Do you know that in addition to viruses that humans can get, plants can also get a virus? And it is also very difficult to get rid of or for the plant to recover. The similarities between human viruses and plant viruses are very great. Plant viruses are very contagious and in most cases the plant may die or become very weak.

The first plant virus ever described was tobacco mosaic virus, or TMV. The term "virus" was derived from the original description of the causal agent of TMV--contagium vivum fluidum, or contagious living fluid. It was discovered in 1898 by Martinus W. Beijerinck, a Dutch microbiologist. And because virus particles are so small it wasn't seen by scientists until 1941 with the invention of powerful electron transmission microscopes. This visual proof dispelled any doubt of the existence of the tiny virus. Images showed that viruses are simple structures made of genetic material wrapped in a solid coat of protein molecules.

Viruses are obligate parasites; that is, they require a living host in order to grow and multiply. They can only grow and multiply in the cells of the host plant (or human). They "hijack" the cell's synthetic mechanisms to produce more virus particles, causing a variety of symptoms in the process. In humans, viruses cause chickenpox, herpes, flu, AIDS and our current scourge--the coronavirus. Most people are aware that antibiotics can't cure a virus, hence the current problem with the coronavirus. But our topic today concerns plant viruses.

One of the ways that plants become infected with a virus is through a wound in the plant. This can occur naturally as in the branching of lateral roots. Pruning instruments that aren't disinfected between use can spread the virus. Insects can also be to blame in wounding a plant; sucking insects such as aphids or leafhoppers and white flies with piercing, sucking mouthparts are very common and economically important vectors of plant viruses. Once in a wounded cell the virus particle sheds its protein coat and the nucleic acid then directs the production of multiple copies of itself and related proteins leading to the development of new virus particles. Once plants are infected, little can be done to free them from the virus. Recent advances in plant cell molecular biology and virology have led to the development of genetically modified plants with superior resistance to some viruses. Johnny's Selected Seeds catalog lists eighteen different diseases that tomatoes are prone to and six of them are viruses. Be sure to pay attention to the seed packet for the veggie you want to grow. How did we ever grow our favorite vegetable before these advances in science?

The following cultural practices can be a great help in reducing plant losses due to virus infection: removal of symptomatic plants or known alternative weed or volunteer plants that may serve as a reservoir for a given virus; clean or sanitize your tools and equipment; wash your hands frequently (what a familiar suggestion); rotate your crops if possible; and isolate newly received plant material prior to its introduction into the rest of a production system to minimize the unintentional introduction of pathogens.

The accompanying photograph shows raspberry leaves infected with a virus. The symptoms are yellow or pale green spotting, blotching, mottling or flecking of leaves; downward curling of the leaf margins; puckering or crumpling of the leaf blade. Not shown, but probable other symptoms are proliferation of weak, spindly shoots; stunted growth and reduced fruit production; dry, "crumbly" fruit; and death of canes. The only remedy would be to destroy the entire planting and, if possible, finding a new location for new plants.