Necrotic leaf blotch
Medium to large, irregular necrotic lesions occur on the foliage of mature leaves during mid- to late summer. The remaining green tissue generally turns yellow shortly after the appearance of symptoms (A, B). The onset of symptoms is often sudden and occurs in 2 to 4 waves during the summer. Defoliation rapidly follows the onset of symptoms and may affect fruit quality if it is extensive.
This is a physiological disorder affecting only Golden Delicious and its bud sports.
This disorder can be confused with Magnesium deficiency (C).
Necrotic leaf blotch is caused by the rapid synthesis of gibberellins triggered by environmental factors. Treating trees with zinc-containing fungicides (e.g., Ziram) or foliar sprays containing zinc nutrients can decrease the severity of necrotic leaf blotch. Zinc oxide applied every 2 weeks from budbreak to harvest can diminish symptoms.
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