Viral Diseases

Compiled by Ned Tisserat, Colorado State University:

Hosts

Probably every plant species has at least one virus that can infect plants and adversely affect the growth and / or production of infected plants. Plant viruses as a rule are named after the first plant on which they are found.

Diagnosis and Damage

Viruses are infectious particles that consist of a nucleic acid core with a protein coat. They replicate in nature only within living cells, controlling synthetic processes of the host so that it produces more virus particles. Most viral diseases of trees and shrubs cannot be diagnosed on the basis of symptoms alone. A few plant symptoms, such as mosaic patterns on leaves and chlorotic or necrotic ring spots, can be attributed to viruses with some degree of confidence. Most other symptoms caused by viruses resemble those caused by numerous other factors. Virus and virus-like symptoms are relatively well known and understood in species cultivated for orchard crops, but most viral disorders in forest, shade and ornamental trees and shrubs have not been studied. Viruses in trees and shrubs often cause no visible symptoms, or they may cause symptoms ranging from slight foliar markings and mild growth suppression to dramatic colorful patterns on leaves, stem distortions and cankers, twig and branch dieback, graft union necrosis, and slow or rapid decline of plant. Two or more viruses or viruses plus other pathogens may infect a plant simultaneously, which complicates both symptoms and diagnosis.

Biology and Disease Cycle

Plant virus particles, which are composed of a nucleic acid core with a protein coat, are very small, and only visible with an electron microscope. They are obligate parasites and cannot survive outside of their host, therefore they must be moved from one plant to another by insects, grafting, or in seed from infected parent plant. Viruses do not divide and do not produce special reproductive structures, they multiply by inducing host cells to form more virus. Plant viruses that cause significant disease become distributed systemically throughout the plant. Most plant viruses invade phloem and parenchyma tissues, but some are limited to phloem. A virus that is transmitted by one group of organisms (e.g. aphids) is not transmitted by other kinds of organisms.

Management

Viruses cannot be controlled with pesticides, but instead must be controlled through other management practices. Management options include prevention of introduction of virus into plant material. Propagation facilities should, if possible, use propagation stock and/ or seed that is certified virus free. Because viruses are transported widely within infected, often-symptomless plants, known weedy hosts of viral disease in question should be eradicated from the area of host plant. Health and vigor of a plant does not help it resist infection by a virus, although plant varieties resistant to certain viral diseases do exist.

Virus Diagnostic Guide

Host	Symptoms	Disease
Stone fruits, rose species	Symptoms vary depending on host but may include shot holes on young leaves, chlorotic leaf mosaic and line patterns, leaf necrosis, leaf deformity, delayed fruit maturity.	Prunus necrotic ring spot virus (PNRSV)
Apple, prune plum	On apple the first symptom is pitting at the graft union that become deep invaginations in the trunk.  Structurally weak tissues are laid down and trees may break off at the union.  On prune, symptoms include weak growth with sparse foliage. At graft union removal of bark reveals a brown line (necrotic tissue).	Tomato ringspot virus (causes apple union necrosis, and prune brown line) (ToRSV)
Grape	Variable symptoms are possible.  Leaves may show a green or yellow mosaic, rings, line patterns or flecks, and exhibit a fan-like appearance.	Grapevine fanleaf virus
Apple	Expanding leaves develop pale yellow to cream-colored areas, appearing as spots, flecks, blotches, vein-net, line patterns, or bands along major veins. Areas turn chrome yellow to white, become necrotic. Leaves may drop prematurely veins	Apple mosaic virus (ApMV)
Sweet cherry, peach, apricot,	Leaves show light green or whitish, diffuse discoloration between the secondary leaf veins, affected leaves often develop an irregular shape, even appearing shredded.  May reduce fruit size.	Cherry mottle leaf virus
Peach, apricot, plum	Symptoms vary with host.  Color-breaking of flower petals that may be smaller, crinkled and deformed.  Leaves show chlorotic spots that vary from tiny flecks to streaks to vein feathering or bold blotches.  These areas become necrotic and drop out.	Peach mosaic virus
Cherry	Leaf-like projections develop along the midrib on the underside of affected leaves.  In severe cases, the whole leaf can be affected.  Symptoms begin in lower part of tree because disease is introduced by nematode vector into the roots.	Cherry rasp leaf virus (CRLV)
Peach, plum	Peach trees produce only mild stunting, and no leaf symptoms.  But peaches infected with both PDV and PNRSV showed severely retarded growth and reduced yields.  Infected plum and prune develop narrow, strap-like leaves that are thicker than normal and early season growth appears bunched due to shortly spaced early season internodes.	Prune dwarf virus (PDV)
Rose	Symptoms vary dependent on rose species or cultivar and conditions.  Foliar symptoms range from chlorotic mottles and ringspots to light green or chlorotic line patterns to vein clearing (yellow net), vein banding, and mosaics of green and yellow or white.  Leaflets may pucker. Symptom expression is dependent on temperature.  Diseased rose bushes are less vigorous and more susceptible to winter injury.	Rose mosaic complex, caused by a complex of viruses, PNRSV and ApMV appear to be the principal pathogens.
Sweet cherry	Delayed budbreak, leaves are small and more erect than those are on healthy limbs or trees.  Lower portions of affected trees often are bare of leaves. Diseased fruit is small and off color.  Wood beneath bark may exhibit pits and grooves near the soil line.	Cherry stem pitting, caused by a graft-transmissible agent of unknown etiology that appears to be soilborne.

GO TO Disorders Associated with Flowers, Fruits, and Seeds

The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.