Bacterial Leaf Spot

Bacterial leaf spot primarily affects poinsettias in propagation, but it may persist in later stages of production when there are favorable conditions for disease development.

Xanthomonas leaf spot on poinsettias begins with tiny, dull gray, water-soaked areas on the leaves. Small, angular tan spots (1 mm in size) with a yellow halo (surrounding the tan spot) develop. As the disease progresses, spots turn darker brown and coalesce and might be confused with fungal diseases such as poinsettia scab or Alternaria leaf spot.

It is a good idea to send samples to the diagnostic laboratory if you see leaf spot symptoms, in order to determine the exact cause of the leaf spots (fungal or bacterial).

Like all bacterial diseases, Xanthomonas leaf spot of poinsettia is spread by water splash from overhead watering (especially boom irrigation during propagation). High humidity, warm temperatures and close plant spacing favor rapid disease development and spread. To reduce spread, it is critical to keep leaves as dry as possible.

Source: Leanne Pundt, University of Connecticut, Extension Educator, Commercial Horticulture