Evaluation of Fire Blight Resistance of Malus sieversii Populations from Central Asia

in HortScience

Malus sieversii is one of the primary progenitors of the cultivated apple. Since 1989, several collecting trips have been made to central Asia by personnel of the USDA and Cornell Univ. to collect seeds of wild Malus sieversii from many diverse ecosystems. In 1992, an ex situ plot in Geneva, N.Y., was established 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 different populations were inoculated by hypodermic syringe with 5 × 108 cfu/ml of Erwinia amylovora strain Ea273. Seedlings from the 1989 collection were in the fourth and fifth field-growing seasons, with some beginning to bear fruit. Seedlings from the 1993 collection were in first and second field-growing seasons. Results from both seasons indicated that individuals within each of the 79 populations of M. sieversii are 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 the seedlings resistant. The range of resistance is quite similar to that seen among apple cultivars from North America and Europe. In another test, some accessions from 1989 collection had sufficient bloom for inoculation in 1995 and 1996. At full bloom, blossoms on these trees were inoculated with the E. amylovora suspensions (5 × 107 cfu/ml) using a backpack sprayer. These also gave diverse resistant reactions.

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