Hi,

From the picture, I can't tell if it is a disease or not but in all likelihood it is a response to the dry conditions we have been experiencing so probably it is leaf scorch. If you really want to know if it is a disease, you can call the horticulturists at the UConn Home & Garden Education Center<personal data hidden> and ask them how to submit samples for disease diagnosis to the UConn Plant Diagnostic Lab. Otherwise, I would make sure the plant gets at least an inch of water per week and use a mulch around it to keep the moisture in.