Plant viruses are extremely minute and consist of a protein coat and a core of nucleic acid. They have no means of self-dispersal, but rely on various vectors 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.
Viruses are commonly found and transmitted in bulb plants which are multiplied via vegetative propagation year after year. As such, there is a much greater risk of virus infection levels building up. This is particularly the case for older Narcissus varieties. Where cross-pollination techniques have been used to breed new varieties, these are less likely to be affected.
Once in the plant, Narcissus yellow stripe virus and several other Narcissus viruses can be aphid-transmitted. Aphids do not breed on daffodils, but winged forms do visit and feed and can therefore transmit viruses. Narcissus species are also affected by several viruses that are vectored by plant parasitic nematodes (eelworms) which live in the soil. One of the more important viruses of this type is Arabis mosaic virus, but this is hard to detect without specialist techniques because the symptom is simply a lack of vigour.