Sycamore scale (Stomacoccus platani), which occurs only on sycamore, is the most damaging insect pest of these trees in California. Its hosts include the native California sycamore (Platanus racemosa); Oriental sycamore (P. orientalis), which was introduced from Eurasia; and the London plane tree (Platanus x acerifolia), a commonly planted hybrid. Sycamore scale reportedly is not a pest on American sycamore (P. occidentalis), which is native to the Eastern United States.

Leaves that become infested before they have developed to their full size often are distorted and remain smaller than healthy ones. Where each scale feeds, brown spots form on the leaf petiole, and yellow or white spots develop on the leaf blade. The pale leaf spots gradually turn brown as the affected tissue dies. Infested leaves can drop prematurely.

- S. H. Dreistadt: (2010) Pest Notes: Sycamore Scale^©

Reference: http://ipm.ucanr.edu/PMG/PESTNOTES/pn7409.html