![]() ![]() Compared to Cercospora leaf blight, the lesions of Alternaria dauci are more evident initially on the lower, older leaves while the lesions of Cercospora carotae are more numerous on the younger as well as older leaves. Large lesions can also develop on the petioles and may girdle and kill the leaves. Eventually, the leaflets may shrivel and die. Under favorable conditions, the lesions become numerous and continue to expand until they ultimately coalesce giving the leaf tissue a blighted (burned) appearance. The lesions are irregular in shape and size, dark brown to black in color (Figures 3 and 3a). Symptoms and SignsĪlternaria leaf blight lesions are small and commonly found on the margins and tips of carrot leaflets. View of carrot field infected with carrot leaf blight. Each can also infect and be harbored in wild carrot and Queen-Anne’s Lace ( Daucus carota var. All three pathogens are known to be seedborne and are often found in and/or on seeds. Prior to the 2000 growing season, Alternaria leaf blight was the most prevalent in carrot fields in New York, but Cercospora and bacterial leaf blights have been more frequently observed since then. In addition, severe defoliation and weakened foliage results in reduced harvesting efficiency. Yield loss due to these foliar diseases can be considerable, especially if they occur early in the season and are ineffectively managed (Figures 1 and 2). Since bacterial leaf blight is an explosive disease that develops rapidly under hot, wet and windy conditions, the presence of a trace level in the field requires action. For the two fungal pathogens, a 25% disease incidence level (25% of leaves examined show one or more lesions) is generally recommended to trigger the first fungicidal spray. Since any combination of the three pathogens may occur in a field, proper identification is important for employing the proper management strategies. Carrot leaf blights are caused by two fungal pathogens, Alternaria dauci and Cercospora carotae and one bacterial pathogen, Xanthomonas campestris pv. ![]()
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