Hylophobia is the abnormal fear of forests; it may only apply to deep forests (which is the most common form of hylophobia), or to any wooded area regardless of size.
It generally arises from a traumatic experience with the wild animals inhabiting forests, such as an encounter with an aggressive bear. Also, the risk of contracting dangerous parasites, such as ticks, can motivate the phobia.
Sometimes, the phobia can stem from the trees themselves (dendrophobia), especially from the risk of a branch fall, or even a tree fall. Other times, it is due to local folklore and superstitions, which was popularized by movies such as The Blair Witch Project.
Contrast this with nyctohylophobia, which is the vastly more widespread fear of dark wooded areas at night.