Certified GKH Gardening Expert
This can happen for many, many reasons, unfortunately! Lack of humidity, high heat, and lack of fertilizer are most common. This does not look like disease, but the picture is not quite clear enough to make out much detail.
You could take samples and photos up to your local extension service, and they can be of greater assistance. It will help to be able to see the issue in person.
This article will help you with proper care: