Few things are more upsetting than seeing your homegrown apples (Malus domestica) fall from the tree without knowing why they are dropping. The quickest way to discover the cause of the sudden drop is to cut open a fallen apple to see if there is any sign of rot or mold in the core. Moldy core is a fungal disease that affects an apple tree’s fruit, rotting the apples on the inside. Apples grow in U.S. Department of Agriculture plant hardiness zones 3 through 8 and many apple cultivars or varieties are susceptible to moldy core.
Several different fungal pathogens can cause moldy core in apples, including fungi from the Alternaria, Stemphylium and Cladosporium genera. These fungal pathogens become a problem when the apple trees are blooming during wet weather or when there is dry weather in the early summer that is followed by a rainy late summer. Lush tree growth can also help harbor the detrimental fungus.
While infection can occur at any time, even after the apples are picked and placed in cold storage, it is most common for the fungus to infect the apple’s core or seed cavity by first colonizing the blossoms after they have just opened. The fungus enters into the developing fruit through the calyx or bottom opening. The moldy fungus usually stays contained within the apple's core, but if the moldy core fungus penetrates into the apple’s flesh, dry core rot will infect the entire apple, causing it to rot.
It is nearly impossible to detect moldy core just by looking at the tree and its fruit. The disease is not spotted on the outside of the fruit, on its skin, and it is not detected until the fruit is cut open. Sometimes the apples infected with moldy core will ripen prematurely and drop from the tree, but to find the cause of the problem, the fruit must be inspected for symptoms of moldy core.
There is very little you can do to save the fruit after moldy core has been detected. Attempts have been made to fight off the fungus with fungicides during the tree’s blooming cycle, but the results have been erratic and the practice is not recommended. The best method for combatting moldy core is to prevent or decrease further fungal problems. Pruning and tree training are recommended to improve the tree’s air circulation and sunlight.