Good Fruit Grower

January 2017

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8 JANUARY 1, 2017 GOOD FRUIT GROWER O roville, Washington, grower Dave Taber is relieved he didn't plant more Royal Red Honeycrisp apples on Geneva 935 rootstocks. As it was this year, he tore out 3,000 trees — two acres worth — from a commercial block after an unknown disorder left many of them stunted, small and sickly. Only a few of the trees reached the top wire of his trellis while the roots strug- gled to grow. "We are quite small farmers. If we would have done that on the 30-some acres of high density that we did, it'd probably (darn) near break me," Taber told a collec- tion of about 60 growers and nursery owners visiting his North Central Washington orchard during a mid-Octo- ber rootstock tour. Certain strains of some varieties have been showing symptoms of tree decline on G.935, one of several new apple rootstocks released by Cornell University. Both researchers and growers are noticing the problems, though they don't have clear answers as to what's caus- ing them yet. "As for the G.935 performance, yes, to the grower, I'm sorry," said Gennaro Fazio, U.S. Department of Agriculture geneticist and breeder in Geneva, New York, shaking his head. "It was supposed to be the best thing." Then again, he feels just as sorry when entire orchards planted on Malling 9 rootstocks are pulled because of fi re blight, he said. The head-scratching problems may be an unintended consequence of the industry's drive to renovate older orchards and increase the number of higher-value vari- eties with high-performing rootstocks. The eight-mem- ber Geneva series rootstocks are noted for high precocity, improving tree growth on replant sites and resisting fi re blight. Five of the series also resist woolly aphid. However, with a seemingly infi nite combination of scion, rootstock, soil and climate, unexpected things have happened. Maybe blame a virus One theory is a virus or combination of viruses. Tom Auvil, project manager for the Washington State Tree Fruit Research Commission and organizer of the tour, compared the problem to getting a fl u shot only to fall ill to a previously unknown strain. "We just have so many combinations of scions and rootstocks that we can't test everything to the degree that we would like to," Auvil said. In the 1970s, the growers worked with three major scion varieties on only three or four rootstocks. Today, there are about 20 commercial rootstocks with more coming and nearly 40 active scion varieties. The virus theory goes like this: Scion wood from a few varieties developed from domestic sources may have contracted a combination of latent viruses that showed up when grafted to G.935. Researchers have reported that apple chlorotic leaf spot and apple stem pitting viruses created a similar decline in Red Delicious planted in New York, also on G.935, though researchers have not confi rmed that in Taber's orchard. So far, the mysterious problems affect relatively few combinations. Several scions have grown well on G.935 with no problems, Auvil said. If viruses are the cause, uncertifi ed sources of wood may be the culprit, Fazio said. In Europe as well as in other apple growing regions of the world, the industry has partnered to produce trees made with certified sources of budwood and rootstocks. Meanwhile, ELISA tests aren't fail-safe. The immuno- assay tests for apples only detect one specifi c virus per test and sometimes report false negative results, leading Problematic pairings Certain scion strains show mysterious tree decline symptoms when combined with Geneva 935 rootstocks. by Ross Courtney ONLINE For more information about rootstock trial results, visit to certifi cation of infected material and giving growers false security. With so many scions and rootstocks, to run all plant material through the virus lab for deep sequencing would take considerable cash. Some nurseries are con- sidering budding all of their scion wood sources across all rootstocks and growing the trees for four years to rule out latent viruses in their own fi eld trials, Auvil said. Researcher Tom Auvil discusses the pros and cons of different rootstocks at test blocks in Wapato, Washington, at the end of a research trial tour last fall.

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