Phytoplasma Resource Center

Phytoplasmas are very small bacteria that are enveloped only by a single membrane and do not possess a cell wall like typical bacteria. According to results from phylogenetic studies of various genes, phytoplasmas descended from ancestors that did possess walls. In their descent from walled bacteria in the Bacillus/Clostridium group, the genomes of phytoplasmas became greatly reduced in size. Phytoplasmas thus lack some biosynthetic pathways for the synthesis of compounds necessary for their survival, and they must obtain those substances from plants and insects in which they are parasites. Many bacteria can be isolated and grown in artificial media in the laboratory, but so far, no one has been able to accomplish such a feat with any phytoplasma.

Phytoplasmas are parasites and pathogens of plants and insects. All known phytoplasmas infect plants and are spread from plant to plant by insects, mainly leafhoppers, that feed in the phloem tissue of the plant's veins. A leafhopper can become infected by a phytoplasma when it feeds on a plant that has a disease caused by the phytoplasma. The phytoplasma grows (multiplies) in the body of the insect (vector) and eventually reaches the insect's salivary glands, where it becomes incorporated into the insect's saliva, and is injected into the phloem of a plant when the insect feeds.

In the inoculated plant, the phytoplasma lives mainly, or perhaps only, in the sieve cells of phloem tissue. The sieve cells are highly specialized cells that are living but do not contain nuclei when they are mature. Infection of a plant by a phytoplasma often results in disease. Worldwide, many plant diseases are caused by phytoplasmas. Symptoms that are characteristic of diseases caused by phytoplasmas include yellowing of leaves, reduced size of leaves, stunting of the plant, and proliferation of axillary buds (buds in the leaf axil where the leaf's petiole is attached to the stem). Such growth from axillary buds often results in a witches' broom appearance, or symptom. Other symptoms may be dieback (death) of terminal portions of branches on trees and other woody plants, poor growth of roots, and bunchy growth at ends of branches. In some cases, death of the plant may occur within six months of becoming infected, as is the case in palm lethal yellowing disese of coconut trees. For more complete descriptions of phytoplasmas and the diseases they cause, please refer to references listed elswhere in this website.
-R.E. Davis