Poinsettia scab caused by the fungus Sphaceloma poinsettiae, is not a common disease on greenhouse grown poinsettias, although it is regularly found outdoors in Florida, Hawaii, Puerto Rico and other areas of outdoor production. It was originally discovered in Florida in 1940. The disease has been occasionally seen in greenhouses in North America but with early detection, roguing and treatment, it can be eliminated quickly.
Symptoms of the disease on stems and leaf petioles include:
- Scab-like or raised lesions with tan centers, often surrounded by white, red, or purple margins
- Heavily infected branches exhibiting a gibberellin-like effect that may grow twice as long as other branches
On leaves, symptoms include:
- Round to angular spots with purple margins
- Spots, raised or scab-like on the leaf
- Tissue near the leaf spot generally bleached our or yellowish in color
- Leaves with spots are often distorted in growth, with a puckered appearance
In 2000, Margery Daughtrey at Cornell University evaluated 42 poinsettia cultivars for susceptibility to poinsettia scab. All cultivars were susceptible to the disease to some degree. In general, the dark leaf cultivars tended to be slightly less susceptible to poinsettia scab than the green leaf cultivars.
Hot, wet conditions favor development of this disease. The fungus is spread primarily by splashing water, such as during propagation or with overhead irrigation. This organism is known to infect only poinsettias or other members of the Euphorbia family. Infected plants will not necessarily show all the symptoms. Scab does not appear to be highly contagious after the cuttings are moved out of propagation conditions.
Poinsettias may express symptoms during propagation and early crop development that mimic the symptoms of scab. However, these symptoms may be due to other factors, which commonly occur under propagation conditions, such as leaf scarring, Odema, Alternaria leaf spot and environmental disorders. As with all diseases, it is important to scout the crop on a regular basis and take the appropriate action. If in doubt, isolate plants and observe them. If you have any questions, please consult our technical advisory staff or your local pathology laboratory.
If this disease is found:
- Spray the entire crop thoroughly with a fungicide to insure coverage to upper and lower leaf surfaces as well as to stems
- If symptoms persist, spray again then rogue the affected plants, 2 or 3 fungicide applications at weekly intervals may be required
- Place affected plants in plastic bags and remove from the greenhouse
- Disinfect hands, tools, or other equipment periodically and immediately after handling plants with symptoms of the disease
- Carefully inspect adjacent plants for early symptoms of the disease
- Adopt cultural practices that minimize splashing and leaf wetness
- If possible, avoid overhead irrigation or syringing of the crop
- Continue to scout the crop each week
Trials conducted in 2000 at Cornell University and at Chase Research Gardens have found that pesticides containing mancozeb (Dithane®, Protect™), mancozeb plus thiophanate methyl (Zyban®), mancozeb plus copper hydroxide (Junction), thiophanate methyl plus chlorothalonil (Spectro™), triazoles (Systhane® and Terraguard®), strobilurins (Compass™ and Heritage®), and copper-based (Phyton 27) all gave good to excellent control of poinsettia scab. As with other diseases, optimal control is achieved when the fungicides are applied preventatively. Some of the listed fungicides, especially those containing mancozeb, tend to leave a significant amount of residue and may not be appropriate for use late in the production of a poinsettia crop. Read and follow the instructions on the label before using any pesticide. Before using a pesticide on the crop for the first time or on a new cultivar, treat a few plants and check for phytotoxicity.
©2011 Ecke Ranch