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GULF COAST RESEARCH & EDUCATION CENTER
IFAS, UNIVERSITY OF FLORIDA
5007 60TH ST. E.
BRADENTON. FL 34203
Bradenton GCREC Research Report 8RA1985-24 October 1985
POINSETTIA: ALTERNARIA LEAF SPOT AND BLIGHT
Arthur W. Engelhard and Jeffrey B. Jones
The poinsettia (Euphorbia pulcherrima Willd. ex Klotsch) is a popular short day
plant grown for its colorful bracts, and marketed during late November through
Christmas, Poinsettias are raised commercially in greenhouses, and outdoors
under saran in the southern parts of Florida. Landscape plantings of
poinsettia are popular in central and southern Florida. Dozens of both
standard and multi-flowering varieties are available which can be finished in
many forms: trees, pixies, hanging baskets, planters, and traditional 6-8"
pots. Rhizoctonia, Phytophthora, Pythium, Corynebacterium, Botrt~is, and
Sphaceloma are common and destructive pathogens on poinsettia in Florida,
requiring constant vigilance in the cultural program to minimize losses.
A new disease caused by Alternaria euphorbiicola Simmons and Engelhard (1, 2)
caused symptoms on leaves, bracts, petioles, cyathia and stems. It was
observed on poinsettia plants in Florida in 1984 and 1985. It was very
destructive and caused losses in commercial nurseries in both years. Spores
are produced by the fungus on diseased tissue on the plant and on the ground.
The spores are spread by wind and water. Environmental conditions are more
conducive for spread and development of the disease outdoors than indoors
(greenhouses). Spread and development of the disease occurred during a dry
period in March-June 1985 on plants grown outdoors. Presumably, disease
developed because plants frequently were wet with dew from evening until about
10 AM the next day, Spores of Alternaria are xerophilic, indicating they can
survive in dry weather, but develop and cause disease when some moisture is
The disease causes a variety of symptoms on the plants, and some symptoms are
similar to those associated with bacterial canker (Corynebacterium
flaccumfaciens pv. poinsettia) and scab (Sphaceloma poinsettiae. The disease
therefore may be misidentified.
Leaf symptoms caused by spores
Leaf spots induced by spores may be 1-2 mm (less than 1/10 of an inch) in
diameter, have tan centers, dark, thin margins, and usually chlorotic halos.
Spots enlarge and become irregularly shaped necrotic lesions 12-25 mm (I to 1
inch) or larger in the largest dimension. When several spots are present, the
leaf becomes chlorotic and may fall off. Dark, thin (1-2 mm wide), necrotic
lesions up to 25 mm (1 inch) in length occur along lateral veins, and sometimes
main veins, on leaves. The leaf tissue adjacent to the elongate lesions is
chlorotic, Because the affected tissue does not grow, leaf distortion occurs
when the surrounding tissue grows. These diseased areas also increase in size
with time. Flowering, bract development and bract coloration are delayed or
fail to occur, depending on the severity of the disease.
Leaf and bract symptoms induced by metabolites or toxins
Other leaf symptoms probably are induced by or are the result of toxins or
metabolites produced by host/parasite in the infections on stems and petioles.
The metabolites are translocated upward to the foliage. Large areas.
one-fourth or more of a leaf, develop a stippled necrosis, followed by a
general necrosis of the area. The affected area tends to dry and/or curl and
wrinkle. It is usually delimited by the main vein down the middle of the leaf.
On occasion the predominant symptom may be chlorosis rather than the stippled
necrosis. Affected leaves fall prematurely. Developing leaves near the
apex may have unilateral chlorosis and an accompanying curvature of the
Stem and petiole infection
Lesions caused by developing spores on stems and petioles are round, dark, 1-2
mm diameter, and sunken. They grow and elongate and may reach 25 mm (1 inch)
or more in length on stems. The terminal end of the stem may shrivel and die
when stem lesions develop close to or at the apex. Flower parts are killed.
The plants usually are stunted and remain so with infections in the growing
points, Leaves with petiole lesions become chlorotic and dehisce. Leaves in
close proximity to stem infections usually become chlorotic or necrotic and
The V-14. cultivars (Glory, White and Jingle Bells II) and Eckespoint C-1 Red
are very susceptible. V-10 Amy appears intermediate to tolerant in
susceptibility and the Annette Hegg cultivars (Dark Red, Top White, Brilliant
Diamond, Hot Pink) quite tolerant in the field, Usually only small leaf spots
with tan centers and slight or no chlorotic halos form on the latter, or
elongate lesions form along the veins of leaves.
Alternaria leaf blight is extremely difficult to control on very disease-
susceptible cultivars grown outdoors. Therefore, effective disease control
requires using a combination of thorough fungicide spraying, proper cultural
practices, sanitation, proper cultivar selection, and scouting the crop,
Chemical Control. Good disease.control-.was obtained with weekly sprays in
experiments on outdoor grown plants sprayed with 1.5 lb/100 gal mancozeb 80W,
iprodione 26019 50W, or chlorothalonil 75W. Copper hydroxide 101 1.5 Ib/100
gal provided good disease control, but repeated applications caused leaf burn.
Aqua-Gro wetting agent at 4 liq. oz./100 gal added to the above sprays reduced
the splotchy residue problem on the leaves and bracts without reducing disease
control. Benomyl 50W alone does not control Alternaria blight, but mancozeb
does. When the two are tank-mixed at 0.25 lb/100 benomyl 50W and 0.75 lb/100
mancozeb 80W, the combination controls Alternaria blight. When scab
(Sphaceloma poinsettia) is a problem (all cultivars are susceptible) the
benomyl-mancozeb combination is effective for both blight and scab. Captan 503
gives moderate control of Alternaria blight and good control of scab when it is
tank-mixed with benomyl. Iprodione 26019 50W gives good control of Alternaria
blight but poor control of scab.
Use of common names or trade names of pesticides in this report does not
indicate that the products are labelled for poinsettia plants. Please check
product labels before using a compound.
Good disease control can be obtained by spraying susceptible cultivars grown
outdoors once or twice weekly depending on disease pressure, but only if
coverage of the foliage (both upper and lower leaf surfaces) is complete.
It appears the difficulty in getting Alternaria blight control is related to
the difficulty in covering the lower leaf surfaces with fungicide spray. The
lower surfaces of leaves should be examined immediately after spraying to
determine if they are getting wet with spray. The large, closely spaced leaves
fold together during spray application, making good spray coverage difficult.
Poor coverage indicates a need to change the spraying procedure. When the
lower surfaces of some leaves are not covered, the airborne spores will find a
place to land and grow, allowing disease to develop and spread; coverage of all
plant tissue is essential for disease control,
Cultural. Free water on plant parts is required for spore germination.
Therefore, an excellent control of Alternaria blight is to grow plants in
greenhouses and/or under cover where the foliage stays dry. Overhead watering
should be avoided as wetting the foliage is a practice conducive to the
development of the disease, Spacing the plants to permit quick drying also
reduces disease hazard.
Do not take cuttings from diseased plants as disease builds up with the
abundant moisture required in the mist propagation.
Sanitation. Severely diseased plants should be removed carefully from the
growing area and buried. Plants with elongate stem lesions usually are
stunted, and do not recover as toxin is translocated upward causing stunting,
chlorosis, and leaf drop. Plants with severe spotting of the leaves, bracts
and petioles should be discarded. Plants and all fallen leaves and plant parts
should be discarded in a plastic bag, Airborne spores are produced on diseased
plant parts and fallen leaves so all plant debris should be removed. Good
disease control is difficult when a continuous supply of airborne spores is
Cultivar selection. The Hegg cultivars (Dark Red, Brilliant Diamond, Top
White, Hot Pink) and V-10 Amy are disease tolerant. They become infected, but
the spots generally stay small and are less abundant than on the susceptible
V-14 cultivars. Disease control with chemicals also is easier than on the very
susceptible V-14 cultivars. Including disease tolerant (more resistant)
cultivars in the cultivar mix should decrease potential losses and increase
overall product quality if Alternaria blight is a problem.
Scouting the Plants. The crop should be examined closely two times each week
for early symptoms of Alternaria blight, and more frequently if disease was
found previously. The susceptible V-14 cultivars (Glory, Top White, Jingle
Bells II) and C-1 Red should be examined very carefully as they would be
expected to show disease first.
1. Engelhard, A. 14. and J. B. Jones. A leaf, petriole and stem lesion
disease of poinsettia incited by Alternaria sp. Phytopathology 75:
(in press). Abst.
2. Simmonds, E. G, 1986. Alternaria themes and variations. (14-16).
Mycotaxon (in press).