Introduction

Plum pox virus is a serious virus disease facing the European stonefruit industry. The virus is known to affect plum, peach, nectarine and apricot crops causing high yield losses, a reduction in fruit quality and has resulted in large areas of stonefruit in Europe being removed. The virus has never been found in NZ, however, if it did manage to get into New Zealand it would have a serious impact on the New Zealand summerfruit industry. NZ plant quarantine regulations are very stringent to deal with the threat of importing infected summerfruit plant material from Europe.

What is plum pox?

Plum pox virus was first discovered in Bulgaria in 1917. In the 1920s and 1930s it became widespread in eastern Europe and after the World War II spread to Germany, Switzerland, The Netherlands, France, Italy and the United Kingdom. It is now believed to be in all European countries, but not all orchards, and occurs in several strains. It affects plums, apricots, peaches, nectarines and a range of Prunus ornamental and hedging trees and shrubs. Some weed species and ornamentals in orchards near infected Prunus trees have been found to have the virus, including henbit, red dead nettle, lupins, black medick, bittersweet, clover and bladder campion. The virus is one of the most destructive plant viruses found in Europe. This is because of the heavy orchard losses it causes, its widespread distribution and its ease of spread.

Symptoms

A wide range of symptoms are found depending on the susceptibility of the variety to the virus. Symptoms, found on the leaves and fruit are often indistinct. They may be restricted to part of the tree or even a single branch, especially if the tree has been infected for a short period. Fruit symptoms are more easily observed than leaf symptoms.

Leaf Symptoms

Symptoms on the leaves consist of diffuse or blurred pale green or occasionally yellow rings, lines, spots. Blotches or flecks are normally found between the main veins but sometimes following or running parallel to the veins.

Fruit Symptoms

The fruit symptoms can be so severe fruit is unsaleable either for the fresh market or for processing. Fruit symptoms include engraved rings, lines or bands, sometimes sharply defined on one side of the fruit, but blurred on the other. External pitting and grooving of the fruit, which has given the virus its name, may be associated with internal tissue browning and markings on the stone.

Fruit size, sugar content and anthocyanin levels can be lowered, reducing fruit quality. Fruit flavour is altered resulting in tasteless, bitter fruit. The keeping quality, even of unmarked fruit, is often also reduced. Partial or complete fruit drop, 2-3 weeks before the normal harvest date, can occur, depending on the susceptibility of the affected cultivar.

Virus Spread

The virus is usually introduced into a nursery or orchard by either infected planting material or aphid vectors from infected trees on nearby orchards. Once in the orchard or nursery the virus is spread rapidly by aphid vectors, unless control measures are introduced. Many aphid species are efficient vectors of plum pox. Some researchers have reported transmission of the virus by pollen and seed, while others have not found this.

Control

Fruit trees in the orchard infected with plum pox cannot be cured. In Europe, the spread of plum pox is reduced by using only certified virus free plants, maintaining tight aphid control spray programmes and eliminating weeds which can be infected with the virus and act as reservoirs of infection. Once the virus is detected in the orchard, the major control method is to immediately rogue out trees which show symptoms and burn the plant material. Whole orchards have had to be removed. For control in the longer term, most European countries are placing strong emphasis on breeding programmes to find plum pox resistant varieties.

Implications for New Zealand

The introduction of plum pox into New Zealand would have a devastating affect on the New Zealand Summerfruit industry. Plum, apricot, peach and nectarine crops would all be affected as we grow susceptible varieties. The likelihood of plum pox being introduced is low, given NZ’s strict plant quarantine regulations. Under the quarantine regulations all Prunus plant material including budding and grafting scion wood, pollen and seed, is required to go through plant quarantine for careful screening.

Further reading

Plum Pox (sharka disease) 1980. Ministry of Agriculture, Fisheries and Food, UK, Leaflet 611.

Nemeth M 1986. Virus, Mycoplasma and Rickettsia Diseases of Fruit Trees. Martinus Nijhoff Publishers. Pp. 463-479.

Acknowledgements

Prof L. Giunchedi, University of Bologna for permission to publish plum pox pictures.