Plant viruses are extremely minute infectious particles consisting a protein coat and a core of nucleic acid. They have no means of self-dispersal, but rely on various vectors (including humans) to transmit them from infected to healthy plants. Once viruses penetrate into the plant cells they take over the cells’ nucleic acid and protein synthesis systems and ‘hijack’ them to produce more virus.
Viruses are frequently transmitted through propagated material but, depending on the virus, can also be transmitted via insect or mite vectors, pollen, mechanical transfer via contaminated hands and tools, or nematode vectors in the soil. Some viruses can be transmitted via seed, but generally these are a minority and therefore seed propagation is often a useful way to ensure virus free plant material.
The vector of CaYMV is not known, but it is thought it may be transmitted mechanically, for example on propagating tools. BYMV and CaYSV are vectored by aphids, which become contaminated with virus-containing sap when feeding on infected tissues, then fly to healthy plants and transfer the virus infection during feeding. BYMV may also be transmitted through infected seed.