Poinsettia Scab Alert!By Dr. S. Nameth If most of you hadn't already heard, scab disease has reared its ugly head in the early stages of this seasons poinsettia production. Scab is a relatively rare disease that was first reported on poinsettia in the 1940's. Since then the disease has "popped up" every now and then just to let us know it's still alive and well. The disease was last seen in 1996. Scab has been identified in at least two greenhouses in Ohio and a few other greenhouses scattered throughout the U.S. Scab is caused by the fungus Sphaceloma poinsettiae. The disease is called scab because it produces raised elliptical scab-like lesions on the stems and leaf midribs of the affected poinsettia plant. The center of these "scabs" can often appear salmon colored or pinkish. On the leaves the fungus manifests itself in leaf spots. These spots may be round or angular with a purple-ish margin. Beyond the margin the leaf tissue may appear to be yellow or "bleached white" in color. If many spots are together, a large area of the leaf surface will become necrotic (dead). Leaf spots may also be raised, however, it is not always the case and non-raised spots should not be taken as a lack of scab. Other symptoms include the elongation or stretching of infected stems which may be caused by some sort of plant hormone effect. If you suspect you have scab you need to get in touch with your county Extension agent or your university Extension personnel so that the suspect material can be positively identified. Once identified the proper control measures should be taken. The causal fungus is relatively host specific and will only infect poinsettia or other plants in the family Euphorbiaceae. Infected plants should be removed from the greenhouse as soon as possible. The best way to remove infected plants without causing spread of the disease is to cover the entire plant with plastic bag and remove the bagged plant from the greenhouse. This will eliminate the possibility of spreading fungal spores about the house. Once the symptomatic plants have been removed, pay close attention to plants adjacent to the one you removed. These plants are the ones most likely to become infected in the near future and they should be monitored for disease daily. At the first sign of scab the plants should be removed. To inhibit any further spread of the disease by splashing, overhead watering should be curtailed and plants should be watered early in the day so that the leaves do not stay wet for extended periods. Fungicide applications should start immediately. Preliminary fungicide trials with myclobutanil (Systane) have been very promising, showing both preventative and curative activity. This chemical is currently registered for control of powdery mildew on poinsettias throughout the United States, with the exception of select counties of Long Island, New York (see label for details). Systane has been used without phytotoxicity or significant residue on poinsettias and because it is systemic, may be the best new fungicide for protection of your poinsettia crop. Products such as Cleary's 3336, Fungo (thiophanate-methyl), Daconil (chlorothalonil), Dithane, Fore T/O (mancozeb), and combination products such as, Spectro have all been shown to be affective against scab. Regardless of the product continual daily monitoring of the crop for scab needs to continue until a week has passed without signs of new infection sites. For pictures of symptomatic plants you can access the Ecke web site at www.ecke.com/scabupdate/ |
