text: Jun 26, 2024
  Virus transmission: Part II (2024)
  Fungal transmission of viruses:
  Several viruses have been shown to be transmitted by soil-inhabiting fungi and protists. Two of the common fungal species that transmit viruses in vegetable cropping system are Olpidium spp. and Polymyxa spp. There is specificity between the zoospores of the fungi and virus particles for successful transmission to occur. Not all soilborne fungi can transmit viruses.
  Olipdium brassicae is an obligate parasite, meaning the fungus always needs a host plant to survive (brassicae, cucumber, carrot, lettuce). The fungius that act as vector of s viruses survive the time in between crops by producing resting spores. A virus that we often see in the desert, Lettuce big vein virus, is transmitted fungus Olpidium brassicae . Lettuce big vein virus is acquired by the zoospore (motile spores) as well as resting spores of the fungus.
  The other soilborne virus we see in the desert it Tomato bushy stunt virus is soilborne but has no known vectors. Thought it was predicted to be transmitted by a fungus Olpidium brassicae, new research findings have rules out the particular fungal vector. But there is a possibility that the virus has some soilborne vector.
  Common viruses with fungal vector:
  Cucumber necrosis virus: Olpidium bornovanus
  Beet necrotic yellow vein virus: Polymyxa betae
  Beet soilborne mosaic virus: Polymyxa betae
  Beet soilborne virus:Polymyxa betae
  Beet virus Q: Polymyxa betae
  Nematode transmission of viruses
  Two genera of viruses; Nepoviruses (Nematode transmitted polyhedral viruses) and Tobraviuses (Tobacco rattle virus) are transmitted by nematodes. Xiphinema (Dagger nematodes), Longidorus (needle nematodes), Trichodorus and Paratrichodorus (stubby root nematode) vector transmit different species of viruses.
  Virus transmission by nematodes happens in 7 different but interrelated steps.
  - Ingestion: The process where the nematode ingests the virus particle along with the plant material/sap.
  - Acquisiton: The unique receptor in the nematode identifies the virus, so he virus particles stay intact within the nematode.
  - Adsorption: The phenomena of the virus being intact instead of disintegrated in the nematode digestive system.
  - Retention: Once adsorption, the virus particle can be retained in nematodes anywhere from months to even years.
  - Release: The virus particles are released onto new plants when nematodes commences feeding on new plant.
  - Transfer: The virus particles get transferred to new plant.
  - Establishment: The virus particles establish themselves in the new host plant.
  It is a sophisticated and thankfully a very specific relationship. Not all nematodes transmit virus, neither do nematodes vector all plant viruses. Nematodes and nematode transmitted viruses are of a bigger problem in perennial agriculture system such as citrus orchards and grapevines.
  Viruses transmitted by nematodes:
  tobacco rattle virus: beans, beets, peppers, potatoes, and spinach and 100 others
  tomato ringspot virus-tomato, cucumbers,
  peach rosette mosaic virus: peach, blueberries
  cherry rasp leaf virus: cherry, apple
  grapevine fanleaf virus: grapes and grape hybrids
  Seed transmission: About 1/7 th of the known plant viruses are transmitted through seeds. Different viruses have different host ranges (the plants that they can infect). Tobacco mosaic virus, Cucumber mosaic virus are some viruses with a very wide host range and they may not be seed transmissible in all plants they infect. Seed transmission plays a huge role in virus epidemiology. Not only they can be a primary source of infection, leading to an epidemic in the field upon conducible environment, seed transmission is an effective way for long distance travel of the virus, thus introducing the virus to new places. You have heard of USDA regulations/restrictions on different crops, from certain foreign countries to avoid introduction of infected seeds/plant materials.
  Seed transmission can occur simply by contamination of seeds, as in tomato seeds by Tobacco mosaic virus. This can be readily inactivated by seed treatments.
  The second type of seed transmission occurs when the virus is present in the embryo tissue that can happen prior to fertilization or takes place at pollination. Pea seed-borne mosaic virus is a well studied plant virus in this category.
  Pollen Transmisison: Some viruses are transmitted from plant to plant via pollen. As in seed transmission, pollen transmission has two mechanisms, gametic infection of embryo and direct infection of mother plant.
  Vegetative propagation: An important horticultural practice, and unfortunately a very effective method for perpetuating and spreading viruses. In clonally propagated plants, an infected mother plant which could be asymptomatic could be used to make hundreds and thousands of daughter plants, which will all have the virus. Any vegetative parts such as bulbs, corms, runners, and cutting will be infected.
  Grafting: Essentially a form of vegetative propagation, once the organic union has been established and plants (Scion and Stock) function as a single plant. In experimental front, grafting is used as a virus transmission methods, when all other methods fail.
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