Exotic plant pests - Plum pox (sharka)
Officers of Biosecurity
What is it?
Plum pox is caused by a virus and is considered one of the most serious diseases of stonefruit. It has not been detected in Australia, but could be devastating if it established here. The virus affects all Prunus species, including the cultivated stonefruit plum, peach, nectarine and apricot. It also affects a range of weed hosts such as white clover and nightshade. Plum pox causes high yield losses and has resulted in large areas of tree removal in Europe, where it is well established. In the last two years the disease has been subjected to eradication programs in the USA and Canada.
What does it look like?
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Figures 1 and 2 (above): Leaf symptoms; Figures 3 and 4 (above): Fruit symptoms
Figure 5. Leaf symptoms and discolouration of stone
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 (Figure 1, 2 and 5). 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).
What are the implications of an outbreak in Australia?
In areas where plum pox is established it is 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 completed removed from affected orchards in Europe and North America.
How does plum pox spread?
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 has restrictions on entry of stonefruit plants and plant material to Australia because of this risk.
Controlling plum pox
Quarantine and certification
Plum pox is extremely difficult to eradicate from a growing district once it becomes established. But 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 the local area. 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 ten percent to one percent in an orchard over a three-year period.
Aphid control
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.
Resistance
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.
Further information
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If you would like further information or think that you may have seen plum pox, contact your nearest DPI&F plant health inspector or the DPI&F Call Centre on 13 25 23 (free call for Queensland residents; 8 a.m. to 6 p.m. weekdays) for advice on what to do next. Non-Queensland residents: phone 07 3404 6999. E-mail the DPI&F Call Centre: callweb@dpi.qld.gov.au.
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DPI&F Internet site: www.dpi.qld.gov.au.
This DPI Note is also published on the DPI's PrimeNotes CD-ROM.
Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries and Fisheries Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.
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Last reviewed 31 August 2005
