Practical 8 : Plant Virology

INTRODUCTION 

Plant viruses, as obligate parasites of host plants, can cause extreme injury to plants. They contain nucleic acid (ribonucleic acid [RNA] or deoxyribonucleic acid [DNA]) and a protein capsid. Viruses can be classified on the basis of their chemical compounds and morphology, biologic, pathologic, and molecular characteristics. Viruses can penetrate a host plant cell only in a passive way, through a microscab or with the help of different vectors, such as insects. The majority of plant viruses have a broad range of hosts (e.g. Cucumber mosaic virus [CMV], Tomato spotted wilt virus [TSWV]), whereas others have a narrower host range (e.g. Sowbane mosaic virus [SoMV]). Viral infections can be identified on the basis of the symptoms appearing on test plants (for more exact and qualitative identification, other methods, such as serologic and molecular ones, are necessary.

AIM 

1) To study the virus morphology

2) To determine the disease symptoms caused by virus

3) To demonstrate the mechanical inoculation

EXERCISE 

A. VIRUS MORPHOLGY 

Rigid rods

Flexous thread

Bacillus-like rods

Spherical

Isometric

Twin isometric

B. DISEASE SYMPTOMS 

a) Chlorosis

• an even change of color (yellow) on the inside with distinct spotting

b) Necrosis

• cell death, appears as general necrosis or in the form of spots

c) Mosaic

• Characterized by light green yellow or white areas intermingled with the normal green of the leaves or fruit
• Pattern and intensity of discoloration differ (mottling, streak, vein clearing, vein banding) 
• For example, mosaic of chilli, tobacco and ground nuts

Mosaic symptoms that occur in tobacco 

Mosaic symptom that occur in chilli

d) Ringspot 

• chlorotic or necrotic rings on leaves, fruits and stem

Papaya ring spot 

source : https://www.google.com/search?q=ringspot+virus&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiy-e-32oDbAhXEto8KHeARBl0Q_AUICigB#imgrc=UimST6D6zpGvDM:

e) Veinclearing

• chlorosis of the leaf vein

Veinclearing that caused by virus Tristeza

f) Veinbanding

• severe chlorosis of the leaf veins. 

g) 'Penyakit Merah' of paddy

• stunting, mottling, yellow orange discoloration of the leaves

C. MECHANICAL INOCULATION

Inoculation steps : 

1. Material / equipment that required is diseased leaf, test host plant, mortar and pestle, carborundum, buffer/ distilled water. 
2. The diseased leaf were crushed using mortar and pestle until unifrom extract obtained. 
3. Two volumes of (0.5 to 1 mL) of distilled water were added to the extract.
4. Carborundum were applied on the leaf surface of test host plant (healthy plant) by using soft piece of cloth. The function of  carborundum in this experiment is to cause injuries on the leaf resulting in minute openings on leaf surface. 
5. The leaves were gently inoculated with inoculum (crushed diseased leaf) across upper epidermis 3-4 times so that the leaves appear to be uniformly wetted.
6. Below is the video on the process of virus inoculation.

The left side is the host plant and the right side is the control
(before inoculation process)

The effect of inoculation towards the plant after a week
(after inoculation process)

CONCLUSION

In conclusion, viruses severely affect the host both in quantity, quality and longevity. Symptoms may often be very characteristics for a specific virus on specific host.