Haustoria of Blumeria (Erysiphe) graminis in cells of grass host
In this parasite the hyphae are superficial and run over the surface of the grass leaf. The only part of the fungus inside the host is the feeding cell ( = haustorium). A narrow penetration peg punctures through the grass cuticle into an epidermal cell. This enlarges and branches inside the host cell to produce a multi-armed, octopus-like haustorium. The haustorium does not actually penetrate into the living cell but invaginates the cell membrane and the multibranched nature of the haustorium gives it a large surface area for nutrient exchange so it can steal the host's nutrients more efficiently. If you look carefully you will see the delicate outline of the host membrane enveloping the haustorium (arrows).
Blumeria graminis is an epiparasite on grass blades. The entire blade of the grass may turn white and appears as if painted. The hyphae of the parasite run over the surface of the host and produce masses of spores to give the typical 'Powdery Mildew' symptom. The so-called Powdery Mildews are a very successful group of fungal parasites that attack tens of thousands of species of wild and domesticated higher plants many of which are crop plants, fruits, and ornamentals e.g. roses, lilacs etc.