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| Purple Coneflower wtih Aster Yellows disease. The plant reproductive parts are converted to leaf-like structures. Photo by Dale Hoyt |
Pathogen: Candidatus Phytoplasma asteris; Aster Yellows. A bacterium that lacks a cell wall and has greatly reduced genome size.
Plant Host: Wide variety of plants in families Asteraceae, Apiaceae, Brassicaceae, Cucurbitaceae, Fabaceae. Can infect multiple host species.
Vector Host: Phloem-feeding insects. The Aster Leafhopper, Macrosteles quadrilineatus (Hemiptera: Cicadellidae) is considered the principal vector species in North American agricultural fields, but the phytoplasma can be transmitted by many other planthoppers, leafhoppers or psyllids.
Region affected: Worldwide, but more extensively in tropical and subtropical areas.
Problem: In the plant the phytoplasma is restricted to the phloem and is both intra- and extracellular, making in vitro cultivation difficult. The inability to culture the disease organism makes it difficult determine how it interacts with the host defenses, which, in turn, creates difficulty in breeding plants for resistance.
The insect vectors are phloem feeders and acquire the phytoplasma by feeding on an infected plant. The phytoplasma then travels from the gut to the salivary glands where it can reach the saliva. The phytoplasma is transmitted via the saliva when the infected insect feeds. To complicate matters further, the host insects are more attracted to infected plants and have higher fecundity when feeding on them.
It is suspected, based on the aberrations in normal growth that are induced, that the phytoplasma interferes with plant developmental signaling. Symptoms include stunting and yellowing of leaves and stems, transformation of floral parts to leaf-like structures, greening of non-green tissues and formation of witches' brooms. Large yield losses have been experienced in some vegetable crops: 60% in tomato, 93% in pepper, 30 to 80% in potatoes and 100% in cucumbers. Ornamental plants experience phyllody, conversion of flowers to green, leafy structures.
The insect vectors are phloem feeders and acquire the phytoplasma by feeding on an infected plant. The phytoplasma then travels from the gut to the salivary glands where it can reach the saliva. The phytoplasma is transmitted via the saliva when the infected insect feeds. To complicate matters further, the host insects are more attracted to infected plants and have higher fecundity when feeding on them.
It is suspected, based on the aberrations in normal growth that are induced, that the phytoplasma interferes with plant developmental signaling. Symptoms include stunting and yellowing of leaves and stems, transformation of floral parts to leaf-like structures, greening of non-green tissues and formation of witches' brooms. Large yield losses have been experienced in some vegetable crops: 60% in tomato, 93% in pepper, 30 to 80% in potatoes and 100% in cucumbers. Ornamental plants experience phyllody, conversion of flowers to green, leafy structures.
Who is affected,
Growers of ornamental plants, vegetables, grapes, onions, coconuts have all experienced outbreaks attributable to Aster Yellows or related phytoplasmas. The principal vector is not limited to feeding on a single host species, so the phytoplasma can be easily transmitted to a new host.
Growers of ornamental plants, vegetables, grapes, onions, coconuts have all experienced outbreaks attributable to Aster Yellows or related phytoplasmas. The principal vector is not limited to feeding on a single host species, so the phytoplasma can be easily transmitted to a new host.
What can be done to manage the disease? Breeding for resistance is difficult without knowledge of how host's defenses are overcome. The ease of spreading to new hosts presents another difficulty. The only practical solution seems to be spraying insecticides to control the insect vector and removing affected plants to reduce transmission.
Sources:
Kumari, S., Nagendran, K., Rai, A.B., Singh, B., Rao, G.P., and Bertaccini, A. (2019). Global Status of Phytoplasma Diseases in Vegetable Crops. Front. Microbiol. 10, Article 1439. doi: 10.3389/fmicb.2019.01349
Kumari, S., Nagendran, K., Rai, A.B., Singh, B., Rao, G.P., and Bertaccini, A. (2019). Global Status of Phytoplasma Diseases in Vegetable Crops. Front. Microbiol. 10, Article 1439. doi: 10.3389/fmicb.2019.01349
Jiang, Y., Zhang, C.-X., Chen, R., and He, S.Y. (2019). Challenging battles of plants with phloem-feeding insects and prokaryotic pathogens. Proc Natl Acad Sci USA 116, 23390-23397. doi:10.1073/pnas.1915396116
