It is caused by the plum pox virus (PPV).

A wide range of symptoms will be seen depending on the susceptibility of the affected variety. Leaf symptoms include diffuse or blurred pale green or occasionally yellow rings, lines or spots. Generally these symptoms are barely seen with the naked eye and will be found at the bottom of a branch, but not the tip. Symptoms on fruit are more obvious, particularly on plum, and include the development of discoloured rings, spots, or bands on the skin (Figure 3). External pitting, grooving (Figure 4), or deformation of the fruit may also occur as well as internal flesh discolouration and marking on the stone (Figure 5).

Aphids are small sucking insect pests that are capable of spreading the disease over short distances, such as in or between orchards. The green peach aphid, Myzus persicae, which is common in Australia, is one of the more efficient of these plum pox vectors.

Long-distance spread and introduction of plum pox to new regions can occur through the human movement of infected plants or plant parts. The discovery of plum pox virus in several European countries has been associated with introduction of infested nursery stock. The Australian Quarantine and Inspection Service has restrictions on entry of stonefruit plants and plant material to Australia because of this risk.

Plum pox can be extremely damaging to fruit production. Tree yields can be severely affected and up to 100% premature fruit drop has been reported in some plum varieties. Infected fruit are unsightly and misshapen and are generally unmarketable. Because of the seriousness of the disease, quarantine restrictions on movement of fruit and plant material would be likely in the event of an outbreak to prevent further spread of the organism. As a control measure, infected trees are normally completely removed from affected orchards in Europe and North America.

Quarantine and certification
Plumpox is extremely difficult to eradicate from a growing district once it becomes established. Because the disease is easily spread on plant material, unaffected areas and districts can be protected through the implementation and enforcement of quarantines on movement of these items. Spread can also be controlled though the use of planting material that is certified free of plum pox.

Tree removal
Infected trees cannot be cured of the disease, and in a newly affected orchard they are normally destroyed as quickly as possible to minimise spread. In France, where plum pox is well established, infected trees are removed within one week of the virus being detected. This method can reduce the incidence of the disease from 10% to 1% percent in an orchard over a three-year period.

Aphid control in stonefruit orchards may have some effect on preventing plum pox spread, but will not completely stop it. While insecticide applications will reduce total aphid populations in orchards, 100% control of this insect cannot be achieved and it may only take one or two aphids to infect a tree.

The ability to breed stonefruit cultivars that are resistant to plum pox holds the key to controlling this disease in the future. Some cultivars are known to be more tolerant but as yet none have been identified that are completely resistant to the disease. However, some promising work is occurring in this area in Europe.

If you would like further information or think that you may have seen plum pox:

• contact your nearest Queensland Primary Industries and Fisheries (QPIF) plant health inspector
• contact the QPIF Business Information Centre
• Department of Employment, Economic Development & Innovation (inc. QPIF) website.