EVALUATION OF BUFFER ZONE SIZE AND INSPECTION NUMBER REDUCTION ON PHYTOSANITARY RISK ASSOCIATED WITH FIRE BLIGHT AND EXPORT OF MATURE APPLE FRUIT

Apple fruit from Washington State were first exported to Japan in 1995 under export regulations for fire blight that included a 500-meter (radius) buffer zone around export blocks, three orchard inspections, and disqualification of the export block if fire blight is found in the block or buffer zone. In 1999, the U.S. proposed program changes to reduce the buffer to 10 meters and to reduce the number of inspections to one. In 2000, experiments were conducted jointly by Japan MAFF and USDA-ARS to determine the effect of buffer size on the incidence of Erwinia amylovora in mature apple fruit. Two experimental sites were used from which 30900 mature apple fruit 0, 10, 25, 50, 100 or 300 meters from the source of fire blight inoculum were harvested for analysis. Nine hundred fruit were analyzed directly after harvest for internal populations of E. amylovora. No E. amylovora was detected in any fruit, even when harvested from trees with or directly adjacent to fire blight. Of 30000 fruit stored in cold storage for three months, none developed fire blight. These results indicate that a buffer of any size provides no phytosanitary security. To determine whether early season fire blight could be confirmed at harvest, 120 shoots of four varieties were inoculated at petalfall with E. amylovora, tagged, wetted and bagged in plastic for 4-6 days. After disease development, half of the strikes were pruned out. At harvest no recurring symptoms were observed at the pruned stubs. At harvest time all the remaining strikes were cut out and confirmed as being caused by E. amylovora using either CCT medium or with E. amylovora-specific PCR. These results demonstrate that strikes on all varieties were accurately and reliably confirmed at harvest, although they developed much earlier in the growing season, and that a single inspection at harvest can confirm early-season blight.