Aster Yellows (Phytoplasma Disease)

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Aster Yellows (Phytoplasma Disease)

Aster yellows is caused by phytoplasmas, which are specialized bacteria that lack cell walls and live in the phloem of plants.

Caused by Candidatus Phytoplasma asteris.



Taxonomy:

  • Kingdom: Bacteria
  • Phylum: Tenericutes
  • Class: Mollicutes
  • Order: Acholeplasmatales
  • Family: Acholeplasmataceae
  • Genus: Candidatus Phytoplasma
  • Species: Candidatus Phytoplasma asteris      

    • Shape: Pleomorphic (varied shapes, but often round or elongated).
    • Size: Very small, ranging from 200-800 nm in diameter, making them smaller than most bacterial cells.
    • Lack of Cell Wall: Unlike many bacteria, phytoplasmas lack a rigid cell wall and are instead bound by a flexible plasma membrane.

Transmission:

  • Insect Vectors:
    • Primarily spread by leafhoppers (Macrosteles quadrilineatus and others).
    • The phytoplasma is transmitted from plant to plant through feeding by infected leafhoppers.
  • Persistent Transmission: Once leafhoppers acquire the pathogen, they can transmit it throughout their lifespan.

Symptoms:

  • Chlorosis (Yellowing): Leaves turn yellow or pale green, often starting from the veins and extending outward, giving the plant a yellow or light green appearance.
  • Leaf Deformation: Leaves may become stunted, twisted, or form rosettes.
  • Flower Abnormalities:
    • Flower petals may turn green and become leaf-like (a condition known as phyllody).
    • Flowers may fail to form or be deformed, leading to reduced seed and fruit production.
  • Stunted Growth: Plants may exhibit overall reduced growth and become bushy or dwarf-like.
  • Witches' Broom: Some infected plants develop clusters of abnormal, bushy shoots from the same point.
  • Premature Reddening and Necrosis: Older leaves may show early reddening and die prematurely.

Commonly Infected Plants:

  • Asters, Marigolds, Daisies, Chrysanthemums: Common in ornamental plants.
  • Lettuce, Carrots, Celery, Potatoes, Onions: Common in vegetables.
  • Grains and Cereals: Can also affect some cereals and grasses.

Disease Cycle:

  1. Leafhoppers acquire the phytoplasma from infected plants by feeding on their phloem.
  2. Latent Period: The phytoplasma multiplies within the insect vector and becomes transmissible.
  3. The infected leafhopper transmits the phytoplasma to healthy plants during feeding.
  4. The phytoplasma spreads within the plant through the phloem, causing disease symptoms.

Management Strategies:

  1. Insect Vector Control:
    • Use insecticides to control leafhopper populations.
    • Employ reflective mulches to deter leafhoppers from settling on plants.
    • Utilize natural predators and beneficial insects to reduce leafhopper numbers.
  2. Use of Resistant Varieties:
    • Plant varieties that show resistance to phytoplasmas or are less attractive to leafhoppers.
  3. Rogueing and Removal of Infected Plants:
    • Remove and destroy infected plants to reduce the source of phytoplasma for leafhoppers.
  4. Barrier Crops:
    • Use barrier crops or trap crops to prevent leafhoppers from reaching target crops.
  5. Monitoring and Early Detection:
    • Regularly monitor leafhopper populations using sticky traps or sweep nets.
    • Inspect crops for early symptoms of infection and take action promptly.
  6. Crop Rotation:
    • Rotate crops with non-susceptible plants to break the disease cycle.
  7. Cultural Practices:
    • Ensure good plant spacing to promote air circulation and reduce leafhopper activity.
    • Maintain field sanitation by removing weeds that can serve as alternate hosts for both phytoplasmas and leafhoppers.

 

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