschkeCadisChionoidesCynthiaMrs. FurnivalR. impeditum (dwarf)Many of the Gable hybrid azaleasLee's Dark Purple Purple SplendorScintillationTrilby

Many, many varieties.A little care in the selection of

the proper variety for the given lo-cation and attention to the detailsof correct soil preparation will prevent many problems.

H-3-Hardy to - 5 ° F

AliceBrittania CaryAnn ChristmasCheer CrestJockLavender Girl LoderiKing George LeoPink PearlPuget SoundRosamundiSappho

H-4-Hardy to + 15 ° F

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HOW TO IDENTIFY RHODODENDRON AND AZALEA PROBLEMS

This guide is based on generaldescriptions of plant problem cate-gories which are listed below. Ex-amine the plant carefully anddetermine which of the symptomsbest describe the condition ob-served. Then look at the picturesand detailed descriptions until youidentify the causes. When diagnos-ing plant problems, it is important

to assess the plant's habitat. Forexample, note such things as ex-posure to sun, soil moisture overlong periods, pesticide usage, fer-tilizers applied, or any other en-vironmental condition you mayobserve. If the plant is not yours,ask the owner to summarize thesegrowing conditions as thoroughlyas possible. For specific solutions

to correct maladies, refer to otherpublications or confer with aqualified plant diagnostician or in-dividual who is knowledgeableabout this plant group. Further in-formation is available fromWashington State UniversityCooperative Extension offices ineach county.

MISSING PORTIONS OF LEAVES

Fig. 1. Adultroot weevilnotching.

ROOT WEEVILS

Symptoms: Small, irregular orsemicircular notches on leaf edges,sometimes running together intolarger jagged notches (Fig. 1).

Cause: Adult root weevil (Fig.2) feeding. Root weevils (Otiorhyn-chus spp. and others) normally feedat night. Their damage can be con-firmed by checking plants at nightwith a flashlight for the presence ofthe weevils. Damage to leaves byweevil adults is not normally a seri-ous threat to the plants, but heavyfeeding results in an unsightly plant.Some chemicals are effective incontrolling adult weevils on theplants (see EB 0970), but chemicalapplications only prevent futurenotching. They cannot eliminatenotching already present. Thesenotches will remain until the leaffalls off. Also, many rhododendronand azalea varieties or speciesdemonstrate considerable weevil re-sistance. Selecting resistant varietieswill minimize maintenance anddamage (Appendices A and B). NOTE:Pesticides registered for root weevilcontrol will normallyreduce aphids and many other leaf Fig. 2. Obsci, re root weevil adult. Other weevils may also notch rhododendrons. Thesefeeding insects as well. include black vine weevil, strawberry roof weevil,-woods-weevil,-and clay-colored-weevil.

Adults range from 1/4-1/2 inch long.

CATERPILLARS

Symptoms: Large, irregular,chewed leaf areas.

Cause: Several species of cater-pillars, cutworms, and loopers (Figs.3-5). Many of these are nocturnalfeeders and can be seen at nightwith the aid of a flashlight. Whendamage becomes too unsightly, orthe plant is threatened, select aregistered pesticide, or pick off thecaterpillars and destroy them.

Fig. 3. Rusty tussock moth caterpillars onrhododendron. These caterpillars can beseen feeding during the day. They often stripthe leaves down to nothing. Caterpillars,cocoons, egg masses, or wingless grayfemales are signs of this insect. Caterpillarsare approximately 1 inch long.

Fig. 4. Damage torhododendron typicalof climbing cutworms.

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Fig. 5. A looper andits damage (similar tocutworm damage).Loopers may be up to1 1/2 inches long de-pending on species.

SAWFLIES

Symptoms: Leaves badlychewed, sometimes strippeddown to midrib.

Cause: Sawfly larvae (notflies) belonging to the orderHymenoptera. These greencaterpillar-like larvae blend withthe leaves (Fig. 6.) They can beseen during the day and areeasily controlled with pesticides (when the larvae are present).They can also be picked off anddestroyed without usingchemicals if only a few plants areinvolved.

Fig. 6. Saufly larva damage.Note caterpillar-like insect incenter of picture. Sawfly larvae1/2-3/4 inch long.

SPOTTING, DISCOLORATION, OR DEAD AREAS ON PLANT PARTS

PHYSIOLOGICALLEAF SPOT

Fig. 8. Fungus leaf spot. Several species offungi cause this disease.

Symptoms: Variously colored (generally dark purple), discrete ordiffuse spots, discoloration andblotches occur on leaves as a resultof environmental and cultural stress (Fig. 7.) Some varieties (Mrs. G. W.Leak) are known for their spots.

Cause: Actual causes are notknown. If the problem is severe orpersistent, consider replacing therhododendron with a more desirablevariety.Fig. 7. Physiologicalleaf spot. Note diffuseblotches on leaves.

FUNGUS LEAF SPOTSymptoms: Spots are irregular

in size and color (Fig. 8). Some havered-brown borders with silvery graycenters. Very small black dots (fruiting structures of fungi) aresometimes visible in the center ofthe spot or in concentric rings. Thesefungi commonly enter throughwounds.

Cause: The fungi Phyllosticta,Septoria, Pestalotia are commonlythe causal agents of this malady.Remove and destroy affected leaves.Spray with a registered fungicide afterflowering and repeat at 10-14 dayintervals until dry weather begins. It isusually advisable to apply a spreader-sticker with the fungicide to hold thefungicide on the leaves during rains.

AZALEA LEAFMINER

Symptoms: Brown, blister-likemines on leaves. Leaves maybe tightly rolled and skeletonizedfollowed by premature leaf drop.Plants look thin and scraggly.

Cause: Azalea leaf miner,Caloptilia azaleella, (Fig. 9). Smallyellowish caterpillars mine insideleaf tissues and later roll theleaves. Chemical controls maybe used if the infestation is severe.If only a few leaves are involved,squeezing the insect within its minemay decrease damage to anacceptable level. Since the larvaepupate in leaf debris, rake and de-stroy the leaves in the fall. Do notcompost.

Fig. 9. Azalea leafrninerdamage.

Fig. 10. Mine and damage ofrhododendron leafminer (middle leaf).

RHODODENDRON

LEAFMINER

Symptom: A serpentine or fairlystraight mine starting at the leafedge and eventually going verticallyto, into, or across the midribcausing all leaf tissue from thatpoint to the tip to die.

Cause: Rhododendron leafminer(Fig. 10). Seldom causes enoughdamage to warrant control. Removeand destroy infested leaves.

Fig. 11. Marginalleaf necrosis.

Fig. 12. Marginal leafnecrosis. Plant showssyrnptoins on both tipand edges of leaves.

MARGINAL LEAFNECROSIS

Symptoms: Upper leaves brown,burned back (necrotic) from tipsand/or edges toward the midrib ormiddle of the leaf (Figs. 11, 12).

Causes:1. Cold damage occurs when tem-peratures dip to near or below thehardiness limit of the plant. May beaccentuated by wind anddrought, especially in easternWashington. 2. Drought, especiallywhile the plant is in active growth orthe foliage is in soft growth, and onnewly established plants. 3. Highamounts of salts in the soil caused byexcessive use of soluble fertilizers.Very common close to the housewhere the eaves protect soil fromrain, and along the dripline of thehouse where salt (fertilizer)concentrations are high whenthe plant is in soft growth. 4. Rootdamage due to poor drainage,planting too deep, physical injury toroot system, or disease. 5.Girdling due to weevil feeding onbark and/or roots. 6. Nutrientdeficiency.

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IRON OR MANGANESE DEFICIENCY

Symptoms: Marked yellowing (chlorosis) of leafparts, primarily between veins of new leaves. Theyellowing (from very pale yellow to intense brightyellow) will vary with severity of the symptoms andthe cause (Fig. 13).

Causes:1. An overly alkal ine soi l (should have apH of 4.5-6.0).2. Lack of sufficient iron or manganese in the soil (uncommon).3. Lack of sufficient air space and/or lack of gooddrainage (soil constantly waterlogged or compacted).4. Excessive amounts of some herbicides may causesimilar symptoms (see E131048, Leaf Scorch ofRhododendron).

Fig. 13. Iron chlorosis at left, leaves on theright show probable manganese deficiency. (Note: notching on the leaves is from insectfeeding and has nothing to do with mineraldeficiency.)

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Fig. 15b. Leaf yellowing causedby wet feet.

Fig. 14. Sunscaldon rhododendronlr

Fig. 15a. Marginalyellowing and brown-ing can indicate prob-lems with the roots.

HEAT DAMAGESymptoms: Brown, indistinct blotches of varying

degrees, originating mostly on the central portionsof the top leaves. Symptoms are usually more severeon the south and southwest side of the plant and onleaves oriented perpendicular to the sun.

Cause: Heat and/or sun scald symptoms developrapidly after a hot spell with intense sunlight (Fig.14). Plant a sun tolerant rhododendron or providepartial shade. Cooling the foliage with water on thehottest afternoons may help temporarily.

NITROGEN DEFICIENCY/WET FEET

Symptoms: Overall yellowing ofleaves, generally more prevalent onolder or lower leaves (Fig. 15b).

Causes:1. Nitrogen deficiency. Lack ofnitrogen or available nitrogen tied upin decomposing organic matter, suchas sawdust in the soil around theplant.2. Wet feet. An early symptom ofpoor drainage is yellowing andwilting of newly emerging growth. Ifthe plant is wilted and the soil isvery wet, there is probably a rootproblem or insufficient air space and/or drainage in the soil.

ROOT PROBLEMSSymptoms: Marginal yellowing of leaves; may be

followed by browning (necrosis) (Fig. 15a).Cause: Root problems are caused by poor drainage,

disease, soil compaction, heavy clay soils or being bu-ried too deep in planting hole or by mulch buildup.Early symptoms include yellowing and wilting of newgrowth and irregular yellowing along margins of olderleaves. Roots need abundant air spaces to functionproperly. (See Root Rots, page 24.)

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Fig. 16. Virus symptomson rhododendron.

VIRUS DISEASESSymptoms: Bright yellow to

red-brown rings, spots, andblotches on rhododendron leaves.The patterns are distinct butmargins of the patterns are vague.

Cause: Probably severaldifferent viruses involved (Fig. 16). They are not common and oftenare not severe. If severe, removeand replant with other varieties.Virus disease cannot be cured.Once a plant is infected, itremains infected.

BUD BLIGHT

Symptoms: Flower buds turn brown andfail to open in the spring. Later, tiny black bristlescover their surface.

Cause: The fungus Briosia azaleae causes budblight (Fig. 17). Remove and destroy (do not com-post) affected buds. Fungicide applications areusually not necessary.

Fig. 17. Bud blight. Note the tiny blackbristles or spores on the bud surface.

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Fig. 18. Flower budcold injury.

Fig. 19. Flower bud coldinjury. Note differentialbud development withshriveled buds.

Fig. 19b. Frost damage tonewly emerged shoots.

COLD INJURY/BUDSSymptoms: Flower buds turn brown and fail to

open, or only partially open. If they open, onlysome of the flowers develop, while the underde-veloped flowers turn brown (Figs. 18, 19).

Cause: Freezing temperatures in late fall, win-ter, or early spring. Replant with a more hardy

FROST INJURY/NEW GROWTH

Symptoms: Newly emerged growth killed orpartially killed in the spring of the year. Most of thedamage is to the top of the plant. Severity ofsymptoms vary with cultivar and species. Partiallyaffected leaves will eventually be distorted.

Cause: Late spring frost. Newly emerged leavesand stems are not as hardy as older foliage andstems. Varieties which start to grow early are more

variety if problem occurs regularly. susceptible.

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Fig. 20. Normal leafsenescence.

LEAF SENESCENCE

Symptoms: Older leaves (1-2years old) will turn yellow and/orbrown and fall off the plant (Fig.20) .

Cause: Like all evergreens,rhododendrons eventually lose theirleaves one, two, or more years afterthey first emerge. When older leavesturn yellow and/or brown and fall off inlarge numbers, this is usually normalleaf senescence. Some varieties willlose their older leaves just before theonset of winter, and some varietieswill lose their leaves during summerdrought periods. This is normal.

NORMAL BARKDEVELOPMENT

Symptoms: Emerging rhododendron shoots are, fora period of time, very green. They eventually turnbrown (bark color). Where this color change takesplace on the stem there is usually a sharp contrastbetween the green and the brown(Fig. 21) .

Cause: Normal development. However, thecontrast between new and old bark sometimes leadspeople to think a disease is involved.

Fig. 21. Normal barkdevelopment.

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Fig. 22. Yellow speck-ling caused byrhododendronlacebug.

Fig. 23. Adult lacebugs.Note the fecal drops or"tar" spots on leafRhododendron lacebugis approximately 1/8inch long.

LACEBUGSymptoms: Leaves show a

yellowish speckling on the topsurface (Fig. 22). Brown and blacktar spots are present on theunderside of the leaf.

Cause: Rhododendron lacebug,Stephanitis rhododendri, (Fig. 23).When speckling becomes apparent,control is advised. Most materialsregistered for root weevils alsocontrol lacebugs.

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Fig. 24. Chemical injuryto leaves (possiblyherbicide).

CHEMICAL INJURY

Symptoms: A variety of leafdiscolorations that do not fit wellinto previously listed symptoms.These are often attributed to chemi-cal burn (Figs. 24, 25).

Cause: Herbicides and otherchemicals. The plant diagnosticianmust also be a good detective andask the right questions. People areoften reluctant to admit to chemicalmisuse or are simply unaware of theeffects of such chemicals as roofpreservatives, demossing chemicals,paint thinners, or herbicides onornamental plants.

Fig. 25. Chemical injury. In this case, pentachlorophenoldrift from nearby roof Damage is on older leaves but noton new leaves. May indicate treatment occurred two orthree years earlier and is only now being noticed.

Fig. 25b. An overzealous application ofCasoron (Dichlobenil) resulted ininterveinal and marginal yellowing.

CURLING, DISTORTION, OR MISSHAPEN PLANT PARTS

LEAF ROLLERSymptoms: Leaves rolled, webbed, and

chewed (although significant chewing maynot yet be apparent at the time ofobservation).

Cause: Several species of leafrollercaterpillars (Fig. 26). If damage is signifi-cant, then a registered pesticide may beapplied. Be sure the chemical gets into the "rolled" leaves or control may not becomplete. Look carefully for thecaterpillars, or at least their damage.Harmless spiders sometimes web leavestogether, but they do not chew leaves.

Fig. 26. Leafroller damage.Leafroller larvae about 1/2 inch long.

Fig. 27. Drooping rolledleaves caused by coldweather.

COLD RESPONSE/LEAVESSymptoms: Drooping, rolled leaves (

Fig. 27) in winter.Cause: Some rhododendrons will have

curled leaves (rolled towards the bottom ofthe leaf) due to cold weather. This will varywith the species or variety. It is a normalreaction of the plant which serves tominimize damage, and the leaves will recoverwith warmer weather. In summer, suchsymptoms can be a sign of root rot orextreme lack of water.

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AZALEA LEAF GALL

Symptoms: On azaleas, light green, fleshy, bladder-shaped galls appear on new leaves in late spring andearly summer. The galls change from red to brown,become hard, and are covered with a powdery whitebloom. On rhododendrons, leaves may be entirely orpartially chlorotic (often bleached to a creamy off-white or pinkish color) and may be somewhatthickened. Part ial ly affected leaves have adistinct line between the healthy green and the chlo-rotic tissue. A white fungal growth develops on theunderside of the leaves. A rosette of affected leavesmay occur at the ends of branches. Blossoms and seedpods can also be affected.

Cause: Azalea leaf gall (Figs. 28, 29). The fungusExobasidiuna vaccinii is responsible for this disease.Remove and destroy affected leaves. Spray withregistered fungicide just prior to bud break andrepeat two or three weeks later (see EB 1052).

Fig. 28. Azalea leafgall on azalea.

Fig. 29. Azalea leaf gallon rhododendron.

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Fig. 30. Twisted, puckered newgrowth on rhododendron causedby aphid feeding. If you examinethe photograph carefully, youwill see the white cast skins ofthe aphid.

APHIDSSymptoms: Leaves, particu-

larly new growth, appear twisted,puckered, or curled (Fig. 30).

Cause: Several species ofaphids. Either the aphids or theircast skins can be found on theleaves, usually on the undersides. Asticky, shiny material calledhoneydew is deposited by theaphids onto the plant. Honeydewdeposits offer a medium for blacksooty mold, which may also bepresent. Late frost damage onnewly emerged leaves is similar,except that the insects and/or castaphid skins will not be present.Varieties which start to grow early (such as Christmas Cheer) aresusceptible to late frost damage.

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POWDERY, GRANULAR, OR OTHER UNUSUALMATERIAL ON LEAVES AND/OR STEMS

RUSTSymptoms: Light green to yel-

low, small, diffuse spots randomlydistributed on the leaf. Associatedwith spots on the underside of theleaf are pustules producing yellowto orange powdery spore masses.Later in the season, dark brownspores are produced in thesepustules.

Cause: The fungus Chrysomyxaledi is the most common cause ofrust in the Pacific Northwest (Fig. 31). Sitka spruce is the alternate host.To avoid this problem, use resistantvarieties. Avoid planting Sitka sprucein close vicinity. Preventativeapplications of a registeredfungicide may be necessary whenrust continues to be a yearlyproblem.

POWDERY MILDEWSymptoms: Leaves may be off-

color, and are covered with fungusgrowth which is often powdery.Black pepper-sized structures of thefungus (cleistothecia) are visible laterin the season. Plants in the shade mayshow more symptoms.

Cause: The fungusMicrosphaera azalae causes thisdisease (Fig. 32a). To reduce the in-cidence of this disease, rake up anddestroy fallen leaves. Spray with aregistered fungicide at the first sign ofdisease.

Fig. 31. Rust onrhododendron.

Fig. 32a. Powdery mildewon rhododendron.

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Fig 32b. Powdery mildew symptoms on upper leaf surface seen on somerhododendron varieties.

POWDERY MILDEWSymptoms: Some rhododendrons

have unusual symptoms and lack thewhite powdery appearance typicalof powdery mildew diseases. Themost common symptoms are diffuseyellow spots on the upper leafsurface and various discolored areason the lower surface (Figs. 32b, c).Other varieties may show purple-brown spots, purple ring spots, orpurple discoloration along the leafveins. Infected leaves may dropprematurely.

Cause: The fungus Microsphaeraazalae causes this disease. To control,rake up and destroy fallen leaves, andspray a registered fungicide at firstsign of disease and continueapplications during periods favorablefor disease. Plant resistant varieties.

Fig. 32c. Lower leaf surface of rhododendrons showing diversity of powdery mildew symptoms.

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SOOTY MOLD/BARK SCALE

Symptoms: A black sooty growth ispresent on the leaf surfaces and bark,which is easily wiped off. It is associatedwith aphid or other sucking insectactivity, either on the plant or ontrees or shrubs overhead.

Cause: Sooty mold (Fig. 33). Sootymold is the fungus which develops onthe honeydew excreted by suckinginsects . Control of these insectswill eliminate the problem. Sootymold can be partially washed off withwater.

Symptoms: Plant appears thin andunthrifty and sooty mold is often present(see EB1051). Small white cottonymasses are present on the bark (Fig. 33).

Cause: Azalea bark scale,Eriococcus azaleae. Certain insecti-cides will control this pest if appliedwhen crawlers (newly hatchednymphs) are present.

LECANIUM SCALE

Symptoms: Plant appears thin andunthrifty; sooty mold is oftenpresent. Tortoise-like bumps (soft)are present on the bark. The "bumps" may be brown or marbledwhite and brown (Fig. 34).

Cause: Lecanium scale,Lecanium spp. If infestation is se-vere or plant is showing signs ofstress, chemical control will benecessary (see EB0746). Note:Oystershell and possibly otherscales may also attack this plantgroup.

Fig. 33. Azalea bark scale. Sooty mold is also present (on bark). Pink eggsand young crawlers are present in the scale that has been broken open.

Fig. 34. Lecanium scale.

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INDUMENTUM

Symptoms: A matted wooliness ispresent on the surface of the leaves (often the underside) or the twigs. Thehairs range in color from white to avery bright tan-brown or rusty brown.

Cause: Indumentum (Fig. 35). Thewoolly hairs are normal (and often theprized feature) of some species andvarieties of rhododendrons.

Fig. 35. Indumentum, a normal phenomenon which in this case the brownfuzziness is on underside of the leaves. Other types of indumenturn may benearly white.

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LICHENS

Symptom: Yellow, gray, green,orange, or black fleshy or paperygrowth on bark.

Cause: Lichens are a fungi/algaeassociation which use the bark as agrowth site, not a feeding site. Theycan be controlled chemically, ifdesired, for appearance. Althoughoften associated with spindly, un-thrifty plants, lichens cause nodamage to the plant (see E131050) (Fig. 36).

ALGAESymptoms: Green felt-like ma-

terial on leaf surfaces, which can berubbed or scraped off.

Cause: Algae (Fig. 37) cause noapparent injury to plant, mostly aconcern of unsightliness.

Fig. 36 Lichens. Note yellow and graygrowths on stems. Leaves are exhibitingnormal fall color of a deciduousazalea.

Fig. 37. Algae onrhododendron leaves.

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DIEBACK, TOTAL DECLINE, OR POOR PERFORMANCE

This category is a "catch-all" andmay overlap with other categories.Hopefully, you have had theopportunity to observe the plantearlier in its decline, have seen it atits planting site, or have a thoroughknowledge of the maintenance (orlack of maintenance) program. Someof the causes of such symptomsfollow.

Fig. 38. Crown girdlingby root weevil larvae.

CROWN GIRDLINGCrown girdling or root

feeding: Plants appear unthrifty withno apparent symptoms on foliageexcept full or partial loss of color inthe leaves. Over time, plant declinesand dies. This can be the result ofroot weevil larvae feeding on theroots or base of the plant. Thelegless, cream colored, C-shapedlarvae live in the soil and feed on thesmall roots through the winter. Inthe spring, they attack larger rootsand may even girdle the plant (bychewing away bark) just below thesoil or mulch line. A girdled plantmay still get some water andnutrients to the leaves, so the deathprocess is prolonged. Look for apoor root system, girdled crown (Fig. 38), or white larvae in the soil (Fig. 39).

Fig. 39. Root weevillarvae. These are thelarvae of the samegroup of weevils thatnotch the leaves.

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ROOT ROT AND STEM DISEASESRoot rots: Leaves (Fig. 40) wilt (roll downward),

turn yellow, and eventually the plant dies. Dead leavesremain attached to the plant. Small, fibrous roots rotfirst. Rot progresses to large roots, and finally, the en-tire root system and lower stems develop a brown dis-coloration. The fungus Phytophthora cinnainomi is acommon cause of root rot, but other species ofPhytophthora and other fungi can also be responsible.Poorly drained soil can also be responsible as rootssuffocate in water and are then invaded by rot organ-isms. Remove and destroy infected plant. Do not replantinto the same hole. To avoid root rots, provide goodsoil drainage, maintain proper nutrition and soil pH, andpurchase healthy plants from reputable outlets. Severaleffective fungicides are available for use in greenhouses andnurseries.

Stem diseases: In contrast to root rots, where theentire plant is affected, diseases causing stem diebackusually only affect some of the twigs and branches,while others will appear normal. Two fungi,Phytophthora and Botryosphaeria, are common causes ofthis problem. Reddish brown to black sunken cankersdevelop and girdle the stem. Leaves and stems above thecanker wilt and die. Diseased stems should be prunedout well below the cankered area. The prunings shouldbe destroyed and the pruning shears disinfected.Applications of an appropriate fungicide may also behelpful.

Broken branches: Branches are sometimes ac-cidentally broken by animals or people, resulting indead spots in the plant canopy. Look closely at suchsymptoms to be certain the cause is properly diagnosed.

Mountain beaver: These rodent-like creatures areburrowers and occasionally eat away part or all of thesubterranean portion of the plant. They are also inor-dinately fond of rhododendrons and will cut offbranches and drag them back to their burrows. Thebranches may also be left near the plant of origin.Trapping for these destructive animals is the only wayto avoid further plant and landscape damage.

Small leaves: Small rhododendron leaves are com-mon on plants suffering from cultural or environmentalstress. Although leaves may look fairly normal, com-pare them to the leaves of previous years. They maybe smaller or less green, a sign that the plant needsattention. Sometimes the newer leaves are bigger than inpast years, which indicates there has been a past prob-

lem and the plant is revovering.

Fig. 40. Root rotresponse. Could bemistaken for cold injury, sobe certain to check allaspects mentioned in text.

Planted too deep: If rhododendrons are plantedtoo deep, the symptoms on the plant will besimilar to lack of drainage with reduced topgrowth, smaller yellow leaves, and a possibility ofdeveloping root rots. Frequently, shrubs planted atthe proper depth (crown even with the soil) willsink lower as the loosened soil in the planting holebegins to settle. Adding mulches year after year mayalso bury roots too deep.

SELECTED REFERENCESAmerican Rhododendron Society, Quarterly Bulletin, Portland,

Oregon. 1947 to date.

Antonelli, A., R. L. Campbell. Root Weevil Control on Rhododendrons.EB0970. Cooperative Extension, WSU. 1993. 2 pp. 504=.

Antonelli, A., S. Collman. Lecanium Scale. EB0746. Cooperative Exten-s ion, WSU. 1991. 2 pp. 50C.

Byther, R. Azalea Leaf Gall. EB 1052. Cooperative Extension, WSU.1992. 2 pp. 504=.

Byther, R. Sooty Mold. EB 1051. Cooperative Extension, WSU. 1992. 2pp. 50'.

Clarke, J. Harold (ed). Rhododendrons in Your Garden. AmericanRhododendron Society, Portland, Oregon. 1961. 251 pp.

Cox. P. A. Dwarf Rhododendrons. Macmillan, New York. 1973.296 pp.

Davison, A. D. Algae, Lichens and Mosses on Plants. EB1050. Coopera-t ive Extension, WSU. 1992. 2 pp. 50C.

Davison, A. D., R. Davidson, B. G. Wesenberg. Leaf Scorch ofRhododendron. EB1048. Cooperative Extension, WSU. 1992. 50C.

Davison, A. D., B. G. Wesenberg, O. C. Maloy, Maladies of OrnamentalPlants. EB0608. Cooperative Extension, WSU. 1980. 16 pp. 50C.

Greer, H. E. Greer's Guidebook to Available Rhododendrons: Speciesand Hybrids . Offshoot Publ icat ions, Eugene, Oregon.1982. 152 pp.

Johnson, W. T. and H. H. Lyon. Insects that Feed on Trees and Shrubs.Comstock Publishing Associates, Cornell University Press. 1976. 464pp.

Leach, D. G. Rhododendrons of the World. Allen and Unwin, London.1962. 544 pp.

Lee, F. P. The Azalea Book. Van Nostrand, Princeton, N.J. 1965.435 pp.

Van Veen, Ted. Rhododendrons in America. Sweeney, Krist, andDimm. Portland, Oregon. 176 pp.

MISCELLANEOUS

The American Rhodeety. 14635 S. W. BulTigard, Oregon 9723639-5922. (Publishesbulletins and other ir.information texts. ExRhododendron Hand]

28

GLOSSARY

Algae-low forms of plant life that never form trueroots, leaves, or stems.

Chlorosis-yellow coloration usually resulting fromreduced development or loss of chlorophyll.

Crawlers (young)-the mobile nymphal stage of cer-tain insects, such as scales, mealybugs, etc. Laternymphal stages become somewhat or completely im-mobile.

Crown-the junction of major roots and stem.

Fungi-small plants that are unable to produce theirown food because they lack chlorophyll. They feed onliving or dead plants and animals, resulting in diseaseor decay.

Fungicides-chemicals that prevent or retard fungaldevelopment.

Girdling-an encircling destruction of a plant stemor root, resulting in restrictive flow of water andnutrients.

Herbicides-Specialized chemicals that kill plants.

Honeydew-name given to the excretory material ofaphids and many other sucking pests.

Hymenoptera-order of insects that includes thebees, ants, wasps, and sawflies.

Larva-the immature stage of insects possessing anegg-larval-pupal-adult progression. Example:caterpillars.

Mine-the condition seen after an insect (usually theimmature stage) feeds between the epidermal (surface)tissue of leaves or just under the bark.

Necrotic-dead or dying (usually in reference totissue).

Nymph-The immature stage of insects possessing anegg-nymph (young adult)-adult progression. Exam-ple: grasshoppers.

Pentachlorophenol-a chemical frequently used onstructures (fence posts, etc.) to retard or prevent thegrowth of fungi or other plant life.

pH-a measurement relating to the acidity (low pH)or alkalinity (high pH) of a medium such as soil orwater.

Pustules-a blister-like disruption on a plant surfacecontaining fungus spores.

Registered insecticide-a chemical that is registeredby a company with the federal government for killingspecific insects on specific plants. The word registeredimplies specific pest and host and may also precedefungicide or herbicide.

Spores-minute reproductive bodies produced byfungi and some other plants.

Symptoms-abnormal appearances of a plant.

Virus-submicroscopic infective particles which canonly multiply in living cells, often detrimental tothese invaded cells.

29

APPENDIX A

Species Rhododendrons ShowingResistance to Feeding by Adult Root Weevils

Species Series Possible Blossom Colors Ratingheliolepis Heliolepis white, rose 100

impeditum Lapponicurn purplish blue 100

scintillans Lapponicum purplish blue 100

burmanicum Maddenii yellow to greenish white 100

dauricum Dauricum lavender-rose 97

intricatum Lapponicum mauve 97

minus Carolinianum rose, white 93

desquamaturn Heliolepis rose, violet 93

ferrugineum Ferrugineum rose, white 93

hemsleyanum Fortunei white 93

cuneatum Lapponicum rose 90

fastigiatum Lapponicum lilac, purple 90

yakusimanum Ponticum white, rose 90

ungernii Ponticum white, pale pink 83

rubiginosum Heliolepis pink, rose 83

irroratum Irroratum white, ivory, rose 83

racemosum Virgatum white, rose 80

russatum Lapponicum blue-purple 80

carolinianum Carolinianum pink, rose, white 80

oreodoxa Fortunei rose, white 80

oreotrephes Trflorum mauve, purple, rosy red 77

vernicosum Fortunei white, rose 77

adenophorum Taliense rose 77

campylogynum Campylogynum pink, purple, crimson 77

xanthocodon Cinnaborinum ivory, yellow 77

diaprepes Fortunei white, pale rose 73

pubescens Scabrifolium white, rose 73

lepidastylum Trichocladum pale yellow 73

pemokoense Uniflorum lilac-pink 73

arizelum Falconeri white, yellow, rose, crimson 73

glaucophyllum Glaucophyllum white, rose 73

decorum Fortunei white, pink, chartreuse 73

cardiobasis Fortunei white, rose 73

praestans Grande magenta-rose, pink 73

hippophaeoides Lapponicum lilac, rose 73

eurysiphon Thomsonii ivory, rose 73

imperator Uniflorum pink, rose 70

concatenans Cinnaborinum apricot, yellow 70

yunnanense Triflorum white, lavender, pink 70

ciliatum Maddenii white, rose 70

discolor Fortunei white, pink 70

davidsonianum Trflorum white, pink, rose 70

*The higher the number, the less feeding is expected_ A 100 rating indicates-complete-resistance.

30

APPENDIX B

Hybrid Rhododendrons ShowingResistance to Feeding by Adult Root Weevils

Hybrid Possible Blossom Colors Rating*

P.J. Mezzitt (P.J.M.) pink 100Jock pink 92

Sapphire blue 90

Rose Elf white, flushed violet-pink 89

Cilpinense white 88Lucky Strike deep salmon-pink 83

Exbury Naomi lilac tinged yellow 81

Virginia Richards Chinese yellow with crimson blotch 81Cowslip cream, pink 80Pride of Leonards lee (Luscombei) rose-pink 80

Vanessa soft pink 80

Oceanlake deep violet-blue 80Dora Amateis white, lightly spotted green 79

Crest yellow 79

Rainbow carmine-pink 76

Point Defiance pink 76

Naomi pink 76

Pilgrim rich pink 76Letty Edwards yellow 76

Odee Wright yellow 76

Moonstone yellow 73

Lady Clementine Mitford pink 72Candi bright rose 72

Graf Zeppelin bright pink 71

Snow Lady pure white 71

Loderi Pink Diamond delicate pink 71

Faggetter's Favourite cream with pink 70

*The higher the number, the less feeding is expected. A 100 rat ing indicates complete res is tance.

Additional copies of this publication are available by mail from Bulletin Office,

Cooper Publications Building, Washington State University, Pullman, Washington

99164-5912. For order information, call 509-335-2857.

Washington State University Cooperative Extension also has hundreds of other

publications available of interest to gardeners of many different levels of skill,

experience, and ability, as well as publications in a wide range of other subject

areas. For a complete list, request Educational Materials, C0506, from anycounty Extension office, or from the address above. It is free.

Cover photograph is courtesy of Jay Lunn, Hillsboro, Oregon.

College of Agriculture and Home Economics, Pullman, Washington

Issued by Washington State University Cooperative Extension, Harry B. Burcalow, InterimDirector, and the U.S. Department of Agriculture in furtherance of the Acts of May 8and June 30, 1914. Cooperative Extension programs and policies are consistent withfederal and state laws and regulations on nondiscrimination regarding race, color, gender,national origin, religion, age, disability, and sexual orientation. Evidence ofnoncompliance may be reported through your local Cooperative Extension office.Trade names have been used to simplify information; no endorsement is intended.Slightly revised July 1993. Reprinted July 1995. Subject Code 541. H. EB 1229

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