Black Root Rot (Thielaviopsis basicola)
This can be a problem disease on carrots grown on muck or high organic soils. Washed carrots may show black scattered lesions when stored in polythene bags.
Root damage and high storage temperatures appear to favor disease development.
Cavity Spot (Pythium spp.)
Cavity spot occurs on both parsnips and carrots. Tonnage of the crop is not reduced but the cavities make the roots unmarketable. The disease occurs on carrots grown on mineral or peat (muck) soils.
No control for this disease is known other than growing carrots on new land free of this problem disease.
Common Scab (Streptomyces scabies)
This is the same scab that attacks potatoes and other root crops. Alkaline soils and dryish high organic soils are conducive to the development of the superficial disease. Scab lesions on the carrot surface.
Avoid potato fields and lower the soil pH if above 7 or more.
Cercospora Leaf Spot (Cercospora carotae)
This disease can be as destructive as Alternaria leaf blight. Again the disease is both seed- and residue-borne. Younger leaves are more susceptible than older leaves.
Ploughing under crop residues, crop rotation and foliar fungicides are all control recommendations.
Sclerotinia Rot - White Mold (Sclerotinia sclerotiorum)
The infectious ascospores of this fungus are abundant as a consequence of the build-up of the disease on millions of acres of canola and bean crops. Carrots may show little or no damage incidence in the field but following washing and storage white mold outbreaks often occur on the stored roots. Only a small percentage of the roots may be initially infected but the fungus mycelium can move very rapidly from carrot to carrot. In a matter of weeks the whole storage container may become a mass of white mold and black sclerotia surrounding each and every carrot.
Frequent inspection in storage, low temperatures, aeration.