Serious diseases of poinsettia, especially powdery mildew, scab,
and bacterial blight, can arrive at your greenhouse on cuttings that
are already infected. If you do not keep poinsettias all year and
have a several month period when poinsettias are not in your greenhouse,
diseases on a new crop probably came on infected cuttings. Therefore,
inspect cuttings carefully when they arrive for the symptoms described
in this fact sheet. Set aside any cuttings with symptoms and obtain
a diagnosis of the problem PROMPTLY. When plants have been in the
greenhouse for several days or weeks, it is not likely that a diagnostician
will conclude that the disease definitely came on the cuttings. After
planting is done, scout the crop frequently and thoroughly early
in the season for symptoms. This will help you find problems and
avoid serious disease losses later in the season. Encourage all workers
to watch for symptoms.
| Disease |
Symptoms |
Pathogen/Cause |
Management |
| Ammonium Toxicity |
Root and top growth is restricted. Lower leaves yellow while
leaf margins burn. |
Favored by low light and low temperature in late fall, acidic
potting mix, and exclusive use of ammonium nitrogen source. |
Do not use ammonium nitrogen sources exclusively. Do not over
water during periods of low light and low temperature. |
| Bacterial Canker |
Longitudinal water-soaked streaks on stems and spots on leaves.
Defoliation and plant death follow. |
Corynebacterium poinsettia |
Destroy infected plants. Avoid overhead irrigation. |
| Bacterial Stem Rot |
Cuttings develop a soft rot at the base which moves upward
quickly and kills the cutting. Wounded stems of older plants
develop soft rot and lodge. |
Erwinia carotovora |
Use pasteurized propagation media. Discard infected plants,
crop debris, and infested media. Disinfest tools. |
| Botrytis Flower Blight |
Brown spots form
on flower, leaf, or stem tissue. |
Botrytis cinerea |
Avoid damaging plants. Remove damaged tissues. Space plants
to provide good air circulation. Heat and vent to reduce humidity.
If these practices are followed, then fungicides can help in
management. Apply chlorothalonil (thermal smoke) to dry foliage
not blossoms; or fenhexamid, or fludioxonil to foliage. |
| Botrytis Stem Canker |
Large, light brown to tan, slightly sunken cankers form on older
stems especially near large branches or crotches. Defoliation
and death of branches occur above cankers that girdle stems. |
Botrytis cinerea |
Avoid damaging established, well-branched plants. Apply fludioxonil
to stems. |
| Bract Burn |
Dead spots form
along the margins of the bracts. |
Low calcium (Ca); imbalance between
Ca, potassium (K), and magnesium (Mg); ammonium-Ca antagonism |
Maintain a 2:3:1 ratio of K:Ca:Mg. Use at least 60% nitrate
nitrogen after first color appears to avoid ammonium-Ca antagonism.
Reduce fertilizer rates after first color. Apply 400 ppm calcium
chloride sprays weekly beginning at first color. |
| Leaf Drop |
Defoliation. |
Root rot, over fertilization, low light intensity, or lack
of moisture. |
Avoid root rots and Clean the greenhouse covering and space
plants well. Maintain even soil moisture levels. |
| Magnesium Deficiency |
Yellowing develops between the veins of mid and lower leaves. |
Insufficient magnesium. |
Use of magnesium-containing limestone. Apply a magnesium-containing
fertilizer. |
| Molybdenum Deficiency |
Yellowing is followed by burning of lower leaf margins. Lower
leaf margins are cupped downward. |
Lack of molybdenum or acidic pH potting mixes (pH 4.5; 5.5
on some cultivars) |
Lime to adjust the soil pH above 5.5. Use fertilizer containing
molybdenum or use a complete minor element supplement or add
sodium molybdate to soluble fertilizer. |
| Over fertilization |
Plants are stunted. Lower leaves yellow and fall. Leaf margins
yellow and burn. |
The combined use of slow-release and soluble fertilizer or
soluble fertilizer use with no leaching favors this. Conductivity
readings at or above 100 mhos x 10 to the -5/cm for soil or 250
mhos x 10 to the -5/cm for soilless mixes (1:5 dilution) are
excessive. |
Do not combine the use of slow-release and soluble fertilizers.
In continuous feed programs, 10% of the soluble fertilizer applied
should leach out the bottom or clear tap water should be applied
every second or third watering. If soluble salts become excessive,
leach heavily, wait overnight, and leach again. |
| Powdery Mildew |
Yellow spots form on the upper surface of leaves. White
fungal growth in patches is seen on leaves and bracts. |
Oidium |
Scout stock plants frequently and inspect cuttings as soon
as they arrive. Immediately remove any infected leaves. Continue
scouting throughout the season, Apply kresoxim methyl, copper,
thiophanate methyl + mancozeb, thiophanate methyl + iprodione,
chlorothalonil + thiophanate methyl, iprodione, horticultural
oil, neem oil, piperalin, triadimefon, Ampelomyces,
myclobutanil, or triflumizole. Prevent disease before bracts
form because fungicides do not remove the white fungal growth. |
| Pythium Root Rot |
Early in season,
the rooted cuttings are stunted, yellow, and wilting. Roots
are dark brown and the outer layers of root tissue strip off leaving
a bare strand of inner vascular tissue exposed. Later in the
season, plants defoliate and flower prematurely. |
Pythium sp. |
Use pasteurized soil and clean pots and tools. Keep hose ends
off the ground. Do not over water or over fertilize plants. Apply
mefenoxam, etridiazole, metalaxyl, etridiazole + thiophanate
methyl, or propamocarb as a soil drench. Control fungus gnats
and shoreflies. |
| Rhizoctonia Root Rot |
Early in the season, cuttings wilt and yellow. Roots are rotted.
Lower stems below ground may have a shredded appearance. Later,
stunted plants defoliate, flower prematurely, and die. Sunken
dark brown areas on stem may reach slightly above soil line. |
Rhizoctonia solani |
Use pasteurized soil and clean tools, and hang up hose ends.
Do not overfertilize. Apply flutolanil, fludioxonil, PCNB, etridiazole
+ thiophanate methyl, triflumizole, or iprodione at planting. |
| Scab |
Leaves have small, raised blister-like,
circular spots that may grow to be 0.5" in diameter. Spots
are light tan to brown, often with a reddish-purple line surrounding
them. Infected
leaves may fall. Gray to tan spots form on stems. Young infected
stems elongate rapidly above healthy stems, making the infected
plants obvious on the bench. Spores are spread by splashing. |
Sphaceloma poinsettiae |
Inspect new cuttings for symptoms. Reject the shipment or discard
infected plants. Water in a manner that keeps leaf surfaces dry.
This prevents spreading the spores and inhibits infection by
spores already on the plant surface. If few leaves are affected,
remove them. Apply myclobutanil, trifloxystrobin copper, chlorothalonil,
or mancozeb + thiophanate methyl to protect leaves and stems. |
| Thielaviopsis Root Rot |
Late in the season, roots turn black as dark
microscopic spores from in the cells. Plants wilt. Longitudinal
splits form at the stem base at and below soil line. Leaves
yellow and fall. |
Thielaviopsis basicola |
Use pasteurized soil and clean tools, and hang up hose ends.
Apply thiophanate methyl or etridiazole + thiophanate methyl. |
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