Reading:  Chapter 14


I Definitions

II Properties of viruses

III Taxonomy

IV Taxonomic criteria

V Composition and Structure

VI Transmission - Virus Vectors

VII Virus Replication

VIII Detection

IX Physiology of virus-infected plants

X Symptoms

XI Control

XII Examples

I Definitions:

A virus is a subcellular, submicroscopic, infectious obligate parasite composted of nucleic acid within a protein coat that uses the host cellular machinery to replicate itself.

A viroid is a low molecular weight covalently closed circular RNA molecule that may cause disease. Viroids are ssRNA, but have a high level of secondary folding. An example is potato spindle tuber viroid.

II Properties of viruses


III Taxonomy:

Plant viruses are named on the basis of the symptoms induced on the first reported host plant.

IV Taxonomic criteria: (see handout)

1. type, size and number of nucleic acid units

2. architecture, symmetry, size and number of virus particles

3. transmission

4. replication strategy

5. genome organization


1a) Nucleic acid

1b) Genome segments

    1. monopartite (genome in one segment)
    2. bipartite (genome divided in two segments)
    3. multipartite (genome divided in three or more segments)


V Composition and Structure:

1) Particle morphology:

Geminated particles: i.e. Tobacco yellow dwarf virus


VI Transmission - Virus Vectors:

    1. Mechanical/pruning
    2. Vegetative propagation
    3. seed
    4. pollen
    5. fungi

-virus can be found external and/or internal to fungus.

-Example: Big vein of lettuce is vectored by the fungus Olpidium.

    1. nematodes

-Transmit 27 known viruses

-Nematode-virus relationships tend to be specific.

-Transmit by feeding - virus carried on the stylet/foregut region.

    1. eriophyid mites
    2. parasitic higher plants - dodder

-Dodder can transmit viruses through twining stems

- Dodder makes vascular connection with the plant, so virus can move through the phloem.

    1. Insects

o      types: aphids, leafhoppers, whiteflies, thrips, beetles, mealybugs, treehoppers

       Homoptera: aphids, leafhoppers and whiteflies. - Most common and important of the insect vectors.

       Piercing/sucking mouthparts - makes virus transmission more efficient as virus is placed directly into the plant.

       Virus-vector relationships can be general in which the insect is able to transmit a number of viruses, or may be specific, where the insect only transmits one virus.

      1. Parasitic higher plants:

VII Virus Replication

A. Infectious virus RNA acts like mRNA, + sense RNA

  1. Viral RNA polymerase copies + sense RNA into complementary strand
  2. Complementary strands, - strand of RNA acts as a template for replication of + strand RNA
  3. + strand RNA serves as messenger RNA for translation of viral proteins, including capsid protein.
  4. Capsid proteins polymerizes on the viral (+) RNA -> virion produced capsid - protein coat.

VIII Detection



Transmission, mechanical or vector

Nucleic acid analysis

IX Physiology of virus - infected plants:

    1. Decrease in photosynthesis.
    2. Decrease the production of some growth regulators: gibberellin and auxin.
    3. Increase the production of other growth regulators: ethylene and ABA (Absisic Acid).
    4. Respiration is generally increased immediately after infection as the plant works to replicate the virus. Respiration may then remain high, return to normal or may decrease.


X Symptoms:

-Symptoms produced by viruses may be difficult to distinguish between symptoms caused by other organisms, nutrient deficiencies, chemical toxicities, etc.

-Occur on the entire plant, on leaves, stems, fruit, and roots.

-Reduced growth - stunting, dwarfing

-Necrotic or chlorotic spots (local lesions)

-Mosaics - yellow and green mottling of foliage, occur in what can appear to be a pattern or may appear as streaks.

-Ringspots - chlorotic rings on leaves, fruit, stems.

-Vein clearing - the veins turn yellow or white while the rest of the leaves stay green. May be the first symptom which occurs and as the infection ages, the vein clearing turns into a mosaic type symptom or general chlorosis.

-Leaf and stem distortion - curling, rolling, twisting, rippling, small leaves, bushiness at terminals, enations.

-Fruit distortion - bumps, deformation, streaks.



-Stem pitting - cankers

-Viruses can infect locally (local lesions) or may be systemic (move throughout the vascular system in the plant).

-Viruses can infect plants causing visible symptoms. In this situation the virus is called a latent virus and the host is known as a symptomless carrier. In addition, plants which usually develop symptoms may be temporarily symptomless under certain environmental conditions. In this case, the symptoms are masked.

XI Control:

 XII Examples:

Virus- host-vector


- Alfalfa Mosaic Virus (AMV) on pepper - Calico virus:

Cucumber Mosaic Virus (CMV):

- Beet Curly-Top Virus (BCTV) on peppers:

-Pepper Mottle Virus. - aphid.:

-Tomato Spotted Wilt Virus - Thrips:

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