In order to rotate crops to advantage as a disease control measure, the interrelations of the major diseases of carrots and other crops must be understood. So far as is known, bacterial blight of carrots does not affect any other crop plants. Gray mould (Botrytis), which causes a very destructive root rot of carrots, is important also on lettuce, beans, and peas. Sclerotinia, which is sometimes of importance as a root rot of carrots, is the cause of the worst disease of lettuce, and may cause severe damage to celery, cabbage and related crops, beans and peas. Bacterial soft rot affects not only carrots, but also potatoes and beets, and to a lesser extent, other vegetable crops. Botrytis, sclerotinia, and bacterial soft rot are soil-borne. All may be introduced into a field on rotting roots. Sclerotinia may be seed-borne in beans and possibly in other crops. Sclerotinia and Botrytis are spread also by air-borne spores which may be scattered far and wide by the wind from piles of susceptible, decaying vegetable refuse.
From the standpoint of disease control, small grains, grasses, corn, alfalfa, and sweet clover are the most desirable crops for rotation with carrots. Potatoes and sugar beets are satisfactory, while lettuce, peas, beans, and celery are the most objectionable.
Infected carrots are one of the chief sources of the serious aster yellow disease of the lettuce seed crops. The disease is not soil-borne but, because infection spreads to weeds along the head lands and ditch banks, crop rotation is of some importance in its control.