I found the following inforamtion from Parks Seed Co newletter:
Lettuce and Greens don't tend to have problems with pests as they grow quickly, often in cooler weather when insects are less likely to be active. You still need to regularly check them for pests, however, to prevent a small problem from becoming a disaster. Do not use chemical pesticides since you will be eating the leaves. Instead, remove pests with your hands or wash them off with a stream of water. Encourage the presence of beneficial bugs such as ladybugs, lacewings, and other "good bugs" that prey on aphids and mites.Grow your Leafy Greens in well-drained, fertile soil, and allow good spacing. This will prevent most diseases.Spinach can be susceptible to downy mildew (Blue mold). This is a fungal disease that produces slightly yellow lesions on the top of the leaves and purplish sporulation (small spores) on the underside. The best way to prevent this is to space the plants far enough apart they receive good air circulation, and when watering, wet the ground not the foliage. Rotate your Spinach crops each year to prevent soil-borne diseases.