Plum Pox Virus

Plum Pox Virus

Plum pox virus (also known as sharka) was first seen in southwest Bulgaria about 1918. Since then, the disease has been widely disseminated in planting material to most of Europe with the exception of Scandinavia. It causes serious losses in plum (Prunus domestica), peach, nectarine, and apricot.

Symptoms

Foliar symptoms in plum consist of pale green chlorotic spots, rings, and lines that are visible from early summer onward (Plate 119). These markings can become necrotic. Symptoms frequently are restricted to only a few leaves per shoot. Infected trees usually are not stunted and are difficult to identify.

Fruit symptoms in red and dark-colored plums consist of rings and blotches in fully expanded unripe fruit (Plate 120). These markings tend to disappear as the fruits color and reach maturity. Sunken rings and spots occur in some cultivars, overlying discolored flesh. Red rings and spots can occur on the stones. Affected fruit are low in sugar and tasteless; they drop from the tree prematurely. An abundance of abscised fruit helps in locating suspect trees in infected orchards.

In peach, symptoms appear in the first leaves to expand and consist of chlorotic veinclearing and veinbanding, with twisting and distortion of the Iamina. Symptoms in peach fruit (pale or dark rings, lines, and spots) may disappear at ripening. Affected fruits sometimes are deformed.

In apricot, leaf symptoms are similar to those in plum but are less conspicuous. Fruit of some cultivars, however, are severely marked and may be misshapen, with flavorless, dry flesh around the stone, which is usually marked with pale rings, lines, or spots (Plate 121).

In general, cultivars of each host show differential responses to infection.

Causal Agent

Plum pox virus is a member of the potyvirus (potato virus Y) group, characterized by its flexuous, filamentous particles (approximately 750 nm long) that contain a single-stranded RNA with a molecular weight of 3.5 x 10^6. The virus is aphid-transmitted in a nonpersistent manner, and infection is associated with pinwheel inclusion bodies in the cytoplasm.

Disease Cycle and Epidemiology

The most important means of spread is by diseased plant material, which accounts for much of the rapid spread among and within European countries during the last 20 years. Once established, plum pox virus is extremely difficult to eradicate. Spread by aphids usually occurs along a comparatively steep gradient; i.e., new infections next to infected sources and occasionally at longer distances. Infection is slow to become systemic and is often confined to one or two limbs of a tree. Because symptoms can be mild or transient, considerable spread can occur from new disease foci before infections are noticed. There is no detailed information on the epidemiology of plum pox virus (e.g., the relative importance of different vector aphids, seasonal variation in spread, or the effect of pesticides).

Within the genus Prunus, the virus infects most species in the subgenera Prunophora (plums and apricots) and Amygdalus (almonds and peaches). Some members of the microcerasus section of the subgenus Cerasus can be infected experimentally, but not members of the eucerasus (cherries), pseudocerasus (ornamentals), padus (bird cherries), or laurocerasus (cherry laurel) sections. There are, however, reports of the virus occurring naturally in sweet and sour cherries in Moldavia and Bulgaria. Few species other than Prunus species have been naturally infected, but the virus has a wide experimental host range, including Chenopodium foetidum Schrad., Lamium amplexicaule L., Nicotiana clevelandii Gray, N. megalosiphon Heurck & Muell., Pisum sativum L., Ranunculus arvensis L., Senecio sylvaticus L., and Zinnia elegans Jacq.

Control

Control depends on obtaining healthy plant material, achieved in countries in Europe by certification programs. Trees are grown from tested propagation material in isolation from potential sources of disease and are inspected before distribution to growers. The most widely used test for plum pox virus is enzyme-linked immunosorbent assay (ELISA), but results should be interpreted cautiously because distribution of virus is unpredictable and uneven in infected trees. Besides ELISA, molecular-based assays such as polymerase chain reaction are being done in many countries for plum pox virus detection and identification.

Alternatively, standard biological indicators of plum pox virus have included seedlings of either peach cultivars GF 305 and Siberian C or Nanking cherry (P. tomentosa). Recently, eight plum pox isolates were compared in controlled inoculations under standard growing conditions in a quarantine containment facility in Frederick, Maryland. In Nanking cherry, leaf symptoms-consisting of chlorotic veinclearing (Plate 122), chlorotic oak-leaf patterns, and mild distortion among scattered leaves developed in 30 days. Nanking cherry was found to be superior to the peach indicators in detecting all virus isolates tested.

The productivity of orchards with low levels of infection can be prolonged by frequent inspection and removal of infected trees. Rigorous spraying for aphids may delay the spread of virus. In areas where plum pox virus is common, the use of tolerant cultivars is the only effective control. Although a few immune or highly resistant plum and apricot cultivars exist, they are not in use commercially.

Selected References

Adams, A. N. 1978. The detection of plum pox virus in Prunus species by ELISA. Ann. Appl. Biol. 90:215-221.

Maiss, E., Timpe, U., Brisske, A., Jelkmann, W., Casper, R., Himmler, G., Mattanovich, D., and Katinger, H. W. D. 1989. The complete nucleotide sequence of plum pox virus RNA. J. Gen. Virol. 70:513-524.

Mathys, G., ed. 1974. Progress realises dans la connaissance de la sharka. OEPP Bull. 4. 125 pp.

Van Oosten, H. J. 1975. Susceptibility of some woody plant species, mainly Prunus spp., to sharka (plum pox) virus. Neth. J. Plant Pathol. 81:199-203.

Varveri, C., Ravelonandro, M., and Dunez, J. 1987. Construction and use of a cloned cDNA probe for the detection of plum pox virus in plants. Phytopathology 77:1221-1224.

(Prepared by A. N. Adams; information on the Nanking cherry test for plum pox virus was supplied by V. Damsteegt)

(From the Compendium of Stone Fruit Diseases. 1995. J. M. Ogawa, E. I. Zehr, G. W. Bird, D. F. Ritchie, K. Uriu, and J. K. Uyemoto, Editors. APS Press, St. Paul, MN)

American Phytopathological Society

Web Site Author: Alan R. Biggs
11/10/1999 06:46:43 AM