Apr 19, 2019

Tissue cultivation helps quick adaptation of Geneva rootstocks

Last summer, when Cornell University applied fruit crop physiologist Terence Robinson returned from a three-year sabbatical from Northeast apple orchards, a surprise was waiting for him.

Adoption of Geneva rootstocks released by Cornell had exploded. And yet growers in Cornell’s backyard of west New York – some of whom had their own nurseries – weren’t trying to grow them.

Geneva rootstocks are bred for productivity and disease resistance, including replant disease complex and fire blight, through a joint program of Cornell and USDA’s Agricultural Research Service. A few years ago, just a couple of million Geneva rootstocks were sold each year – now, about eight million rootstocks were sold during the 2018 calendar year. The G.31 variety was the most popular with about four million sold, while the G.35 variety sold 2.5 million.

And yet, seemingly, growers can’t get them quickly enough. That’s something Robinson has tried to remedy.

The relationship of Cornell-bred rootstocks and the west New York growers who don’t use them seemed somewhat ironic at the latest International Fruit Tree Association (IFTA) winter conference and tours held in February in Rochester, New York. The day after a full afternoon of presentations on rootstocks by academics – most of whom touted the many benefits of Geneva rootstocks – attendees toured some orchards of prominent IFTA growers who were not using them.

“Don’t plant another B.9, or don’t plant another M.9 or another M.26,” Robinson said during his own presentation, referencing some industry classics. He emphasized the superior traits of Geneva rootstocks, especially increased yields that, he said, growers could hardly afford to refuse. Geneva rootstocks include royalty fees; but a Cornell economist gave a separate presentation at the IFTA conference attempting to show growers it was more than worth the investment, even if the royalty rates were increased. Another common complaint from growers has been that the rootstocks are not readily available.

“Some of you have planted rootstocks just because you could get them, or because you didn’t want to pay that 25-cent royalty to Cornell,” Robinson said. “That’s peanuts, nothing. We’re talking millions of dollars here.”

It’s not just about the raw yields. High-value varieties are important; Robinson urged growers to choose apple varieties that will pay more than $300 a bin. But getting those cultivars to grow vigorously enough is also an issue. Slow-growing trees or new plantings affected by orchard replant disease, mean lost harvests.

“Planting rootstocks that can fill the space in 2-3 years is critical,” Robinson said. “Pick the rootstocks that will fill the space you’ve given that tree by the end of the second year or by worst at the end of the third year.

“It’s not just that I’m from Cornell,” Robinson said. “These are clearly superior genetics. They’re thoroughbreds. Now, like any child, they all have their problems. Not one of them is perfect – G.41 has a brittleness, you have to treat it with kid gloves; G.935 seems to have trouble with some varieties that have either rubbery wood or some other viroid in them – but the kind of dollars that are involved would lead us to try to figure out how to grow these rootstocks.”

A large number of Geneva rootstocks have been released so that growers can pick one ideal for their soil, climate and apple-bearing cultivar, but the other side is there’s no one-size-fits-all.

“Some people are tired of so many Geneva rootstocks,” Robinson said. “Nurserymen hate the idea because it requires them to carry lots of inventory.” Robinson said that 20 nurseries are licensed to grow Geneva rootstocks, but stool beds for growing them need three years of setup time, and most growers don’t place orders that far in advance.

But Robinson said there’s now something of a shortcut. A number of tissue cultivators in North America can deliver plants to a nursery within eight or nine months. The plants are roughly a foot tall, either with a few leaves or a dormant stick.

“The ability to grow rootstocks by tissue culture has opened up the panorama to be able to order essentially order any rootstock you want,” Robinson said. “It’s something you can do quicker. The turning around time is shorter.”

Robinson listed a number of producers who do tissue cultivation: North American Plants of Lafayette, Oregon; Sierra Gold Nurseries of Yuba City, California; Phytelligence of Seattle, Washington; and Skagit Horticulture of Mabton, Washington.

“It’s no longer a situation where you call up and the nurseryman says, ‘Sorry, don’t have it,’” Robinson said. “Every grower should be able to get the rootstock they want.”

— Stephen Kloosterman, associate editor