Aster yellows

Aster yellows is a destructive systemic plant disease caused by phytoplasmas, which are specialized bacteria lacking a cell wall. These pathogens inhabit the phloem tissue of the host plant, severely disrupting the transport of photosynthates and hormones, which leads to stunted growth and physiological decline.

The host range of this disease is extensive, impacting numerous economically significant plants. Susceptible species include onions, garlic, celery, watermelons, annual sunflowers, Jerusalem artichokes, winter barley, and linseed. The pathogen often persists in perennial weeds that serve as reservoirs in field margins.

The primary mode of transmission is through insect vectors, specifically leafhoppers (such as Macrosteles quadrilineatus). Once a leafhopper feeds on an infected plant, the phytoplasma colonizes the insect's salivary glands, allowing for transmission to healthy plants during subsequent feeding events.

Symptoms are distinct and often include chlorosis (yellowing) of young leaves, stunted internodes, and abnormal proliferation of shoots, often referred to as "witches' brooms." Floral parts may undergo phyllody (reverting to leaf-like structures) or virescence (turning green), preventing normal seed set and crop development.

Management focuses on reducing the population of leafhopper vectors using appropriate insecticides, eliminating weed reservoirs within and around fields, and using certified disease-free planting material. Since there is no cure for infected plants, early detection and rogueing are essential for maintaining field productivity.

• Yellowing (chlorosis) of younger leaves
• Stunted plant growth and shortened internodes
• Virescence and phyllody of flowers
• Excessive branching or "witches' broom" appearance
• Reduced bulb quality and yield failure