Currently there are no commercial replacements for diazinon for soil insects or alternatives for dichloropropene and oxamyl for nematode control. Diseases Major leaf spot diseases include Alternaria and powdery mildew. Some carrot varieties are more tolerant to Alternaria than others. Growers may use gibberellin to regrow new tops if Alternaria or weather severely damages foliage. Soil-borne diseases cotton root rot and Sclerotium rolfsii are favored by high soil temperatures and excessive moisture.
Bacterial pathogens cause root decay in some soil conditions.
Field rotation is practiced to reduce soil-borne problems. Thinner spacings improve air circulation and reduce leaf spot disease. No biological agents control carrot diseases. Weeds Problem weeds include winter annual broadleaves such as ragweed, wild parsley and london rocket plus some perennials such as bermudagrass and yellow nutsedge. Cultural controls include fields rotations and mechanical cultivation.