Phytophthora crown rot has become fairly common on poinsettias. The pathogen is Phytophthora drechsleri, which causes a root rot that may extend upwards into the lower stem. It is not possible to distinguish this injury from that caused by Pythium aphanidermatum, which is also commonly found on poinsettias. Fortunately both are controlled by the same kinds of chemistry: appropriate materials include SubdueMAXX/Subdue; Truban/Terrazole/Banrot and Aliette. Be alert to the possibility that the strain of Phytophthora affecting your crop may be one that is not sensitive to Subdue, and alternate products with different modes of action.