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.
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.
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 85°F 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.
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
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.
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
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.
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.
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
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
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.
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.