Certified GKH Gardening Expert
There are a few things that can cause this, depending on how it appears, and where. Unfortunately your photo did not come through, though.
Most commonly, overwatering, pH imbalances, calcium, or magnesium deficiencies will be likely.
Test your soil nutrient content and pH. Amend accordingly.
Water, only, when the soil is dry COMPLETELY down to 2 inches or so, instead of daily.
Here are some articles that will help: