Fire blight resistance of Malus species from Sichuan (China), Russian Caucasus, Turkey, and Germany

The United States National Plant Germplasm System has sponsored several explorations to collect a diversity of apple germplasm from Asia and Europe, in order to conserve it for posterity, and after evaluation, to make it available to breeders and geneticists worldwide. Germplasm was acquired from Sichuan Province, China (Malus hupehensis, M. kansuensis, M. prattii, M. sieboldii, M. toringoides, M. transitoria, M. yunnanensis, and M. zhaojiaoensis); Russian Caucasus (M. orientalis); Turkey (M. orientalis); and Germany (M. sylvestris). Samples of germplasm were evaluated as seedlings for resistance to fire blight, by inoculating potted plants in the greenhouse with Erwinia amylovora strain Ea273. Plants with < 20 % of shoot length blighted were characterized as resistant. Considerable variability in resistance was observed in seedlings from almost all collections of all species from individual sites. Seedlings from small-fruited Sichuanese species were also evaluated by inoculation of 2nd leaf plants in the field. Greenhouse and field evaluations of the Sichuanese species were in close agreement. Proportion of resistant seedlings ranged, in the different sites collected, from 0 - 3 % of M. yunnanensis to 0 - 100 % of M. hupehensis. Most populations of M. hupehensis, M. sieboldii, M. toringoides, and M. transitoria had a high frequency of resistant seedlings. Seedlings from different populations of trees with - 30 mm fruit size (M. orientalis) from Russian Caucasus were 50 - 93 % resistant, whereas M. orientalis seedlings from Turkey were only 6 - 50 % resistant. Seedlings from small-fruited M. sylvestris from Germany were almost all susceptible. The resistant seedlings from the Sichuanese species may have breeding potential especially for rootstocks and ornamentals. However, Russian M. orientalis may convey resistance to fire blight accompanied by less sensitivity to latent viruses, than small-fruited Malus species. This would be especially useful for breeding of fire blight resistant rootstocks.