The cottage is Joy's home that she breifly shares with Michael. Their relationship is presented as ideal and idyllic. But then Galloway uses the setting to undercut this perfect image and to introduce a sense of trouble developing. The cottage becomes infested with dry rot, a kind of fungal decay that makes wood disintegrate:
The house was being eaten from the inside by this thing. The spores could pass through concrete and plaster and multiplied by the thousand thousand as we slept. They could take over the whole structure if they wanted.
The presence of decay reflects ideas of death and disintegration in the novel. The sudden, unexpected appearance of the dry rot foreshadows Michael’s sudden, inexplicable death.
The fungus is out of control, a symbol of the all-consuming, seemingly uncontrollable grief that triggers Joy’s depression. Galloway uses personification in the way it
threaten[s] to take over emphasising its uncontrollable nature.
Just as the dry rot threatens to destroy the infrastructure of the house, so Joy’s depression threatens her mind and with it the
infrastructure of her very existence.