Alternaria leaf spot is one of several fungal diseases that can pounce on your poinsettias when your attention is diverted. The pathogen, Alternaria euphorbiicola (or certain other Alternaria species) most likely travels along with poinsettia cuttings, and attacks when the environmental conditions favor infection.
Susceptibility to this poinsettia disease varies from one cultivar to another; in the past, some of the V-14 types were seen to be particularly susceptible in comparison to the Heggs, which developed only tiny leaf spots. Unfortunately, modern cultivars have not been trialed for their relative susceptibility. This leaf spot problem is most likely to appear on outdoor grown crops in hot, humid environments, but there have been several outbreaks in greenhouses in the Northeast in recent years.
Because the disease is unfamiliar, there is a chance that it will be mistaken for Botrytis blight. Especially on the less-susceptible cultivars, the symptoms could be confused with those of bacterial leaf spot caused by Xanthomonas axonopodis pv. poinsettiicola, or with poinsettia scab caused by Sphaceloma poinsettiae. The large, brown, multi-celled Alternaria spores can be moved on air currents or splashed to nearby plants by overhead watering.
Wet leaf surfaces, high humidity and warm temperatures are needed for infection. The symptoms appear not only on leaves, but also on stems, petioles, cyathia and bracts, so that the disease quickly has an impact on saleability of a crop.
Alternaria leaf spot is not often mentioned as a propagation disease, where it might be mistaken for Botrytis blight; it tends to be noticed in late summer or early fall. Alternaria leaf spot is obvious on a highly susceptible host: spots are purplish-black and become irregularly shaped as they expand to become an inch across or more. Leaves can be distorted if infections land on veins, and there may be yellowing and leaf drop. Spots on stems are elongated and sunken, up to an inch long.