Carrot leaf blight

Symptoms of A. dauci appear first as greenish-brown, then water-soaked, and finally necrotic lesions 8–10 days following an infection event. These lesions will appear on carrot leaflets and petioles, and have a characteristic chlorotic, yellow halo. The lesions can be irregularly shaped, and will often appear on older leaves first. Older leaves are the most susceptible to infection; when approximately 40% of the leaf surface area has become infected by Alternaria dauci, the leaf will completely yellow, collapse, and die. It is during extended conditions of warm, moist weather that lesions can coalesce and cause entire tops of carrot plants to die off, a phenomenon that is sometimes mistaken for frost damage.