Problem type: Disease
Name of problem: Cercospora leaf blight
Plant name(s): Carrot
Symptoms / Characteristics:
Elongate lesions form on leaf edges (causing the leaves to curl) and more rounded lesions form on inner leaf surfaces. The lesions are initially small, yellow flecks that enlarge as the disease progresses. The lesions eventually coalesce and the entire leaf senesces. Older lesions can range from tan to almost black. They have dead, dry centers and are framed by an undefined yellowish area. Younger leaves tend to be more susceptible to the disease than older leaves. The deterioration of the leaves causes a reduction in photosynthesis and a subsequent reduction in carrot size. Lesions on leaf stalks and stems are similar in shape to those on the leaf edges. They are typically brown with light centers but may darken and take on a silvery appearance under humid conditions. If stem lesions coalesce, the stem becomes girdled and the leaves collapse. Infected stems are extremely weak and easily broken. The root is essentially the only part of the carrot that does not become infected. The edible portion is unharmed but harvesting is extremely difficult because of the brittle stems. Floral parts are also targeted. Early attacks cause a failure in seed production. If seed had already been produced, it too is invaded and can lead to future seed-borne infections.
Cercospora leaf blight is a fungal disease caused by Cercospora carotae. The fungus overwinters in infected plant debris and seed. Wind, rain, people and equipment all facilitate the spread of the disease to healthy host plants. The most severe outbreaks occur in late summer when hot and humid conditions persist. Alternaria blight produces similar lesions, but the pathogen attacks mostly older leaves. As a result, this disease occurs later than Cercospora leaf blight. Alternaria blight also attacks carrot roots, unlike Cercospora leaf blight.
Control / Preventions:
Implementing a crop rotation that includes non-host plants will help to reduce disease incidence. Apply a fungicide to carrot seed prior to planting to prevent seed-borne infection. High-density plantings create a warm, moist microclimate that is ideal for the development of Cercospora leaf blight. Ensure adequate spacing between plants and rows in order to improve air circulation and reduce disease incidence. Keep plants as healthy as possible throughout the growing season by implementing a balanced fertilizer program and effective water management. This helps to build a plant's immunity to potentially harmful infections. Remove and destroy plant debris and residue after harvest to reduce potential inoculum sources and overwintering sites. Registered fungicides are available and are most effective when applied early. Early maturing carrots typically do not require fungicidal applications. There are several cultivars that are resistant to Cercospora leaf blight, but there are no cultivars with complete immunity.