With all of the talk about the Zika virus these days, I thought I would do a brief blog about understanding plant virus diseases. Viruses are among the most primitive forms of life. Basically, what they are is a piece of DNA or RNA, usually surrounded by a coat of protein. Many scientists do not even consider viruses to be living things, for two reasons. One, they don’t have a cell structure like other microbes, higher plants and animals. Second, viruses cannot reproduce on their own. They generally invade the DNA of cells of their host, and trick the genes into running off numerous copies of themselves. This is how after exposure to a common cold virus, numerous viruses are created which can take over your body and make you sick in a matter of just days.
Viruses pretty much cannot move on their own. They have to be moved through physical contact or the bodily fluids of plants and animals. In the case of plant viruses, most are spread or vectored by insects, frequently aphids, thrips or whiteflies. The insect will penetrate the cells of a virus infected plant, sucking out some of the infected plant sap. It will then penetrate a healthy plant, and thus transfer the virus into the plant cells, where the viruses are then replicated into large numbers. Some plant viruses are mechanically transmitted, moving by physical contact from cutting tools, hands, or even water dripping from one plant to another.
Virus diseases in trees, shrubs and turfgrass are relatively rare. Plant types most commonly affected by viruses include vegetables, orchids, and certain tropicals. Many viruses are host specific, meaning they attack a few types of plants but not most others. They are usually named after their primary or first discovered host plant, such as Tobacco Mosaic Virus (TMV) or Tomato Spotted Wilt Virus (TSWV). Most of them only attack a few related plant species, though there are others capable of infecting hundreds of different plant types.
Plant virus symptoms vary, from whitish mosaic spots to odd chlorotic patterns to plant stunting, leaf distortion or necrotic spots. In some situations, no symptoms are evident at all. Virus symptoms can be confused with phytotoxicity, herbicide injury, microscopic mite injury, or heat or cold stress. If you suspect a virus may be infecting a crop, the 1st thing you should do is see how that virus is vectored, how it is transferred from one plant to another. Controlling the vector is the key. This may involve stepping up insect control measures and usually sanitation practices as well. You can’t really “kill” plant viruses. They are technically not even alive anyway. There are no “viruscides) for most horticultural situations. Your best bet is to try to catch an infestation early, discard the infected plants where appropriate, and limit the spread by controlling the vectors.
To confirm a virus infestation in plants, you must send fresh plant tissue to a lab that specializes in virus work. Many of these are university labs, though there are a few good private ones as well. Many basic plant pathology labs who check for fungus and bacterial diseases may not be set up to test for viral diseases. Diagnostic techniques vary among labs. Some test kits are also available for purchase which you can use yourself if you suspect a particular type of plant virus.