If I can add a little to this discussion, part of the reason is the tissue that virus requires to replicate in the plant.  Mechanically transmitted viruses are viruses that can replicate easily in the epidermis (even if they replicate in other tissues also).  Thus, they depend on vectors whose feeding includes the epidermis. Thus, viruses, like maize dwarf mosaic virus or soybean mosaic virus, depend on nonpersistent aphid transmission, because it is based on aphid sampling behavior that involves fast probes into the epidermal layer. Other viruses, such as barley yellow dwarf virus, are phloem limited and depend on vectors that feed from the phloem, such as phloem probes by aphids for barley yellow dwarf virus.

Determining how and why one vector is able to transmit one virus and not another also depends of which insect it can interact with and what part of the insect that it interacts with.  For the four categories of aphid transmission, each interacts with a different portion of the aphid's anatomy in some way, such as two sites on the stylet for nonpersistent transmission, the foregut for semipersistent transmission, and the hindgut and salivary gland for persistent transmission.      Sometimes, it depends on the virus behavior after it is inoculated to the healthy plant as in beetle transmission of viruses.

Back to your original question, R. Louie had some success with vascular puncture (Phytopathology 85:139-143) to transmit viruses that are not easily transmitted.  I know that it has been used for other viruses, but they don't immediately pop into my memory.

I hope that this helps,

Marie Langham

Plant viruses need vectors to enter plant because the host has a thick cuticle (protective cover), plant viruses also lack enzymes to break the barrier,. so they need some agents (vectors) to do this job. I'm explaining a part of your question. thank you!

Yes, Mathew. I understand that viruses require a vector to get from one plant to another. My question is why some viruses require a specific vector and cannot be transmitted by any other method, while others may be transmitted fairly easily by rubbing with sand or other wounding.

Sir, there is considerable specificity in the mechanism by which any one virus is naturally transmitted. Plant viruses have very specific and  intricate interactions with their biological vectors.

Obviously. The question is why.

If I can add a little to this discussion, part of the reason is the tissue that virus requires to replicate in the plant.  Mechanically transmitted viruses are viruses that can replicate easily in the epidermis (even if they replicate in other tissues also).  Thus, they depend on vectors whose feeding includes the epidermis. Thus, viruses, like maize dwarf mosaic virus or soybean mosaic virus, depend on nonpersistent aphid transmission, because it is based on aphid sampling behavior that involves fast probes into the epidermal layer. Other viruses, such as barley yellow dwarf virus, are phloem limited and depend on vectors that feed from the phloem, such as phloem probes by aphids for barley yellow dwarf virus.

Determining how and why one vector is able to transmit one virus and not another also depends of which insect it can interact with and what part of the insect that it interacts with.  For the four categories of aphid transmission, each interacts with a different portion of the aphid's anatomy in some way, such as two sites on the stylet for nonpersistent transmission, the foregut for semipersistent transmission, and the hindgut and salivary gland for persistent transmission.      Sometimes, it depends on the virus behavior after it is inoculated to the healthy plant as in beetle transmission of viruses.

Back to your original question, R. Louie had some success with vascular puncture (Phytopathology 85:139-143) to transmit viruses that are not easily transmitted.  I know that it has been used for other viruses, but they don't immediately pop into my memory.

I hope that this helps,

Marie Langham

That's very helpful. Thank you very much for your answer, Marie!

It depend on the properties of plant viruses, especially on their coat protein and some proteins encoded by virus genomes which will be reacted with some specific proteins found in particular insect vector. In addition with the behavior of plant virus which accumulation in different  plant tissue, such as white fly transmitted viruses are located in phloem while mechanical transmitted viruses (easily transmission by rubbing on leaf) are located in epidermis cells.

Hi, the specificity is explained by how the plant virus interacts with its vector, which is mediated by vector and virus protein binding and recognition. Some viruses have to circulate through the vector tissues to be delivered into another plant. In this process, the virus has to trespass physical barriers, from the gut to the hemolymph and from the hemolymph to the salivary glands. Non vectors usually have physical barriers that prevent the passage of viruses from one tissue to the other and so blocking its transmission. The vectoring ability phenotype in some insect species is regulated by a number of  genes in an additive fashion. For more information, please see the publication attached here on genetic regulation of Luteovirids transmission by aphids.

http://www.ncbi.nlm.nih.gov/pubmed/18943747

Jack, I think you expected an answer that these two groups of viruses differ in tolerance to environmental factors: vector-transmitted ones can stay viable outside host organism (vector or plant) for short time only, when viruses like TMV can be viable even after several months in soil/in dried plants..