Avoiding Top Problems of Poinsettias
Poinsettias can, at times, be a problematic crop, but beingaware of the more common nutritional, physiological and insect anddisease-related problems encountered during production can help reduce theheadaches. By watching for potential problems, understanding their causes andknowing the proper corrective actions to take, one can avoid most of theseissues.
Nutritional and physiological disorders
High pH. High substrate pH can induce nutrient problems inpoinsettias. The recommended pH range in a soilless substrate is 5.8-6.5. Irondeficiency is the most common problem at a substrate pH above 6.5, at whichpoint new leaves will exhibit interveinal chlorosis (yellowing). Lower the pHwith an iron sulfate drench or by using an acidic fertilizer. Iron deficiencycan also occur with root death, over-irrigation, poor substrate drainage orinsect damage. Inspecting the roots will help determine the cause of theproblem.
Low Electrical Conductivity.
Calcium Deficiency. Symptoms appear at the top of the plant. Young leavesmay develop variable patterns of chlorosis and distortion such as dwarfing,strapping or marginal leaf burn. Leaf edges may become necrotic. Typically,symptoms in young plants occur during periods of overcast weather when theplant’s ability to uptake calcium is inhibited. Bracts can also develop amarginal necrosis. Sunny weather, avoiding water-logged substrates, lowhumidity and calcium chloride or calcium nitrate foliar sprays will helpprevent the problem.
Magnesium Deficiency. Lower leaves develop interveinal chlorosis; underadvanced conditions, the leaf margins turn necrotic. On younger Áplants, symptoms appear on the lower leaves. On flowering poinsettias, symptomstend to develop on the top half of the plant. One or two applications ofmagnesium sulfate (1 lb. per 100 gal. of water) or a magnesium-containingfertilizer will help prevent the problem.
Molybdenum Deficiency. Symptoms appear as chlorosis (yellowing) of therecently mature leaves (middle of the plant), rolling of the leaves and leafedge burn. The leaf chlorosis of molybdenum deficiency resembles magnesiumdeficiency, except that the thin, marginal band of chlorosis is expressed fromthe leaf tip to the leaf base. Molybdenum deficiencies can cause distortedleaves due to the failure of the interveinal areas to expand normally. Monthlymolybdenum applications should be made to poinsettias.
Leaf Distortion. Plants develop distorted or cupped leaves, most oftenduring the early stages of the crop (within a few weeks of being pinched). Mostpoinsettias will outgrow this condition, but shoots with extreme distortion maynot improve. It is unclear what causes this disorder, but thought that rapidchanges in humidity, which can occur early in the morning when the vent fanscome on, may lead to an accumulation of salts along the leaf margins and veins,resulting in leaf injury. Leaf distortion becomes apparent as these injuredleaves grow and expand.
Excessive Plant Stretch.
Fungus Gnats and Shoreflies.
Western Flower Thrips. Less than one-sixteenth of an inch long, westernflower thrips have piercing-sucking mouthparts that cause direct damage topoinsettias by feeding on leaf buds before they open. This results in leaf scarringand new growth distortion, as the remainder of the tissue expands around areasfed upon by the thrips. Poinsettias are not susceptible to the virusestransmitted by thrips, including impatiens necrotic spot virus (INSV). Althoughnot a primary host, thrips will feed on poinsettias if no other food source isavailable in the greenhouse. Control measures include spinosad (Conserve),abamectin (Avid) and methiocarb (Mesurol).
Whiteflies. Whitefly adults are white to slightly yellowishin color, narrow-shaped and approximately one-sixteenth to one-eighth of aninch long. Most of the whitefly stages are located on the undersides ofpoinsettia leaves. Whitefly “crawlers” hatch and crawl about,inserting their threadlike mouthparts into the lower leaf surface to feed onplant fluids. This may result in leaf yellowing, plant stunting, plant wiltingand plant death (if populations are high enough). Whiteflies produce honeydew,a clear, sticky liquid, during feeding that serves as an excellent medium forblack, sooty mold fungi. Control measures include imidacloprid (Marathon),pymetrozine (Endeavor), pyriproxyfen (Distance)*, potassium salts of fattyacids (Insecticidal Soap), Beauveria bassiana (Botanigard/Naturalis),bifenthrin (Talstar), cyfluthrin (Decathlon), kinoprene (Enstar II) andpyridaben (Sanmite). (*Distance cannot be used on poinsettias when bracts arevisible.)
Spider Mites. Spider mites are oval-shaped and can be yellow-orange,green or red. Adult mites have two dark spots on both sides of the abdomen.Spider mites feed primarily on leaf undersides and remove chlorophyll (greenpigment) with their stylet-like mouthparts. Damaged leaves appear stippled or”dirty” with small, silvery-gray to yellowish speckles. Webbing maybe present if populations are high. Spider mites prefer warm, dry conditionswith low relative humidity. Lewis mite injury to poinsettias is similar to thatof the Á two-spotted spider mite. Lewis mites are smaller and haveseveral small black spots on both sides of the body. Control measures includeabamectin (Avid), bifenazate (Floramite), bifenthrin (Talstar), pyridaben(Sanmite), clofentezine (Ovation), hexythiazox (Hexygon), fenpyroximate (Akari)and chlorfenapyr (Pylon).
Botrytis. (Gray Mold) quickly colonizes damaged or senescing plant tissuesand initially appears as brown spots on the leaves and flower bracts. Largenumbers of gray to olivaceous green spores are produced on the infectedtissues. This fungal pathogen thrives under cool temperatures around 68-77°F. The relative humidity within the plant canopy should be kept below 93percent throughout the production cycle. Large, tan, sunken cankers may form onthe older stems, girdling the stem. Promptly remove fading flowers and deadplant tissue from the greenhouse. Fungicides such as chlorothalonil,fenhexamide and fludioxonil, in conjunction with good cultural practices, canhelp prevent this disease. Carefully read and follow label precautions to avoiddamaging flower bracts.
Powdery Mildew. This pathogen can be moved from greenhouse to greenhouse oninfected cuttings or plants. Yellow spots form on the upper surface of leavesindicating the location of grayish-white fungal colonies on the undersides ofthe leaves. Characteristic white, powder-like colonies may also be present onboth leaves and flower bracts. However, it is important to control this diseasebefore the bracts become infected, as the white fungal patches are not”erased” by fungicide applications. Frequent and careful scoutingof the crop for powdery mildew symptoms should begin when greenhousetemperatures begin to drop below a daytime high of approximately 86° F.Temperatures above this point are inhibitory to the infection process. Powderymildew can be controlled with the application of fungicides such asmyclobutanil, kresoxim-methyl, piperalin, triadimefon, triflumizole andtrifloxystrobin when fungal colonies are first observed. Piperalin does havecurative activity, but like all fungicides, it cannot erase the damage alreadydone.
Pythium Root Rot. This is most likely to show up early in the season,soon after cuttings are planted. The base of the cutting will appear brown, andseverely infected cuttings will wilt and quickly die. Infected plants areusually stunted and show signs of wilt during the heat of the day. Infectedroots are dark brown, and the outer cortex of the root is easily slipped off,leaving a thread-like strand of vascular tissue. Plants that survive untilflowering will often flower prematurely and defoliate. Control of this pathogenis difficult once infection has begun. Sterile potting mix and disinfestationof work surfaces will help avoid the contamination and spread of this fungalpathogen. Fungicides and biological control agents can be applied asprotectants to help manage this disease. Preventative applications ofmefenoxam, propamocarb or etridiazole may be applied.
Rhizoctonia stem rot. An important disease most likely to occur duringpropagation, Rhizoctonia stem rot causes small lesions to develop at the pointon the stem that is even with the top of the rooting cube. If the rooting cubeis later transplanted too deeply, it may appear that the lesion is originatingat the roots. Stem lesions have a dry appearance, with a tan center and darkborder. The stem lesion may expand, girdling and quickly killing the cutting.Web-like hyphae can sometimes be seen at the base of the rooting cube. Leavesmay become infected when they contact the potting mix or bench surface and canbecome quickly colonized. The fungal infection can move down the leaf andinfect the stem. Once a cutting strip is infected, the fungus can move throughthe entire strip and infect other cuttings. The entire strip should bediscarded if any diseased cuttings are found. Fungicides such as chlorothalonil,flutolanil and iprodione have been shown to prevent colonization of the rootingcube by this fungus.
Scab (Sphaceloma poinsettiae).