Plant Pathogens: Mollicutes and Fruit Tree Viruses

Class Mollicutes and the Spiroplasmataceae Family

The Class Mollicutes (Division Tenericutes) includes the family Spiroplasmataceae. Within this family, the genus Spiroplasma consists of helical forms that lack flagella. A notable species is Spiroplasma citri, which is a quarantine pest for Chile.

Characteristics of Spiroplasma

• Cell Structure: They lack a cell wall and are highly pleomorphic.
• Morphology: Found as spiral-shaped bodies in the vessels of infected plants.
• Mobility: They are highly mobile, exhibiting wave-like and helical motions.
• Cultivation: They may develop in culture on special media, where they appear rolled.
• Membrane: The cell membrane can consist of up to three layers.
• Growth Requirements: They require sterols in the culture medium for growth.
• Antibiotic Sensitivity: They are resistant to penicillin but sensitive to tetracycline and other antibiotics. Treatment with tetracycline may inhibit growth but does not heal the plant.
• Testing: ELISA reagents are available for diagnostic testing.
• Specificity: They are specific to their hosts.

Phytoplasma Symptoms and Effects

Phytoplasma infections manifest through various symptoms, including:

• Redness or yellowing of leaves.
• Shortening of internodes and stunted growth.
• Smaller leaves and excessive proliferation of shoots (witches' broom).
• Phyllody, virescence, and flower sterility.
• Phloem tissue necrosis and dieback of branches in woody plants.
• General decline and eventual plant death.

Viral Diseases in Citrus and Fruit Trees

Citrus Psorosis Ophiovirus (Spirovirus)

There are two primary types of Citrus Psorosis: Psorosis A and B. Symptoms include:

• Peeling of the trunk bark, with or without gum exudation. This can be localized (Type A) or generalized (Type B).
• Chlorotic, mottled, and banded venal designs along the tertiary veins.

Citrus Tristeza Virus (CTV)

Belonging to the genus Closterovirus (Family Closteroviridae), the Citrus Tristeza Virus has numerous strains classified by virulence: low, medium, and severe (the latter present in Chile). Transmission occurs via:

• Grafting buds or spikes from diseased plants or latent infections.
• Aphid vectors: Aphis craccivora, A. gossypii, A. spiraecola, Toxoptera citricida, and T. aurantii (transmitted in a semi-persistent form).
• Contaminated budding utensils.

Apple Mosaic Virus

The Apple Mosaic Virus is transmitted through:

• Grafting and root grafting (contact between diseased and healthy trees).
• Mechanical transmission to herbaceous hosts.

Plum Pox Virus (PPV)

Also known as Sharka, the Plum Pox Virus has four main groups or races identified to date:

• PPV-D: Found in apricots in southeastern France.
• PPV-M: Found in peach trees in Greece.
• PPV-EA: Found in El Amar apricots, Egypt.
• PPV-C: Found in sour cherry in Moldova.

Hosts: Severe symptoms are observed in apricots, plums, and peaches. It tends to be latent in cherry trees, which rarely show symptoms.