Spread Leaf blight spores are spread by water, wind and machinery. The spores may come from other diseased fields or from debris of decomposing carrot leaves. Alternaria dauci can be introduced on infested carrot seed. Spores produced on infected plants are spread rapidly during wet, windy weather. Dusting is not as effective as soaking.
Most seed companies take precautions against seed-borne disease.
Check the seed tin label or with your seed company representatives. Rotary hoe the harvested leaves from old crops, since these contain fungal spores that can spread the disease to neighbouring, younger crops. Good crop nutrition will help limit leaf blight. Avoid excessive nitrogen fertiliser use, since very lush tops make blight difficult to control. Some carrot varieties are more tolerant of leaf blight than others.