The potential for spread of Erwinia amylovora and fire blight via commercial apple fruit; a critical review and risk assessment

Abstract

The biology of Erwinia amylovora is reviewed with reference to the phytosanitary risk associated with the movement of export-quality apple fruit to countries where fire blight does not occur or is not widely established. The low epiphytic fitness of E. amylovora on apple fruit, the low incidence of viable E. amylovora populations on mature apple fruit, and the lack of a documented pathway by which susceptible host material could become inoculated and infected from fruit-borne inoculum all support the view that movement of E. amylovora via commercial apple fruit is highly unlikely. A simple linear model using published data for estimation of fruit contamination levels was used to estimate the likelihood of fire blight outbreaks in unaffected areas as a result of commercial fruit shipment. We estimate the likelihood of a new outbreak of fire blight in a previously blight-free area caused by movement of E. amylovora on commercial apple fruit to be one outbreak every 38462 years under the current (United States and New Zealand) apple export programs for Japan. Under a ‘relaxed’ program without buffer zones, with one preharvest orchard inspection and allowing a low incidence of fire blight in export orchards, one outbreak in 35971 years may occur. If fruit were to be exported from any area, one outbreak in 11364 years may occur. By using the published data and making assumptions based upon documented pathogen biology, the model indicates that the risk of importing E. amylovora on commercial apple fruit and the concomitant risk of establishing new outbreaks of fire blight is so small as to be insignificant.