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VIRUS IN ORCHIDS by Anne McKenzie and Mark Clements

The following article is another in an on-going series that forms part of the Australian Orchid Foundation Awareness Campaign.

Viruses are parasites - they reproduce only within cells of the host plant. They take over normal cell functions to divert the metabolism of the host plant into virus production.

A single virus consists of a minimum number of genes encased in a protein structure, and its replication inside the plant cell is absolutely dependent on the metabolic system of the host plant. For this reason there is NO KNOWN CURE for virus infection, as any chemical/drug that will stop virus reproduction will also disrupt the metabolism of the plant cell and probably lead to death of the plant.

Transmission of infection from infected to healthy plants relies on physical damage to the plant tissues, which causes a 'break' in the plant cell wall. This is caused either by mechanical damage or by the vector associated with transmission.

Mechanical transmission can occur during vegetative propagation or pruning when

  1. secateurs or other cutting instruments are used,
  2. when roots are damaged during division or re-potting,
  3. when leaves rub together in an overcrowded glasshouse, or
  4. when there is wind damage to any part of the plant.

However, most plant viruses are transmitted by organisms or 'vectors' that feed on and move between plants. They may be insects (aphids, whitefly, leafhoppers, plant hoppers, thrips, beetles), mites or soil-inhabiting organisms (nematodes, fungi). Some viruses are seed-borne but rarely are all the seedlings reared from one seed lot infected. A few viruses are soil-borne.

Initially, most viruses multiply at the site of infection, causing localised symptoms such as necrotic spots on the leaves due to damage to the plant cells in the immediate vicinity.

Systemic infection results when the virus then spreads to all parts of the plant, either by the plant's vascular system or by direct cell-to-cell spread. Multiplication of the virus in the plant cells leads to changes that cause many characteristic symptoms, such as stunting, distortion of leaves and flowers, mosaic patterns or striping of leaves and flowers, yellowing, wilting and infertility.

For accurate confirmation of virus infection, specialised tests MUST be carried out in a laboratory. Tests may involve electron microscopy, serological testing using antibodies to specific viruses, or advanced techniques using molecular genetic techniques.

So far, NO KIT has been developed to allow easy detection and diagnosis of virus infection by the orchid grower.

The most effective way of preventing virus infection is to destroy infected plants (or isolate them if seed is required), to control potential vectors of virus transmission, and to wash hands and sterilise tools before each plant is handled.

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