Bacterial leaf spot on poinsettias
Monday, 26 July 2010 15:15
Watch for bacterial leaf spots (due to a pathovar of Xanthomonas) on poinsettias. This disease primarily affects poinsettias in propagation, but it may persist in later stages of production when there are favorable conditions for disease development. The bacteria are not the same ones that cause bacterial blight on geraniums (Xanthomonas campestris pv. pelargonii) and, fortunately, the infection of poinsettia leaves is NOT systemic. 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.
If detected early, discarding plants showing symptoms and reducing leaf wetness will help to reduce disease spread and protect the crop from widespread injury. The use of copper bactericides (e.g. Phyton-27, which has no objectionable residue) may also help reduce disease spread. Some poinsettia cultivars are prone to injury from copper sprays, and this injury will be more pronounced if there is good spray coverage on the underside of the leaves. Phyton 27 should be applied only when there are good drying conditions, to reduce the potential for plant injury. Some growers alternate copper sprays with Bacillus subtilis (e.g. Cease). Bacillus subtilis treatments provide some biological control of the leaf-spotting bacteria and reduce the likelihood of leaf yellowing from excessive copper exposure by allowing more time between copper treatments. The effectiveness of B. subtilis against this particular Xanthomonas leaf disease has not been assessed, but research trials on a similar disease of zinnia have shown control benefits. Keeping the leaf surfaces dry is the most effective way to curb bacterial leaf spot, but prompt roguing of diseased plants combined with copper and B. subtilis treatments can help.
Leanne Pundt, University of Connecticut
Margery Daughtrey, Cornell University
