Virus diseases cause economic losses through lower yields and reduced quality of apples and apple products. In general, virus diseases in perennial crop plants, such as apples, are more potentially damaging than in annual crops. Viruses can remain latent, spreading through an orchard and inflicting damage, often without the growers’ knowledge. Latent virus infection can produce small to moderate losses in fruit production. Often growers can maintain the productivity of diseased orchards at a profitable level by removing infected plant parts and replacing dead trees to limit the spread of the virus. Sometimes, however, losses are severe, and an acute viral infection can require tree removal.
Apple virus diseases are mainly spread thorough vegetative propagation. Because of this, in apple production, two methods are used to control virus diseases: adoption of virus-free propagation materials and eradication of contaminated trees. In addition, in areas where tomato ringspot virus is endemic, practices may be employed to minimize alternative virus hosts (i.e., deciduous weeds) and reduce nematode vector populations.
Some virus infections cause incompatibility between apple rootstock and scion cultivars, affecting yields and profitability of apple nurseries. A low success rate of successful grafting in nurseries may be attributed to virus infection. Reductions in bud survival have been reported to range from 20% to 67%. The degree of impact depends on the pathogen or its strain in combination with fruit type, cultivar, rootstock, nutrient supply, and tree age.
Virus testing of imported propagation materials into the United States has been the most important measure used to prevent the introduction and spread of unwanted viruses.
The testing and use of virus-free materials have different benefits for each of the three sectors involved: nurseries, growers, and consumers.
Growers and nurseries benefit from virus testing in three ways:
Consumers benefit from lower prices and more abundant fruit.
There are two facilities that tests for viruses in fruit trees in the United States. One is the National Research Support Project 5 (NRSP-5). The NRSP-5, located at the Washington State University Irrigated Research and Extension Center in Prosser, Washington, implements the nation’s virus protection program for all deciduous fruit trees. NRSP-5 is responsible for providing sources of deciduous fruit tree propagation materials free of virus and virus-like diseases. NRSP-5 also develops, evaluates, and implements new technologies for virus detection and the elimination of viruses and virus-like agents from commercially important cultivars. Prior to the establishment of NRSP-5, viruses abounded in every fruit-growing region in the United States. The success of this project has resulted in a dramatic reduction in the incidence of viral diseases. The other, according to Renee DeVries, is the Plant Germplasm Quarantine Office (PGQO), Beltsville Agricultural Research Center, Beltsville, Maryland. For more information on Plant Quarantine Programs Managed by APHIS-PPQ, click here.
A short list and description of potential viruses are provided in Table 1. The list is not exhaustive, as many other viruses and strains of known viruses are not listed here. In addition, many unconfirmed graft-transmissible causal agents may ultimately be shown to be viruses or phytoplasma. Table 2 provides information on yield reduction caused by virus diseases in apple.
The National Clean Plant Network has recommendations for minimum testing standards for foundation and certification materials (test recommendations marked with an asterisk below are those acknowledged by the International Working Group on Fruit Tree Viruses (Acta Horticulturae 657:2004, pp. 575) and for which adequate testing exists with ELISA, PCR, or PT-PCR. Items identified as agents are graft-transmissible but are incompletely characterized.
|Capillovirus||Apple stem grooving virus||ASGV||Stem grooves, abnormal graft union||Me/Gr/Se||Uyemoto and Gilmer, 1971; Plese et al., 1975|
|Ilarvirus||Apple mosaic virus||ApMV||Mosaic, mottling, necrotic ring spots||Me/Gr/Po||Gotlieb and Berbee, 1973; Wood et al., 1975|
|Nepovirus||Tomato ringspot virus (apple union necrosis)||ToRSV||Mosaic or ringspots, rasp leaf, yellow bud or vein, ringspots and chlorosis||Me/Gr/Se/Po/Ne||Stouffer et al., 1977; Parish and Converse, 1981|
|Sobemovirus||Apple latent virus||ALV (type II)||Latent infection||Me/Gr/Se/Po/In||Franki and Miles, 1985|
|Trichovirus||Apple chlorotic leaf spot virus||ACLSV (ALV type I)||Chlorotic leaf spots or rings, stem pitting, stunning, line patterns, chlorosis||Me/Gr/Ne||Chairez and Lister, 1973; Dunez et al., 1975|
|Foveavirus||Apple stem pitting virus||ASPV||Die back, inner bark necrosis, decline, epinasty, vein yellowing, latent infection||Me/Gr||Fridlund and Aichele, 1987; Kogenezawa and Yanase, 1990|
|Phytoplasma||Apple rubbery wood||Causal agent is a phytoplasma and is included here because it was first investigated as a possible virus disease||Abnormal flexibility of stems and branches, shortened internodes, reduced or stunted annual growth, rootstock productivity, tree vigor and yield may be reduced||Me/Gr||NRSP-5|
|*Possible transmission patterns:
Me = mechanical.
Gr = grafting.
Se = seeds.
Po = pollen.
In = insects.
Ne = nematodes.
|Apple Cultivars||Virus Strain||Yield Reduction (%)||References|
|Golden Delicious||Apple mosaic virus (AMV)||46||Baumann and Bonn, 1988|
|Golden Delicious||AMV, rubbery wood disease agent (RW)||21-67||Baumann and Bonn, 1988|
|Golden Delicious||RW||46||Wood, 1978|
|Golden Delicious||Apple stem grooving virus (ASGV), apple stem pitting virus (ASPV), apple chloritic leaf spot virus (ACLSV)||12||Meijnske et al., 1975|
|Golden Delicious||ASGV, ASPV, ACLSV||30||van Oosten et al., 1982|
|Red Delicious||AMV||42||Zawadzka, 1983|
|Red Delicious||RW||20||Zawadzka, 1983|
Alan R. Biggs, West Virginia University