Effect of the Environment on Powdery Mildew of Poinsettia
Powdery mildew of poinsettia is caused by a fungus (Oidium sp.) that appears as white talcum-like spots on leaves, bracts and stems. It begins with a conidium (spore) that germinates to form a germ tube. The fungus attaches itself to the plant surface through a structure that acts like a suction cup called the "appressorium." Under the appressorium, the fungus penetrates the plant via an infection peg, and a specialized sac, the "haustorium," forms inside the plant cell which allows the fungus to take up nutrients from the plant. Under favorable conditions, sporulation occurs as the fungus produces abundant chains of conidia on stalks ( conidiophores) which give the infected plant a white, and powdery or fluffy appearance.
Source: • American Floral Endowment Special Research Reports #116