Leaf yellowing may be the result of multiple problems. If these are old leaves it may be due to nitrogen, magnesium or potassium (major elements) deficiency, overwatering, natural aging of leaves, or root rot. I would recommend a soil test if the condition persists or worsens. You can obtain a soil kit from your local Penn State extension office. I've also listed a link to other common maple diseases, their symptoms, and possible treatments. Verticillium wilt was the only diagnosis consistent with the symptoms you describe but there may be other symptoms evident on other parts of the tree. http://extension.psu.edu/pests/plant-diseases/all-fact-sheets/maple-diseases Feel free to send additional information if the condition persists or spreads to other parts of the tree.