Occurrence and importance
The term "cork" as used here applies to boron-deficiency symptoms on apples. There are two phases of the disease on the fruits: namely, external cork, characterized by surface spots, and internal cork, characterized by lesions in the core or core and flesh. Before 1936, external cork was usually referred to as "drought spot" and internal cork was known as "corky core". The cause of the disorder was not understood at that time, and losses both in the orchard and after harvest were often extensive. Boron-deficiency cork has occurred probably in all of the important apple-growing regions of the world.
In New York, Cortland and McIntosh are among the varieties most severely affected. Wealthy is moderately affected; and Baldwin, Northern Spy, Jonathan, Rhode Island Greening, Golden Delicious, and Delicious are less commonly affected.
In the Shenandoah-Potomac Valley districts, orchards of Ben Davis, Gano, and Oldenburg are most frequently affected. Yellow Transparent, Jonathan, Rome Beauty, and Grimes Golden have also been found affected in these districts.
In California, cork occurred on Spitzenberg, Wagener, Rome Beauty, and Gravenstein, but Spitzenberg and Wagener were the most susceptible. The disorder was especially bad in the Sebastopol area.
In the Pacific Northwest, cork has been observed on most varieties. It is probably most serious on Yellow Newtown, Rome Beauty, Jonathan, and Winesap. The Delicious variety may in some instances benefit from boron fertilization even though none of the usual deficiency symptoms are evident. It has been found that where boron is lacking the fruits do not attain their characteristic shape. The apples are inclined to be flat, with little tendency toward prominent lobing at the calyx end.
Apples affected with cork usually ripen a week or two earlier than normal fruits, and their color is dull. When stored they may develop characteristic symptoms of internal breakdown like fruit that is overripe when stored.
Now that boron-deficiency cork can readily be prevented by the periodic application of boron to deficient soils, the disease is rare and apples with cork are seldom found on the market.
Fruits with serious external symptoms of boron deficiency are affected quite young and are strictly an orchard problem because they would never reach the market. A brief account is included here to show the range of symptoms of boron deficiency cork.
Mild and severe forms of external cork symptoms may develop on the fruits. The severe form may develop within 2 weeks after petal fall, or as late as 8 weeks after bloom. The spots appear water-soaked at first; then they rapidly turn light brown and become wrinkled, and droplets of sap exude over the surface. The exudate dries, becomes hard and brittle, and is lost through weathering. The affected areas are 1/2 to 1 inch across, and round to irregular in shape with rounded margins. Generally the lesions are superficial, but they may extend to a depth of 1/16 inch or more. As the fruits develop, the affected areas crack and become corky, the less damaged ones assuming the appearance of severe spray injury such as sulfur burn. Seriously affected fruits frequently drop before reaching maturity, and those remaining on the tree are likely to be deformed and unmarketable.
Whether external or internal cork develops depends upon the variety of apple and the date at which the disease starts. When the disease is initiated within 6 to 8 weeks after the petals fall, the variety largely determines whether the lesions will be predominantly external or internal. In New York under the conditions just mentioned, Baldwin, Rome Beauty, and Jonathan develop mostly external cork, but some internal cork may occur. In Cortland and Rhode Island Greening, internal cork predominates and the surface of the apple remains normal. Wealthy, McIntosh, and Northern Spy usually have both external and internal cork. Regardless of variety, when the disease develops 8 weeks or more after petal fall, cork is usually entirely internal. In some varieties internal cork is accompanied by certain external symptoms, less severe and of a different character than those described for external cork.
In mild cases of internal cork, there are no external symptoms. In certain varieties with serious internal cork, the external appearance of the fruit is rough and pebbly (top photo), but without pits. This is typical of varieties such as Ben Davis and Gano. In some varieties there are no external signs.
The surest way to diagnose boron-deficiency cork at the market is to cut suspected fruits and examine them for internal cork. Internal cork always affects the core area. In mild cases there may be one to several brown corky spots of varying size within the core area. There is a tendency for these corky spots to appear nearer the stem end than the calyx end, especially in Rome Beauty. In seriously affected fruits, most or all of the core area and part or most of the flesh may be affected. When the flesh is affected, brown, corky, diffuse streaks or wedge-like areas extend from the core area out into the flesh in certain varieties (bottom photo). In other varieties there may be isolated spots in the flesh but always of the same texture as the corky tissues in the core. There is a tendency for dead tissues to lose moisture and appear firm to tough or corky.
The symptoms of boron-deficiency internal cork are distinct from those of typical cork spot (York spot). Cork spot does not occur within the core area. Some early publications, however, show illustrations of cross-section of apples that appear to have both cork spot and internal cork.
Internal cork should not be confused with brown core as typically found in McIntosh. The texture of internal cork is firm, whereas in brown core the texture is mealy.
External and internal cork of apples are symptoms of a physiological disease caused by a deficiency of boron in the fruits at some stage in their development. Some soils are deficient in boron even under favorable moisture conditions. Others become deficient during periods of drought or in some instances where feeder roots are damaged by waterlogged soil or by disking.
The disorder is not nearly as complex, however, as that found in bitter pit and cork spot, because external and internal boron deficiency cork can readily be corrected by adding boron to the soil around the trees.
External and internal cork can be controlled by periodic application of boron to the soil. Consult your State agricultural extension service or experiment station.
Boron-deficiency cork on
Typical severe internal boron-deficiency cork