Leaf spot of Sunflowers

Pathogen. Sunflower (Helianthus annus L.) is a crop of importance for oil production, human consumption of the seeds, and as an ornamental flower. A major threat to this crop is sunflower blight disease, also known as sunflower leaf spot. The disease is caused by the fungus Alternaria helianthi. Yield losses of up to 80% have been reported. The disease is most severe in warm and humid climates. The main host is sunflower, but it has proven capable of infecting safflower (Charthamus tinctorius), noogoora burr (Xanthium pungens), cocklebur (Xanthium strumarium),and bathurst burr (Xanthium spinosum), all of which can serve as alternative hosts.
Symptoms and Signs. Symptoms initiate with chlorotic spots on the leaves that later turn dark brown with irregular size and shape. Some may show a yellow halo around the spot. On stems, lesions are dark and elongated. Severely affected stems will ultimately break. Lesions also occur on the petals, sepals, and head. The infection causes blight and defoliation, destruction of the flowers, and early senescence.