Disease Spread or Transmission
Spread or transmission is the transfer of a pathogen from an infected plant to a healthy one. Transmission may occur by means of contact among plants (e.g. leaves touching each other), mechanical (a contaminated tool or hand wounding a healthy plant), animal (e.g. aphid, beetles, nematodes), grafting, seed, soil or water.
Seed – Almost one-fifth of known plant viruses may be seed transmitted. Such viruses are in the seed embryo, often infecting through the ovary of pollen. Some viruses may infect seed through the endosperm. Others, mostly members of the Tobamovirusgenus (e.g. Tobacco mosaic virusand Tomato mosaic virus) are found in the seed coat or testa and infect the young seedlings through wounds caused by soil, etc. External seed treatment may inactivate some of these viruses.
Contact – The movement of a pathogen from an infected plant to a healthy adjacent plant by contact of plant parts, such as leaves.
Mechanical – The movement of a virus from an infected plant to a healthy plant by an object contaminated with infected plant juices entering a wound in the healthy plant.