When these plants were examined, the roots appeared to be rotted. When the root tissue was examined under the microscope, there were oospores present. This is a sign of Pythium root rot. The visual symptoms of the poinsettia roots also appeared to be those associated with Pythium root rot at various stages of infection.
Pythium can be introduced to plants by infested crop debris in the greenhouse, transplanting infected cuttings, or recycling potting mix or containers. In addition, in some cases, this pathogen can be introduced by contaminated irrigation water. The problem could also be that the potting medium is not draining properly. Remember, environmental conditions can cause soils not to dry out properly.
This disease is favored by high soil temperatures. Another thing that seems to favor this disease is high soluble salts in the root substrate. I also read where excess fertilization can promote the growth of Pythium.
Good sanitation is the best defense against this disease. Make sure that the potting mix is sterile and disinfect work surfaces. Unfortunately, once Pythium infection takes place, control of the pathogen can be very difficult. Disease development can be reduced by lowering pH to 5.5 or below; however, this can cause nutrient disorders to develop in plants.
A preventative fungicide application may be necessary the following year, in addition to improved sanitation.
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