Pythium root rot on poinsettias
The most common root-rot pathogen that infects poinsettias late in the production cycle is Pythium.
Not all Pythium species are the same. Although there are many types of Pythium, the two main species that infect poinsettias are Pythium aphanidermatum and Pythium ultimum. Pythium aphanidermatum prefers growing medium temperatures above 77°F (25°C), so it can be an issue early in the crop cycle when temperatures are warm.
Pythium ultimum prefers colder growing media and has been found to cause the greatest amount of root damage at 62°F (17°C), but no serious root damage at 80°F (26°C). This species typically attacks poinsettias later in the crop cycle when both the outside temperature and growing medium are cold. What are the cultural conditions that exist during the cool, short days of the late fall that favor Pythium ultimum?
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