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Poinsettia Problem Diagnostic Key - Corrective Measures

Pythium and Phytophthora Root Rot

Root rot of poinsettia can be caused by Pythium aphanidermatum, P. debaryanum, P. irregular, P. megalacanthum, P. oligandum, P. perniciorum, P. polymastum and P. utimum. Poinsettia root rot is also caused by the closely related fungus Phytophthora parasitica. Disease severity may vary between the above species but the conditions for disease development and symptoms are quite similar. Root rot caused by these fungi may be the common disease on poinsettia worldwide. It can occur at any stage of production but often shows up in November or early December.

Pythium root rot.
Click on image for larger version. (image Ron Jones, NCSU)

Stunted growth of plants infected with Pythium root rot.
Click on image for larger version. (image Brian Whipker, NCSU)

Plants in an entire flat killed by Pythium root rot.
Click on image for larger version. (image Brian Whipker, NCSU)

Wilted plant with rotten roots of poinsettia plant infected with Pythium root rot.
Click on image for larger version. (image Brian Whipker, NCSU)

Plants infected with Pythium (left) and Botrytis (right). Note the healthy white root system present on the Botrytis infected plant.
Click on image for larger version. (image Brian Whipker, NCSU)

(insert XX10. Phytophthora root rot. )

The primary symptoms of root rot are badly rotted dark wet roots. The rot may affect stems to varying degrees depending upon the age of plant and upon cultural conditions. If roots of very young succulent plants become infected, the pathogen may proceed up the stem several inches and cause the entire stem to become water-soaked, then turn dark brown to black. Stems may collapse at the soil line and fall over.

Infected plants can be stunted, have chlorotic leaves and lower leaves abscise. Plants infected with Pythium or Phytophthora that survive the disease in early growth stages may bloom early or wilt, collapse, and die just before sales.

Pythium and Phytophthora root rot is favored by high soil temperature. Disease development may be reduced by lowering pH to 5.5 or below (but the at pH 5.5 or below other nutrient disorders are possible). Moderate to cool media temperatures are best for disease development for Pythium where as Phytophthora root rot is usually more severe under warm conditions. Pythium and Phytophthora root rot is also favored by high soluble salts in the root substrate (>3.0 mS/cm based on the saturated paste extraction method).

 

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