Dana, not sure where you are located but it may be basil downy mildew, a serious disease that is most damaging on sweet basil types. Symptoms are yellow areas on the leaves that are often delimited by the veins. After wet or humid weather, look for dark 'dirty looking' sporulation on the underside of the leaf in the same area as the yellowing to confirm downy mildew. The best control methods are to minimize leaf wetness by planting to allow space for airflow around the plants and avoid wetting the leaves when irrigating. This is because the spores of the fungus-like pathogen require a film of moisture to germinate and cause a new infection. Moisture also encourages more spore production. Orient rows parallel to the prevailing wind direction to additionally speed up drying of dew or raindrops. Some fungicides can aid in control of this disease but are preventive not curative. Look for basil or herbs and downy mildews on the label. Lemon and spice basils are resistant.