Symptom and Disease Match-Up
Please note that this is not a tool for professional diagnosis.  It is meant only to provide some clues as to what might be wrong with a poinsettia, based on the symptoms it is expressing. 
How to use match-up:
Symptoms are divided first according to their location, and then by symptom type.  For example, if you observe distorted leaf growth on your plant, you would look first for the "LEAVES" category and then for the "distorted growth"  listing within that category.  Diseases listed below "LEAVES...distorted growth" are those which sometimes result in that symptom.  Click on disease names for more information and to further compare symptoms. 
Tip:
The most common poinsettia diseases are starred (*).  You may want to start your search by reading about these diseases.  A plus sign (+) next to a disease name indicates that its information page includes images.
Symptom Locations include:
1) BRACTS, 2) BUDS, 3) CYATHIA, 4) CUTTINGS, 5) LEAF VEINS, 6) LEAVES, 7)ROOTS, 8) STEM, 9) VASCULAR SYSTEM / PITH / CORTEX, 10) WHOLE PLANT.
 BRACTS
blight:
Greasy Canker
Rhizopus Blight

distorted growth:
Phytophthora Crown Rot
* + Splitting = Premature Flower Initiation

latex exuded:
* + Botrytis Blight

necrosis:
lesions-
Alternaria Leaf Spot & Blight
* + Botrytis Blight
Corynespora Leaf Spot
marginal (small spots or outline)-
* + Bract Edge Burn = Bract Necrosis
Calcium Deficiency

 poor color:
+ Poor Bracht Coor
Phytophthora Crown Rot

premature development:
* + Splitting= Premature Flower Initiation

revert to vegetative growth following flower initiation:
* + Splitting=Premature Flower Initiation

silver spots next to main vein:
Rabbit Tracks

visible sporulation/mold (grey and fuzzy):
* + Botrytis Blight
 
BUDS
blight:
* + Botrytis Blight
Greasy Canker
CYATHIA
blight:
Greasy Canker

drop:
Center Bud Drop
Negative DIF

 necrotic infection:
Alternaria Leaf Spot & Blight
CUTTINGS
blight:
Rhizopus Blight

collapse/death:
Choanephora Wet Rot
* + Pythium Root Rot
* + Rhizoctonia Stem and Root Rot

distorted leaf growth:
Leaf Crippling

failure to root:
Fungus Gnat Injury

 fungal growth/sporulation:
Rhizopus Blight (white webbing with black specks)

leaf blight:
* + Botrytis Blight

premature flower initation:
* + Splitting = Premature Flower Initiation

rot/soft rot: 
* Botrytis Blight
Choanephora Wet Rot
+ Erwinia Soft Rot = Bacterial Soft Rot (soft rot, odorous)
Fungus Gnat Injury
LEAF VEINS
necrotic lesions:
Alternaria Leaf Spot & Blight
Scab= Spot Anthracnose
LEAVES
amber droplets visible:
Bacterial Blight =Bacterial Canker

blight:
Rhizopus Blight

chlorosis: 
halos around necrotic spots/lesions-
Alternaria Leaf Spot & Blight
Greasy Canker
Scab = Spot Anthracnose
Xanthomonas Leaf Spot = Bacterial Leaf Spot (symptom is actually a pale green halo)
interveinal-
Magnesium Deficiency
Soluble Salt Toxicity
irregular patches-
+ Cycocel Damage
marginal-
Boron Toxicity
+ Cycocel Damage
Magnesium Deficiency
Molybdenum Deficiency
mottled-
+ Poinsettia Mosaic & Poinsettia Cryptic Viruses
Magnesium Deficiency
spots-
+ Powdery Mildew
throughout-
Alternaria Leaf Spot & Blight
Magnesium Deficiency
* + Pythium Root Rot
* + Rhizoctonia Stem and Root Rot
Scab = Spot Anthracnose
+ Thielaviopsis Root Rot = Black Root Rot
Xanrhomonas Leaf Spot = Bacterial Leaf Spot

curl/roll upward:
Molybdenum Deficiency

discoloration:
+ Erwinia Soft Rot= Bacterial Soft Rot (darkened, watersoaked throughout) 

distorted growth:
Leaf Crippling
Alternaria Leaf Spot & Blight
Bacterial Blight = Bacterial Canker
Boron Deficiency
+ Poinsettia Mosaic & Poinsettia Cryptic Viruses

drop/abscission:
Alternaria Leaf Spot & Blight
Bacterial Blight = Bacterial Canker
+ Cycocel Damage
Leaf Drop
* + Pythium Root Rot
* + Rhizoctonia Stem and Root Rot
Scab = Spot Anthracnose
Soluble Salt Toxicity
+ Thielaviopsis Root Rot = Black Root Rot
Xanthomonas Leaf Spot

 epinasty (twisting, downward growth):
Ethylene Injury

fungal growth/ sporulation:
* + Botrytis Blight (grey & fuzzy)
Choanephora Wet Rot (white, whisker-like)
+ Powdery Mildew (white, powdery)
Rhizopus Blight (whilte webbing w/ black specks)
Rust (cinnamon-colored spores)

hardened appearance (accompanies severe chlorosis):
Magnesium Deficiency

necrosis:
lesions-
Alternaria Leaf Spot & Blight
Bacterial Blight =Bacterial Canker
* + Botrytis Blight
Corynespora Leaf Spot
Greasy Canker
Phytophthora Crown Rot
Scab=Spot Anthracnose
marginal-
Boron Toxicity
+ Bract Edge Burn = Bract Necrosis
Calcium Deficiency
+ Cycocel Damage
Molybdenum Deficiency
Scab = Spot Anthracnose
Soluble Salt Toxicity
mottled-
+ Poinsettia Mosaic & Poinsettia Cryptic Viruses
spots-
+ Bract Edge Burn = Bract Necrosis
Greasy Canker
Scab = Spot Anthracnose
+ Thielaviopsis Root Rot = Black Root Rot
Xanthomonas Leaf Spot = Bacterial Leaf Spot (spots are cholcolate-brown to rust colored)
throughout-
* + Botrytis Blight
Potassium Deficiency
water-soaked lesions-
+ Erwinia Soft Rot = Bacterial Soft Rot
Xanthomonas Leaf Spot = Bacterial Leaf Spot (very tiny, grey to brown, underside of leaf)

rot/soft rot: 
* + Botrytis Blight
Choanephora Wet Rot
* + Rhizoctonia Stem and Root Rot

wilting:
+ Erwinia Soft Rot = Bacterial Soft Rot
 
 

 
ROOTS
gall formation:
Crown Gall

necrotic lesions:
* + Rhizoctonia Stem and Root Rot
+ Thielaviopsis Root Rot = Black Root Rot

 rot/ soft rot:
brown or blackened-
+ Thielaviopsis Root Rot = Black Root Rot
dark brown-
* + Pythium Root Rot
* + Rhizoctonia Stem and Root Rot
softened & brown to black-
Fusarium Root Rot
any of above-
Fungus Gnat Injury
STEM
amber droplets:
Bacterial Blight = Bacterial Canker

blight:
* + Rhizoctonia Stem and Root Rot

collapse/wilt:
+ Erwinia Soft Rot = Bacterial Soft Rot

cracking:
Bacterial Blight = Bacterial Canker
+ Thielaviopsis Root Rot = Black Root Rot

discoloration:
+ Erwinia Soft Rot = Bacterial Soft Rot (darkened & watersoaked)
Rhizopus Blight (dark, greasy, extending 4" up from base)
+ Thielaviopsis Root Rot = Black Root Rot (blackening at soil line)

distorted growth:
Bacterial Blight = Bacterial Canker
Boron Deficiency

divides as if pinched (into 3 whorls & one flower bud):
* + Splitting = Premature Flower Initiation

droop:
Choanephora Wet Rot

elongated (abnormally):
Scab = Spot Anthracnose

fungal growth/ sporulation:
cream to orange-
Fusarium Root Rot
grey & fuzzy-
* + Botrytis Blight
white webbing w/ black specks-
Rhizopus Blight
white & whiskery-
Choanephora Wet Rot

 gall:
Crown Gall

growth tip ceases to grow:
Boron Deficiency
* + Splitting = Premature Flower Initiation

laterals breaking:
Lateral Branches Breaking

necrosis:
streaks-
Bacterial Blight = Bacterial Canker
Phytophthora Crown Rot
lesions/cankers-
Alternaria Leaf Spot & Blight
* + Botrytis Blight
Greasy Canker
Phytophthora Crown Rot
* + Rhizoctonia Stem and Root Rot (dry-looking, may crack)
Spot Anthracnose = Scab (scab-like or sunken)
water-soaked lesions/cankers-
Bacterial Blight = Bacterial Canker
+ Erwinia Soft Rot = Bacterial Soft Rot
Phytophthora Crown Rot (grey)
* + Pythium Root Rot

rot/soft rot:
* + Botrytis Blight
Choanephora Wet Rot
+ Erwinia Soft Rot = Bacterial Soft Rot
Rhizopus Blight (dark, greasy, extending 4" up from base)

shrivelled:
Phytophthora Crown Rot

 
VASCULAR SYSTEM / PITH / CORTEX
cortex rotted:
* + Pythium Root Rot

pith discolorartion:
black-
+ Thielaviopsis Root Rot = Black Root Rot
dark brown-
Phytophthora Crown Rot

 vascular system discoloration:
+ Erwinia Soft Rot = Bacterial Soft Rot (brown)
WHOLE PLANT
collapse/wilt/death:
+  Erwinia Soft Rot=Bacterial Soft Rot
Phytophthora Crown Rot
* + Pythium Root Rot
* + Rhizoctonia Stem and Root Rot
Rhizopus Blight
+ Thielaviopsis Root Rot = Black Root Rot		 stunting:
+ Erwinia Soft Rot= Bacterial Soft Rot
+ Poinsettia Mosaic & Poinsettia Cryptic Viruses
* +Pythium Root Rot
* +Rhizoctonia Stem and Root Rot
+ Thielaviopsis Root Rot = Black Root Rot

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