Phytoplasma infection

Systematic position and nature of the pathogen. Phytoplasmas are specialized, cell-wall-less prokaryotes belonging to the class Mollicutes. As obligate intracellular parasites, they reside and multiply strictly within the phloem tissues of their host plants. These pathogens are notoriously difficult to study because they cannot be cultured in standard laboratory media, requiring molecular techniques for precise identification.

Diseases and susceptible crops. Phytoplasmas cause a wide array of systemic diseases globally, commonly referred to as "yellows" or "witches' broom." A diverse range of crops is susceptible, including potatoes, tomatoes, grapevines, fruit trees, and various ornamentals. Characteristic symptoms include chlorosis, stunting, floral greening (virescence), proliferation of shoots, and abnormal leaf development, often leading to total yield loss.

Biology and life cycle. The transmission of phytoplasmas is entirely dependent on sap-sucking insect vectors, primarily leafhoppers, planthoppers, and psyllids. When an insect feeds on an infected plant, it ingests the phytoplasmas. After a latent period during which the pathogen replicates within the insect's body, the vector becomes capable of transmitting the infection to healthy plants during subsequent feeding events.

Conditions for spread and economic impact. The spread of phytoplasma diseases is heavily influenced by the population dynamics of insect vectors and environmental conditions. Warm weather typically accelerates insect activity and multiplication, leading to rapid disease transmission. The economic impact is devastating, as infected plants remain lifelong carriers, often resulting in systemic decline, lack of fruit production, and significant financial loss for farmers.

Control and prevention measures. Since there is no cure for phytoplasma-infected plants, control strategies focus on prevention and integrated pest management:

• Using only certified virus-free planting material.
• Eliminating weed reservoirs that harbor both the pathogen and the insect vectors.
• Applying insecticides to manage and reduce populations of leafhoppers and other vectors.
• Rouging and destroying infected plants promptly to prevent further spread.
• Implementing physical barriers or netting in high-value greenhouses to exclude insects.