Plants, like humans, can become sick from viruses. A virus is a small, parasitic particle that causes infection. Most viruses are host specific, meaning a plant cannot transmit a virus to humans or another kind of plant, according to scientists at Rothamsted Research. Symptoms include yellowing or distorted leaves and sometimes stunted growth of the entire plant or abnormal formation of flowers or fruit. Over 400 plant viruses are known to exist.
Apple Chlorotic Leaf Spot Virus
This virus affects apple trees, pears, plums, peaches, apricots and cherries. It causes many symptoms, including yellow spots on apple leaves, ring patterns on pear, green mottle on peach. This virus can occur anywhere in the world that apples grow and has been reported in numerous countries. It has not been shown to have symptoms in apples that are farmed commercially.
Banana Bunchy Top Virus
One of the most serious banana viruses, bunchy top has existed in Hawaii since 1989. The University of Hawaii reports that this disease is very difficult to get rid of or control after it infects a plant. Aphids are responsible for spreading the virus among plants, and humans often inadvertently spread it by moving infected plants from one location to another. Controlling aphids is one of the only ways to prevent this virus. All banana varieties can be affected and no cure exists: infected plants must be destroyed. UH concludes, "BBTV is a serious threat to Hawaii's banana industry and to... banana plantings in home gardens."
Beet Curly Top Virus
Leafhopper insects spread this virus, which has adversely affected beet production in the western United States. It causes leaf curling and distortion, and can result in rigidity, dwarfing and eventual death in not only beets, but also Swiss chard, spinach, tomatoes, beans, peppers, cucumbers and squash, flax and over 300 other plant species. Occurring in dry areas ranging from Mexico to Canada and the Mediterranean basin, the beet curly top virus has no cure: when it occurs, all of the plants in a field must be destroyed.
Tobacco Mosaic Virus
Many types of tobacco viruses exist. Tobacco mosaic virus affects about 200 plant species, including turnips, watermelons, wheat, clover, cucumbers, yams, zucchini and others. Tobacco mosaic virus is common in tomatoes grown in home gardens. It occurs around the world and causes a mosaic pattern on plant leaves, as well as a "necrotic symptom" called mosaic burn, according to scientists who contribute data to the website DPVweb. Although no insects have been specifically implicated in spreading this disease, it can cling to clothing and greenhouse structures. The use of tobacco in areas near tomato plants is discouraged, because tobacco can contain the pathogen that causes TMV. Smokers are also advised to wash their hands before handling tomato plants in order to prevent the disease.