carrot diseases

Late blight

Leaf blight-necrotic lesions may make carrot ferns appear brown or black. Lesions are irregular in shape. 
Figure 1. Late blight of carrot. Note necrotic leaves. Figure 1. Late blight of carrot. Note necrotic leaves.
Figure 2. Late blight of carrot. Figure 2. Late blight of carrot.
Figure 3. Late blight of carrot. Figure 3. Late blight of carrot.
Figure 4. Late blight of carrot. Figure 4. Late blight of carrot.

Root knot nematode

Root-knot nematode-infection on carrot may result in the decline of the plant. But also, since the root is the marketable portion of the plant, the galls will affect marketability directly. 
Figure 1. Root knot of carrot. Figure 1. Root knot of carrot.