Pest risk assessment of apple proliferation phytoplasma (“Candidatus Phytoplasma mali”)
Report no: 2012: 08
The Norwegian Scientific Committee for Food Safety (VKM) considers that there is an overall probability that the pathogen “Candidatus Phytoplaasma mali” (AP) might spread in Norway and cause extensive economic damage to apple producers and nurseries.
Stricter measures are necessary to avoid the disease to spread to districts which are not known to be contaminated, VKM concludes in a pest risk assessment commissioned by The Norwegian Food Safety Authority.
The most serious effects of AP infection are secondary shoots forming “witches’ brooms” on apple trees, and crops consisting of few, small, tasteless and non-marketable fruits.
AP may rapidly spread over large distances by use of infected propagation material, and locally also by insect vectors and natural root bridges.
In the 2010 season there was a significant increase in the number of AP detections, mostly in the fruit-producing districts in south-western Norway. The number of farms with infected trees and the extent of infection at each farm were significantly higher than before.
The disease was registered for the first time in Norway in 1996, but it may have been introduced in the 1970s. The climatic conditions are considered to be suitable for AP, and establishment is not restrained by any competitors or natural enemies.
VKM states that the present control measures have not been sufficient to eradicate the disease. In the infected areas it will probably be possible to keep AP at a low level, if the current control measures are combined with permanent survey programs, testing of trees, and extensive use of disease-free planting material.
These measures, however, would be at a high cost for the producers as well as the public sector.
VKM is of the opinion that it should be possible to prevent the spread of the disease to districts where it is not registered by allowing the use of only certified disease-free planting material, whether it is domestically produced or imported.
Important measures according to present regulations are annihilation of sick trees, prohibition on using plant material from orchards or nurseries where the disease has been detected, and imposing pesticide programs against the insect vectors.
The pest risk assessment will be used by The Norwegian Food Safety Authority as background for its regulatory and administrative work.