Many causes exist for premature fruit ripening or drop in cherry trees and other stone fruits. Some fruit are shed by healthy plants as a simple act of self management where a tree knows it cannot support all of the fruit that is developing. Other causes are environmental such as late frost and insufficient irrigation. Pest and insect activity can also play a role but fungal disease and viruses are more common to this type of problem.
The Monilinia fructicola virus causes young cherry fruits to develop reddish colored halos on the skin that mimic natural ripening over some portion of the fruits. The pits can begin to rot, which resembles the natural degradation process on more mature fruit. Mature cherry fruit infected with brown rot will appear rotted and drop from the tree.
Sour Cherry Yellows Virus
The Yellows Virus affects mostly sour cherry and choke cherry cultivars. When this virus is present branching and fruiting becomes stunted and the cherry fruits are aborted. The virus remains in the tree year after year and will advance to thwart fruiting almost entirely over time.
The pseudomonas syringae bacteria produces a toxin that decays plant tissues on cherry and other stone fruit trees. One of the ways this bacteria can manifest itself is by by creating rotting spots on the cherries that give the appearance of aged rotting fruit on what is actually young fruit. Bacterial canker can also cause the cherries to drop to the surface of the soil.