How to Manage Pests
UC Pest Management Guidelines
Virus diseases are recognized by several characteristic symptoms. Light and dark green mosaic patterns, mottles, ringspots, vein clearing, and vein enations are some of the symptoms seen in the leaves. Deformed yellow, stunted growth or overall stunting are additional symptoms that can be encountered.
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 but no protein coat. Chrysanthemum stunt and chrysanthemum chlorotic mottle are examples of diseases caused by viroids.
Positive identification of virus infection involves 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 a large number of viruses. Thrips vector tomato spotted wilt and other Tospoviruses. Mites, nematodes, and lower fungi also serve as vectors of a few viruses. Once an insect has acquired a virus, it may retain it in a persistent (for its lifetime) or nonpersistent (usually means minutes to hours only) manner.
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 virus in muskmelons, tomato mosaic virus in tomato, and others). Many are transmitted by vegetative propagation of plant material from infected plants.
Control of virus diseases is a matter of prevention and the use of virus-free planting stock. Once a plant is infected by a virus it usually remains infected for the life of the plant. Plants vegetatively propagated from such material are usually infected. However, virus-free plants can be obtained from infected plants by a combination of heat treatment and shoot tip 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 only) 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. In any case, remove weeds that may harbor the virus; sometimes nematode control may reduce spread as well.
Disinfection of pruning or propagation tools between plants or at least between different varieties or species and the use of disposable gloves can help reduce the cross contamination of virus diseases in a greenhouse operation. The use of a 1 to 5 dilution of household bleach in water (1% sodium hypochlorite) for 5 minutes acts as an effective disinfectant for virus-contaminated materials (tools, benches, etc.). Bleach solutions must be rinsed off using clean water to avoid toxicity to plants. See MANAGEMENT OF SOILBORNE PATHOGENS section for more details.
UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries