Food & Environment Research Authority (FERA) bacteriologist Andy Aspin said of bacterial angular leaf spot (xanthomonas axonopodis): "It is a quarantine disease and we have found it in the UK over the past few years. Poinsettia is big around Christmas and this is the time when people should be keeping an eye on it."
FERA senior plant pathologist Dr. Paul Beales said: "There are two types of powdery mildew on poinsettia that we take action on - American powdery mildew and African powdery mildew. It's worth looking out for the American one as it is quite devastating. The symptoms are a real covering of a white dust, like icing sugar, on the upper and lower surface of the plant."
He said any growers concerned about their crop can send samples to the FERA plant clinic for identification, but warned that white residue may also appear as a result of the plants being watered with hard water or if remnants of the plant's internal white resin have seeped out because of damage.