But when the bath was filled we found a fur, (line 18)
Here's the first blotch on their otherwise successful berry hunt. The boys are probably as proud as peacocks that they've picked enough berries to fill a whole tub and now they're finding out that the beautiful fruit's about to spoil. Just around the corner of the height of satisfaction is disappointment.
A rat-grey fungus, glutting on our cache. (line 19)
You can almost hear the anger in his voice. It's not a cloud-gray fungus, it's "rat-grey." Rats just about never mean anything good or positive (unless you're a hungry snake). The rat seems to be robbing these boys of something, ushering in their disappointment.
The juice was stinking too. (line 20)
So, the berries are spoiled and there isn't even a consolation in the juice; that's spoiled too. "Stinking" works like "rat" does in the previous line. It's used as an insult in our everyday language – "stinking liar," "stinking jerk," "I hate eating stinking Corn Flakes every morning." The speaker feels disappointed and hurt, and the language of the poem reflects that.
I always felt like crying. It wasn't fair (line 22)
Well this one's pretty self-explanatory. He's so disappointed about the rotting berries that he wants to cry. And it's not fair, is it? Why can't any of the good stuff ever last?