New Zealand pipfruit was granted access into Japan in 1993 but exports of apples have not grown to expected volumes. This is partly due to strict regulations put in place by the Japan MAFF to ensure fire blight (Erwinia amylovora) does not enter their country. These regulations are costly and time-consuming and ultimately difficult for the grower to meet. The export of apples from New Zealand to Australia and South Africa continues to be prohibited because of the presence of fire blight in New Zealand.

Yet research by Chris Hale and his team at HortResearch, Mt Albert, has shown that it is very difficult to infect apples with the fire blight disease (even when fire blight is deliberately inoculated ! ). Mature, healthy fruit from orchards with no fire blight symptoms are unlikely to transmit fire blight from one country to another.

During the 1994/95 season, the team at Mt Albert physically inoculated Gala flowers with fire blight bacteria. Only at high inoculum concentrations did the flowers show symptoms of fire blight. Likewise when cotoneaster, another susceptible host of fire blight, was artificially inoculated, the symptoms only developed from bacterial concentrations above a certain level (> 10⁴ colony forming units).

Most of the blossoms which did develop fire blight symptoms aborted before reaching the fruitlet stage. Fruitlets and fruit developing close to the clusters which had been inoculated were tested regularly up until four months after blossom. None of these fruit contained the fire blight bacterium whether in the calyx or on the surface (see Table 1). The results were double-checked with the sophisticated DNA probe which can detect very low levels of bacteria. The results were even more meaningful as they were obtained in a year when conditions were warm and wet and ideal for fire blight development.

When mature Gala apples from coolstore were inoculated with, or dipped in, Erwinia amylovora and then hung in the tree next to the blossoms, no infection of the blossom occurred. In other words, even if an export apple happened to be infected with fire blight it would be unable to infect apple trees in Australia, Japan or any other importing country. Of course, it is very unlikely that an infested apple would be packed in the first place due to stringent checking (and elimination of any orchard with fire blight symptoms), followed by a chlorine dip.

The research has been done and the next step is to convince the regulatory authorities in the importing countries that the possibility of fire blight entering their shores via a carton of New Zealand apples is virtually impossible. At the same time some of the requirements for the Japanese market, such as a 500m buffer zone and three orchard inspections may prove unnecessary.

If successful, the potential return for New Zealand pipfruit growers could be enormous and well worth the investment in research.

Table 1: Number of positive detections of Erwinia amylovora in calyxes and surfaces of fruit samples from inoculated trees.

References

HALE. C.N, R.K. TAYLOR and R.G. CLARK, (1996) Ecology and Epidemiology of Fire Blight in New Zealand. Acta Horticulturae. In press.

HALE. C.N, R.K. TAYLOR, R.G. CLARK and T.A. BATCHELOR (1996) Quarantine and Market Access. Acta Horticulturae. In press.

HortResearch acknowledges the support of ENZA New Zealand (International) and Foundation for Research, Science and Technology with this article.