Bacteria Genomes - ASTER YELLOWS WITCHES'- BROOM PHYTOPLASMA

Phytoplasmas have diverged from gram-positive eubacteria, and belong to the genus Phytoplasma within the Class Mollicutes. Phytoplasmas can only survive and reproduce in live plant tissue and cannot be isolated and cultured using conventional laboratory methods. Phytoplasmas lack cell walls and live in the phloem (food- and hormone-conducting passages) of plants.

Phytoplasmas associated with yellows diseases were firstly described in 1926 by Kunkle in American aster plants showing virescence and flower malformations. Initially it was believed to be a virus. Transmission of phytoplasmas to plants occurs when leafhoppers feed from plant phloem. The bacteria are introduced into plant phloem with insect saliva and then systemically spread throughout the plant. Leafhoppers usually transmit phytoplasmas most efficiently to plants preferred for feeding. They feed on a variety of plants and can spread phytoplasma pathogens from wild to cultivated plants and vice versa. Infected biennial and perennial plants may allow phytoplasmas to persist from one year to another in fields and natural areas.

Affected plants generally show leaf yellowing, reddening and stunting in the early stages of disease development. The yellowing symptom can be quite striking, and the name "yellows" is often used to describe these diseased plants. Plants with yellows sometimes have extremely numerous, small, branched, axillary shoots coming from the stem nodes, giving them a bunchy or witches'-broom appearance.

References:

http://www.oardc.ohio-state.edu/phytoplasma/what.htm
http://web.uniud.it/phytoplasma/pap/bert8310.Html
http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex616?opendocument