Could be the fuzzy down around the bottom of the plant is possibly a mildew
Basil downy mildew can be transferred by seed, insects, rain or fresh growing leaves. Infected transplants and leaves may not show symptoms if maintained in cool, dry conditions.
Spray with a 50-50 mixture of water/whole milk- 3x's per week. The fatty milk will help control the mildew and pick out the leaves that are effected.
