Mar 23, 2019
Cornell/USDA rootstock program reimagines how apples are grown

Bite into an apple, and you’re tasting the fruit of scientific progress with deep roots. Apple-growing operations over the past half-century have undergone a stark transformation. Large, stately trees have given way to diminutive ones spread across wires like grapevines. But the story of the modern apple orchard begins in many ways below ground.

Almost all commercial growers now graft branches of a preferred apple variety– say Gala or SnapDragon or any one of the thousands of other options – onto chosen rootstocks. These rootstocks provide optimal attributes below ground while allowing growth of any fruit variety above. Rootstock breeding has made a dramatic shift in orchards around the world, influencing attributes such as the size and shape of the trees and their resiliency in withstanding disease.

The Cornell University/U.S. Department of Agriculture apple rootstock breeding program at Cornell AgriTech in Geneva is one of only six functioning programs worldwide and the only one in the western hemisphere. The Cornell program has released 15 apple rootstocks since 1982, and it continues to influence major changes in orchards across New York, the U.S. and around the world.

“Dwarfing rootstocks have allowed fruit growers to plant apple trees only two to three feet apart and to develop tall, slender, spindle-type trees that have much higher yield and better fruit quality resulting in much greater profitability for New York apple growers,” said Terence Robinson, professor in the School of Integrative Plant Science and co-leader of the program since 1994. “The dwarfing rootstocks are central to this progress.”

The first shift in apple orchards started more than a century ago when scientists at East Malling Research in the United Kingdom discovered rootstocks that produced shallower roots. In turn, the tree above grew shorter and produced earlier than traditional apple trees. The resulting trees, which look more like bushes, made harvest simpler and production less expensive. East Malling’s standard-bearer rootstock, the M9, is still planted in orchards throughout Europe and America.

But while that rootstock improved production in many ways, it also introduced vulnerabilities, particularly to a disease called fire blight. This bacterial pathogen infects the flowers of apple trees as they bloom. The disease gets its name because of the way leaves and flowers of infected trees look as though they’ve been burned. Plus the pathogen spreads like wildfire: One epidemic in Michigan in 2000 caused an estimated $42 million in damages, and the disease has ravaged orchards from New York to Washington.

“The Washington apple-producing region has experienced increasing fire blight pressure for the last five years,” said Tom Auvil, a grower and researcher from Washington state. “In 2018 the losses have reached epic levels. Several leading firms have been transitioning to new rootstocks from Geneva as the cornerstone of efforts to combat this disease.”

In 1968, visionaries Cornell CALS apple breeder James Cummins and plant pathologist Herbert Aldwinckle recognized the shortcomings of M9 and established the rootstock breeding program at Cornell with the express purpose of producing rootstocks that were resistant to fire blight. The researchers retained the useful “dwarf” feature but worked to find roots that could also withstand real-world orchard villains: bacteria and viruses, fungi, drought and flooding, among others.

The first fire-blight-resistant Geneva rootstock was released in 1991, and 13 more have followed.

One of those, G.41, now makes up 20 percent of the national market for apple rootstocks. Released in 2005, it was selected as an elite variety for its high productivity and resistance to diseases like fire blight and crown rot, and to pests. By 2011, about 1 million plants were in production. G.41’s popularity has since soared, with 5 million plants in production in 2017, and it is the primary rootstock used in the launch of the new apple variety Cosmic Crisp.

A company in Washington planted two orchards of 150,000 trees each—one with G.41 rootstock and another with M9. None of the G.41 trees were lost during the 2018 spring fire blight epidemic, while about 20 percent of the M9 rootstock was lost.

“When you lose a large number of trees like that, you end up having costs for replacements, missed revenue for each tree lost, costs of infrastructure and wasted spray treatments,” said Gennaro Fazio, a USDA-Agricultural Research Service scientist and adjunct Cornell professor who took over as head of the rootstock breeding program in 2002. Fazio estimated that the G.41 trees saved the orchards hit by the blight more than $20 million.

The Geneva rootstock line has flourished in recent years as fruit growers and researchers have discovered additional benefits beyond fire blight resistance. G.41 is also resistant to apple replant disease, which causes new apple trees to grow poorly in soil that previously had apple trees. The rootstock is cold hardy and can withstand the extremely cold winters in northern states and Canada. Additionally, it is resistant to woolly apple aphids, produces almost no root suckers (shoots) and increases yields up to 20 percent. Those characteristics have resulted in worldwide interest in the line.

“Our newest rootstocks produce many more fine roots than normal rootstocks, and we think this allows them to explore the soil more efficiently,” said Fazio.

Apple growing combines the beauty and the patience of good science. The newest rootstocks released in the last few years were first crossed in 1976. The varieties have undergone trials for disease, stress and drought, and have been planted in dozens of test orchards across the country.

“And yet, even after all the testing, we still find new things about these rootstocks,” Fazio said. “We’re always learning new things.”

Krisy Gashler, Cornell University





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