Resistant rootstocks key to surviving fireblight
Malling 9 rootstocks are too susceptible to fireblight.
Within the next two years, Cornell University plant pathologist Herb Aldwinckle hopes scientists and the tree fruit nurseries will have solved the problems of propagating Cornell’s Geneva 41 rootstock—because, he thinks, that rootstock, and maybe others, is the answer to the problems of Malling 9.
Malling 9 dwarfs trees to the size that growers want, but the rootstock is susceptible to fireblight infection and to replant disease. Fireblight is an especially severe threat in the Midwest, Northeast, and Mid-Atlantic areas, and replant disease is a major headache in the Pacific Northwest. G.41, which is resistant to both diseases and the size of the medium-sized clones of M.9, could be the solution across the apple-production regions of North America, perhaps the world. Budagovsky 9, as a rootstock, is also resistant to fireblight, but it is not as good horticulturally as G.41 or M.9, Dr. Aldwinckle said.
“The only guaranteed method of fireblight control is the use of resistant rootstocks,” he said.
Right now, G.41 is in short supply because nurseries have trouble propagating it in stoolbeds. “We’re making a considerable effort to get supplies pumped up,” Aldwinckle said. “Supplies of G.41 are still not adequate to meet demand, and it will be a couple more years before they are.”
Aldwinckle was one of the original researchers who began working on rootstock breeding at Cornell 40 years ago. It was, he said, the first rootstock breeding program to focus on fireblight resistance as a top priority. But the breeders also wanted to avoid other problems—like excessive sucker production and burr knots. It turns out, he said, that the characteristics that discourage burr knots and suckers also make those rootstocks harder to propagate.
“The goal was to make rootstocks that would help growers, and they didn’t want suckers and burr knots,” he said. Now, work at Cornell is looking at ways to make G.41 easier to propagate in the nurseries. The rootstock breeding program includes teams led by plant pathologist Aldwinckle, horticulturist Dr. Terence Robinson, and USDA plant geneticist Dr. Gennaro Fazio.
Also part of the breeding program was a focus on Phytopthora, which causes root and crown rot. That focus apparently also gave a significant level of resistance to replant disease, of which Phytopthora can be one element.
Since the breeding work began, the problems of fireblight and replant disease have gotten worse, not better. Most of the modern, new high-quality varieties are more susceptible to fireblight than is Red Delicious, a variety they are replacing. Replacing orchards means replanting on previously used sites, so replant disease is more of a problem now than it used to be.
In addition, the new high-density plantings have made these diseases much more expensive in their impact. The new shape of these trees—the short scaffold limbs—have made it more likely that fireblight strikes on the blossoms and shoots will make it to the trunks, where they can kill the tree or travel down the trunk and kill the rootstock.
“With smaller trees, fireblight goes further into the structure—and there is less tree to be killed,” Aldwinckle said.
Resistant rootstocks will not protect susceptible scion varieties, he said, but a dead rootstock won’t support any scion variety at all. If the rootstock survives after fireblight infection, it limits the damage if the tree can be pruned or topworked and restored to production. The use of renewal pruning, as used in the high-density plantings, also makes it easier to take out infected shoots without affecting the overall structure of the tree.
Aldwinckle has a sizable planting of trees at Cornell’s New York State Agricultural Experiment Station in Geneva, New York. There, he evaluates rootstocks and conducts studies on how fireblight kills trees. He showed visitors his fireblight-infested orchard during the International Fruit Tree Association tour there last summer.
He has found that the bacteria that causes fireblight, Erwinia amylovora, can get to the rootstock by any of several avenues. It can move from blossom infections or shoot strikes to the trunk, and then it moves downward. “Rootstock blight symptoms will generally persist only in the main shank and crown of the rootstock and usually do not progress into the outlying root system or up into the scion,” he said.
“Bacteria move within the vascular system of the tree without causing visible necrosis. Once bacteria gain entry into the rootstock, no treatment is available to prevent the development of rootstock blight.”
Bacteria can enter the rootstock through open wounds or insect feeding sites, and through rootstock suckers, but the most significant source, he said, is migration from infected blossoms in April and May and infected shoots in June.
There are several tools growers can use to reduce the risk of fireblight.
Forecasting systems can tell growers when infection is most likely, and they can apply antibiotic sprays. Shoot blight can be reduced by applying the plant growth regulator Apogee (prohexadione calcium) to reduce shoot growth.
Suckers should be removed to avoid the risk of rootstock infection, he said. “We also need to be serious about borers,” he said. “We need to use trunk sprays at the time of borer flight to prevent the bacteria from entering the rootstock through those wounds.”
Geneva 41 is not the only rootstock that is resistant to fireblight. In evaluations Aldwinckle has done at Cornell, the rootstock Budagovsky 9 has proven resistant in orchards, although for some unexplained reason it did not show resistance in screening tests. “Absence of rootstock blight in the field is so consistent that we now recommend B.9 to growers as a fireblight-resistant rootstock to replace M.9,” he said.
Geneva 11, another Geneva rootstock close to M.9 in size, is rated as tolerant to fireblight (it is not quite as resistant as G.41), and G.16 is resistant. Other Geneva rootstocks of other sizes are also resistant.
The Cornell program is evaluating rootstocks from Vineland in Canada, from Poland, from Pillnitz in Germany, from Marioka in Japan, and others. Two M.7-size rootstocks in the Pillnitz program, PiAu-51-4 and PiAu-56-83, have been found to be resistant to fireblight, as has the M.26-sized B.62.396.