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How to Manage Pests

UC Pest Management Guidelines

Foliage damage by impatiens necrotic spot virus.

Floriculture and Ornamental Nurseries

Viruses and Viroid Diseases

(Reviewed 3/09, updated 3/09)

In this Guideline:


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.

Cultural Control
In open 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.

Virus Transmission Ornamental hosts Crop plant hosts Weed and native plant hosts
Bean yellow mosaic (potyvirus group) aphids; mechanically to an extent in gladiolus gladiolus, sweet pea, violets legumes, bean, clovers, fava bean, pea, soybean, sweet clover legumes, Chenopodium, clovers, sweet clover
Beet curly top (rhabdovirus group) leafhoppers cosmos, coreopsis, geranium, nasturtium, petunia, strawflower, stock, viola, zinnia bean, beets, borago, buckwheat, celery, clovers, cress, cucurbits, fava bean, fennel, flax, horseradish, pepper, potato, radish, rhubarb, tobacco, tomato, vetch Atriplex spp.,Chenopodium spp., clovers, Polygonium spp., Rumex spp., Russian thistle, shepherd's-purse
Cauliflower mosaic (caulimovirus group) aphids honesty (lunaria), stock crucifers, broccoli, cabbage, cauliflower, Chinese cabbage, mustard mustard, Raphanus spp., shepherd's-purse
Cucumber mosaic (cucumovirus group) aphids; mechanically in a few hosts begonia, buddleia, calendula, china aster, columbine, dahlia, daphne, delphinium, geranium, gerbera, gladiolus, ligustrum, lily, lobelia, nasturtium, passionvine, primula, snapdragon, vinca, viola, zinnia buckwheat, carrot, celery, cucurbits, cowpea, pepper, potato, tobacco, tomato commelina, lambsquarter, lupine, milkweed, nightshade, penstemon, pigweed, pokeweed, tree tobacco (Nicotiana glauca)
Prunus necrotic ringspot
(ilarvirus group)
grafting; pollen Prunus spp., rose apple, hops, Prunus spp. Prunus spp.
Tobacco mosaic (tobamovirus group) mechanical; seeds may be externally contaminated, can be soilborne delphinium, petunia, phlox, wisteria, flowering tobacco beans, tobacco, tomato Emilia, tree tobacco
Impatiens necrotic spot virus and tomato spotted wilt (tospovirus group) thrips amaryllis, aster, ageratum, begonia, calendula, calla, chrysanthemum, coreopsis, cosmos, dahlia, forget-me-not, gerbera, gladiolus, gloxinia, gypsophila, impatiens, kalanchoe, lily, nasturtium, papaver, petunia, phlox, primula, ranunculus, salvia, stock, sweet pea, tagetes, verbena, zinnia, and others artichoke, bean, cauliflower, celery, clover, cowpea, endive, fava bean, lettuce, papaya, pea, peanut, pepper, pineapple, spinach, tobacco, tomato, and others bindweed, chickweed, emilia, jimsonweed, knotweed, lupine, malva, Mesembryanthemum, miner's lettuce, physalis, pigweed, nightshade, shepherd's-purse, and others
Turnip mosaic (potyvirus group) aphids anemone, nasturtium, petunia, statice, stock, sweet william, wallflower, zinnia Brussels sprouts, cabbage, cauliflower, cress, horseradish, mustard, radish, rape, rhubarb, swede turnip cruciferous weeds


[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
S. T. Koike, UC Cooperative Extension Monterey County
C. A. Wilen, UC IPM Program, UC Cooperative Extension San Diego County
Acknowledgment for contributions to Diseases:
R. D. Raabe, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
A. H. McCain, (emeritus) Environmental Science, Policy, and Management (ESPM), UC Berkeley
M. E. Grebus, Plant Pathology, UC Riverside

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