Diseases of Poinsettia
Part II

Leaf and Bract Blight and Stem Canker

Poinsettia leaf and bract blight is one of the most common diseases associated with poinsettia production. This disease is caused by the fungus Botrytis sp. This fungus is probably the most widespread pathogen associated with greenhouse production of floral crops. It is everywhere in every greenhouse, and, given the right environmental conditions, it can spread like wildfire through a crop. This disease thrives in conditions of high humidity (at or above 85%) and stagnant air. The disease usually starts at the margin of a leaf or bract (in a wound or burn scar) and then moves quickly through the leaf, down to the petiole, and in some cases into the stem. In most cases, the gray, fluffy, growth of the fungus can be seen sporulating on the leaf, bract or stem. The fungus produces enzymes that break down the plant cells and leaves the infected tissue mushy and water soaked in appearance. The fungus can advance rapidly in green tissue, but it really takes hold in dead or declining plant parts such as spent bracts or leafs.

Cultural controls, such as keeping the relative humidity in the greenhouse at 85 percent, allowing for good air circulation and ventilation, keeping plant wounding to a minimum, and not allowing free water to sit on plant parts for extended periods of time help keep the threat of this disease to a minimum. Removing spent tissue from developing plants is a must for proper disease control. Chemicals, if applied in a timely manner, are effective avenues for curtailing this disease. Many chemicals are labeled for Botrytis control, and they all show some degree of efficacy depending on the type of chemical and the formulation. Some of the best contain Chlorothalonil as their active ingredient. These include the Daconil and Exotherm Termil line of products. When you treat poinsettia for Botrytis, be careful not to apply chemicals that will burn or discollor the bracts. Once bracts are fully developed, the only product labeled for effective Botrytis control is copper-based Phyton-27. If used as directed by the manufacturer, this is one of the safest and most effective methods of Botrytis control on finished poinsettia.

Leaf Spots

Powdery Mildew. This is the newest in the onslaught of diseases that attack poinsettia. This disease is caused by the fungus Oidium sp., and it affects poinsettia at its most vulnerable time-during bract production. The disease is easy to diagnose because it produces white fluffy lesions or spots on the affected bracts. Due to color contrast, it is most obvious in the reds and pinks, but will infect the whites with just as much fervor. Recent work by Celio and Hausbeck at Michigan State University indicates that the non-red varieties appear to be less susceptible to infection; however, at 31 days post infection, none of the cultivars tested were considered saleable. Fungicides shown to be effective against powdery mildew include: Phyton-27, Chipco 26019, Domain, Strike 25, and Terraguard. Again. Be careful if you apply any of these chemicals when the bracts are fully developed.

Other diseases

Approximately a dozen other disease affect the production of quality poinsettia, although most of these play somewhat a minor role compared to the previous mentioned diseases. Most notable would be bacterial stem and cutting rot caused by the bacterium Erwinia sp. Controls include: not wounding plants and cuttings, avoiding production of "soft" plants with over fertilization, making sure that all surfaces of cuttings come in contact with are disinfected and bacteria-free, and applying bactericides such as copper-containing compounds in a timely manner.

Knowing what to look for and when to look for it are major factors in timely disease control. Using cultural or non-chemical controls whenever possible will help control disease and reduce chemical fungicides and bactericides needed for effective disease suppression.