Leaving stains upon the tongue and lust for
Picking. Then red ones inked up and that hunger
Sent us out with milk cans, pea tins, jam-pots
Where briars scratched and wet grass bleached our boots.
Round hayfields, cornfields and potato-drills
We trekked and picked until the cans were full,
Until the tinkling bottom had been covered
With green ones, and on top big dark blobs burned
Like a plate of eyes. Our hands were peppered
With thorn pricks, our palms sticky as Bluebeard's.
We hoarded the fresh berries in the byre.
But when the bath was filled we found a fur,
A rat-grey fungus, glutting on our cache.
The juice was stinking too. Once off the bush
The fruit fermented, the sweet flesh would turn sour.
I always felt like crying. It wasn't fair
Heaney endows the fungus with the intelligence of a plague-spreading rat. 
 
 “Glutting” is an especially well chosen word to convey this ugly kind of decay. It feels like the young speaker, whom we can assume will go on to be a poet, is learning about the imaginative possibilities the natural world holds. Note that ‘glutting’ means gorging or stuffing, from which the word ‘gluttony’ derives. Therefore, the fungus was gobbling the ‘cache’ and destroying it. 
 Note also that ‘cache’ echoes the earlier word ‘hoard’, suggesting things greedily hidden away.
Blackberry Picking