appressorium

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The flattened tip of a hypha in various species of parasitic fungi, from which a filament emerges that penetrates the host's tissues.

from The Century Dictionary.

• noun The organ by which parasitic fungi attach themselves to their hosts, consisting usually of the flattened or swollen end of a hypha.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a bulbous formation produced by parasitic fungi that is well attached to the cuticle of the host and from where a peg-shaped hypha is formed that penetrates the cuticle