Citrus stubborn disease

Citrus stubborn disease, caused by the mollicute Spiroplasma citri, is a significant constraint in citrus production worldwide. This pathogen is a wall-less, helical, and motile bacterium belonging to the genus Spiroplasma. Unlike other plant-pathogenic mollicutes, it is culturable in artificial media, which facilitates its study and detection in laboratory environments.

The primary host range of Spiroplasma citri includes various citrus species such as sweet orange, grapefruit, and mandarin. Furthermore, it is known to infect a wide variety of non-citrus plants, including crucifers and other agricultural weeds. This broad host range makes it challenging to eradicate the pathogen from agricultural landscapes once it becomes established.

The transmission of the disease is strictly dependent on insect vectors, primarily the leafhopper Circulifer tenellus. The pathogen is acquired by the leafhopper while feeding on the phloem sap of an infected plant. After a latent incubation period within the insect, the leafhopper becomes a lifelong carrier, effectively spreading the spiroplasma to healthy trees during subsequent feeding events.

Pathogen activity is closely linked to environmental conditions, particularly heat and dryness, which favor the breeding and movement of the leafhopper vector. Upon systemic colonization of the citrus tree, Spiroplasma citri disrupts nutrient translocation. Typical symptoms include severe stunting, short internodes, leaf chlorosis, and the development of small, bitter, and lopsided fruits that often drop prematurely.

Managing Citrus stubborn disease requires an integrated approach focused on minimizing transmission and eradicating sources of infection:

• Establishment of orchards using certified disease-free nursery stock.
• Stringent vector control programs using appropriate insecticides to suppress Circulifer tenellus populations.
• Removal and destruction of symptomatic trees that serve as local inoculum sources.
• Weed management in and around orchards to eliminate alternate hosts for both the pathogen and its vector.
• Strict quarantine regulations regarding the movement of citrus propagative material from affected areas.