Certified GKH Gardening Expert
Unfortunately, your photo did not come through. Treatment will depend on the exact damage and disease in question.
If this is on the plant, itself, then it could be downy mildew. This article will help:
It could be one of several other tomato diseases, as well:
In many cases, it could be the start of a calcium deficiency. This article will help:
Here is an article that will give you all of the information that you could need to grow tomatoes: