Poinsettia Disease Primer:
Dr. Larry Barnes
Professor of Plant Pathology and Extension Specialist
Texas A&M University System
College Station, Texas
Poinsettia production is in full swing and growers should be aware of several disease problems that can cause at various production phases. Careful attention to sanitation, irrigation, spacing, pesticide application, and environmental conditions in the greenhouse can help minimize disease problems.
The number one most serious disease problem during cutting and propagation is bacterial soft rot caused by Erwinia carotovora and Erwinia chrysanthemi. The Erwinia bacteria are wound pathogens that are very aggressive on wounded tissue (cuttings). Warm greenhouse temperatures that are common in Texas during the cutting/propagation phase favor bacterial soft rot. Soft rotting bacteria are also greatly favored by wet conditions that are common during mist propagation. Soft rotting bacteria can easily be splashed from plant to plant resulting in extensive cutting loss.
Control- sanitation is the most important factor in minimizing bacterial soft rot problems. Strict attention to clean tools and aggressive removal of infected cuttings at the first indication of soft rot is mandatory. Chemicals are generally ineffective in controlling bacterial soft rot.
Transplant/Early Vegetative Growth:
Rhizoctonia stem and root rot can be a problem anytime in poinsettia production but it usually is more common during the first few weeks following transplanting, when temperatures are still warm. Poor vigor and/or plant wilt, usually accompanied by an obvious sunken brown lesion at and just above the soil line, is suggestive of Rhizoctonia stem and root rot. Poinsettia leaves that touch the surface of the medium can also become infected and rot, frequently with brown fungal strands of the fungus visible from the surface of the potting medium to the infected leaves.
Control consists of drench-applied fungicides, either preventatively or at the first sign of infection. Effective fungicides include Chipco 26019, Cleary's 3336, Domain, Terraclor, and Terraguard.
Problems that occur during this phase of production are more commonly related to various aspects of cultural problems such as nutritional stresses (most notably molybdenum deficiency), soluble salts, temperature stresses, etc.
Although not too common, fungal leaf spots do sometimes develop during this phase of production. Scab, which develops as circular spots with a distinctive brown margins surrounded by an obvious yellow halo, can cause problems as can Alternaria leaf spot, which develops as irregular brown-to-black leaf spots. Both have occurred in Texas production but are not common problems. Wet foliage seems to favor both of these fungal leaf spotting diseases; tube-watered plants are likely at minimal risk. Chipco 26109 would likely be effective for control of either if needed.
The most consistent and serious disease problem at this stage of poinsettia production is Pythium root rot. Symptoms usually become apparent when plants wilt, either rapidly or slowly with associated poor growth. Examination of the root system shows blackened, deteriorated roots, with a mushy consistency. Greenhouse temperatures during this phase of poinsettia production are usually cooler to enhance bract color intensity, but cool greenhouse temperatures favor cool soils and reduced water usage by the plant, and these conditions favor Pythium root rot. Control of Pythium root rot consists of careful irrigation scheduling during this time of reduced water usage as well as the use of fungicide drenches if needed. Subdue, Terrazole, Truban, and Aliette are normally effective for Pythium control if used preventatively or at the first indication of disease.
Other Troubling Diseases:
Botrytis blight or grey mold is probably the most persistent and potentially damaging problem in poinsettia production because it can cause extensive damage to all to all phases of production. Botrytis can infect all above-ground parts of the poinsettia plant and is usually readily seen as a fuzzy grey-to- brownish growth on infected plant tissue. Poor air circulation/air stagnation, poor sanitation practices, tissue stress or wounding, and wet tissue all favor the development of Botrytis infection. Infection can occur rapidly and can devastate a crop in a short period of time under favorable conditions. Stressed cuttings under mist, plant crowding, and leaf and bract weakened by nutritional stress or excessive soluble salts all favor Botrytis development. Mature cyathia are especially vulnerable to Botrytis infection the plants are usually do not remain in the production greenhouse at this stage of production. Effective control of Botrytis must consider sanitation and environmental modification, with fungicide applicationunder high disease pressure. Chipco 26019, Daconil 2787, Exotherm termil, Mancozeb, Phyton 27, and Ornalin should help manage Botrytis problems but be sure to read the fungicide label carefully to avoid plant injury. Most fungicides should not be used on poinsettias showing bract color. Be sure to rotate fungicides used for Botrytis control to minimize the chances for resistance development.
Rhizopus blight does not occur often but when it does, it can be extremely damaging. The Rhizopus fungus grows as a grayish mass, causing rapid tissue collapse and plant loss. Its growth and development is favored by conditions that also favor Botrytis blight. Rhizopus blight usually occurs late in production when plants spacing is tight and air circulation throughout plant canopies is reduced. Rhizopus blight can be particularly damaging on sleeved plants and may not be obvious until the sleeves are removed. Growers should encourage customers to remove the sleeves as soon as possible after the plant shipment is received. There is no effective fungicide for Rhizopus blight.
For Additional Help...
Growers needing assistance in the diagnosis or confirmation of these or other nursery/floral crop diseases can send samples to the:
Room 101 Peterson Bldg,
Texas A&M University
College Station, TX 77843-2132
It is best to send several plants showing a range of symptoms but avoid plants that are in an advanced stage of infection. Plants (entire plants are best) submitted for diagnosis should be wrapped in newspaper, transferred to a plastic bag , and then packed securely in a sturdy cardboard box. Send the specimen(s) by overnight mail or by the bus.
For specific questions concerning specimen submission, contact
Dr. Larry W. Barnes
or E-mail at BARNES@PPSERVER.TAMU.EDU
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