Moldy core of apples (also referred to as core rot) is a fungal disease which affects many commercially popular cultivars of apples. Fungi known to cause the disease include species in the genuses Coniothyrium, Alternaria, Cladosporium, Stemphylium, and Botryosphaeria. Fungal-feeding mites in the families Tarsonemidae and Histiostomatidae are also known to be secondary vectors for the disease.
The disease infects trees early in the development of fruit, and is often worsened by common agricultural practices.
When apple blossoms first open, fungi (or infected mites) are able to penetrate openings in the calyx, the green leaf-like tissue protecting the budding flowers. These fungi are most common in years with wet weather during the period of bloom, as well as when dry early summers are followed by late rains.
Some popular commercial cultivars, such as Fuji, and Red and Gold Delicious, are particularly vulnerable to infection. They have long sinuses (openings) extending through the calyx into the core of the developing fruit. Cultivars with smaller sinuses, such as Granny Smith, are not nearly as vulnerable.
Industrialized growing practices often contribute to the frequency and severity of moldy core. Growers have been informed in recent years of the dangers of overapplication of nitrogen—a topic which we have previously discussed at length. This can be addressed through the implementation of balanced nutrition programs, but, growers often undersupply nitrogen instead.
When nitrogen levels in plant tissues run low, especially during bloom and seed set, the locules where seeds develop in healthy growing fruit may instead mature without developed seeds. This presents a serious issue, as apple trees respond to the presence of the seedless locules by forming callose, a form of plant tissue often produced as a protective wall in response to stress.
Callose tissue is a figurative petri dish for fungal growth, as it has very low resistance to fungal colonization. Fungi penetrates the calyx, finds a ready home in the callose tissue, and does a slow burn until the apple is more fully formed, upon which the fungi increases its reproduction rate and expands. The use of overly specific pesticides can also allow fungal-feeding mites to access trees and pass the fungi on to developing fruit.
Moldy core is a frustrating and sometimes difficult to catch disease, as apples often appear healthy even on store shelves.
As noted above, once the fungi colonizes the developing apple core, it grows at a reduced rate until the apple has more or less fully developed, upon which the rate of growth increases. The apple will appear normal when harvested, shipped, and placed on store shelves. But all the meanwhile, the fungal infection spreads through the inner lining of the core, and sometimes into the flesh.
When infection is limited to the core tissues of the apple, the disease is referred to as ‘moldy core.’ But in situations where infection spreads into the flesh of the apple, the disease is called ‘core rot.’ When infected apples are cut open or eaten, the presence of the infection is revealed. This often results in unhappy retail or commercial customers, and potentially significant financial losses.
Treating and managing moldy core requires the implantation of a balanced nutrition program and removal of dead tissues.
Do not skimp on application of nitrogen, especially immediately prior to or during fruit set. Maintain a balanced nutrition program that provides often undersupplied nutrients serving to balance out applications of nitrogen. Pay particular attention to levels of phosphorus, potassium, calcium and boron. Throughout the growing season, ensure that the ratio of nitrogen to calcium in apple flesh is below 10:1, and preferably below 6:1.
It’s also important to sanitize orchards by removing dead and dying tissues from trees as well as the ground. This is especially the case with mummy fruit lying on the ground from summer prunings, as well as large mummies still on trees. These can harbor fungi and mites, and should be removed from orchards, shredded, and decomposed.
The use of fungicides in periods of wet weather can be quite effective in staving off infection. Captan and Mancozeb are useful, while more recent fungicides in the SDHI and strobilurin chemical families have shown a great deal of effectiveness.
Lastly, control populations of fungal-feeding mites with period sprays of suitable pesticides. Diazinon can be effective (while unfortunately, longtime favorite Thiodan proved to be too toxic for continued use).
Implementing a comprehensive, multi-stage approach to preventing moldy core infection is the most effective means of ensuring that vulnerable apple cultivars remain free of disease.