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Plant Diseases Transmitted by Auchenorrhyncha & Sternorrhynch

Phytoplasma

Phytoplasmas are specialised bacteria that are obligate parasites of plant phloem tissue and transmitting insects (vectors). They were originally named mycoplasma-like organisms or MLOs after their discovery in 1967. They are characterised by their lack of a cell wall, a pleiomorphic or filamentous shape, normally with a diameter less than 1 micrometer, and their very small genomes. They cannot be cultured in vitro in cell-free media.

Phytoplasmas are mainly spread by insects in the families Cicadellidae (leafhoppers), Fulgoroidea (planthoppers) and Psyllidae (jumping plant lice), which feed on the phloem tissues of infected plants, picking up the phytoplasmas and transmitting them to the next plant they feed on. For this reason the host range of phytoplasmas is strongly dependent upon its insect vector. Phytoplasmas may overwinter either in insect vectors or perennial plants. Phytoplasmas enter the insect's body through the stylet, move through the intestine, and are then absorbed into the haemolymph. From here they proceed to colonise the salivary glands, a process that can take up to three weeks. Once established, phytoplasmas will be found in most major organs of an infected insect host.

Phytoplasmas are pathogens of important crops, including coconuts and sugarcane, causing a wide variety of symptoms that range from mild yellowing to death of infected plants. They are most prevalent in tropical and sub-tropical regions of the world.

Viruses

Many plant viruses need to be transmitted by an insect vector insect. Often these vectors are aphids and whiteflies (Sternorrhyncha) but some very important plant diseases are transmitted by leafhoppers and delphacid planthoppers.. A virus's host range may be determined by the host range of the vector - it can only infect plants that the insect vector feeds upon.

Plant viruses are divided into non-persistent, semi-persistent and persistent, depending on the way they are transmitted. In non-persistent transmission, viruses become attached to the distal tip of the insect's stylet and the next plant it feeds on is inoculated with the virus. Semi-persistent viral transmission involves the virus entering the foregut of the insect. Those viruses that manage to pass through the gut into the haemolymph and then to the salivary glands are known as persistent. There are two sub-classes of persistent viruses: propagative and circulative. Propagative viruses are able to replicate in both the plant and the insect whereas circulative cannot achieve this.

Spiroplasma

The best-known species of spiroplasma are Spiroplasma citri, the causative agent of Citrus Stuborn Disease, and Spiroplasma kunkelii, the causative agent of Corn Stunt Disease. Spiroplasma citri, the type species of the genus Spiroplasma (Spiroplasmataceae, Mollicutes), is restricted to the phloem sieve tubes and transmitted by phloem sap-feeding insects, as is characteristic of the phytopathogenic mollicutes.

Bacteria

Candidatus Liberibacter

Citrus greening, also called huanglongbing or Yellow Shoot/Dragon Disease, is a plant disease caused by a bacterium Candidatus Liberibacter. The bacterium, is spread by the psyllid Diaphorina citri and carried by the psyllid from host plant to host plant, where it resides exclusively in the phloem tissues.

Xylella fastidiosa

The bacterium, Xylella fastidiosa, is the most important bacterial disease transmitted by Auchenorrhyncha, it is xylem-limited so that only xylem feeding species are able to transmit it from diseased to healthy plants. X. fastidiosa causes a variety of plant diseases: Pierce's Disease, Oleander Leaf Scorch, Phony Peach Disease, Almond Leaf Scorch, Alfalfa Dwarf, Citrus Variegated Chlorosis, bacterial leaf scorch of oak, leaf scorch disease in pear, bacterial leaf scorch of coffee, maple leaf scorch, mulberry leaf scorch, and bacterial leaf scorch of elm. X. fastidiosa proliferates only in xylem vessels, in roots, stems and leaves. The vessels are ultimately blocked by bacterial aggregates and by tyloses and gums formed by the plant. The bacterium is efficiently acquired by vector insects, with no latent period, and persists indefinitely in infective adult insects. A number species of cicadelline leafhoppers and some Cercopidae are known to be vectors of X. fastidiosa. Pierce's Disease has become a major problem for the grape industry in the USA because of its invasive insect host, theglassy-winged sharpshooter (Homalodisca vitripennis).

Clove disease in Indonesia

Cercopidae of the genus Hindola are known to spread clove disease in Indonesia