The fungus that causes brown rot does not reside on weeds. Rather the infected fruit that drops from the plants harbors the fungus over the winter. Tiny "mushroom-like" fruiting bodies develop on the mummies in spring and produce spores which are splashed and blown to developing shoots. Fungicides must be applied to the beach plum plant to be effective in controlling this diseases.
Phytophthora root and crown rot is quite destructive on a number of plants in the Prunus genus. Look for red-brown discoloration under the bark on affected and dying trees. This disease is particularly a problem in wet years and on wet sites.