This condition is best described in a publication of the Ontario Ministry of Agriculture. Relevant portions of the bulletin follow. Click the link to go directly to the information, including a photo of leaves exhibiting the disorder.
Leaf disorder
“A uniqueness of this cultivar is that it commonly develops a leaf complex indicated by interveinal chlorosis and leaf blotchiness — not very dissimilar to a nutrient disorder (Figure 9). It is not known what causes these symptoms, or whether it is of any physiological consequence to the tree or fruit. However, indications thus far suggest that it is not environmentally based because it is found wherever ‘Honeycrisp™’ is grown. The leaf complex symptoms appear on extension shoot leaves early, worsen throughout the growing season, and persist until leaf drop when leaves often become much browner in comparison with other cultivars.
Its presence is more prevalent on trees with lighter crops and trees with less vigor. Although the symptoms resemble potato leafhopper damage, a nutrient deficiency, or the presence of a virus, these are unlikely to be the primary cause. Some have suggested that the symptoms may be a result of excessive starch accumulation in the chloroplasts of leaves, which impairs the movement of starch from leaves to other parts of the plant such as fruits or roots. Regardless of the cause, orchardists growing Honeycrisp™ will soon discover these symptoms — and might initially be quite concerned. Growers can be assured that this phenomenon is common and does not appear to affect tree performance.”
