Common diseases and disorders of Poinsettia

A.R. Chase is a Plant Pathologist and President of Chase Research Gardens, Inc.
C.R.G. is a private research and consulting corporation specializing in ornamental plants.
Both hourly and contract services are available.
You can reach C.R.G. at P.O. Box 168, Mt. Aukum, CA 95656 Phone and Fax: 916-620-1624 - E-mail: MTAUKUM@AOL.COM


Alternaria leaf spot
Pathogen- Alternaria euphorbiae
Symptoms - Alternaria leaf spot of poinsettias is characterized by small (less than 1 mm in diameter) lesions which are initially water-soaked. These lesions turn reddish-brown, may reach 1/8 inch in diameter and are roughly circular. Lesions generally do not have any type of halo.
Control - Elimination of water on leaves is important to completely control Alternaria leaf spot of poinsettias. Iprodione (Chipco 26019) and the combination product of thiophanate methyl and mancozeb (Zyban) are effective in controlling Alternaria leaf spot. Annette Hegg cultivars have been reported as very resistant to Alternaria leaf spot as well.

Corynespora leaf spot
Pathogen - Corynespora cassiicola
Symptoms - Spots appear on leaves as well as bracts. These areas enlarge to about 1 inch in diameter and are tan to dark brown.
Control - Keep bracts and leaves as dry as possible to minimize conditions for pathogen spread and infection. The fungicides which are mentioned for control of Alternaria leaf spot should work equally well on Corynespora leaf and bract spot.

Myrothecium cutting rot
Pathogen - Myrothecium roridum
Symptoms - Lesions generally appear at cutting bases and result in poor rooting and even loss of cuttings. Necrotic areas are dark-brown and mushy. Examination of the rotted areas reveals sporodochia which are irregularly shaped, black and have a white fringe of mycelium.
Control - Using fungicides when temperatures are between 70 and 85F and fertilizing at recommended levels contribute to minimizing severity of Myrothecium diseases. Chlorothalonil (Daconil 2787) has been effective for Myrothecium control on many foliage plants and mancozeb compounds (such as Dithane M45) also provide excellent control of this disease. Iprodione (Chipco 26019) may also be effective in controlling this disease.

Powdery mildew
Pathogen - Oidium sp.
Symptoms - A white powdery coating covers the top and sometimes the bottom leaves as well as bracts of the affected plants. The covering sometimes forms in circular lesions and sometimes covers the entire surface of the leaf. Older spots may appear grayish around their edges.
Control - This disease is a serious problem throughout the United States, especially where the relative humidity is low which favors disease development. Fungicides containing thiophanate methyl (Cleary's 3336, Topsin M and Domain) as well as the combination of thiophanate methyl and mancozeb (Zyban) are registered for poinsettias and should aid in disease control.

Pythium root and stem rot
Pathogen - Pythium spp.
Symptoms - Roots rot caused by Pythium spp. starts as brownish root tips that rapidly disintegrate and cause the upper portions of the plant to yellow and wilt. The lower leaves of badly rotted cuttings drop off and the bases of these cuttings are black or brown and mushy. This disease is common in poorly aerated, water-logged soils.
Control - Always use clean or new pots and potting medium to reduce the chances of introducing Pythium spp. into the production. Growing plants away from the native soil is also a good idea since the pathogens can be transferred readily to your poinsettia crop. The combination product of etridiazol and thiophanate methyl (Banrot), metalaxyl (Subdue), fosetyl aluminum (Aliette) and etridiazol (Terrazole and Truban) are all effective in controlling Pythium and Phytophthora.

Rhizoctonia cutting rot
Pathogen - Rhizoctonia solani
Symptoms - A brownish rot starts on cuttings at the soil line. It is also common for leaves to become infected and under high temperatures and high moisture the mycelia of the pathogen covers the infected cutting. Rhizoctonia mycelia are usually reddish-brown in color and have the consistency of a spider web.
Control - Chemical control of diseases caused by Rhizoctonia has been investigated on many plants using a variety of fungicides. The fungicide most widely for soil drenches control of Rhizoctonia diseases is thiophanate methyl (Domain or Cleary 3336). In addition etridiazole and thiophanate methyl (Banrot), iprodione (Chipco 26019) and PCNB (Terraclor) also aid in control of Rhizoctonia diseases. Since the pathogen is commonly soil borne these chemicals should be applied as drenches.

Rhizopus blight
Pathogen - Rhizopus sp.
Symptoms - A soft, mushy brown rot can start anywhere on infected plants including cutting bases, leaves, flowers and shoot tips. The white mycelium and black sporangia of the pathogen form rapidly on all infected plant parts, giving them a fuzzy or bearded look.
Control - This disease can spread by air movement as well as splashing from rainfall or irrigation practices. It is generally only a problem during the conditions with high temperatures and relative humidities. Keep plant stress as low as feasible to aid in resisting this disease. Extensive and serious cultural controls have proven effective in controlling this disease on some floricultural crops. There are no fungicides labeled for this use although dicloran (Botran) is used in postharvest control of a similar disease on stone fruits.

Sooty Mold
Pathogen - saprophytic fungi growing on insect honeydew
Symptoms - White, gray or most commonly black powdery coating especially on lower leaves.
Control - These saprophytic fungi grow on the sugary honeydew produced by insects such as white flies, mealybugs and aphids. Controlling these insects will eliminate the food source for the sooty mold fungi. Removing existing sooty mold infestations may be difficult due to the wide variety of saprophytes that cause the condition.

Pathogen - Sphaceloma poinsettiae
Symptoms - Scab lesions appear on stems, petioles and leaves. They are tan, slightly raised and corky. If the lesion encompasses the stem it can result in dieback above the lesion. A bright yellow halo can develop around leaf spots.
Control - Never use cuttings from plants with scab symptoms as the disease is easily carried over to the next crop in this manner. Mancozeb products such as Dithane M45 and Zyban (combined with thiophanate methyl) are effective in controlling scab on poinsettias.

Xanthomonas leaf spot
Pathogen- Xanthomonas campestris pv. poinsettiicola
Symptoms - Symptoms are generally confined to pinpoint yellow to tan lesions scattered across the leaf surface, although they can become large and confined between leaf veins. Lesions are mostly 1/8 inch wide with irregularly raised edges. Severe infections can cause distortion of new leaves as well as complete chlorosis and finally abscission of older leaves.
Control - Eliminate all stock plants which have Xanthomonas leaf spot. The disease is very difficult to control unless plants are produced without overhead watering or exposure to rainfall. Bactericides such as copper containing compounds may be somewhat effective if used on a preventative and regular basis. Acid solutions such as vinegar (1 gal/100 gal) and fosetyl aluminum (Aliette 80WP 2 lb/100 gal) also give control of Xanthomonas leaf spot on some ornamentals. Always test your plants for sensitivity to an unfamiliar chemical application before using on a broad scale.


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