|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.||Curtobacterium 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 sterile propagation media. Discard infected cuttings and infested media. Destroy infected plants and crop debris. 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 a fungicide to protect plants.|
|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 a fungicide to protect plants.|
|Leaf Drop||Defoliation.||Root rot, over-fertilization, low light intensity, or lack of moisture.||Avoid root rots and over-fertilization. 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 of molybdenum-containing fertilizer or 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 1.0 mS for soil or 2.5 mS for soilless mixes (saturated paste extract) 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. Continue scouting throughout the season, especially as plants are being spaced. Apply a fungicide to protect plants as soon as any mildew is found. Control must prevent disease before bracts form. Fungicides do not make the white fungus go away after it is dead.|
|Phytophthora Root and Stem Rot||The base of infected stems appear soft and wet. Roots are brown and water soaked. Cuttings wilt and die rapidly. Under dry conditions, the pith of the lower stem is brown and the stem has a gray canker.||Phytophthora nicotianae||Immediately discard infected plants. Use pasteurized soil and clean pots and tools. Keep hose ends off the ground. Apply a fungicide to protect plants.|
|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||Use only sterile soil and clean pots and tools. Keep hose ends off the ground. Do not over-water or over-fertilize plants. Apply a fungicide to protect plants.|
|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 only sterile soil and clean tools, and hang us hose ends. Do not over-fertilize. Apply a fungicide to protect plants at planting.|
|Scab||Small, light-colored, round spots with yellow halos form on the leaf, particularly along the main vein. Infected stems stretch several inches above the normal crop.||Sphaceloma poinsettiae||Maintain low relative humidity in the crop canopy. Do not wet the foliage when irrigating. Apply a fungicide to protect plants.|
|Thielaviopsis Root Rot||Late in the season, roots turn black. Plant wilt. Longitudinal splits form at the stem base at and below soil line. Leaves yellow and fall.||Thielaviopsis basicola||Use only sterile soil and clean tools, and hang up hose ends. Apply a fungicide to protect plants.|
Botrytis stem canker.
Botrytis flower blight. Molybdenum deficiency
Thielaviopsis stem and root rot.
Prepared by Gary W. Moorman, Professor of Plant Pathology
This publication is available in alternative media on request.