Aster Yellows
Aster Yellows
Aster yellows can be found in North and South America, although it is relatively uncommon. 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 is 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 phenomena termed phyllody (Fig. 3).
 
aster yellows
Figure 1. Aster yellows head symptoms with bracts and ray petals appearing in center of head
aster yellows
Figure 2. Aster yellows head symptoms with bracts and ray petals appearing in center of head
aster yellows
Figure 3. Wedge shaped portion of sunflower head showing aster yellows damage
Aster yellows may be confused with infection from a virus or insects. In Midwestern U.S. states, some head-feeding insects, such as the sunflower bud moth and the sunflower head maggot, cause abnormalities on heads.  However, symptoms among aster yellows, viruses and insects are different, and close examination of suspected infected heads should be done to determine causal agent(s).

Aster yellows is insect transmitted, and the aster or six-spotted leafhopper is the primary vector. Once a leafhopper carrying the phytoplasma feeds on sunflower, the plant becomes systemically infected. The phytoplasma overwinters in the leafhopper or in infected plant residue, but being an “obligate pathogen,” it cannot overwinter in soil without infected plant debris. The incidence of aster yellows is sporadic and low, and thus no control measures are justified, although resistant lines have been identified.
Images
Figure 1.  Aster yellows head symptoms with bracts and ray petals appearing in center of head (Tom Gulya, USDA).
Figure 2.  Aster yellows head symptoms with bracts and ray petals appearing in center of head (Tom Gulya, USDA).
Figure 3.  Wedge shaped portion of sunflower head showing aster yellows damage (Tom Gulya, USDA).
Additional Aster Yellows Information
https://www.sunflowernsa.com/magazine/articles/default.aspx?ArticleID=2894

https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/viruses/aster-yellows.aspx

https://vegento.russell.wisc.edu/pests/aster-leafhopper/

https://cropwatch.unl.edu/potato/aster_leafhoppers
Other NSA Resources
Disclaimer statements
Information based in part on and reproduced from Kandel, H., Endres, G. and Buetow, R. 2020. Sunflower Production Guide. North Dakota Extension Publication A1995. Informational updates made possible by the Sunflower Pathology Working Group, and is/was supported by the USDA National Institute of Food and Agriculture, Crop Protection and Pest Management Program through the North Central IPM Center (2018-70006-28883).
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