Greetings Bonnie:
Quite honestly, there are a myriad of reasons why tree leaves may turn brown: too much soil moisture, too little soil moisture, heat stress, soil fertility or soil pH imbalances, insects, pathogens such as verticillium wilt, soil compaction, soil excavation, construction, changes in soil hydrology, storm damage, bird damage, vandalism, and the list goes on.
Inspect the tree for any signs of pests or pathogens. Is the tree in good health? Has this happened in the past? There are so many variables to consider that one could not possibly identify the cause or extent without an on-site assessment.
Please feel free to contact your local University of Illinois Extension office or contact a local certified arborist for further assistance and an on-site evaluation.
