Spread of Erwinia amylovora via commercial apple fruit: an insignificant risk
A team of American and New Zealander researchers have recently tried to assess the risk of introduction of Erwinia amylovora (EPPO A2 quarantine pest) associated with the movement of commercial apple fruits. It can be recalled that EPPO has always taken the view that this risk was negligible, but some countries like Australia, Japan South Africa, some South American countries have taken a totally different attitude and prohibit imports of pome fruits from countries where the disease occurs. The authors have made a critical review of the literature to evaluate the risk of introduction and establishment of E. amylovora via the movement of consignments of mature, symptomless apples, and have also tried to use a linear and simple model to study probabilities of such an event.
Their conclusion from the literature review is that considering: 1) the low epiphytic fitness (short survival, no multiplication) of E. amylovora on apple fruit; 2) the low incidence of viable populations on mature apple fruit (note: populations of E. amylovora on fruit have only been found in heavily infected trees or in the immediate vicinity of another infected host, generally pear); 3) and the lack of a documented pathway by which susceptible host material could become infected by fruit-borne inoculum, the risk of introduction and establishment of E. amylovora through trade of apples is extremely low.
By using the linear model, it was estimated that the probability of introduction into a new area followed by an outbreak, via infected apples, would be 1 outbreak every 38462 years under the current US and New Zealand apple export programs to Japan (which includes many requirements: free buffer zones around place of production, several growing season inspections, consignment treatments, consignment inspections before export and at place of destination). Under an export regime with less stringent phytosanitary requirements (no free buffer zones requested, one growing season inspection, low incidence of fireblight in orchards tolerated), it was estimated that only 1 outbreak in 11365 years may occur. The authors felt that the results of these estimations are further corroborated by the absence of known outbreaks of fireblight, despite the huge quantities of apples being traded around the world over many years.
Sources
Roberts, R.G.; Hale, C.N.; van der Zwet, T.; Miller, C.E.; Redlin, S.C. (1998) The potential for spread of Erwinia amylovora and fire blight via commercial apple fruit; a critical review and risk assessment.
Crop Protection, 17(1), 19-28.