The plight of the poinsettia with aerial blight

Poinsettia growers should be on the lookout for aerial Phytophthora, an uncommon greenhouse production pathogen.

Poinsettia’s (Euphorbia pulcherrima) are 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” from Michigan State University Extension,  However, certain species of the pathogen Phytophthora, that 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 help 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, rouging all infected plants and those around them may be necessary.

Some fungicides that have been found to be effective in MSU trials on aerial Phytophthora include:

  • FenStop (fenamidone)
  • Micora (mandipropamid)
  • Orvego (ametoctradin + dimethomorph)
  • Segway (cyazofamid)
  • Subdue MAXX (mefenoxam) as a drench or spray
  • Stature SC (dimethomorph)

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