Apple Physiological Disorder - Necrotic Leaf Blotch

Symptoms

The disorder is characterized by irregularly shaped nectrotic blotches on the leaves on older leaves, limited by the veins. These necrotic areas do not have spores, unlike necrotic spots caused by pathogens resulting in frogeye leaf spot, Marssonina blotch, and Alternaria spot.  Necrotic areas can range in size and midshoot leaves are most affected. Affected leaves turn yellow in about four days and subsequently fall off the tree. Other characteristics of the disorder are that the symptoms develop suddenly, appearing overnight, and it can occur in waves from July through August. The disorder tends to be more common and severe later in the summer; however, usually appearing 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.

Management

Although no bacteria or fungi have been associated with the necrotic leaf blotch, the disorder has been reduced where the dithiocarbamate (EBDC) fungicide ziram is used in the summer spray program. Foliar applications of zinc oxide also have been effective in reducing the severity of the disorder.