Aster yellows is a common and destructive disease on many hosts worldwide and may be found on sunflower in North and South America. Aster yellows is the most common disease of sunflower caused by a phytoplasma, which is a specialized, wall-less bacteria-like organism that can only live either within a plant or the insect vector. Despite its name, aster yellows phytoplasma can infect hundreds of plant species and many important crops, including ornamental flowers related to asters, many weed hosts, and crops including barley, canola, chickpeas, flax, peas, potato, wheat.
On sunflower, symptoms of aster yellows are mainly seen on the head. The disease is commonly observed when bracts, leaves or ray petals form in the middle of the head (Fig 1 and 2) or in wedge-shaped sectors within the head; a phenomenon termed phyllody (Fig. 3).