Pythium is a common root infection in poinsettias. The following is more information on this disease from the University of Maryland.
Pythium root rot is one of the most problematic diseases on poinsettias throughout the crop production cycle. The pathogen is really a number of species with different temperature requirements, so growers can see Pythium problems regardless of the season. Sanitation is critical to help reduce Pythium problems – the fungus-like pathogen is commonly associated with soil, dirty pots and untreated pond water. At this point in the poinsettia crop, Pythium can infect roots damaged from water stress or high soluble salts from too much fertilizer. Look for watersoaked, brown roots with vascular tissue intact (commonly called “rat-tailing”) as evidence of Pythium root rot infection. Infected plants may be smaller in size, with off-color leaves, and in extreme cases may show wilt symptoms even though the potting medium is moist. Plants with severe root rot should be discarded. Soil drench application of fungicides may be needed to protect uninfected plants against Pythium root rot. Products effective in managing this disease include FenStop (fenamidone), Banol (propamacarb), Banrot (etridiazole + thiophanate-methyl), Truban or Terrazole (etridiazole) and Subdue Maxx (mefenoxam). Some Pythium strains may be insensitive to Subdue Maxx. It is always a good idea to rotate products with different active ingredients to avoid resistance in Pythium populations.
Information from the current edition of the Greenhouse TPM/IPM Bi-Weekly Report from the Central Maryland Research and Education Center, University of Maryland Extension http://www.ipmnet.umd.edu/09Nov13G.pdf