I. Introduction. The cause of necrotic leaf blotch (NLB) of apple is not known. It is apparently a physiological disorder whose occurrence is related to air temperature, light intensity and soil moisture. A hormonal imbalance may be involved because symptoms are enhanced by gibberellins and reduced by abscisic acid. The disorder occurs worldwide on Golden Delicious and its bud sports. Seedlings out of Golden Delicious vary in susceptibility. Prime Gold and Nugget are also affected.

II. Symptoms and Disease Development: The disorder is characterized by the development of necrotic blotches or irregular areas of dead tissue in mature leaves (Photo 72). Midshoot leaves are most often affected (Photo 73). Blotches are limited by the leaf veins and vary greatly in size. Affected leaves begin to turn yellow after about 4 days and abscise a few days later. Some green leaves with NLB are also lost by abscission.

Two distinctive characteristics of NLB are that the symptoms develop suddenly, almost overnight, and in waves, generally from June through August. The disorder tends to be more common and severe later in the summer, however, and usually appears when a cool, rainy period is followed by hot summer weather. Some orchards or trees within an orchard may show little or no defoliation, while other orchards or trees reach 50 percent defoliation or more.

III. Amelioration: Although it appears that NLB is not caused by a fungus, bacterium or certain air pollutants, the disorder is reduced where the dithiocarbamate fungicides ziram or thiram are used in the summer spray program. Foliar applications of zinc oxide also have been effective in reducing the severity of the disorder.

Text prepared by A. L. Jones and T. B. Sutton

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