There is a disease of hackberry leaves that appears as blocky yellow spots. The spots often have green tissue surrounding them (but not always), so the disease has earned the name of island chlorosis. The image shows sumptoms on hackberry leaves.
The disease has been blamed on a phytoplasma, a virus, and a physiological abnormality; but until lately no one was able to pinpoint the cause. Benham Lockhart, professor of plant pathology at University of Minnesota, has identified a virus responsible for this condition. It is likely that a research report will be available later in the year. We will keep readers posted.
As far as we have been able to determine, the disorder is a curiosity but does not cause significant damage to mature trees. In any event, the symptoms occur on hackberry, are certainly less of an eyesore than witches’-brooms found on hackberry, and can become intense enough to give the entire tree a yellowed appearance.