Brown spots can be some type of disease. A serious disease affecting basil is basil downy mildew. The mildew develops on the lower surface. Eventually you get necrotic areas on the leaves. Plants will look chlorotic as if from nutrient deficiency. The leaves then fall off and the plant dies. If this is the case, destroy the plant. Brown spots are usually not a sign of a need for water. That usually results in wilting. Regardless of whether it is downy mildew, affected leaves can be picked off - don't use them.
