Poinsettia Diseases and Their Control

oinsettias are very popular seasonal plants available for sale from early November through December. Many of these plants are displayed in businesses and homes. Production of poinsettias begins long before the holiday season. In late summer, unrooted cuttings or rooted cuttings are sold to greenhouse operations where they are planted and grown to finished products. From late summer to late September, greenhouse operations purchase poinsettias at any stage of development from unrooted cuttings to rooted cuttings or as potted, prefinished products. Disease problems can occur at any stage of poinsettia production. Environmental or cultural problems, which may include nutritional damage, are described in Extension publication ANR-1222, “Greenhouse Production of Poinsettias.” This publication concerns the common diseases of poinsettias, which are caused by fungi or bacteria. Early disease detection and identification followed by rapid implementation of control measures are necessary for production of a marketable crop.

Foliage Diseases

Botrytis cinerea causes one of the most common diseases of all greenhouse crops. The fungus may cause brown-gray spots and blight on leaves and bracts (Figures 1 and 2), tan-brown stem cankers (Figure 3), and cutting and crown rots (Figure 4). Wounded or senescent plant parts are especially susceptible (Figure 5). This fungus can often be identified by its gray, fuzzy sporulation that will develop on an infected surface when the temperature is moderate (60 to 80 degrees F) and humidity is high. Some leaf lesions may have a zonate pattern. Poinsettia leaves, bracts, and stems are susceptible at all stages of production. Spores of the fungus are often released into air currents after a rapid change in relative humidity and when the plants are handled. Water splash will also carry spores to new plant surfaces where infection will take place.

Control of Botrytis requires cleaning up diseased plant material, applying protective treatments of fungicide to prevent new infections, and reducing relative humidity levels, especially at night. When cleaning the area, remove all diseased leaves, bracts, and cankered stems from the plant growing area. Debris should also be removed from plant branches. Many fungicides are labeled for protective control of Botrytis on poinsettia. See Table 1 for a product list. Some strains of Botrytis have shown resistance to iprodione and thiophanate-methyl fungicides. Preventing the initial development of Botrytis involves proper environment management of the crop. Horizontal air circulation should be maintained by spacing and open mesh benching. Keep leaf wetness and relative humidity low with ventilation and increased temperatures during early evening hours. About 1 to 2 hours after sunset, heat the greenhouse air for a short time with active ventilation to the outside air. With the venting out of warm, moist air, the cooler, moist air is brought in. The complete air exchange usually requires about 1 hour. The vents should then be closed and the air temperature brought up to the desired level. The cooler, moist air will warm up and have a lower relative humidity than the original air in the greenhouse. Follow this procedure each evening 1 to 2 hours after sunset, or activate with a humidistat.

Table 1. Chemical Control of Botrytis Blight on Poinsettia

Fungicide	Rate per 100 gallons	Comments
copper sulfate pentahydrate    Phyton 27	13 to 15 fl oz	Apply as a full cover spray to the foliage when conditions in greenhouse favor disease development and repeat applications at 7- to 14-day intervals as needed to prevent disease outbreak. See product labels for further instructions and use restrictions.
fluidioxonil    Medallion	2 to 4 oz
iprodione    Chipco 26 GT    Sextant	1.0 to 2.75 qt/A 1.0 to 2.75 qt
kresoxim-methyl    Cygnus	1.6 to 3.2 oz	Preventative: Apply lower rate at 7- to 10-day intervals when conditions favor disease. Curative: Apply at higher rate at shorter intervals. See label for further instructions and list of Cygnus-tolerant poinsettia cultivars.
thiophanate-methyl    3336 4.5F    3336 50W    OHP 6672 50W    OHP 6672 4.5F	10 to 20 fl oz 12 to 16 oz 12 to 16 oz 10 to 12 fl oz	For suppression of Botrytis blight, apply every 2 to 4 weeks.
trifloxystrobin    Compass O	2 to 4 oz	Apply as a full cover spray to the foliage every 7 to 14 days when conditions favor disease. See label for further instructions.
chlorothalonil    Exotherm Termil	See label.	Smoke Fumigation in Greenhouse: One can treats 1,000 cubic feet. Do not use if temperatures exceed 75 degrees F or on poinsettias in bloom. See label for use restrictions.

Choanephora wet rot of poinsettia is caused by C. cucurbitarum. Infections of leaves, petioles, and stems develop into a soft, wet rot. Symptoms resemble those of Rhizopus stolonifer. Infected green stems may droop before they totally collapse. The fungus requires high humidity for disease development and sporulation. Young plants are more susceptible than older plants.

When conditions are warm and humid, abundant sporulation is produced on diseased tissues (Figure 6). Masses of visible 10 millimeter long, whitish stalks develop. Each fungal stalk supports compound “heads” of 6 or more clusters of spores, which are brown or red brown, ellipsoidal, and usually ridged. C. cucurbitarum will also infect and cause a rot of many fruits and vegetables and a blight of flowers of Hibiscus rosa-sinensis and petunia. The disease spreads rapidly when conditions are warm and humid. Control of this disease requires (1) plant spacing to promote good air circulation, (2) avoidance of prolonged leaf wetness, and (3) sanitation, including removal of diseased plants.

Rhizopus stolonifer stem blight is a disease of poinsettias as well as a storage mold of many ornamentals, soft fruits, and vegetables. It also causes damage on Catharanthus roseus (annual periwinkle) and Sinningia speciosa (gloxinia). Gerbera jamesonii, crossandra, and crown of thorns are also susceptible. Annette Hegg cvs are reported to be less susceptible than V-14 Glory and V-14 White. R. stolonifer also causes common bread mold. Symptoms on poinsettias may involve a dark, greasy or glassy looking discoloration of the infected stem (Figures 7 and 8). Bracts and leaves may also develop a dark, water-soaked blight. Symptoms may begin during propagation or soon after potting. Lower stems may become diseased. Potted plants may suddenly wilt and collapse after infection (Figure 9). This disease can sometimes be confused with bacterial soft rot. With humid conditions, whitish webs of the fungus may develop on dead tissues. The fungal straight white stalks all support black round bodies, which contain the spores of the fungus. The fungus grows rapidly between 70 and 90 degrees F and a relative humidity above 75 percent. R. stolonifer may colonize wounds or develop on stressed tissues. Infections may begin in bracts and spread downward. Control of this disease involves methods of cultural management. Maintain plants under optimal conditions for poinsettias. Avoid stress and promptly remove infected plant debris. Cutting wounds often serve as points of entry for the fungus so take measures to promote rapid callusing.

Scab or spot anthracnose is caused by the fungus Sphaceloma poinsettiae. Symptoms include raised brown spots, up to 4 millimeters in diameter. Yellow zones 1 to 2 millimeters wide may surround the brown spots (Figure 10). Large brown areas may develop at leaf tips or margins, and whole leaves may become yellow and drop off (Figure 11). The scabby, raised, circular or elongated lesions can develop on stems, petioles, and leaf midribs. The spots are tan colored and may have a red or purple border. Stem lesions may be sunken, coalesce, and girdle the stems. Fungal growth and spores may develop on the lesions. The spores develop in tiny, brown, swollen, flat bodies. All poinsettia cultivars are susceptible. For disease control, use disease-free cuttings and remove all fallen diseased leaves. Some fungicides, such as azoxystrobin and thiophanate-methyl, used for powdery mildew control have good activity against scab.

Powdery mildew, caused by the fungus Erysiphe or Microsphaera (Oidium), does not usually cause plant death, but it makes the plant unmarketable. Symptoms include yellow and brown blotches on upper leaf surfaces. Typically, the white powdery dusting of the fungus develops on the lower leaf surfaces, but it may eventually develop on the upper leaf surfaces and bracts (Figures 12 and 13). The disease develops and spreads mostly when day and night temperatures fluctuate around 68 degrees F and relative humidity is high (above 95 percent). Spores are moved about by air currents and water splash. Disease control requires removal of fallen diseased foliage and severely diseased plants. Air movement around the plants should be increased to reduce relative humidity. Ambient air temperatures in Alabama greenhouses typically are high enough to stop disease development. Protective fungicide sprays are recommended only in those settings where powdery mildew previously caused extensive damage. See Table 2 for a list of some labeled fungicides.

Table 2. Chemical Control of Powdery Mildew on Poinsettia

Fungicide	Rate per 100 gallons	Comments
azoxystrobin    Heritage 50W	1 to 4 oz	Apply at first sign of powdery mildew colonies on the leaves. Repeat applications every 7 to 28 days as needed to prevent further disease spread.
thiophanate-methyl    3336 4.5F    3336 50W    OHP 6672 50W    OHP 6672 4.5F	10 to 20 fl oz 12 to 16 oz 12 to 16 oz 10 to 20 fl oz	Apply at first sign of powdery mildew on leaves and repeat applications at 7- to 14-day intervals until threat of disease is over. See label for additional instructions.
triadimefon    Strike 50W	4 to 8 oz	Repeated applications of Strike may suppress shoot elongation.
trifloxystrobin    Compass O	2 to 4 oz

Rhizoctonia will cause a leaf spot and stem canker of poinsettia (Figure 14). Lesions are brown, dry, and irregularly shaped blotches. Disease is favored by moderate temperatures and high humidity. Control of this disease involves removal of diseased plant parts and application of protective fungicide sprays. With cankers, make cuts at least 2 inches beyond lesion edges. Dip shears into a 10 percent bleach solution between cuts. See Table 3 for fungicides recommended for Rhizoctonia crown and root rot control. (See page 6 for a discussion of crown and root rot.)

Table 3. Chemical Control of Rhizoctonia Crown and Root Rot

Fungicide	Rate per 100 gallons	Comments
fluidioxonil    Medallion	1 to 2 oz	Apply as a drench immediately after sticking cuttings and repeat as needed every 21 to 28 days as needed to prevent disease.
flutolanil    Contrast 70W	3 to 6 oz	Apply 1 to 2 pints of fungicide suspension per square foot of bed or bench area. Reapply every 21 to 28 days as needed to prevent disease. See label for additional application methods and instructions.
iprodione    Chipco 26GT    Sextant	13 fl oz 13 fl oz	Apply as a drench at rate of 1 to 2 pints of fungicide suspension per square foot of mixture and repeat applications every 14 days as needed to prevent disease.
thiophanate-methyl    3336 4.5F    3336 50W    OHP 6672 50W    OHP 6672 4.5F	16 to 20 fl oz 12 to 16 oz 12 to 16 oz 16 to 20 fl oz	Apply as drench to 400- to 800-square-foot area or 1 to 3 pints of fungicide suspension per container after transplanting. Repeat as needed at 21- to 28-day intervals. Also has activity against Fusarium and Thielaviopsis root rot.
thiophanate methyl+ etridiazole    Banrot 40W    Banrot 8G	4 to 8 oz 8 oz/cu yd	Apply as drench at 1 to 2 pints of fungicide suspension per container after transplanting. Irrigate immediately. Repeat at 28- to 56-day intervals. Also has activity against Fusarium and Thielaviopsis root rot. Thoroughly mix into potting media before transplanting. Retreat with a soil drench as needed after 1 month.
triflumizole    Terraguard 50W	4 to 8 oz	Apply as drench at transplating and repeat at 21- to 28-day intervals.

Phytophthora foliage blight is caused by the fungus Phytophthora nicotiana. All foliage of poinsettia is susceptible. Stems develop gray, wet lesions, and crowns display brown rot (Figure 15). With woody stems, infection is not apparent; the pith becomes brown and individual stems die. Leaves may develop small, angular tan spots, which become large tan areas. Leaves and petioles of infected terminals bend down sharply. This fungus has a very large host range that includes many flowering potted plants. Disease develops rapidly above 82.4 degrees F. This fungus requires wet conditions. Spores are spread by splashing and running water. P. nicotiana is soilborne. Movement of soil, water, and plant material will spread the fungus. Control requires plant removal, soil or media removal, sanitation of benches, reduction of watering (if appropriate), and application of protective fungicide drenches. See Table 5 for fungicides registered for the control of Pythium root rot.

Bacterial leaf spots on poinsettia may be caused by one of several bacteria including Pseudomonas cichorii, Pseudomonas syringae pv. syringae, Xanthomonas campestris pv. Euphorbiae, manihotis, and poinsetticola. The Pseudomonas spp. leaf spots are generally seen as watersoaked, dark brown–black, gray, or tan angular spots. In some cases a yellow halo may develop around the spot edge (Figure 16). As the spots dry out, the lesions become thin and papery. Spot cracking and splitting may occur. The bacteria spread by water droplet splashes. High humidity and long, wet periods favor disease development. Disease management is difficult: (1) remove infected plants and debris; (2) wash hands and tools after handling infected plants; (3) do not handle plants when wet; (4) avoid overhead irrigation; and (5) do not maintain high levels of nitrogen, which may increase disease susceptibility. Bactericides are not 100 percent effective. Some of the bacterial leaf spot diseases are reported to be seedborne.

The Xanthomonas campestris leaf spot diseases develop initially as lower leaf surface, gray-brown, angular water-soaked spots. As the spots develop to 2 to 3 millimeters, chocolate brown–rust colored spots become visible on upper leaf surfaces (Figure 17). Pale green zones around the spots may develop. Spots may coalesce into large, blighted areas. Where disease is severe, leaves become yellow and drop. The bacteria are spread by water splashing. Disease management requires strict sanitation. Discard all diseased plants and wipe off bench surfaces with a disinfectant. Avoid wetting the foliage.
 

Bacterial canker caused by Corynebacterium flaccumfaciens pv. poinsettia may result in severe damage. Branch cankers may only develop or spots/blight may develop on stems, leaves, and bracts (Figure 18). Elongated, water-soaked, brown streaks may develop on stem surfaces. Stem lesions may girdle the stems and stem cracking may occur. Defoliation and plant death may follow. Disease occurs when conditions are warm and nitrogen levels are high. Workers may spread the bacteria by handling and with tools. The disease is not usually a problem until the plants are close to finishing. Disease control requires strict sanitation. Remove and destroy all diseased plants. Workers should not work with wet plants. Do not use overhead irrigation. Workers should wash their hands between handling plants. Disinfect tools after working with diseased plants. Discard diseased plants.

Bacterial stem rot or bacterial soft rot is caused by Erwinia carotovora or E. chrysanthemi. The bacteria may cause serious losses in only a few days. Poinsettia cuttings are very susceptible to both E. chrysanthemi and E. carotovora (Figure 19). Several centimeters of the cutting base may develop soft rot. On older plants, stems may develop 5 to 10 centimeters of soft rot. The soft, rotted tissues develop into a mushy consistency. With optimal conditions of warm temperatures and high humidity, symptoms may develop in 24 to 48 hours. Usually the soft rot will develop at the base of the cutting and move upward. Wounded stems of older plants may develop soft rot and collapse. The bacteria may develop in the vascular system and cause vascular browning, wilt, watersoaking of stems, and complete collapse of the plant. Soft rot bacteria may be associated with plants, plant debris, water, soil, or pot media. Surface and underground water has been shown to contain Erwinia spp. Irrigation water may be a source as well as a method of spread. Insects may disseminate and also help provide a wound entry by their feeding action. The bacteria may also be moved by transport of plants and media, water splash, contaminated tools, and the handling of plants by workers. Weakened plants are more susceptible. High nitrogen levels in the poinsettia results in an increased disease susceptibility. Soilless media can become contaminated and hold bacteria for several months. To manage the disease, use pasteurized propagation media. Discard all diseased plants, crop debris, and infected media. Disinfest tools and knives with a commercial disinfestant. Disinfest benches.

Crown and Root Diseases

Fusarium spp. root and crown rots are caused by F. solani, F. moniliforme, and other Fusarium spp. Roots and crowns develop brown-black discoloration and become soft (Figure 20). Rotted areas may develop cream-orange, wetlooking, spore masses. This fungus survives in the soil as specialized spores. Spore survival is longer in dry soil than it is in wet soil. Spores may be spread by air, insects, diseased plants, and irrigation water. For disease management, prevent the entry of contaminated media, soiled containers, and diseased plant material. Prevent the entry of fungus gnats and shore flies that may introduce the fungus into the growing media. Follow continuous sanitation measures at propagation and through the growth of the plants. These measures should include washing hands and tools, cleaning up plant growing areas, removing diseased plants as they appear, increasing spacing to promote aeration, and avoiding splashing water. Discard diseased plants. Fungicides with thiophanate methyl have some protective activity against Fusariumincited disease.

Rhizoctonia solani crown and root rot may develop on poinsettia and other plants including many flowering potted plants. Symptoms include lower stems becoming dark brown and shrunken. The shrunken condition is often referred to as wire stem. When warm, humid conditions exist, the fungus will grow over the lesion surface as a light brown fungal mat, closely appressed to the surface of the plant and nearby soil. With seedlings, damping-off (collapse of the plant) may occur above the lesion. On larger plants, brown stem lesions develop on lower stems and some stems dieback; wilt may occur. The root rot phase of the disease causes discrete brown lesions with rotting of the surface tissues (Figure 21). New root development is inhibited. Whole roots will become brown and dead. Cutting ends may also develop Rhizoctonia rot, which is a brown, dry rot. Foliage symptoms that develop as a result of the crown and root decay are yellowing, wilt, loss of lower leaves, stunting, and plant death.

Poinsettias are most susceptible to Rhizoctonia just before or soon after rooting and just before plant maturity. R. solani is most active in soils/media with even moisture at moderately warm (63 to 79 degrees F) temperatures. Management involves sanitation practices such as removal of plants and avoidance of the accidental introduction of the causal fungus. See Table 3 for labeled protective drenches and sprays.

Thielaviopsis basicola or black root rot is a problem on poinsettia and several other plants including Helleri holly, cotton, pansy, and petunia. This is usually a late-season disease. Roots become black and die. The black color is due to the presence of masses of black spores on the root surfaces (Figure 22). Above-ground symptoms include wilting, yellowing, leaf drop, and stunting. Stems may split just above or below the soil line. The spores of the fungus will survive for several years in the soil or potting media. The spores will germinate over a wide range of potting media or soil pH of 5.0 to 8.5, but the disease is more serious when plants are growing in neutral or alkaline pH. At a lower pH of 5.5, disease is much more reduced. Temperatures of 55 to 63 degrees F favor disease development. Also the disease is more serious in moist soils (more than 70 percent moisture holding capacity). Disease management requires sanitation. All discolored, slow growing plants should be removed. Apply fungicide protective soil drenches when disease is detected. See Table 4 for recommended fungicides.

Table 4. Chemical Control of Thielaviopsis Root Rot of Poinsettia

Fungicide	Rate per 100 gallons	Comments
thiophanate-methyl    3336 4.5F    3336 50W    OHP 6672 50W    OHP 6672 4.5F	16 to 20 fl oz 12 to 16 oz 12 to 16 oz 16 to 20 fl oz	Apply as drench to 400- to 800-square-foot area or 1 to 3 pints of fungicide suspension per square foot after transplanting. Repeat as needed at 21- to 28-day intervals. Also has activity against Fusarium and Rhizoctonia crown and root rot.

Pythium crown and root rot and also Phytophthora nicotiana crown and root rot are common problems with many potted plants. These diseases occur where potting media or soils are kept continually wet. Several species of Pythium will cause root decay of poinsettia. Root and crown rot symptoms include a light-dark brown, wet, glassy discoloration of the root and possibly crown (lower stem) tissues (Figure 23). Infected areas may eventually become dry and shriveled. The decayed root surface layer (cortex) will easily slide away from the inner root core or vascular tissues. Root decay may move upward into the lower stem. The base of unrooted cuttings may become infected and rotted. Rooted cuttings become stunted, yellowed, and wilted. Older plants develop yellowed leaves in the lower canopy first; leaves drop prematurely. Plants may flower prematurely. The fungus can survive as spores in soil, media, or dust on greenhouse floors, flats, and pots. Pythium may be more aggressive in pasteurized soil or soilless media due to the lack of competing microorganisms. Abundant soil or media moisture is needed for the disease to develop. High fertilizer rates may cause poinsettias to be more susceptible to Pythium. Pythium ultimum is active at temperatures ranging from 50 to 86 degrees F. A moisture holding capacity above 70 percent favors growth of Pythium. Disease management requires sanitation to remove diseased plants and infected plant media. Benches and pots should be disinfested. Clean and disinfest floors under benches. Watering practices should be evaluated and reduced, if appropriate. Protective fungicide drenches should be applied. See Table 5 for labeled fungicides.

Table 5. Chemical Control of Pythium Root Rot

Fungicide	Rate per 100 gallons	Comments
etridiazole    Truban 30W    Terrazole 35W    Truban 25E    Truban 30W	4 to 6 oz 4 to 6 oz 3 to 4 fl oz 1.5 to 3 oz/cu yd	Apply as media drench at 1 to 2 pints of fungicide suspension per container after transplanting. Irrigate immediately. Repeat at 30- to 60-day intervals. See label for additional instructions. Mix thoroughly in dry potting media. Irrigate immediately after transplanting.
thiophanate methyl+etridiazole    Banrot 40W    Banrot 8G	4 to 8 oz 8 oz/cu yd	Apply as drench at 1 to 2 pints of fungicide suspension per container after transplanting. Irrigate immediately. Repeat at 28- to 56-day intervals. Also has activity against Fusarium and Thielaviopsis root rot. See label for additional instructions. Thoroughly mix into potting media before transplanting. Retreat with a soil drench as needed after one month. See label for additional instructions.
fosetyl-A1    Aliette T/O	1.24 to 4.0 lb	Apply as a foliar spray using no more than 400 gallons of water per treated acre. Repeat as needed at monthly intervals. See label for additional instructions.
mefenoxam    Subdue MAXX	0.5 to 2 fl oz	Apply as drench at transplanting at rate of 1 to 2 pints of fungicide solution per container. Repeat as needed at 1- to 2-month intervals. See label for additional instructions.
Subdue GR	1.6 to 8 oz/cu yd	Mix thoroughly in dry potting media and then irrigate with enough water to wet the root zone. Retreat with a soil drench as needed after 1 to 2 months. See label for additional instructions.
Potassium salts of phosphorous acid    Allude	6.7 to 12.7 fl oz	Apply as drench at rate of 1 to 2 pints of fungicide solution per container. Repeat monthly as needed to control disease. See label for additional instructions.
propamocarb    Banol 67S	20 fl oz	Apply as drench to potting media at rate of 3.5 fl oz of fungicide solution per 4-inch container. Repeat applications at 3- to 6-week intervals as needed. See label for additional instructions.