By Pam Fisher, Berry Crop Specialist, Ontario Ministry of Agriculture Food and Rural Affairs
A virus is a very small infectious agent made mostly of nucleic acid and protein. Viruses are parasitic and multiply only in living cells. They cause disease by disrupting cell metabolism. They are spread by vectors or when infected plants are propagated and distributed.
A phytoplasma is a tiny, specialised bacteria that acts like a virus and causes virus-like symptoms in host plants. They are also spread by insect vectors, usually leaf hoppers.
A vector is a living organism such as an aphid or leafhopper or nematode, which is specialised to spread disease from plant to plant. Different viruses, or types of viruses, have specific types of vectors. The strawberry aphid (Figure 1) is an important vector of virus diseases in Ontario strawberries. Some species of leafhoppers are vectors of phytoplasmas.
A host is a plant in which a virus or phytoplasma replicates . A host can be cultivated, or wild. It can have symptoms of virus or be symptomless. Most strawberry viruses have a limited host range, infecting only strawberry or closely related species.
A symptom is abnormal plant growth or appearance due to virus infection . Strawberry viruses can cause symptoms such as stunting, crinkled leaves, mottled leaves , vein-banding , chlorosis and yellow leaf edges and abnormal or assymetrical growth. Similar adjectives can be used to describe symptoms and damage from other pests and disorders, making virus diagnosis difficult. Phytoplasmas can cause leafiness ( phylody) and green petals.
Plants infected with a single virus may not show symptoms. Symptoms are generally more severe in plants with multiple virus infections.
Diagnosing viruses in plants: Viruses are too small to be seen under an ordinary microscope, electron microscopy is needed to see virus particles. Unlike bacteria or fungi, viruses can’t be cultured in the lab. Some viruses are identified by grafting or inoculating indicator plants with virus- infected tissue and looking for symptoms which develop in the indicator plant. Fortunately new technology makes detecting viruses a bit more precise.
ELISA: The enzyme-linked immunosorbent assay (ELISA) is a test that uses antibodies and color change to identify the presence of certain viruses in plant tissue. Although relatively inexpensive, only some viruses can be identified with this technique.
RT-PCR : Reverse transcription polymerase chain reaction (RT-PCR) is a technique that can test for and measure RNA in plant tissue , and use this to identify viruses for which the genetic code is known.
Next generation sequencing: Advanced technology which allows faster and less expensive processing of multiple samples for virus diseases, by extracting and identifying all genetic material ( DNA or RNA) in plant tissue and identifying patterns associated with specific viruses.