Take care not to damage the growing tips of plants when you harvest side leaves. Aphids can sometimes be a problem, and may spread diseases like mosaic virus. Use a hose to spray the tiny white insects off the leaves, or spray with a very dilute solution of dish soapy in water. It will last though very cold weather. For a great late season garden salad, steam it lightly, shred in a few carrots, and pour on salad dressing an hour or two before serving.
Soil borne diseases occasionally show up as a soft, rotten patch on the stems.
Diseases plants should be promptly removed and burned. Do not plant members of the cabbage family in the same spot year after year to minimize this problem. You may often see cabbage butterflies circling your plants. Look very closely and you will see their well camouflaged green larvae. Pick them off, or spray with Bacillus Thuringinsis, a bacteria which attacks the intestinal tract of caterpillars, and makes them stop feeding and die within a few days.