Certified GKH Gardening Expert
This will stem, ultimately, from overwatering.
You will need to correct the issue with a fungicide, then correct the watering habits that lead to this damage.
Make sure that the soil has time to dry down to a depth of 2 or 3 inches between watering. A good rule of thumb is to let the leaves start to droop before you water. This will be a sign that they are starting to lack moisture, though they can droop from overwatering. You will have to use your best judgement to know which it is. Generally, if the soil doesn't get bone dry down to an acceptable depth, it will be an overwatering issue.
Here are some articles that will help: