Plant viruses are extremely minute infectious particles consisting of 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, and 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 of tulips are vectored in a number of ways
- CMV, LSV and TBV are vectored by aphids
- ArMV and TRV are vectored by soil-inhabiting nematodes
- TNV is vectored by the microscopic fungal root pathogen Olpidium brassicae
Other viruses may be vectored by other insects, mites or by mechanical transmission on pruning tools and hands.
Tulip breaking virus (TBV) played an important role in 'tulipomania', which occurred in the Netherlands in the seventeenth century. The flower break patterns in affected bulbs were highly prized, and they commanded huge prices. Unfortunately, the effect was often not stable and affected bulbs did not repeat the patterns reliably in future years. The vigour of the virus-affected stock also frequently deteriorated over time. Tulip varieties with these patterns are available today, but the flower breaks result from stable genetic mutation and not virus infection. The English florists' tulips are an exception, in which the breaks still result from infection with TBV and are remarkably stable, although these plants are seldom available for sale.