Alternaria Leaf Spot of Sunflower

The primary pathogen of the disease is the specialized species Alternaria helianthi, although the pathogenic complex also includes the species Alternaria alternata, A. tenuissima, A. helianthiinficiens, A. zinniae, A. arborescens, and species of the A. infectoria group. The disease manifests on cotyledons as round yellow spots, on leaves as spotting with a yellow halo leading to necrosis, and on stems and flower heads in the form of dark streaks and spots, which disrupt seed nutrition.

In humid conditions, a characteristic olive-velvety bloom of fungal sporulation develops on the affected areas. Specific monogenic loci for resistance to the pathogen have not been identified in the crop; therefore, breeding for immunity is based primarily on interspecific hybridization methods with wild Helianthus species to achieve field resistance.