Four rootstocks were released this winter where they join seven others in the stable of Geneva apple rootstocks. Three of the new ones are more vigorous and larger than Malling 9, and one is in the size class with the M.9 Pajam 2 clone, which is slightly more vigorous than the popular M.9 T337 so many growers are planting.
Dr. Gennaro Fazio, the USDA Agricultural Research Service plant breeder who has headed the program in Geneva, New York, since 2001, described the new rootstocks. The Geneva program, which began in 1968, has focused its efforts on finding replacements for the rootstocks M.9 and M.26, looking for resistance or tolerance to fireblight, woolly apple aphid, crown rot, and apple replant disease.
Other goals, Fazio said, are to find rootstocks that are as precocious as M.9, with yields and production efficiencies of 110 to 160 percent of M.9, a low incidence of burr knot and suckering, and cold hardiness.
In addition, he said, the search is on to find rootstocks that might solve specific problems of some popular scion varieties. “Might we be able to correct Honeycrisp problems with a high calcium-uptake rootstock?” he asked. Rootstocks that have the ability to more efficiently absorb micronutrients like magnesium or that tolerate high or low pH could be useful to growers.
The new rootstock that is in the M.9 Pajam 2 rootstock vigor class is G.214, which Fazio described as highly yield efficient and productive with yields of 100 to 125 percent of M.9 in most U.S. trials. It is precocious, resistant to fireblight, crown rot, and woolly apple aphid, and propagates well in the stool bed.
The three more vigorous new rootstocks, in the size class between M.7 and MM.106, are G.890, G.969, and G.210. They are possible replacements for G.30, which has a brittle graft union and is prone to breakage. All have yield efficiency similar to or better than M.9, Fazio said, plus resistance to woolly apple aphid, fireblight, and crown rot, and tolerance to apple replant disease. They root well in the stool bed.
While rootstock resistance to fireblight does not prevent infection of susceptible scion cultivars, it does prevent total tree loss and gives growers the opportunity to prune out dead limbs and oozing cankers and save afflicted trees.
A single fireblight epidemic in Michigan in 2000 killed 220,000 trees, Fazio said, and caused losses estimated at $42 million. He estimated total annual losses from fireblight in the United States at $100 million. M.9 and M.26 rootstocks are susceptible to fireblight, and all of the Geneva rootstocks are tolerant or immune.
The rootstock G.11, released in 2001, is the same size as M.9 T337 and has gained popularity. Fazio said 220,000 trees have been planted on the rootstock so far.
“We have the capacity to produce more than a half million trees on Geneva rootstocks every year now, and we’re hoping to reach into the millions in a few more years,” he said. “We are micropropagating all of them, and six nurseries are licensed to produce liners of Geneva rootstocks.” Those six are Copenhaven, Cummins, Treco, Wafler’s, Willamette, and Willow Drive. All nurseries in the United States can grow trees on these rootstocks as long as the liners are supplied by licensed nurseries.
The line-up of Geneva rootstocks, several of which were released in 2004 and 2005, includes:
G.65, the smallest, which is M.27 in size.
G.11, just slightly less vigorous than M.9 T337 and similar in productivity, with large fruit size. It is resistant to fireblight but not immune, and resistant to crown rot but not tolerant to the replant disease complex. It roots well in the stool bed.
G.41 and G.16 are intermediate in size between M.9 T337 and M.9 Pajam2. G.41 is highly yield efficient, very precocious, very cold hardy, immune to fireblight, crown rot, and woolly apple aphid, is replant-disease–tolerant, and does well in warmer climates like Mexico as well. “It has some issues with propagation that are being resolved,” Fazio said.
G.935, slightly more vigorous than M.9 Pajam 2 and about the same size as the new G.214 and slightly larger than M.26, is very cold hardy, resistant to fireblight and crown rot, tolerant to replant disease, but is susceptible to woolly apple aphid. “It has been the best semidwarf rootstock in New York trials,” Fazio said.
G.202, similar in size to M.26, is described as precocious and productive, resistant to woolly apple aphid, fireblight, and crown rot, tolerant to apple replant disease, and moderately easy to root in a stool bed. It was released in 2002 in New Zealand, where it has been a top performer, and then in the United States in 2004. “It is a good choice for weak-growing cultivars like Honeycrisp,” Fazio said.
G.30, the size of M.7 to MM.106, has had problems with brittle graft unions, and three of the recent new releases are potential replacements in that size category.