Especially if the weather is warm and humid , the infection rate is very high.
For details, please take a look here : http://www.biology.ed.ac.uk/research/gr ... otroph.htm
The spores germinate on a leaf surface, rapidly forming a haustorium to penetrate
the epidermis cells and develop more mycel too:

Inside the epidermis cells root-like structures are formed, to tap the epidermis cells for nutrients.

Stained with Aniline-blue-Lactophenol, DIC illumination.
Thanks for looking,
Gerd