Occurrence and importance
Cork spot (York spot) is a disease that occurs on apples in the Cumberland-Shenandoah districts, and perhaps elsewhere. Although it is most prevalent on the variety York Imperial, its occurrence on Delicious, Golden Delicious, Stayman, Grimes Golden, and Jonathan has resulted in the selection of the term "cork spot" as the preferred name for the disease. It has been considered by some investigators as a form of bitter pit, but differences in the time of occurrence of symptoms, in the appearance of the affected areas, and in responses to boron sprays suggests that it is not bitter pit, or at least that it should be discussed separately for the present.
The York Imperial is a popular processing variety, and cork spot is a very serious problem. The corky areas add to the expense of paring, and some spots are so deep that they may be overlooked. The corky areas are surrounded by tissues that are tougher than normal tissues. When affected fruits are pared and processed for sauce, these tough areas remain lumpy and make an objectionable product.
Next to York Imperial, the Delicious is most seriously affected by cork spot. Some cork spot occurs every year, especially in York Imperial, but the disorder is much worse in certain seasons.
Cork spot develops on the fruits in the orchard and does not change or increase after harvest. The first symptoms may appear in July or early August as slightly flattened areas that appear to be water-soaked because the pigmentation of the skin over them is slightly darker than that of the surrounding skin. When fully developed, individual spots are flat-bottomed or dimple-like pits (top photo). Severely affected fruits become misshapen during growth. The apples may develop one (middle photo) or more spots on any part of the fruit. The spots may exceed 1/4 inch in diameter.
The surface is never brown or black. The corky areas are usually larger than those in bitter pit. They may lie just below the skin, or they may appear halfway to the core (bottom photo).
Some of the same environmental factors and orchard practices that influence the development of bitter pit also influence the development of cork spot. As in bitter pit, the true cause of cork spot is yet to be determined. More research has been done on cork spot on the York Imperial than any other variety.
Boron sprays at time of full bloom and at successive intervals originally gave an encouraging measure of control if the cork spot problem was serious. In some other years, however, control was less successful, especially if the incidence of cork spot was substantially less.
Although cork spot on York Imperial may be reduced by boron sprays beginning at full bloom, the disorder is not at this time classed as boron-deficiency cork. This position is taken because the disorder cannot be corrected by adding boron to the soil, and also the symptoms are different from those of boron-deficiency cork.
Cork spot of York Imperial and the other varieties affected is a very complex nutritional problem. The addition of calcium nitrate to boron sprays did not improve the control of cork spot. Early to mid-season sprays of calcium chloride and boron, however, were more promising than early-season sprays with boron alone.
More recently, sprays with boron at bloom and at 1 and 6 weeks after bloom; calcium chloride applied at 2, 3, 6, and 7 weeks after bloom; and naphthalene acetamide applied when the fruits reached a diameter of about 1/2 inch gave encouraging results in reducing cork spot on York Imperial apples.
The apparent relationship between calcium and boron in reducing cork spot has stimulated further research. One finding of recent research is that the cell membranes can be preserved by calcium. The most recent research and speculation on the problem based on preliminary tests indicate that boron may influence the availability and movement of calcium. An imbalance of these elements may disturb the permeability of the cell walls and result in the death of certain cells, thus influencing the development of cork spot.
For the latest recommendations on the reduction or control of cork spot, consult your State agricultural extension service or experiment station.
Cork spot on York Imperial
Cork spot on Golden Delicious
Cork spot on Delicious