Poinsettias can be susceptible to a variety of pathogens when grown in a greenhouse. Some common diseases, such as bacterial soft rot, Botrytis blight, poinsettia scab, powdery mildew, and a variety of root rots, as well as insect and production issues, are outlined in “Common greenhouse poinsettia production problems” (http://msue.anr.msu.edu/news/common_greenhouse_poinsettia_production_problems) from Michigan State University Extension.
However, according to an article from Kristin Getter in the Michigan State University Extension Department of Horticulture, certain species of the pathogen Phytophthora, which is typically thought of as a root rot, can also cause issues above-ground.
Phytophthora is a water-mold pathogen that can be splashed onto stems and foliage and cause an aerial blight. Symptoms of aerial Phytophthora include brown or purple-black stem lesions that may spread quickly to bract petioles causing the bracts to wilt. Leaf lesions are also possible and will be grayish brown at first and then turn to brown to black. Growth of this disease is promoted by wet conditions and overhead irrigation spreads the infection from plant to plant.
To prevent aerial blights, use pathogen-free media and follow strict sanitation protocols in the greenhouse. Once an aerial blight is detected, reduce overhead irrigation if possible, as this will reduce the spread of the pathogen from splashing. Increased plant spacing will help the foliage dry more quickly after watering and also helps reduce pathogen development. Prune infected stems or leaves from infected plants as soon as symptoms develop and follow with a fungicide application as the infection is enhanced by wounding from pruning and the new growth is more susceptible to infection as well. For severe epidemics of aerial Phytophthora, removing all infected plants and those around them may be necessary.