Practical 8: Plant Virology

Introduction

Viruses are sub-microscopic, infectious particle that multiply only inside living host cells. Viruses are for the most part beyond the resolution capabilities of a light microscope. Consequently, virus structures have been determined primarily by electron microscopy. The basic shapes of plants viruses are elongate (rigid rods or flexuous threads), cylindrical rods (Bacillus-like) and spherical (isometric and polyhedral). Plant viruses  are made up of two components- a protein coat and the nucleic acid center. the nucleic acid is the infectious component of a virus. Viruses are obligate parasites, meaning that they must be within living tissue before they can reproduce themselves. They require a wound to gain entrance to plant cell. In nature, they depend primarily on biological agents such as nematodes, insects, man  for their dissemination. Once duplication starts, the virus is translocated from cell to cell through the plasmodesmata and to distant plant parts by the phloem. 

In general, viruses severely affect the host both in quantity, quality and longevity. To diagnose plant disease caused by viruses, several characterization aspects have to be done. Symptoms may often be very characteristic for a specific virus on a specific host. Symptoms along with other criteria are used to identify virus diseases, which depends on several factors:

1. Symptoms are important indicators of a plant virus disease
2. Symptoms on a diagnostic indicator plants assists in characterizing the virus.
3. The ability to cause symptoms confirms the biological activity of the viruses

A. Virus Morphology

B. Disease Symptoms

C. Mechanical Inoculation

Virus like other plant disease causing agents can be transmitted from plant to plant by sap (mechanical) inoculation. Every virus has a characteristics range of plants species which it is able to infect and in which it induces characteristics symptoms. Thus, the host range and symptomatology of a virus can provide valuable information about the identity of the virus. 

a. Material/equipment required:

• Diseased leaf
• Test host plants
• Mortar and pestle
• Carborundum
• Buffer/Distilled water

Mortar and pestel, Carborundum

Distilled water

Diseased chilli leaf: Mosaic leaf

Test Host Plant

b. The virus inoculum was prepared by crushing the diseased leaf using mortar and pestle until a uniform extract obtained. One or two volume was added (0.5 to 1 ml) of distilled water/buffer to the extract.

Diseased leaf was crushed and distilled water was added.

The diseased leaf was crushed until it become paste.

c. The host plant leaf surface was dusted (to injure the leaf surface) on which virus mix will be inoculated with carborundum abrasive.

Spread carborundum evenly on leaf surface.

Carborundum causes injury on the leaf result in minute openings on the leaf surface.


 

d. The virus inoculum that has been prepared was applied on the test leaf surfaces of the host plant using soft piece of cloth or forefinger. The leaves was inoculated by gently wipe the inoculum acrross the epidermis several time.

The crushed diseased leaf was spread evenly on healthy host leaf.

Distilled water used to uniformly apply the virus into host plant.

e. The leaves was washed that were inoculated with virus using water. This is to remove the excessive leaf pieces and sap from the leaf surface.

The leaf was washed. 

Result

The host plant is easily infected with virus after one day infected with disease.