Fire blight resistance and horticultural evaluation of wild Malus populations from Central Asia

Since 1989, several collecting trips have been made to Central Asia by the U.S. Department of Agriculture and Cornell University to collect seeds of wild Malus sieversii from many diverse ecosystems. In 1992, an ex situ plot was established in Geneva, NY, with trees grown from seed that was collected in three different habitats in Kazakstan, Tajikistan and Uzbekistan in 1989. In 1995, trees grown from seed that was collected in five additional habitats in Kazakstan and Kyrgyzstan in 1993 were added to the ex situ plot. In the summers of 1995 and 1996, tips of vigorously growing shoots of 1135 seedlings from 79 mother tree sources from 6 sites in Central Asia were inoculated by hypodermic syringe with 5x10(8) cfu of Erwinia amylovora strain Ea273 per mi. Seedlings from the 1989 collection were in their fourth and fifth field-growing seasons, with some beginning to bear fruit. Seedlings from the 1993 collection were in their first and second field-growing seasons. Results from both years indicated that individuals within each of the 79 populations of M. sieversii were resistant to fire blight (defined as < 20 % shoot length infected). Resistance differed among populations, with some populations having no resistant individuals and others having > 80 % of their progeny resistant. The range of resistance was quite similar to that seen among apple cultivars from North America and Europe. A total of 22 % of all seedlings fruited by the fourth leaf, increasing to 37 % by the sixth leaf. Those accessions from Tajikistan and Uzbekistan fruited earlier than those from Kazakstan. Seedlings from Tajikistan had the highest population of fire blight resistant progeny among those that had begun to fruit. Of the 4 seedlings from Uzbekistan that were resistant to fire blight and were fruiting, one accession number had red fruit 56 mm in diameter. These results are significant because we have been able to identify genotypes that are not only resistant to fire blight, but in some cases have desirable horticultural traits and/or resistance to other diseases of apple.