Viruses and Viroid Diseases

Virus diseases are recognized by several characteristic symptoms. Light and dark green mosaic patterns, mottles, ringspots, distortion of leaves and other plant parts, vein clearing, and vein enations are some of the symptoms seen in the leaves. Deformed, yellow, stunted growth, or overall stunting are other possible symptoms.

Viruses multiply only in living cells. They are too small to be seen with a light microscope and are therefore considered to be submicroscopic. Viruses are composed of a nucleic acid (most plant viruses contain ribonucleic acid [RNA]) and are enclosed in a protein coat. The nucleic acid of a few plant viruses (Carnation etched ring virus, Dahlia mosaic virus) is deoxyribonucleic acid (DNA). Viroids consist of low molecular weight RNA that lacks a protein coat. Chrysanthemum stunt viroid and Chrysanthemum chlorotic mottle viroid are examples of viroids.

Positive identification of virus infection involves several procedures, including visualization of virus particles with the electron microscope, serological techniques such as ELISA (enzyme-linked immunosorbance assay), sap inoculations of indicator plants, budding and grafting to indicator plants, microscopic examination for inclusion bodies (aggregates of virus particles or virus-induced protein structures), RNA and DNA hybridization, polymerase chain reactions (PCR), and gel electrophoresis.

Many viruses enter the host plant via the feeding activity of vectors that transmit the virus into plant cells. Insects, especially aphids, whiteflies, and leafhoppers, vector many viruses. Thrips vector Tomato spotted wilt virus and Impatiens necrotic spot virus. Mites, nematodes, and lower fungi also serve as vectors of a few viruses. See the Viruses With Wide Host Ranges table for a list of common plant viruses and their hosts, how they are spread, and links to photographs of their damage.

Most viruses are transmitted by vegetative propagation of new plant material from infected mother plants. Many plant viruses and viroids are spread by physical contact or by tools. Some orchid viruses are spread when healthy plants come in contact with diseased ones. Some viruses are pollenborne (Cherry leaf roll virus, Prunus necrotic ringspot virus). A few viruses are seedborne (Squash mosaic virus in muskmelons, Tomato mosaic virus in tomato, and others). Several viruses, including Tobacco mosaic virus, can survive in water run-off from infected plants, which, when recycled and used as irrigation water, can result in new infections.

Control of virus diseases is a matter of prevention–the use of virus-free planting stock and resistant varieties. Once a plant is infected by a virus it usually remains infected for the life of the plant as there are no available pesticide treatments. Plants vegetatively propagated from infected mother stock are usually infected. However, virus-free plants can be obtained from infected plants by a combination of heat treatment and shoot tip (meristem) culture, and sometimes with the aid of chemical inhibitors of virus multiplication. Some viruses are transmitted from plant to plant by means of the feeding activity of insects. Once an insect has acquired a virus, it may retain it in a persistent (up to lifetime) or non-persistent (usually means minutes to hours) manner. Controlling insect vectors may help in reducing the spread of persistently transmitted viruses; however, with non-persistently transmitted viruses, insects can often spread the virus before they are inactivated by insecticides. Remove weeds and other plants that may harbor the virus and/or the vector.

Disinfection of pruning or propagation tools between cuts, or at least between different plant sets, varieties, or species, and the use of disposable gloves can help reduce the spread of virus diseases in a greenhouse operation. A solution of 1 part household bleach in 4 parts of water, applied for 5 minutes, acts as an effective disinfectant for virus-contaminated materials (tools, benches, etc.). Bleach solutions must be rinsed off with clean water to avoid toxicity to plants. (Note: Bleach treatments are corrosive to metal tools.) See MANAGEMENT OF SOILBORNE PATHOGENS section for more details.

In outdoor field crop production, silver reflective mulch has been shown to repel aphids and whiteflies, thus reducing their numbers in and around plant canopies. In addition, virus transmission by these insects was greatly reduced. For best results, apply mulches at the time of planting or transplanting the crop. Apart from reducing aphid and virus incidence, silver reflective mulch increased cut flower production and reduced the crop requirement for irrigation water and fertilizer. This method is acceptable for organic production.