Root, Crown and Stem Rots Pathogen Symptoms Conditions Favoring Disease Control Measures Rhizoctonia root & stem rot* Brown rot of stems at soil line; sunken, constricted canker of stem; brown lesions on roots; lower leaves yellow and fall off. Excessive soil moisture; high temperatures. Rouge infected plants; fungicide drenches using Terraclor 75WP (PCNB), Banrot or Domain. Pythium root & stem rot* Wet, brown-to-black decay of roots and small roots slough off leaving inner string of stele; lower leaves yellow and drop; plants are stunted. Excessive soil moisture, poor drainage of the medium; active at low temperatures. Rouge infected plants; Drench with Truban 30 WP, Truban 25 EC, Banrot 40 WP, Subdue 2E, Lesan 35 WP. Phytopthora crown & stem rot Stem canker; internal pith tissue appears brown when cut; stunting and wilting can cause leaf rot also. Overhead watering; poor media drainage; high temperatures. Rouge infected plants; Drench with Truban 30 WP, Truban 25 EC, Banrot 40 WP, Subdue 2E, Lesan 35 WP. Erwinia carotovora (Bacterial soft rot) Affected plants wilt. Rooted and unrooted cuttings undergo soft decay, total maceration and deterioration of lower stem. Occurs very rapidly, usually 3 days after sticking cuttings. Primarily a propagation problem. Waterlogged rooting media; can be very severe under warm conditions. Strict sanatation; dip cuttings in 500 ppm chlorine; discard diseased cuttings immediately; Streptomycin can be used but is not very effective.
*Pythium root and stem rot and phytopthora crown and stem rot are caused by water mold fungi that have very similar life cycles. Environmental conditions required for infection and plant damage, as well as control measures are similar.
Foliar Problems Pathogen Symptoms Conditions Favoring Disease Control Measures Botrytis gray mold (most destructive disease of poinsettia) Leaf and flower tissue rots; worse on immature leaves or wounded or stressed tissue. Lesions on leaves and bracts appear as tan-brown spots. Masses of gray spores can be seen under favorable conditions. Poor air circulation in greenhouse resulting in air stagnation and high humidity. Wounded or nutritionally stressed tissue much more susceptible to infection. Adequate air circulation; reduce humidity; avioid plant wounding; strict sanatation to remove dead plant tissue. Fungicide application using Chipco 26019, Ornalin 50 WP (not on plants in color), or Exotherm Termil. Powdery Mildew Dusty white growth on leaves, stems or bracts. Poor air circulation, foliar wetting, crowded plants, stressed plants. Maintain plants in vigorous condition; Avoid foliar wetting; apply preventative fungicides susch as Captan 50 WP or Zyban WP. Fungal leaf spots (several fungal genera) Dry brown-black lesions with or without a yellow halo, usually circular to irregular in shape. Cool to moderate temperature, high relative humidity. Adequate air circulation to prevent periods of high humidity. Remove infected plants/plant parts from greenhouse. Apply Phyton 27, Cleary’s 3336 WP, Domain FL, or Zyban WP. Bacterial leaf spots (several bacterial genera) Watersoaked lesions, usually gray to dull-brown in color, frequently with angularshape and yellow halo. Can usually be seen on both upper and lower surface of the leaf. Leaf wetting, splashing water, usually more severe at temperatures in the 80-90 degree range. Rouging; strict sanatation measures; avoid foliar wetting; Agristrep sprays used a preventative.
Note: Be sure to read and follow all label information when using disease control chemicals.
Remember that effective disease control involves anticipating plant disease, using stringent sanatation methods to exclude pathogen introduction, environmental modification to reduce plant infection by potential pathogens, as well as preventative sprays of effective disease chemicals. Growers should consider all of these disease control steps as essential in an overall disease control program.
The diseases listed in this table are not inclusive of all those that are known to occur on poinsettias. They do, however, represent those diseases that Texas growers experience most frequently. If you are having difficulty diagnosing a specific disease(s) we encourage you to contact the Texas Plant Disease Diagnostic Lab. A form is included (which can be photocopied) to submit samples to the lab.