rust fungus

NOUN /rəst_'fəŋɡəs/
11 LETTERS · STARTS WITH R

rust fungus is a valid 11-letter word worth 14 points in Scrabble and 19 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
R1U1S1T1F4U1N1G2U1S1
1 + 1 + 1 + 1 + 0 + 4 + 1 + 1 + 2 + 1 + 1 =14 PTS
DOUBLE WORD → 28TRIPLE WORD → 42
MEANING

What does rust fungus mean?