Erwinia amylovora populations in apple calyxes do not constitute a pathway for fireblight dissemination
Epiphytic populations of Erwinia amylovora (EPPO A2 list) have been reported to survive on symptomless flowers, twigs and external apple fruit tissues. In particular, E. amylovora has been isolated from a small percentage of calyxes of apple fruit collected from heavily infected orchards. Several countries have raised concerns about the possibility of moving fireblight with infected fruits and imposed trade restrictions, although no clear scientific results could support this. Studies were carried out in New Zealand to know whether E. amylovora populations present in apple calyxes are able to multiply and spread to susceptible hosts, in sufficiently high numbers to cause fireblight infection. 600 apples (Malus domestica cv. Braeburn) were inoculated with a marked strain of E. amylovora. These apples were then discarded in an apple orchard (cvs. Royal Gala, Braeburn, Pacific Rose) which was initially free from fireblight and isolated from possible alternative hosts. The survival and possible spread of E. amylovora was assessed over 20 days in October/November 2000, at flowering (susceptible growth stage). The same experiment was repeated in 2001. During these periods, a model was used to verify that climatic conditions were favourable to disease spread. Results obtained both in 2000 and 2001 showed that bacterial numbers in fruit calyxes decreased significantly in the first 4 days, and persisted at low levels for the following 16 days. Spread of E. amylovora from infected apples was not detected (by isolation on growing medium and PCR) in any of the samples of rainwater, apple flowers, leaves, or insects. This demonstrates that E. amylovora was not transferred to susceptible hosts, even when apples with epiphytic populations of E. amylovora in calyxes were placed in close proximity to susceptible hosts at a receptive stage. The authors concluded that the presence of E. amylovora on calyxes of apple fruit does not provide a pathway for dissemination of fireblight.
Sources
Taylor, R.K.; Hale, C.N.; Gunson, F.A.; Marshall, J.W. (2003) Survival of the fire blight pathogen, Erwinia amylovora, in calyxes of apple fruit discarded in an orchard.
Crop Protection, 22(4), 603-608.