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output, assuming a 70% efficient natural gas AirCirculation. Adequateairmovement heating system would be -60. If you exchange around and within the plant canopy is necessary aireveryeveningduring September 1-December to prevent condensation on foliage that could 15, you have increased your heating bill for the lead to botrytis blight. Also, as plants transpire, entire greenhouse byonly$6.36. Thecost ofone the airimmediately surrounding leavesbecomes fungicide application (material + labor + saturated with water vapor and RH approaches equipment depreciation) for botrytis control in 100%, increasing the chances of botrytis the same greenhouse would be greater than the infection. Proper air circulation can increase increase in the heating cost. foliar transpiration and increase evaporation of Temperature. Temperaturemanipulation soilmoisturefrom the soil surface, thus removing for control ofpests is more difficult than control excess soil moisture. Unfortunately, all the ofrelative humidity. The greenhouse should be additional watervaporincreases the ambientRH keep close to temperatures optimum for plant of the greenhouse, so air exchanges become production. However, growers should be aware even more important. oftemperature and how it affects pesticides and Spacing. Crowding plantsresults in leggy pests. Forexample, pesticide applicationsmade growth and stretching of shoots. Inadequate in the middle of the day when greenhouse spacing also reduces aircirculation, can increase temperaturesarehighestcanleadto phototoxicity disease potential, and decreases pesticide anddamage plants. Ifgreenhousesaremaintained coverage. It is recognizedthat a 15" x 15"is not too cool, much below 60°F, the incidence of aseconomical as 12" x 12" ifthe onlyeconomic Pythium and Thielaviopsis can increase. On the factor considered is $/ft2, but crop quality and other hand, pathogens such zsPhytophthora and cost ofpest controls must also be considered as Rhizoctonia prefer warmer temperatures and economic factors when deciding on spacing. can increase in importance if houses are In summary, effective and efficient pest maintained too warm. Growers can reduce the management addresses pest control from the possibilityofbacterialsoft rotduring propagation pest, plant, and environmental angles in order to by maintaining moderate temperatures; integrate control techniques into a economical evaporative cooling can prevent excessive program that protects crop quality and the temperatures that bacterial soft rot prefers. environment.

POINSETTIA DISEASES AND THEIR MANAGEMENT

Ronald K. Jones, Extension Plant Pathologist

Diseases and theirmanagement must be control and as a last resort, the proper use of an integral part of a poinsettia production pesticides. program. Diseases are a threat to a poinsettia Description ofMaiorPoinsettiaDiseases. crop from the establishment of stock to sale of The importance and control of each of the finished plants. The management of the many following diseases will be discussed for each diseasesthatoccuron poinsettia(Table 1)require phaseof production: stock plants, propagation a totally integrated program that includes and finishing. sanitation, cultural practices, environmental N.C. Flower Growers Page 7

Table 1. Diseases ofPoinsettia.

Common Name ofDisease Pathogen Importance in N.C. * Occurrence**

Stem and Crown Rot Rhizoctonia solani ++++ SJPJ RootRot Pythium spp. ++++ S.PJF Phytophthoraparasitica + Sf Thielaviopsis basicola + F

LeafSpot Alternaria euphorbiicola - S

Bacterial LeafSpot Xanthomonaspoinsettiicola - F Bacterial Soft Rot Erwinia carotovora +++ P Botrytis Blight (Gray Mold) Botrytis cinerea im S.P.F Bacterial Canker Corynebacteriumpoinsettiae + F

Crown Gall Agrobacteriumtumefaciens - S

Rust Uromyces euphorbiae - F f. poinsettiae

Root Knot Nematode Meloidogyne spp. - F

Scab Sphaceloma poinsettiae - S.F

Crown Rot Sclerotinia sclerotiorum - SJ7 Watery Rot Rhizopus spp. ++ S.P Virus Poinsettia mosaic virus + S^JF Wet Rot Choanephora cucurbitarum ++ S,P

Greasy Canker Pseudomonas viridaflava - S.F

♦Relativeimportance in N.C: (++++) most important;(-) very rareor does not occur. **Stage ofproduction: S = Stock plants;P = Propagation; and F = Finish. Diseases Caused by Fungi increases as soil temperature is increased from 17 to 26°C (62-80°F). Soil moisture levels RhizoctoniaStem Rot. This is oneofthe below 40% moisture holding capacity (MHC) most important diseases on poinsettia as it kills are not favorable for Rhizoctonia stemrot and a plants in all stages of production (see Table 1). MHC above 80% retards disease development. Rhizoctonia solani causes a light to dark brown Pythium and Phytophthora Root Rot water soaked lesion on the stem extending just Root rot ofpoinsettia can be caused byPythium above and just below the soil line. The lesion aphanidermatwn, P. debaryanum, P. irregular, gradually expands until it encircles the stem. P. megalacanthum, P. oligandum, P. Thelesion becomes slightly sunkenand remains perniciorum, P. polymastum, and P. ultimum. as a dry rot. Infection often begins during Poinsettia root rot is also caused by the closely rooting and may kill plants before or after related fungusPhytophthoraparasitica. Disease transplanting. Ifthe disease progresses slowly, severity mayvary betweenthe above speciesbut plants may not die until mid- to late-November. the conditions for disease development and Plants with stem rot may be stunted, symptoms are quite similar. Rootrot caused by lower may leaves abscise, upper leaves may be these fungi may be the mostcommondisease on chlorotic and the leaf edge may curl upward poinsettia in North Carolina. It can occur at any along the midrib. If the disease progresses stage of production but often shows up in rapidly,the foliage wiltssuddenlyand plants fall November or early December. over and die. In later stages ofthe disease, roots The primary symptoms of root rot are may also be discolored. badly rotted dark wet roots. The rot mayaffect Development ofRhizoctonia stem rot is stems to varying degrees depending upon the not affected by pH. Disease development age of plant and upon cultural conditions. If N.C. Flower Growers Page 8

roots of very young succulent plants become color. Disease development is favored by soil infected, the pathogen may proceed up the stem moisture holding capacity by 70% or higher. severalinchesandcausetheentirestemtobecome Thielaviopsis root rot is also favored by media water-soaked, then turn dark brown to black. pH of5.5 or above. Stems maycollapse at the soil line and fall over. Botrytis Blight or Gray Mold. Botrytis Infected plants can be stunted, have blight, commonlycalled gray mold,is causedby chloroticleaves,andlowerleavesabscise. Plants the fungus Botrytis cinerea. This is the most infected with Pythium or Phytophthora that importantdisease on floral cropsandit is alsothe survive the disease in early growth stages may most important above ground disease on bloom early or wilt, collapse, and die. poinsettiasgrown in North Carolinagreenhouses. Pythium and Phytophthora root rot is Botrytis blightcan occuron poinsettiaduring all favored by high soil moisture. Disease stages of production. development may be reduced by lowering the This fungus causes brownlesions on leaf media pH to 5.5 or below. Moderate to cool stemorbracttissue. Infection andinitial disease media temperatures are best for disease development almost always occur on wounded, development forPythiumwhere asPhytophthora weakened or dead tissue of leaves, stems or root rot is usually more severe under warm bracts. Healthy tissues that comein contactwith conditions. Pythium and Phytophthora root rot diseased tissues almostalwaysbecomeinfected. is also favored by high soluble salts. Lesions on bracts are initially located near ThielaviopsisRoot Rot (Black Root Roti. margins and are minute in size. First they are This rootrot disease is less common now than it light tan in color butbecomedarker brownas the was 30 to 40 years ago. This is probably due to lesion expands. Latex may be exuded on the theeliminationofsoil from mediaandtowarmer underside ofleaflesions. Under high humidity, growingtemperatures. This disease has become Botrytis produces grayish brown spore masses more important over the past 4 to 5 years on overthe surface ofthe rotting tissue. Millions of pansy and vinca plugs. This raises concern spores may be produced on each rotting lesion about re-introduction of this fungus into and each spore is capable of causing a new greenhouses. infection. Spores areeasilydislodgedandcarried The above ground symptoms of by air currents or splashing water to healthy Thielaviopsis root rot and Pythium root rot are plants. very similar. Affected plants are stunted with Botrytis cinerea attacks over 200 hosts badly rotted roots. Initial root symptoms are and is not host specific. Epidemics of Botrytis speckled light brown lesions. As the disease blight are favored by high relative humidity progresses, the root system may turn brown and (85% or above) and free moisture on the plant later black. Longitudinal black cracks develop surfaces. The fungus is active over a wide on lower portions ofstems. These black cracks temperature range. Growing plants with very on the lower portion ofrotting poinsettia stems tight spacing, plants with dense foliarcanopies, infected with Thielaviopsis distinguish it from plants with excessive shade, and plants with Pythium, Phytophthora and Rhizoctonia root very succulent growth are also favorable for rots. disease development. Thielaviopsis root rot is favored by cool Scab. Scab or spot anthracnoseis caused soil temperatures of13to 16°C (55-60°F). This by the fungus Sphaceloma poinsettiae. Raised root rot is most likely to develop late in the circular to elongated lesions from a few mm to production cycle when plants aregrown cool to one cm appear on stems and midribs of leaves. retard flower development and intensify bract Lesions are usually light tan and often are N.C. Flower Growers Page 9 surrounded by a purplish-red margin. Alternaria Blight or Leaf Spot Anew Occasionally, lesions coalesce resulting in the leafspot or blightdisease ofpoinsettiacausedby girdling and death of stems. Brown spots up to the fungus Alternaria euphorbiicola was first onecm appear on leafblades. These are usually observed in Florida in 1984. Symptoms ofthis concave on the upper surface giving the leaf a disease can be confusedwith bacterial cankeror puckered appearance. Severely affected leaves scab. This disease has not been reported in drop prematurely. A grayish-brown velvety North Carolina. layer offungal growth (spores) may be found in Leafinfections initially appear as 1 to 3 the depressed spots on stems. Conidia arespread mm diameter spots with tan centers, dark thin to healthy plants by splashing water. margins and a chlorotic halo. As spots enlarge, Scab is a more common disease on theyoftenprogress along lateralleafveins. Spots poinsettias grown outside in Florida than it is enlarge to form irregular necrotic lesions, 25 under greenhouse production in North Carolina. mm across or larger. Leaves with several spots Wildhostsoutsidethe production areasin Florida become chlorotic and abscise. are important sources ofthe pathogen. Spores of Alternaria euphorbiicola are Wet Rot. Wet rot caused by the fungus air-borne and can survive dry periods to cause Choanephora cucurbitarumoccurson both stock disease when moisture becomes available. plants and on cuttings under mist propagation. Environmentalconditionsin outdoorproduction The pathogen is capable of killing plants in all areasaremorefavorable fordiseasedevelopment stages of production. than greenhouseconditions, presumablybecause The symptoms of wet rot on poinsettia of frequent prolonged periods of leaf wetness. plants are a soft, wet, mushy decay of leaves, Outdoor production and wild host sources of petioles and stems. If the stems are green and spores makethis animportantdisease in Florida. actively growing, they become soft, wet, flaccid It should be less important in North Carolina if and wiltor droop beforecollapsing. Ifthe stems growers start with disease-free plants and grow are somewhat woody, only necrosis occurs. them in greenhouses using a watering system Small plants and rooting cuttings may be that keeps the foliage dry. destroyed in only a few days when weather conditionsarefavorable for disease development. Diseases Caused by Bacteria On large multi-stemmedstockplants, the fungus may kill the plant stem by stem. Bacterial Canker. Bacterial canker is The pathogen usually sporulates caused by Corynebacterium poinsettiae. It is profusely on diseased plant parts, especially on not as wide spread today as in the past, probably stems. The fungus produces single celled due to healthier rooted cuttings and use of sporangiophores (stalks), each bearing a cluster watering systems that keep above ground plant ofblackspores on the end. The sporangiophores parts dry. arelong andveryabundant, givingthe appearance Themostprominentsymptomsarewater- of a coarse, hairy or "whiskery** growth. The soaked areas on the lower stemornarrow water- spores can be spread bywater, wind, movement soaked streaks often extending several of plants (cuttings), hands, knives and insects. centimeters along branches. These lesions are Under North Carolina conditions, this is associated with exudeddroplets oflatex that dry primarily a disease ofstock plants and cuttings to form pale yellow or golden crusts. Infected under mist This fungus attacks the flowers a plants frequently have malformed, small bracts, wide variety of plants including squash, okra, arestunted andretardedslightlyin development petunia and hibiscus. Under conditions of low moisture, marginal N.C. Flower Growers Page 10

wilting of leaves has been observed. Severely Control of Poinsettia Diseases. diseased branches maybe girdled anddie. Plants may grow out of visible symptoms yet retain Control ofpoinsettia diseases requires a internallow populationsofthebacteria. Cuttings total program starting with goodplanningbefore taken from these plants frequently show disease the plants arrive at the greenhouse and end with symptomsin later stages ofproduction. In North clean up after the last plant has been sold. Steps Carolina, this disease is often seen in late stages in a poinsettia disease control program include: of finishing. 1. Do not carry any plants over from one The disease is spread by vegetative season to the next propagationofcuttings from apparentlyhealthy 2. Clean all benches where poinsettias will plants. The bacteria can also be spreadduring be grown. pinching, by infested irrigation water, and on 3. Have clean pots and media available for knives when taking cuttings. potting. Bacterial SoftRot Softrotcausedbythe 4. Try to avoid any unnecessary wounding bacteriaErvvmiacarotovora andE. chrysanthemi or bruising ofthe plants. is acommon disease problem in North Carolina 5. Purchase plants or cuttings from a duringandjust afterpropagation. These bacteria reputable supplier. are most likely to cause problems on succulent 6. Inspect the plants on arrival for any tissue, under hot wet conditions; wounded or problems. bruised tissue is highly susceptible. Infected 7. Try to grow plants as close to optimum tissue becomes water soaked, and develops into conditionsas possibleto minimizestress. a soft mushy rot. The disease can develop very 8. Keep the relative humidity below 85% rapidlyon cuttings undermist. Afterpropagation, whenever possible. Erwiniachrysanthemicauseschlorotic spotsthat 9. Maintain temperatures below 90-95°F rapidlycoalesce forminglargeirregularchlorotic whenever possible. areas. Petioles turned black, shrivel and 10. Use a watering system that keeps the associated leaves wilt within a few hours. foliage dry. Chlorotic areasturned black, stems collapse and 11. Space plants properly to promote good affected tissues become soft and water soaked. air movement between the plants. GreasyCankerorBacterial StemCanker. 12. Have goodhorizontalairmovementover Greasy canker caused by the bacteria the crop. Pseudomonasviridaflava hasbeenreportedfrom 13. Try to preventcondensation on the plant California and Florida. This bacterial disease surface during the night. appears as rapidly advancing oily or greasy 14. Chemical protection: canker associated with pruning wounds. Leaf (a) fungicides can help prevent root rot spotbractand bud blighthavealsobeenobserved. diseases and Botrytis blight Thisdiseaseoccursat all temperatures as (b) drench with the proper combination long as the relative humidity is high. Disease of fungicides on a preventative program severity increases at higher temperatures (27- for root rot (see Table 2) 32°Cor80-90°F). Disease progress is arrested (c) repeat fungicide drenches monthly when relative humidity is reduced; infected for root rot control tissue then becomes tan to brown and papery. (d) use sprays or fumigate as needed to control Botrytis blight (e) alternate fungicides for Botrytis control to avoid resistance. N.C. Flower Growers Page 11

Table 2. Some fungicidesthat can contribute to a total poinsettiadiseasecontrol program based upon sanitation, sterilization, and cultural practices.

Common and tradename Chemical Comments benomyl methyl l-(butylcarbamoyl)-2- Use as a drench againstRhizoctoniasolani Benlate benzimidazole-carbamate and Thielaviopsis basicola and as a foliar spray againstBotrytis cinerea. chlorothalonil tetrachloroisophthalonitrile Use Daconil as a spray against Botrytis Daconil2787 diseases. Exotherm Termil is heated to Exotherm Termil generate dust for greenhouse space treat ment. Do not apply under slow-drying conditions. metalaxyl N-(2,6-Dimethylphenyl)-N- Specific for water mold. Pythium spp. and Subdue (methoxyacetyl)-alanine Phytophthora spp. methyl ester etridiazole 5-Ethoxy-3-trichloromethyl- Specific for Pythium and Phytophthora Truban 1,2,4-thiadiazole species. etridiazole 5-Ethoxy-3-trichloromethyl- Use as a drench against major root rot 1,2,4-thiadiazole organisms. + thiophanate methyl dimethyl4,4-o-phenylenebis Banrot (3-thioallophanate)

PCNB pentachloronitrobenzene SpecificforRhizoctonia solani. NOT Terraclor effective against water molds. Good residual action.

Vinclozolin 3-(3,5-dichorophenyl)-5- Specific for the control of Botrytis blight. Ornalin vinyl-5-methyl-l,3- oxazolidine-2,4-dione

Fungicide recommendations often change from year toyear. Forthe most current recommendations, consult the latest edition ofthe North CarolinaAgriculturalChemicalsManual.