Aster Yellows - Disease Cycle

Disease Cycle

The Aster yellows disease is caused by the aster yellows phytoplasma (AYP) which is a phloem-limited, bacterium-like organism and is vectored by the aster leafhopper, Macrosteles quadrilineatus, a phloem-feeding insect of the order Hemiptera.

Phytoplasmas are small (0.5-1 micrometer in diameter) prokaryotes that reproduce by division or budding in the phloem sieve cells of the host plants as well as the bodies of their leafhopper vectors. Currently AYP cannot be cultured in cell-free media, making detailed study somewhat more challenging. Interestingly, AYP has the ability to increase the fecundity and lifespan of their insect vector, thus enhancing the ability of the host to transfer AYP from plant to plant. AYP survives in perennial weeds, ornamentals, and vegetables. Some examples of weed host plants are thistle, wild carrot, dandelion, field daisy, black-eyed Susan, and wide-leafed plaintain.

The vector leafhopper feeds on the phloem of aster yellows infected plants by inserting their straw-like mouthpart, a stylet, into the cell and extracting it. Once the phytoplasma is acquired, an incubation period follows in which it multiplies within the leafhopper and then moves to the salivary glands. At this point, the phytoplasma can be transmitted to a new host through the saliva as the leafhopper feeds. Within 8–24 hours after inoculation, the phytoplasma moves out of the leaf into the host plant phloem. Cells adjacent to the phloem enlarge and die while surviving cells begin to divide, but soon die too. Surrounding cells in the region of the necrotic area begin to divide and enlarge, producing abnormal sieve elements, while the phloem elements within the necrotic areas degenerate and collapse. Infected plants usually show symptoms after 8–9 days at 25 degrees Celsius and 18 days at 20 degrees Celsius, with no symptoms developing at 10 degrees Celsius.