These problems, diseases and pests are known to affect Carrot plants:
Powdery mildew, carrot leaf blight, Northern root-knot nematode, bacterial soft rot
Carrot root fly is a real pest, but can be prevented by covering the plants at all. times with fleece
Cucumber Mosaic Virus : Cucumber Mosaic Virus (CMV) was first found in Cucumbers, hence its name - however it is not limited to affecting only that plant. Mainly transmitted by Aphids and spread mechanically by humans. Can also be transmitted by seed. Can overwinter in roots of affected plants. Also known as: banana infectious, chlorosis virus, coleus mosaic virus cowpea banding mosaic virus, cowpea, ringspot virus, cucumber virus 1, lily ringspot virus, pea top necrosis virus, peanut yellow mosaic virus, southern celery mosaic virus, soybean stunt virus, spinach blight virus, tomato fern leaf virus, pea western ringspot virus
Bacterial Canker : Seed borne organism that attacks Tomato plants. Can survive for short periods in soil, and longer periods in leaf debris.
Dieback : One of a number of fungi that cause dieback.
Powdery Mildew : White powdery spot disease that affects a large variety of plants.