Common greenhouse poinsettia production problems

Kristin Getter, Michigan State University Extension, Department of Horticulture, and Tom Dudek, Michigan State University Extension - August 19, 2013

Poinsettia growers should be on the lookout for these common greenhouse production problems.

Poinsettias (Euphorbia pulcherrima) are susceptible to a variety of issues when produced in a greenhouse. It is imperative for growers to monitor their crops carefully to avoid such problems. Proper sanitation and cultural control such as removing diseased or insect infested plants debris from the greenhouse, disinfesting benches and equipment, using well-aerated substrate, spacing plants for proper air movement, and venting to maintain low humidity will also help to reduce problems. Once a problem is identified, prompt corrective measures should be taken to avoid major plant losses.

Common insect pests found during greenhouse production of poinsettia include whiteflies, fungus gnats, thrips, shoreflies and spider mites (Photo 1). Monitor insect populations by placing yellow sticky cards – one card just above the crop every 2,000 square feet, replacing weekly – and scouting the crops regularly, randomly picking up pots and searching all portions of plants for problems. This can help growers determine when and how often insecticides should be applied. See Table 1 for recommended chemical controls for common poinsettia insects and mites.

Photo 1. Common insect pests of greenhouse poinsettia production. Top row, left to right: whitefly, fungus gnat, thrips. Bottom row, left to right: shorefly, spider mites. Photo credit: Dave Smitley, MSU

Table 1. Chemical control recommendations for common poinsettia insects and mites

Pest	Chemical control recommendation*
Fungus gnats	Azatin XL, Benefit, Distance, Marathon, Flagship, Tristar or Safari as a soil drench.
Shore flies	Azatin XL, Distance
Spider mites	Akari, Avid, Floramite, Hexygon, Judo, Kontos, Ovation, ProMite, Pylon, Sanmite, Shuttle-O, Tetrasan
Thrips	Aria, AzaGuard, Axatin XL, Mesurol, Orthene 97, Overture, Pedestal, Pylon, Sanmite
Whiteflies	See Michigan State University Extension article “Whitefly management on poinsettia for fall 2013”

* Follow all label instructions and note warnings; local restrictions may apply. Bracts are especially sensitive to chemical injury, so control of a pest before bract formation is advisable. Test a product that is new to you or to a particular crop on a small number of plants three to five days before treating the whole crop to observe any phytotoxicity symptoms that may occur.

Potential disease problems include a variety of root and stem rots caused by several organisms including Rhizoctonia, Pythium or Thielaviopsis; Botrytis blight; powdery mildew; poinsettia scab; and bacterial soft rot. For descriptions and pictures of the symptoms that these pathogens produce in poinsettia, see Plant Pathology Fact Sheet Poinsettia Diseases by John R. Hartman and Cheryl A. Kaiser, University of Kentucky Cooperative Extension Service, or Poinsettia: Some Common Diseases of the Christmas Flower by Mike Benson, North Carolina State University Plant Disease and Insect Clinic.

For cultural and chemical control recommendations, see Table 2. If you are unsure what is the disease problem, send samples to the MSU Diagnostic Services lab to obtain an accurate diagnosis.

Table 2. Suppression techniques and chemical control recommendations for common poinsettia diseases

Pest/Problem	Suppression techniques	Chemical control recommendation*
Bacterial soft rot	Avoid waterlogging the media and keep propagation temperatures below 90 degrees Fahrenheit.	According to The Ecke Poinsettia Manual (published in 2004 by Ball Publishing), copper-based products are often used to reduce the incidence of this pathogen.
Botrytis blight	Avoid wet foliage during irrigation, reduce greenhouse humidity, increase spacing and air circulation, keep temperatures above 60 F and remove all dead plant material.	See Controlling Other Greenhouse Plant Diseases: Botrytis, Pythium, Powdery Mildew, Rhizoctonia, and Thielaviopsis by Blair R. Harlan, and Dr. Mary K. Hausbeck, Michigan State University.
Poinsettia scab	Avoid high humidity and waterlogged media.	See Sample of the Week: Poinsettia scab by Mike Munster, North Carolina State University Plant Disease and Insect Clinic.
Powdery mildew	Increase air movement and reduce humidity.	See Special Research Report #118: Managing Powdery Mildew on Poinsettia by Mary Hausbeck, Michigan State University.
Pythium	Rogue infected plants, avoid waterlogged media, and reduce fungus gnat and shore fly population that may contribute to the spread of this disease. Avoid high soluble salts.	See Controlling Other Greenhouse Plant Diseases: Botrytis, Pythium, Powdery Mildew, Rhizoctonia, and Thielaviopsis by Blair R. Harlan, and Dr. Mary K. Hausbeck, Michigan State University.
Rhizoctonia	Avoid planting too deeply and injuring the stem while planting. Also avoid high soluble salts.	See Controlling Other Greenhouse Plant Diseases: Botrytis, Pythium, Powdery Mildew, Rhizoctonia, and Thielaviopsis by Blair R. Harlan, and Dr. Mary K. Hausbeck, Michigan State University.
Thielaviopsis	Rogue infected plants, avoid low media temperatures and avoid high soil pH.	See Controlling Other Greenhouse Plant Diseases: Botrytis, Pythium, Powdery Mildew, Rhizoctonia, and Thielaviopsis by Blair R. Harlan, and Dr. Mary K. Hausbeck, Michigan State University.

Other production problems include bract or leaf edge necrosis (burn) that is thought to be caused by a localized calcium deficiency. Another problem is poor and uneven branching that can be caused by waiting too long to pinch, high temperatures, or over-crowded plants on benches or floors. Leaf distortions can also occur especially on young leaves and are likely due to environmental stressors such as physical damage during pinching or handling, excessive phosphorous absorbed by the leaf during overhead fertigation, or large changes in temperature or humidity or insect damage such as from thrips. Split bracts can occur when premature flowers are initiated, such as when a plant is exposed to a period of short days, followed by long days.

For cultural recommendations to control such production problems, see Table 3.

Table 3. Cultural control of common poinsettia production issues

Production problem	Cultural recommendations
Bract or leaf edge necrosis	Increase air circulation so that ample transpiration is occurring for calcium movement inside the plant, use fertilizer with calcium in it, or a calcium spray.
Poor and uneven branching	Avoid late season pinches and high greenhouse air temperatures, and space plants properly to minimize stretch.
Leaf distortions	Avoid physical damage to the plant. Avoid fertilizers with an excessive amount of phosphorous. Avoid large changes in temperature or humidity. Control thrips populations.
Split bracts	Make sure correct photoperiods are being provided.

For more detailed information about these and other poinsettia production problems, see:

Avoiding Top Problems of Poinsettias by Brian E. Whipker, Colleen Warfield, Raymond Cloyd, James L. Gibson, and Todd J. Cavins, North Carolina State University
Poinsettia Production Problems and Disorders by Roberto G. Lopez, Purdue University
Common Problems In Northern Poinsettia Production by John Erwin, Department of Horticultural Sciences, University of Minnesota
Poinsettia Propagation by Roberto G. Lopez, Purdue University

Contact your local MSU Extension floriculture educator for more assistance with this or any other greenhouse-related issues.

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