N.C. Flower Growers Page 6 output, assuming a 70% efficient natural gas AirCirculation. Adequateairmovement heating system would be ~6#. If you exchange around and within the plant canopy is necessary airevery evening duringSeptember 1-December to prevent condensation on foliage that could 15, you have increasedyour heatingbill for the leadto botrytis blight. Also, asplants transpire, entiregreenhouseby only $6.36. The costofone theairimmediately 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 andincrease evaporation of Temperature. Temperature manipulation soil moisturefrom the soil surface, thusremoving forcontrol ofpests is more difficult thancontrol excess soil moisture. Unfortunately, all the ofrelative humidity. The greenhouse should be additional watervaporincreases theambientRH keep close to temperatures optimum for plant of the greenhouse, so air exchanges become production. However, growersshouldbe aware even more important. oftemperature and how it affects pesticides and Spacing. Crowdingplantsresultsinleggy pests. Forexample, pesticide applicationsmade growth and stretching of shoots. Inadequate in the middle of the day when greenhouse spacing also reduces air circulation, canincrease temperatures arehighestcanleadtophytotoxicity disease potential, and decreases pesticide anddamageplants. If greenhouses are maintained coverage. Itisrecognized thata 15" x 15" isnot too cool, much below 60°F, the incidence of aseconomical as 12" x 12"ifthe onlyeconomic Pythiumand Thielaviopsis can increase. On the factor considered is $/ft2, but crop quality and otherhand, pathogens such asPhytophthoraand cost ofpestcontrolsmust alsobe 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 possibilityofbacterialsoftrotduring propagation pest,plant, andenvironmentalanglesin orderto 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 their management 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 DescriptionofMajorPoinsettiaDiseases. 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 diseases thatoccuronpoinsettia (Table 1) require phase of 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 ++++ S.P.F RootRot Pythium spp. ++++ Sff Phytophthoraparasitica + SJF Thielaviopsis basicola + F LeafSpot Alternaria euphorbiicola - S Bacterial LeafSpot Xanthomonaspoinsettiicola - F Bacterial Soft Rot Erwinia carotovora +++ P Botrytis Blight (Gray Mold) Botrytis cinerea ++++ S?F Bacterial Canker Corynebacteriumpoinsettiae + F Crown Gall Agrobacterium tumefaciens - S Rust Uromyces euphorbiae - F f. poinsettiae Root Knot Nematode Meloidogyne spp. - F Scab Sphaceloma poinsettiae - S.F Crown Rot Sclerotinia sclerotiorum - S.F Watery Rot Rhizopus spp. ++ S.P Virus Poinsettia mosaic virus + S?F Wet Rot Choanephora cucurbitarum ++ S.P Greasy Canker Pseudomonasviridaflava - SF ♦Relativeimportance in N.C.: (++++) mostimportant; (-)veryrare ordoes notoccur. ♦♦Stage ofproduction: S = Stock plants; P = Propagation; andF =Finish. Diseases Caused bv 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 stem rot and a plants in all stages ofproduction (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 aphanidermatum, P. debaryanwn, P. irregular, gradually expands until it encircles the stem. P. megalacanthum, P. oligandum, P. Thelesion becomes slightly sunken and 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 fungus Phytophthoraparasitica. Disease transplanting. If the disease progresses slowly, severitymayvary betweentheabove speciesbut plants may notdie until mid- to late-November. the conditions for disease development and Plants with stem rot may be stunted, symptoms arequite similar. Root rot causedby lower may leaves abscise, upper leaves may be these fungi maybe the mostcommondisease on chlorotic and the leaf edge may curl upward poinsettia in North Carolina. It can occurat any along the midrib. If the disease progresses stage of production but often shows up in rapidly, the foliage wilts suddenlyandplants fall November or early December. over anddie. In later stages ofthe disease, roots The primary symptoms of root rot are may also be discolored. badlyrotteddarkwet roots. The rot may affect DevelopmentofRhizoctoniastemrotis 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. Diseasedevelopment is favored by soil infected, the pathogen may proceed up the stem moisture holding capacity by 70% or higher. severalinchesandcausetheentirestemtobecome Thielaviopsis rootrot is also favored by media water-soaked, then turn dark brown to black. pH of 5.5 or above. Stems maycollapse at the soil line and fall over. Botrvtis Blight or Gray Mold. Botrytis Infected plants can be stunted, have blight, commonlycalledgray mold,iscaused by chloroticleaves,andlowerleavesabscise. Plants the fungus Botrytis cinerea. This is the most infected with Pythium or Phytophthora that importantdisease on floralcrops anditisalsothe survive the disease in early growth stages may most important above ground disease on bloom early or wilt, collapse, and die. poinsettias grown inNorth Carolina greenhouses. Pythium and Phytophthora root rot is Botrytis blight can occur onpoinsettia during all favored by high soil moisture. Disease stages ofproduction. development may be reduced by lowering the This fungus causes brown lesions onleaf media pH to 5.5 or below. Moderate to cool stemorbracttissue. Infection andinitial disease media temperatures are best for disease development almost always occur onwounded, developmentforPythiumwhere asPhytophthora weakened or dead tissue of leaves, stems or root rot is usually more severe under warm bracts. Healthy tissuesthatcomeincontact with conditions. Pythium and Phytophthora root rot diseasedtissues almost always becomeinfected. is also favored by high soluble salts. Lesions on bracts are initially located near ThielaviopsisRoot Rot (Black Root Rot). margins and are minute in size. First they are This rootrot disease is less common now than it light tan incolor butbecome darker brown asthe was 30 to 40 years ago. This is probably due to lesion expands. Latex may be exuded on the theeliminationofsoil from mediaandtowarmer underside ofleaf lesions. Under high humidity, growingtemperatures. Thisdisease has become Botrytis produces grayish brown spore masses more important over the past 4 to 5 years on over the surfaceoftherottingtissue. Millionsof 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. Sporesareeasilydislodgedandcarried 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 systemmay 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 of stems. 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