The flesh of the plum becomes brown at the point where growth has manifested. At first the lesions are small, brown spots, with no superficial fungal growth evident but which expands under favourable conditions. The lesions are categorised as being firm and leathery. Monilinia laxa is the causal pathogen.
Spore masses of a tan / light-brown colour, often in the form of tufts / clusters, appear in concentric rings, as the fungus progresses in the host tissue. The skin of the infected fruit does not readily slip away when one’s finger is slid across the lesion. The skin of infected plums may darken during storage, hence the reference to ‘brown rot’. Spread of decay to adjacent fruit is common during storage.