Occurrence and importance
Water core occurs in practically all apple-producing sections of the United States, but it is of greatest importance in those having an arid or semiarid climate. Among the most susceptible varieties are Rambo, Jonathan, Delicious, Stayman, Arkansas (Mammoth Black Twig), and Winesap. In the Northwest, the last-named variety is more often affected than any other.
Water core is a nonparasitic disease characterized by water-soaked regions in the flesh of the apple. When water core is severe, affected areas are hard and glassy (top two photos) and may be externally visible. In mild and moderate cases, the disease is more commonly found near the core and around the primary vascular bundles (third photo), but it may occur in any part of the apple or may involve the whole of it. Heretofore it was impossible to detect the disease in the core area without cutting the fruits. Recently, however, equipment was developed to detect and segregate several classes of water core by measuring light transmittance through intact fruits. Packinghouse equipment using this principle is being developed. Seriously affected fruits are heavier than normal ones and may be segregated by flotation in mixtures of water and alcohols.
Water core is particularly bad in sections where heat and sunlight are intense. High temperatures at the time the apples are approaching maturity favor its development. The disease is most likely to occur in fruits that are freely exposed to the sun. A severe form of the disease, visible water core, is frequently associated with sunburn. Water core often increases rapidly as apples become overmature. Apple tissues affected with the disease are distinctly different in both texture and location from the soft, water-soaked tissues adjacent to spots bruised while apples are frozen.
Water core not only does not develop or spread in transit or storage, but in certain varieties such as Yellow Newtown and Winesap it may actually disappear after a few months' storage, particularly if originally present in only a mild form. When large portions of the flesh are affected, especially in soft-textured varieties like Jonathan, Delicious, Stayman, and Rome Beauty, there is danger of subsequent internal breakdown. Prompt disposal of the fruits at or very soon after harvest is advisable.
Control of the disease is attained mainly by picking fruits before extensive water core develops. Spraying for an increase in red color is not the purpose of stop-drop sprays, but because the spray prevents apples from dropping, the practice of leaving the red varieties on the trees for more red color development has, unfortunately, become a common practice. This permits fruits to become over-mature and often causes a serious problem of water core. Near harvest time, growers should examine the fruits for presence of water core, particularly fruits well exposed on the southwest side of the tree, and start picking if appreciable water core is developing.