Alternaria and Cercospora leaf blights are common fungal diseases of carrot leaves and petioles. While neither disease directly hurts the carrot root, yield loss occurs when petioles become so brittle that they break off during mechanical harvest leaving the carrot in the ground. In warm, moist weather, Alternaria leaf spots develop rapidly so that the entire field may appear to have been injured by frost or chemicals. Crop loss will be more severe when plants become infected early in the season.
Alternaria and Cercospora are difficult to distinguish in the field and often appear on the same plant. The principle difference involves disease timing. Cercospora leaf blight, caused by the fungus Cercospora carotae, attacks young rapidly growing plants; Alternaria leaf blight, caused by the fungus Alternaria dauci, primarily attacks older plants, although seedlings may also be infected.
Symptoms and effects
With both diseases, small, greenish-brown, water-soaked spots appear on leaves and petioles. Leaf lesions are often surrounded by a diffuse yellow halo. During humid weather, the lower surface of Cercospora leaf lesions turn pale gray and are peppered with tiny black spore-producing structures. As the irregularly shaped lesions increase in size and number, the entire leaflet shrivels and dies, creating a burnt appearance.
Petiole lesions caused by Alternaria become brown and irregular. With Cercospora, they become elliptical with tan centers and brown borders. Eventually, the petiole may be girdled and the leaf killed.
The disease begins in small patches within a field. By the end of the growing season, it will be uniformly spread throughout.
Only one other disease, bacterial blight, has symptoms that closely resemble Alternaria and Cercospora. With bacterial blight, leaf spots are angular rather than rounded and are typically confined between leaf veins.
The fungi that cause Alternaria and Cercospora leaf blight overwinter in diseased plant debris and on wild perennial hosts such as Queen Anne’s lace. The fungi can survive in debris for up to 2 years. Alternaria and Cercospora may also be spread on or in contaminated seed—the primary means of transmission to new production areas.
During the growing season, spores are spread by wind, water, and field equipment. Infection requires prolonged leaf wetness, which allows spores to enter through pores in the leaves. Lesions appear 3-5 days later and soon become the source for new inoculum.
A disease severity predictive model based on air temperature and relative humidity is available at the Vegetable Disease and Insect Forecasting Network (VDIFN) website. From the Disease tab, select the “Foliar Disease (Carrot)” model. This model uses Disease Severity Values (DSVs) computed from the last week of gridded NOAA weather data to calculate the risk of Alternaria and Cercospora leaf blights, which is displayed as a colored map overlay. When risk is high, initiate a preventive fungicide spray, or monitor the crop closely for evidence of disease progression. Click any grid point in VDIFN to get more detailed weather and disease severity information for that location.
Alternaria and Cercospora are difficult to control, so prevention is the best strategy. The first and least expensive control tactic is to use disease-tolerant varieties. These include Apache, Bolero, Caro-choice, Caropak, Cellobunch, Early Gold, Enterprise, Kuroda, Magnum, Nevis, SugarSnax 54, Sweet Bites, and others. On tolerant varieties, disease appears later in the season and spreads more slowly. Also, losses are lower and there’s less need for fungicide treatments.
Other prevention strategies include purchasing clean seed from a reputable dealer to assure you’re not bringing spores into a clean field; irrigating early in the day to allow foliage to dry thoroughly; selecting well-drained sites when planting new fields; incorporating plant debris immediately after harvest to hasten decomposition of infected debris; and following 3-year crop rotations.
Each week after crop emergence, randomly collect 50 leaves from the field. If any of the collected leaves have lesions on the leaves or petioles, fungicide treatment should be initiated. Continue to monitor plants weekly to determine the need for additional treatments.
Backyard gardeners can use copper-based fungicides to manage Cercospora and Alternaria leaf blights. Several additional products are available for commercial growers. For Wisconsin-specific fungicide information, refer to Commercial Vegetable Production in Wisconsin (A3422), a guide available through the UW Extension Learning Store website. Or, for home garden fungicide recommendations, see Home Vegetable Garden Fungicides (XHT1211), a fact sheet available through the UW Plant Disease Diagnostic Clinic website. Always follow label directions carefully.