| Disease (causal agent) | Symptoms | Survival of pathogen and effect of environment | Comments on control |
|---|---|---|---|
| Bacterial canker * (Curtobacterium flaccumfaciens pv. poinsettiae) |
Longitudinal water-soaked lesions on stems and petioles. Spotting of leaves and defoliation. In advanced stages, stem lesions split open. | Infected poinsettia plants and debris. Favored by warm, moist conditions. Bacteria spread in water. Bacteria may be present in symptomless plants and cuttings. | Plant pathogen-free cuttings. Avoid overhead irrigation and syringing. Steam soil and disinfect benches with copper naphthenate; disinfect tools. |
| Bacterial leaf spot * (Xanthomonas campestris subsp. poinsettiaecola) | Spots start out on the undersides of leaves, expand, and become visible on the upper surfaces. Spots are grey to brown, becoming darker as they expand. Generally, they are surrounded by a water soaked area of lighter green or yellow. | Infected poinsettia plants and cuttings can survive in dried leaf debris for one year. Favored by high humidity, warm temperatures, and close plant spacing. | Prevention is the best management strategy, primarily by avoiding overhead watering that allows splashing of bacteria to adjacent plants. Once present, discard affected plants and disinfect benches. Control can be difficult, but sprays of copper compounds have shown some level of success. |
| Bacterial stem rot (Dickeya (=Erwinia) chrysanthemi) |
Watery, soft rot of cuttings or stems, resulting in disintegration of infected tissues. Rot develops rapidly and plant collapses. | In diseased plant tissues. Favored by high temperatures (73° to 86°F) and succulent plants. | Avoid high temperatures and practices that produce succulent growth. Careful sanitation practices will minimize spread of bacteria. Disinfect tools with quaternary ammonium. |
| Gray mold * (Botrytis cinerea) |
Blasting of flowers and browning or spotting of bracts. Fuzzy gray fungal growth forms on dead parts. Lesions on stems and leaves. | In plant debris. Favored by cold, moist conditions and condensed moisture on bracts and flowers. | Provide better growing conditions and air circulation. Clean up plant debris. Avoid wetting leaves and try to lower relative humidity. Protect plants with fenhexamid. |
| Greasy canker (Pseudomonas viridiflava) |
Greasy-appearing cankers on stems. Necrotic lesions with chlorotic margins on leaves. Sometimes confused with bacterial canker. | Infected plant debris. Bacterium has a wide host range. Disease favored by high humidity, high temperature, and condensed moisture on plants. | Reduce humidity. Sanitize pruning tools. Avoid wetting foliage. |
| Powdery mildew * (Oidium spp.) | Yellow spots or whitish growth on upper leaf surfaces and bracts. | On living plants. Favored by moderate temperatures and crowded, shaded foliage. | Regularly inspect plants. Remove, bag, and dispose of infected plants at first sign of infection. Apply protectant fungicide when plants are young and more easily sprayed. |
| Root and stem rot * (Thielaviopsis basicola, Rhizoctonia solani, Pythium spp.) |
Plants are stunted and wilt easily. Lower leaves become chlorotic and may fall. Roots are rotted and dark brown, water-soaked stem lesions develop. Large roots and lower stem may be enlarged and ridged. Young plants frequently are killed. Each fungus can cause disease independently, or fungi may interact to produce rapid decline. Symptoms vary with the pathogen. | In soil. Favored by excess moisture and overcrowding of plants. Thielaviopsis and Pythium severe at high (86°F) or low (63°F) soil temperatures, whereas Rhizoctonia develops most rapidly between 63° to 79°F. Disease development most severe at rooting and just before maturing. | Plant disease-free cuttings in heat-treated or chemically treated soil. Before planting, mix into soil mefenoxam plus thiophanate-methyl. If root rots occur after potting, drench with mefenoxam plus thiophanate-methyl. |
| Rust * (Uromyces euphorbiae) |
Pustules of cinnamon-brown spores on both leaf surfaces. | On living plants. Favored by moist conditions. | Pick off and burn diseased leaves. Protective fungicide sprays help in control. |
| Spot anthracnose or scab (Sphaceloma poinsettiae) | Circular, buff-colored spots that develop into scablike lesions on leaves and stems. | Infected plants and debris. Favored by wet conditions. Spores spread in water. | Do not wet foliage. Protective fungicidal sprays should help in control. Avoid splashing water. |
| Virus or viruslike disease | Symptoms | Host range and natural spread | Comments on control |
| Mosaic (Poinsettia mosaic virus) |
Distortion of leaves and bracts. Some bracts may fail to color normally. Mild mottling of leaves. Angular leaf spotting, apparent only under cool temperatures. Symptoms most severe on plants grown at 61° to 68°F. Plants grown at 75° to 82°F appear normal. | May be symptomless in poinsettias. Mechanically transmitted. | Obtain virus-free plants. Grow at higher temperatures. Discard plants with symptoms. |
| Poinsettias are also susceptible to Rhizopus soft rot (Rhizopus stolonifer). | |||
| * For additional information, see section on Key Diseases. |