The fungi enter the developing fruit through an opening in the calyx. Photo by K. Peter.
Moldy core may develop into dry core rot if the pathogen penetrates into the core flesh, but the fungus is generally restricted to the core or carpel region.
Moldy core is characterized by the growth of fungus mycelium within the locules (the seed cavity), without penetration into the flesh of the fruit. External symptoms are rare, except infected fruit may color and fall prematurely.The disease isn't noticed until the fruit are cut open.
The moldy core fungi colonize the flower parts as soon as the blossoms open. The fungi then enter the developing fruit through an opening in the calyx. Moldy core is primarily a problem during years with light fruit set or when dry weather in early summer is followed by heavy rains in late summer. In addition, wet weather during bloom may cause conditions favorable for the fungi to produce spores.
Apple cultivars vary in their susceptibility to moldy core. Those most susceptible, such as "Delicious" or "Idared," have an open sinus extending from the calyx into the core. Use of fungicides to control moldy core during bloom time have had mixed results.
To promote fast drying in the orchard, which discourages spore development and germination, employ cultural practices of orchard training and pruning, which allow for good air movement and light penetration.