Good Fruit Grower

January 2017

Issue link:

Contents of this Issue


Page 8 of 39 GOOD FRUIT GROWER JANUARY 1, 2017 9 Maybe not Viruses, however, are only one educated guess to explain the problems, said Bill Howell, manager of the Northwest Nursery Improvement Institute and retired manager of the Northwest Clean Plant Network at Washington State University's Irrigated Agriculture Research and Extension Center in Prosser, Washington. The virus theory has not been proven. "In police work, we have suspects," Howell said. "That is a suspect." Viruses can indeed hide in tolerant tree varieties, only to manifest when grafted onto a sensitive rootstock. Howell calls that problem virus-induced incompatibility. However, the declining G.935 trees have been show- ing unusual symptoms, Howell said. For example, the graft unions on them appear normal, while virus-tainted combinations typically develop malformed growth around the graft union. Samples from one afflicted orchard have been sent to the Northwest Clean Plant Network, Howell said. Results were due back in mid-December, and Howell suspected they would still be inconclusive. Even if the tests detected a virus, that wouldn't prove the virus caused the decline. The Nursery Institute and the university are teaming up this spring for trials at Van Well Nursery in Wenatchee, Washington, to determine whether the problem is being transmitted through graft unions as a way to test the virus theory. Whatever the cause, the tree decline is real, Howell said. "Some very good growers are having trouble." Even experts struggle Taber's 300-acre diversified farm was the first stop on the two-day rootstock tour in Central Washington, where growers looked over numerous varieties on several differ- ent Geneva rootstocks. Trial blocks in Brewster, Orondo, Wenatchee, Mattawa and Wapato rounded out the field days. Taber participated in the trials, but also used G.935 commercially in another part of his ranch, drawn to the rootstock for its replant tolerance when he planned out roughly 35 commercial acres of Royal Reds. He put about 10 percent of the trees on G.935, the rest on M.9 Nic 29, a more traditional or "legacy" genotype. The traditional 90 percent grew just fine. In a Brewster orchard owned by Jim Divis, growers on the tour looked over wilted trees of the Pazazz variety on G.935. The leaves developed a purple hue and roots didn't grow, giving the tree a look similar to mouse damage. In the summer, leaves had turned pale green or greenish yellow with necrotic margins on the leaves, Auvil said. Even Auvil's own orchard in Orondo had problems when he grafted Granny Smith cuttings from 20-year-old trees onto Gala trunks on G.935. The trees grew poorly compared to similar aged trees nearby. "Looks like I will get to replace them," he said in self-deprecating humor for a collection of nursery owners touring his farm. Don't give up Fazio and Auvil don't suggest growers give up on new rootstocks, even the G.935, a semi-dwarfing rootstock noted for productivity, cold hardiness and resistance to fire blight and crown rot. With other varieties, some of the healthiest, most pro- ductive trees grew on G.935. For example, Galas and Fujis have made it past the four to five-year symptom thresh- old with no apparent trouble, Auvil said. In a rootstock trial in Brewster, Pacific Rose apples grafted onto trees planted as Fujis on G.935 had "no casualties," Auvil said. They recommend making sure scion wood comes from clean material, verified by the grower's local center of the National Clean Plant Network. For example, they expect no problems with Cosmic Crisp, or WA 38, the new variety from Washington State University for which only clean material is available from licensed nurseries. However, Auvil suggests growers with G.935 rootstock planted at nurseries to verify the scion wood's "heritage." If the planned wood is not clean, or has not been through the Clean Plant Center, consider budding WA 38 for it is as clean as the industry can provide. In general, G.935 still works in spite of the hiccups, they said. Indeed, growers on the tour heard plenty of good news about Geneva rootstocks. For example, a "replant in place" trial in Wapato in which G.935, G.214, G.41 and G.969 filled space and produced well in place of dead M.9 trees, probably hit by fire blight, Fazio said. Such news came as a relief to Fazio. "I don't want to talk about the 935 issue any more," he said. The crowd joined him in a sympathetic laugh. Divis, the Brewster orchardist, is a fan of G.969, citing how it produces a lot of fruit but leaves healthy fruiting wood for the following year. Meanwhile, G.41 has shown a high resistance to woolly aphid in trials, Auvil said. Even Taber, who lost 2 acres of trees to this mysterious tree decline, likes some of the results of the new rootstocks in his trial orchard. "There's a lot of promise in some of them," he said. • PHOTOS BY ROSS COURTNEY/GOOD FRUIT GROWER A Pazazz apple tree on a Geneva 935 rootstock shows signs of stress during a rootstock trial tour last fall in a Brewster, Washington, orchard. Researchers and growers have noticed mysterious tree decline when certain varieties and rootstocks are married. Pazazz, Royal Red Honeycrisp and Firestorm Honeycrisp on G.935 are among the problematic pairings. Oroville grower Dave Taber in front of a trial row of what used to be Royal Red Honeycrisps on Geneva 935 rootstock. He lost about 3,000 trees in the commercial portion of his orchard due to the troubled combination. U.S. Department of Agriculture researcher Gennaro Fazio examines a weak root structure of a Granny Smith tree grafted onto a Gala trunk on a Geneva 935 rootstock.

Articles in this issue

Links on this page

Archives of this issue

view archives of Good Fruit Grower - January 2017