Viral botany

Most of us by now are probably fed up to the back teeth with hearing about viruses, the coronaviruses and COVID-19 in particular. However, unless you are a horticulturalist, botanist, or microbiologist, you may not realise just how ubiquitous viruses are in the natural world, and the plant kingdom is no exception. Apart fromconifers which seem to be largely resistant, viruses of many kinds infect a huge range of plants. Although they are usually present throughout an affected plant’stissues, symptoms are often limited to certain parts or the virus may be entirely latent and, the bane of horticulturalists, only become apparent when vegetative propagation such as cuttings or grafting is practised.Transmission between plants may also be by insects, mites, nematodes, and other routes, and there are pollen and seed viruses too.

Like animal viruses, plant viruses are wholly parasitic on plant cells, hijacking the cell’s organelles to replicatetheir genetic material. They may cause foliage yellowing, mottling, mosaics line patterns, vein yellowing/banding, ring spotting and other leaf abnormalities. The picture shows some of the kinds of manifestations leaves may display when a plant is infected, but these can be, and often are, caused by other environmental and constitutional insults and conditions. Necrosis and various malformations such as leaf rolling of affected plants may also be seen as well as general poor growth. Plant viruses do not in themselves cause rotting, although that may occur as a secondary effect. Treatments are limited to prevention of spread – now where have we heard that before?!

This is still a relatively poorly researched topic, especially when it comes to wild flora. There are over seventy genera of known plant viruses, grouped into some fifty families but very many have not been classified, especially those that attack wild plants. You may have heard of tobacco mosaic virus and I have come across viruses in Cymbidium orchids, tomatoes,and passion flowers, but in our native wild flora, the topic is rarely discussed. Nonetheless, viruses are common in wild plants. In oak and ash trees, for example, you may see leaf ring patterns which are probably caused by the arabis mosaic virus that is transmitted by eelworms. Poplars suffer from poplar mosaic virus, in hazels, ringspot virus and apple mosaic virus may be observed and over thirty viruses are known to infect Prunus, including cherry leaf roll virus which also infects birches. Viruses are well known in the lily and onion families, Rubus and elms, and cucumber mosaic virus can infect primulas. Indeed, as in mammals, viruses are ubiquitous in the botanical world.

If you would like to know more about plant viruses, Wikipedia has a useful page at https://en.wikipedia.org/wiki/Plant_virus.