Quick answer
Going mushroom picking is идти́ по грибы́ — an old construction with по plus the accusative, used only for gathering.
The mushrooms
The forest
What you do
A national hobby
From August onwards, whole families disappear into the forest at dawn with baskets and knives. The catch is cleaned, fried, salted or dried the same evening. Knowing which mushrooms are safe is local knowledge passed down in families — and the rule for visitors is simple: do not pick what you cannot identify.
В лесу́, not в ле́се
Forest belongs to the small group of masculine nouns with a stressed -у́ in the prepositional: в лесу́, в саду́, на полу́, в углу́, на берегу́. Learn them as set phrases — the regular в ле́се exists but means something closer to “in forest matters”.
Typical mistakes
в ле́се
в лесу́
лес takes the stressed -у́ ending.
идти́ для грибо́в
идти́ по грибы́
The fixed gathering construction is по + accusative.
Check yourself
Which is correct?