Story and Photographs by Ryan T. Bell

Making apple trees grow shorter over the years has made them easier to pick and ship, but it has also left them more vulnerable to disease.

Last fall, I picked a wild apple from a tree growing in the Ile-Alatau Mountains of southern Kazakhstan. It was a Malus sieversii, a wild ancestor of the cultivated apple. Biting into it, the flavor was bitter—a “spitter,” as apple aficionados call them—nothing like the sweet Fuji apples I eat from the tree on my family’s farm in Washington State.

The difference in flavor represents 2,000 years of agricultural refinement and a journey that took the apple out of central Asia, by way of the Silk Road, to Europe and then across the Atlantic to America. Portable, slow to spoil, and packed with nutrition (around eight percent of the daily dose of fiber, vitamin C, and potassium if eaten with the skin on), it’s easy to understand why travelers went to all the trouble.

Along the way, however, apple trees lost something essential: genetic diversity.

Root Cause

“Most of the apple trees in the world are planted on M.9 rootstock,” says Gennaro Fazio, a plant geneticist with the U.S. Department of Agriculture’s Agricultural Research Service. “They’re clones of a tree that grew in the 17th century. That’s ancient technology.”

Rootstocks are used in grafting when the branch of a desirable fruit tree is spliced into the trunk of another. M.9 is used to dwarf, or shrink, the size of a fruit tree. A Gala, Fuji, or Red Delicious grown from seed will become tall and unmanageable, but if you graft their scions (branches) onto M.9 rootstock, the resulting tree will be only one-third the height. This results in a smaller canopy that allows more sunshine through, creating better and more consistent apples on each tree. And the lower height makes for more efficient harvesting.

The term M.9 is short for Malling 9, a code given to the rootstock in 1912 when it was officially cataloged at the East Malling Research Station in Kent, England.

What those early botanists were doing, unknowingly, was narrowing the apple tree’s genetic pool to the point where a mutation occurred. In this case, in the form of the M.9’s shrimpy size. That process also eliminated genes that helped the tree fight pests and diseases.

“While M.9’s evolution was stopped in time, Mother Nature and all her pests kept on developing ways to attack it,” Fazio says.

The result is that today’s M.9 rootstocks are vulnerable to an array of maladies, such as fire blight, woolly apple aphid, and cedar apple rust.

The disease that really concerns Fazio is replant disease. Wherever multiple generations of M.9 have been replanted, the rootstock is attacked from something in the soil. Scientists don’t fully understand why.

“That’s not a problem for sieversii,” Fazio says. “They’ve been replanting themselves in the wild for thousands of years. That's a trait we want to get back.”

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