Occurrence and importance
Brown core is a non-parasitic disorder of apples grown in the northeastern United States and in Canada. In the United States, brown core occurs principally in New York and New England. It is more serious in McIntosh than in other varieties, but it may occur in Rhode Island Greening, Twenty Ounce Pippin, and Baldwin. The disease occurs throughout Canada on varieties such as McIntosh, Fameuse, Baldwin, Wagener, Grimes Golden, Gravenstein, and Yellow Newtown. It also occurs in Australia, where it is known as core flush. The seriousness of brown core in McIntosh apples has helped to promote controlled-atmosphere storage as the principal method of storing this variety.
Brown core is characterized by a browning of the flesh around the seed cavities (first and third photos). The browning may involve all or part of the core area and may extend as discolored wedges beyond the core line into the surrounding flesh. In McIntosh, a browning of the skin in the stem cavity (second photo) and sometimes in the underlying flesh, properly called stem-cavity browning, may be associated with severe brown core. While these two symptoms may not always occur together, it is believed that they are expressions of the same disease.
Brown core may be confused with boron-deficiency cork. Cork, however, may be distinguished by corky spots or streaks scattered through the flesh in addition to the corky discolored core tissues. The discolored core tissues of brown core are softer and wetter than those of boron-deficiency cork.
Brown core is a low-temperature storage disorder of apples that usually does not develop until January. It becomes most serious in fruits stored for long periods at 31° to 32 °F and becomes more pronounced after the fruits have been removed from the cold. An extended period of cloudy, rainy weather when apples are maturing appears to predispose the fruits to brown core. Delay in harvesting beyond prime maturity and an excess of nitrogen fertilization may also increase susceptibility.
Brown core may be greatly reduced by storing susceptible varieties at 38° or 40 °F, but the storage life of the apples is seriously shortened. About half of the McIntosh apples stored in the United States are stored at 38° in controlled atmosphere to prevent brown core and also to extend the storage life of the apples. The controlled atmosphere contains 2 to 3 percent carbon dioxide for the first month, then 5 percent for the remainder of the storage period. A content of 3 percent oxygen is maintained throughout the storage period. McIntosh apples not stored in controlled atmosphere are usually marketed early.