Carrot Leaf Blight

Causes
Cercospora carotae, Alternaria dauci and Xanthomonas carotae.

Symptoms
Leaf spots caused by Cercospora carotae first appear along the margins of the leaves, often causing the leaves to curl. Spots inside the leaf edges are small, roughly circular and tan or gray to brown with a dead center. As the lesions increase in number and size, the entire leaflet withers and dies. It is likely to occur in July and early August in New York state.

		Alternaria leaf spots first appear at the margin of the leaflets and are dark brown to black and irregular in shape. Lesions produced on the petioles and stems are dark brown and often coalesce and girdle the stems. As the disease progresses, entire leaflets may shrivel and die, appearing scorched. Because cool weather is favorable for its development, Alternaria leaf blight is most severe in late August and September in New York state.
		Early symptoms of leaf blight caused by the bacterium Xanthomonas carotae appear on the leaflets as small yellow areas. The centers of young lesions become brown and dry and are often surrounded by a yellow halo. Well-developed symptoms appear on the leaves as irregular brown spots and on the petioles and stems as brown streaks. It is often difficult to distinguish from Alternaria in the field.
		Management Two-to three-year rotation is recommended to allow for natural decline in the pathogen population. Fall plowing or disking is also recommended to hasten the decomposition of plant residues and to reduce the number of overwintering bacteria and fungal spores. The use of disease-free seed is strongly recommended.

Research has shown that well-fertilized carrots are less likely to develop Alternaria leaf blight. A nitrogen application made in mid-August or early September may be beneficial in promoting foliage development. Cultivars with tolerance to leaf blight are available from seed companies. Carrot producers with a history of leaf blight epidemics should consult their local extension agent to determine which tolerant varieties are adapted to their growing conditions. There are no resistant varieties currently available.

Source
Cornell University Cooperative Extension. Photos courtesy Beth K. Gugino, Department of Plant Pathology, Cornell University.

Published
Fall 2004 issue of Carrot Country.

© 2004 Columbia Publishing