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 pelagoniums can all be transmitted to the next generation of plants through cuttings taken from infected plants. The viruses can also be transmitted by various vectors, as follows:
Beet curly top virus - leaf hoppers
Cucumber mosaic virus - aphids
Impatiens necrotic spot virus and Tomato spotted wilt virus - thrips
Arabis mosaic virus, Tobacco necrosis virus, Tobacco rattle virus, Tomato bushy stunt virus, and Tomato ringspot virus - nematodes
Pelargonium flower break virus, Pelargonium line pattern virus, Tobacco mosaic virus and some of the other viruses mentioned above, can be easily transmitted mechanically on pruning tools and on hands.
These viruses are unlikely to be of importance to most gardeners, because commercial suppliers of plants will normally have taken steps to ensure that their stocks are virus-free. However, gardeners with pelargonium collections are more likely to encounter problems. Viruses are typically much more of a problem when plants are multiplied over many years by vegetative means (cuttings), because infections slowly increase in stock material over the years. Many viruses are not seed borne, so new cultivars produced by hybridisation, and plants normally propagated by seed, are less affected.