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RPD No. 938 - LEAF BLIGHTS OR SPOTS OF CARROT
[ Cercospora Leaf Blight ] [ Alternaria Leaf Blight ] [ Control ]
Round to elongated spots (lesions) with light tan to whitish centers
develop on the leaflets. Primary lesions usually form along the margins
of the leaflets causing them to curl (Figure 1). The lesions may merge
into large blotches that shrivel, blacken, and kill entire leaflets, closely
resembling symptoms of
The Cercospora fungus overseasons on and in seed; in wild carrot or Queen
Anne’s-lace (Daucus carota) and other minor wild hosts including
D. gingidium, D. hispanicus, D. maritimus, D. pulcherimus, and D. pusillus;
in soilborne debris from diseased plants; and in foliage carried into
storage and later
The spores (conidia) of Cercospora are borne on the surface of the leaflets
and petioles and are disseminated by air currents, splashing rain, and
flowing water, on farm equipment and tools, and the clothing of workers.
Hyphae from germinated spores penetrate through stomata and infect the
carrot foliage. Infection occurs over a temperature range of 60° to
92°F (16° to 33°C) with an optimum of
Figure 1. Cercospora leaf blight or spot of carrot (courtesy Dr. A.F. Sherf).
Cercospora carotae, the fungus that causes Cercospora
3. Figure 3. Alternaria leaf blight of carrot (courtesy Dr.
4. Alternaria dauci, the fungus that causes Alternaria leaf
The symptoms of this disease closely resemble those caused by Cercospora.
However, the lesions are more irregular, dark brown to black with yellow
borders, and develop near the margins of the leaflets (Figure 3). In prolonged,
warm, moist weather the enlarging lesions cause entire tops to turn a
yellow-brown, shrivel, and die. These symptoms are often confused with
frost damage. Elongated, girdling lesions may develop on the petioles
killing the leaves. Petiole infection can occur without spots developing
on the individual leaflets. Alternaria dauci can also cause damping-off
of seedlings and a blighting of seed stalks. A closely related fungus,
A. radicina, causes a black decay of the fleshy roots.
The Alternaria fungus overseasons on or in seed and in soilborne debris
from diseased tissue. Besides
The fungus can form conidia on carrot petioles stored dry for 90 days,
but dies under alternating
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1. Plant disease-free seed grown in areas such as semiarid regions of the Pacific Northwest, where Cercospora and Alternaria are absent.
2. Treat all carrot seed with a seed protectant fungicide. Refer to Report
on Plant Diseases No. 915, “Vegetable Seed Treatment,”
3. Whenever possible, plant in raised, welldrained beds. Avoid overcrowding of both plants and rows.
4. Eradicate all weeds, preferably before planting and during the season, particularly those in the carrot family (Umbelliferae). For current herbicide recommendations, refer to Midwest Vegetable Production Guide for Commercial Growers, revised annually. Keep down all weeds as far around the field or garden as is practical.
5. Apply a suggested fungicide at 7- to 10-day intervals starting about June 15 or when the leaf spots are first evident. Spraying is more effective than dusting. If the weather is unusually wet, shorten the interval to 2 to 4 days for dusts and 5 to 7 days for sprays. Thoroughly cover all aboveground plant surfaces with each spray or dust. Try to apply the fungicide just before rainy periods when infections occur. For current fungicide recommendations refer to Midwest Vegetable Production Guide for Commercial Growers. Apply dusts and sprays in the early morning or evening when the wind is usually at a minimum (less than 5 miles per hour for dusting and 10 mph for spraying) and leaf surfaces are damp with dew. Dusts should contain at least 5 to 10 percent fungicide. Be sure to follow all directions and precautions for mixing and applying as printed on the container label.
6. When practical, plow or spade down cleanly, burn, or compost all tops after harvest.
7. Rotate carrots two or three years with other crops, excluding parsley, celery, and celeriac.
8. Some commercial carrot cultivars are less susceptible than others
to Cercospora and Alternaria. For example, the following Spartan cultivars
are partially resistant to Cercospora: ‘Spartan Bonus’, ‘S.
Classic’, ‘S. Delite’, ‘S. Delux’, ‘S.
Fancy’, ‘S. Premium’, and ‘S. Winner’. Tolerant
cultivars to Alternaria include ‘Hi-Color 9', ‘Orlando Gold’,
and ‘Waltham Hi-Color’. For the latest information
Information concerning insecticides, weed control, varieties, and other recommendations can be found in the Illinois Homeowners' Guide to Pest Management, available at your nearest Extension office.
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