A rootstock breeding program that has released seven fireblight-resistant rootstocks shows no signs of slowing down. Thousands more genotypes are in the pipeline for testing as scientists look for rootstocks resistant to other diseases and insects.
Since 1968, Cornell University has supported an apple rootstock breeding program located at Geneva, New York. In the beginning, Drs. James Cummins and Herb Aldwinckle headed the project, searching for rootstocks resistant to fireblight.
Today, the rootstock program is a joint effort between the U.S. Department of Agriculture and Cornell, with Dr. Gennaro Fazio leading the program. Cornell’s Dr. Terence Robinson and Cooperative Extension educators contribute to the program by testing promising selections in field trials.
Rootstock genotypes used in the crosses come from breeding programs around the world. Wild apple planting material is also used, collected from remote forests.
Advanced selections are tested in the United States as part of the NC-140 Regional trials, as well as in foreign countries. Geneva selections are also part of two field trials in Washington State that are part of a collaborative effort between USDA and the Washington Tree Fruit Research Commission.
“The exciting thing now about the rootstocks is that we are testing them in replant situations and are seeing some growth,” said Fazio, who joined the program in 2002.
New material from wild apples of Kazakhstan show strong immunity to Rhizoctonia, which is thought to be one of the causes of replant disease, he explained. Many of the seedling selections in the program also show resistance to crown rot (Phytophthora).
He is pleased with results now coming from some of the new crosses of genotypes. Typically, when seedlings go through the disease screening stage, 90 percent of the seedlings are eliminated when they are exposed to phytophthora. But seedlings from the new Kazakhstan crosses are showing 50 to 60 percent resistance, a marked improvement in the initial screening of potential selections.
While the breeding program is having success with fireblight, crown rot, and now replant diseases, Fazio noted that there is no resistance in apple rootstocks to combat nematodes. “Genetically engineered rootstock will be the only way to deal with nematodes.”
Fireblight is a problem in many apple-producing regions of the world, but is particularly threatening to growers of high-density apple orchards on dwarfing rootstocks. Malling 9 and 26, used in many orchards, are very susceptible to the bacterial disease. In some eastern U.S. growing regions, entire orchards planted to dwarfing rootstocks have been lost to fireblight.
Fireblight bacteria usually infect trees through blossoms, causing shoot dieback. As bacteria move down the shoot and trunk, they infect the union and rootstock. If the rootstock is susceptible, the tree will die. Resistant rootstock won’t protect the scion from infection, but will allow the grower to save the tree. Infected shoots can then be pruned out and the tree regrown.
Of the seven Geneva rootstocks patented and released for commercial propagation by licensed nurseries, three are sold commercially, and two were just released last year. All seven are resistant to fireblight and crown rot, and some show resistance to replant disease and specific pests.
Cornell University’s Robinson, who collects field data on advanced rootstock selections, identified two releases—Geneva 11 and 41—that show potential as replacements for M.9 and M.26 in fireblight-prone areas. They are both dwarfing, producing trees similar in size to M.9 and M.26, with good productivity. Field tests in France show encouraging results.
It will take a while for nurseries to get trees into the hands of growers, Robinson said, but he anticipates that around 200,000 liners of G.11 could be available for sale this winter.
“I hope a few growers throughout the United States, including Washington, will test the rootstocks,” he said.
He expects interest to increase once the industry learns more about the new rootstocks.
Geneva 30, a productive semidwarfing rootstock that is resistant to fireblight, phytophthora, and replant disease, has been out for about 10 years, he noted. However, nurseries only plant about 100,000 rootstocks to be offered for sale each year.
With recent development of disease-resistant rootstocks, growers have options that never before existed. The rootstocks will help minimize some of the risks involved in planting new orchards or replanting old ones.
No single answer
But like all rootstocks, the new ones are not faultless. Each has strengths and weaknesses that must be understood by growers and nurseries. Some are sensitive to latent virus in the scion wood. Others need a multi-wire trellis to support the trees.
“There is no one perfect apple rootstock that will do everything everywhere,” Fazio said, adding that each grower needs to find what works best for his or her situation.
“We do need to have many more than just one rootstock available. If we only plant one rootstock, we are setting ourselves up for problems, like the grape industry had with phylloxera. We need diversity and a range of rootstocks.”
And that is why Fazio continues to look for other valuable traits waiting to be discovered in new rootstock crosses.