Aster Yellows (Mycoplasma): The first symptom of aster yellows is yellowing of the foliage (See Photo) followed by excessive growth and bunching of shoots. Older leaves become twisted and may fall off. Leaves later have a bronzed appearance (See Photo). Roots are misshapen, and are of poor quality with a proliferation of adventitious roots (See Photo). This organism is transmitted by leafhoppers. weeds in and surrounding the crop can serve as a reservoir for this pathogen. The disease usually occurs sporadically with little economic loss.
Black Root Rot (fungus – Thielaviopsis basicola): This is primarily a post-harvest problem, although soil is the source of the pathogen. Black, irregular areas are seen on the roots. (See Photo) Physical injury predisposes carrots to infection, but disease can be minimized by washing soil from them, cooling them as quickly as possible to at least 45o, and by rinsing them in chlorinated water before placing them in bags.
Cotton Root Rot (fungus – Phymatotrichopsis omnivora): Carrots are usually grown during cooler times of the year when this fungus is not active and so, they escape this disease. In geographic areas where this pathogen is present, the disease can be seen later in the spring on carrots growing in lighter soils, particularly following an irrigation. Areas of dead and wilted plants are seen in a field (See Photo). These resemble the dead areas that can also be caused by the southern blight fungus, but there is no fungal growth on the surface of the soil (See Photo). When carrots are pulled out, they are not rotted and a sheath of soil clings to them (See Photo). When this soil is removed, infection cushions and mycelial strands of the fungus can be seen on the surface of the carrot (See Photo). A microscopic examination for the the characteristic cruciform hyphal branching will confirm the diagnosis. Planning for an earlier harvest will prevent disease losses.
Damping-off (fungi- Rhizoctonia, Fusarium, etc.): Like many other vegetables, carrot seedlings are susceptible to several species of soilborne fungi, particularly during the periods of cool, wet weather. Infected seedlings wilt, turn brown and die, resulting in poor stands. To prevent damping-off, seeds should be planted on a bed to allow for drainage. Seedlings should not be overwatered.
Growth Crack (physiological): The length of the carrot is split (See Photo). This is caused mainly by fluctuating soil moisture throughout the growing season. If there is a high incidence in a field, then carrots should be watered more regularly to prevent wide moisture fluctuations.
Leaf Blight (fungus – Alternaria dauci) Infection occurs mostly on older leaves, but younger leaves may also become infected. Leaf blight first appears as indefinite brown to black areas with pale yellow centers (See Photo). Infected leaves shrivel when infection is heavy. Under these conditions foliage appears as if burned by fire (See Photo). The fungus overwinters in infected crop refuse. Air-borne spores of the fungus are produced in large numbers on old lesions during periods of high humidity. Spores may also be carried on seed. The fungus requires moisture on leaves for infection. A preventative fungicide program should be followed where the disease is normally present, using a spray interval of 10 to 14 days.
Leaf Spot (fungus – Cercospora carotae): This disease may occur at any position on the leaf but is most common on margins. Spots are circular in shape and, with age, they coalesce to form larger spots. Lesions on the leaves are, sometimes surrounded by a lighter-colored halo. Lesions on the petioles are enlongated with a pale center and dark margin. Spots on the petiole may encircle it causing defoliation. Leaf spot is distinguished from leaf blight by the nearly circular, sharply defined lesions with a yellow halo. The disease usually occurs during the latter part of the growing season and can be controlled with the same fungicide program as used for leaf blight.
Powdery Mildew (fungus – Erysiphe polygoni): This disease is not usually serious. The surface of affected leaves are covered by a white mass of the fungus, which has a powdery texture (See Photo). Symptoms may also be found on petioles. Fungicide applications at 10 to 14 day intervals will control the disease.
Root Knot Nematode (nematode – Meloidogyne spp.): There are no diagnostic symptoms seen on above-ground plant parts. When infected carrots are examinged, galls can be seen on adventitious roots and there is often multiple tap-root formation (See Photo) or malformed roots (See Photo). Although deformed carrots are culled at the packing shed, nematodes contained in the roots are not harmful to humans when consumed. A soil test for nematodes can be done before planting to determine the potential for a problem. Pre-plant nematicides can be used where the soil is infested. Nematode populations in soil can also be reduced with a weed-free fallow or by rotation with cereals.
Southern Blight (fungus – Sclerotium rolfsii): This disease occurs close to harvest and is associated with warm temperatures that follow heavy rains. Plants die in clusters within a field. A bright, white mycelium is found on the surface of the soil, along with spherical, tan resting bodies of the fungus that resemble a mustard seed (See Photo). By the time the fungus is seen on the surface of the soil, the carrot is rotted. There are no control recommendations. Harvesting should be done as quickly as possible in affected fields. Harvested carrots should be processed and cooled as quickly as possible to minimize post- harvest decay caused by the fungus.