Natural Modes of Transmission of Viruses

Natural Modes of Transmission of Viruses

This includes air borne transmission through insects and mites and soil borne transmission through nematodes and fungus.

A) Air Borne through Insects:
 
The most common and economically most important means of virus transmission in the field.

Members of the order Homoptera- Aphids, Jassids, Leaf hopper, white flies, mealy bug , scale insects.

Thysanoptera – Thrips, coleopteran- Beetles.

Insects with sucking mouth parts carry plant viruses on their stylet- stylet borne or non persistances.

B) Circulative:

1. Circulative Viruses (Persistent): 

Viruses that are acquired by their vectors through their Mouth Parts, accumulate internally, then they are passed through their tissues and introduced into the plants again via mouth parts of the vectors.
 
2. Propagative Viruses:

Some Circulative viruses may multiply in their respective vector are then called Propagative viruses.

C) Insect Vectors:

Virus transmitting insect is called vector Walker (1969) showed that viruses are mostly really on insect for transmission ( 400 species of insect vector transmitting more than 200 viruses).

The viruses are transmitted by diffeent types of insects and most efficient vectorsare of sucking and bitting type of insects.

Group of viruses based upon the action of vector:
 
1. Non persistent viruses
2. Persistant Viruses
3. Latent viruses
4. Bacteriophage
5. Circulative viruses

a. Aphids:

These are the most important insect vector of plant viruses and transmit great majority ( about 170) of the all stylet borne viruses Ex. Soybean mosaic , cowpea aphids, borne mosaic, potato leaf roll, pea enation mosaic, lettuce necrotic yellows, red clover mosaic BYMV, MCMV, AMV.

b. White Flies:

Tobacco leaf curl, yellow vein of okra, pumpin yellow mosaic , mung yellow mosaic, cotton leaf crumple, cassava mosaic, sweet potato mosaic.

c. Leaf Hoppers:

Atleast 40 plant viruses are transmitted by leaf hoppers, all leaf hoppers transmitted viruses are circulatory, several are known to multiply in the Vector ( Propagative), cause disturbance an phloem region ex. Rice, dwarf viruses, Maize mosaic, Rice tungro viruses, Beet curly top, Potato yellow dwarf, Maize Rough dwarf.

d. Thrips:

Tomato spotted wilt virus.

e. Beetles:

Cowpea mosaic virus, cowpea chlorotic motile virus, bean pod mottle, squash mosaic, raddish mosaic.

f. Mites:   

Transmit viruses like fig mosaic, tur sterility mosaic, peach mosaic, wheat streak mosaic.

B) Soil Borne (Nepo Viruses) Nematodes:

1. NEPO:

Nematode transmitted polyhedral soil viruses.
Ex. Tobacco ring spot viruses.

2. NETU:

Nematode transmitted tubular soil viruses- tobacco.

a. Soil Borne Nematodes:

Approximately 20 plant viruses have been shown to be transmitted by one or more species of 4 genera of soil inhabiting Ectoparasitic Nematodes.

b. V Fungus:
 
Root infecting fungus-Olpidium transmits 4 plants viruses.