—by Matt Milkovich
The apple’s greatest genetic diversity can be found in its place of origin, Central Asia. Meanwhile, the greatest genetic diversity of fire blight (Erwinia amylovora), the destructive bacterial disease regarded as one of the apple’s greatest foes, can be found in North America.
Does that mean fire blight originated in North America?
Cornell University geneticist and fruit pathologist Awais Khan suspects so. And if that’s true, fire blight likely coevolved with wild crab apples native to North America (and/or other relatives in the Rosaceae family of flowering plants). One would expect those wild apples had many, many generations to develop resistance to the bacterial disease. Khan believes such deep genetic resistance could be a crucial weapon in the fight against fire blight in domesticated apples.
A North American origin for fire blight would explain why most domesticated apples, first brought to the continent by European settlers and carrying Eurasian genetics, are so susceptible to the bacterial disease. It also would fit the historical evidence: Fire blight was first reported in New York’s Hudson Valley in the 1700s. Cornell researcher J.C. Arthur wrote the first scientific paper on fire blight in 1886, Khan said.
Research has discovered fire blight resistance in wild North American crab apples. Khan’s lab, in collaboration with professor Zhangjun Fei of Cornell’s research-oriented Boyce Thompson Institute, recently sequenced the genomes of two native North American species, Malus coronaria and Malus ioensis. Using these in combination with two previously sequenced North American species, Malus fusca and Malus angustifolia, Khan is identifying and marking genomic regions linked to fire blight resistance. They’re using this information to develop disease-resistant pre-breeding lines, which apple breeders can use to breed resistance into domesticated lines, he said.
But merging disease resistance from wild apples with the fruit quality traits of domesticated apples requires multiple crosses to develop a marketable new cultivar, a process that can take decades. To speed the cross-breeding process, Khan’s lab also uses genetic engineering to bring an early flowering gene from silver birch into the apple lines. Instead of taking years for an apple seedling to start flowering, it now takes months, he said.
“Instead of five years, you can do it in one,” Khan said. “That’s the magic of the early flowering transgene.”
The early flowering trait, not desirable in commercial cultivars, can be removed from pre-breeding lines once the resistance is incorporated.
Khan’s team is also searching for fire blight resistance within the genome of Malus sieversii, the primary ancestor of the domesticated apple and a native of Central Asia. They planted the world’s only Malus sieversii orchard in Geneva, New York, in 2023, with about 300 accessions of the species, he said.
Khan also aims to preserve the wild North American crab apples he’s been studying, which are under threat from climate change, disease and urbanization. As part of a “hobby” conservation project, he’s grafting wild North American species from the U.S. Department of Agriculture’s apple germplasm collection in Geneva and working with city foresters and botanical gardens to plant them in their native habitats.
“The trees have beautiful blooms, foliage and fruit,” Khan said. “They attract native pollinators and birds and increase biodiversity. Why not grow ornamental North American apples?” •
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