What causes peaches and plums to rot and turn brown just as they are ripening?
Brown rot, caused by the fungus Monilinia fructicola, is a devastating disease of peach, nectarine, plum, and cherry. In warm, humid, disease-favorable weather, brown rot disease can destroy a fruit crop practically overnight. The tan, moldy growth seen on some decayed fruit consists of spores (reproductive bodies) and mycelium (fungal threads) of the causal fungus. Rotted fruits left hanging in the tree shrivel up and become mummified, serving as sources of disease the following year.
Sanitation, which is the removal of mummified fruits from the tree and ground before the next growing season, is an essential step in brown rot management. Fungicides can be used to help manage brown rot, but sprays need to be applied from bloom until near harvest. The fungicides are only effective if complete and thorough coverage of the tree(s) can be obtained. Contact your local Extension office for fungicide recommendations.