“Every Fraser fir tree we’ve inoculated, we’ve killed,” stated Dr. John Frampton of NCSU, forest geneticist and Christmas tree researcher. He has long been on a quest to find a Fraser fir with some degree of genetic resistance to the virulent Phytophthora cinnamomi.
Frampton continued, “2012 was a really rainy year, and we saw the disease spread rapidly.” Hurricane remnants can saturate the soil for extended periods in western North Carolina, making conditions perfect for the growth of this water mold and for its colonization of new host trees and the soil around them. Unfortunately, it seems that all Fraser firs, which are native to the area and prized for Christmas trees nationwide, are vulnerable. “We’ve inoculated Fraser firs from diverse backgrounds repeatedly, and they all died.”
For North Carolina’s Christmas tree growers, for the long term, Frampton has high hopes for Turkish fir from Asia. Many individuals of this species have a high degree of resistance to Phytophthora inoculation, despite deliberately harsh conditions designed to make sure the trees that survive are truly resistant.
Frampton has made two trips to Turkey to collect seed from GPS’d mother trees, and NCSU is part of a national replicated trial using some of that seed to see which mother trees produce seedlings that are both Phytophthora resistant and well-adapted to North Carolina growing conditions.
In the meantime, until those studies are complete, Frampton and colleagues have worked out some other possible solutions for their growers with soil badly contaminated by Phytophthora. (The overwintering spores of this water mold are extremely long-lived in the soil.)
In an earlier study, Frampton collected and tested 32 different species of fir trees from around the world for Phytophthora resistance. “The most consistent survivor of inoculation with Phytophthora cinnamomi,” he explained, “was momi fir from Japan.” Growers in North Carolina, as a result of these studies, can take the rootstock of momi fir, which confers Phytophthora resistance, and graft a Fraser fir branch on top to successfully grow Fraser fir Christmas trees in badly contaminated soil.
However, the increased cost for grafted seedlings is a problem for growers already short on profit margin. “Some growers are using grafted trees,” he continued. “Probably they are mostly choose- and-cut growers with a fixed land base, who would be targeting fields where they’re having trouble keeping Frasers alive. Larger wholesalers, faced with Phytophthora-infested soil, would likely just lease a farm somewhere else.”
The resistance Frampton found in momi fir has found other uses among his growers. “In eastern North Carolina, because momi fir also happen to be very heat tolerant, growers in the Piedmont have found momi to be the only fir species they can grow to offer their choose-and-cut customers, planted with mixed white and Virginia pines and Leyland cypress. Although momis stand out as very different, they can be sheared to look like a traditional Christmas tree, and some customers like them, despite their prickly needles.”
Nordmann fir are being used in Phytophthora- infested fields by some North Carolina Christmas tree growers, with some success. “In our testing at the University,” Frampton explained, “some Nordmann seedlings are resistant to our phytophthora here, and some are not. We’ve found that about 40 percent are resistant under our admittedly harsh test conditions. But if a grower has a less severely infested site, with drainage that is not too bad, Nordmanns may offer an alternative fir species that can be grown.”
Frampton, however, is betting on Turkish fir for the future. “We have quantified genetic resistance in Turkish fir. About 60 percent of them are resistant. The rest are completely susceptible. They either survive inoculation, or they die.
“Because we have kept separate the cones that came from each GPS’d mother tree in Turkey, as we learn which mother trees produce a higher percentage of resistant seedlings, we can work with those. We’re still trying to determine the mode of resistance to Phytophthora on a genetic level.”
Frampton hopes in the future to be able to find genetic markers that can be used for an in-the-field test on mature trees in research field trials to determine if a given tree has the desired genetic resistance. (He and his team already appear to be closing in on genetic markers that can be used for an in-the-field test for needle retention.)
Frampton is also working with Trojan fir, although he has found this species to have less resistance than Turkish fir under North Carolina conditions, but more resistance than Nordmann. “We don’t understand why any of these fir trees are resistant,” he continued. “Phytophthora cinnamomi is not native to their natural growing areas in Turkey. There is an interesting geographical pattern to the resistance of seedlings from mother trees in that area. As you go east in the mountain range, the individual trees have a higher frequency of resistance than those from the western part of their range, regardless of the elevation.
“These Asian firs have apparently adapted to some combination of soil type and/or rainfall pattern and/or other pathogen in the soil that helped to confer this resistance. Will Kohlway, the graduate student working on this is making progress. We just got back a large data set of DNA sequences, and we’re doing the bioinformatics on that data.
“It’s a slow process,” continued Frampton. “We inoculate the roots, collect the DNA, and then send that to the lab for sequencing. Since there is no cost-effective cure for Phytophthora, the best way to tackle the problem is to find resistant tree species.”
The severity of problems with a given Phytophthora species varies with the temperature, the amount of rainfall, and the soils, graduate student Katie McKeeever at Washington State University has found in her extensive research on the subject. According to Michigan Christmas Tree Extension Educator Jill O’Donnell, “On our sandy soils that are well drained we can grow Fraser fir with no difficulty. On our heavier soils, our growers can plant other types of Christmas trees. Fortunately, they have a market for a variety of pines, spruces, and other firs for Christmas trees. In addition, they also produce trees for the landscape market.”
It helps that the Phytophthora cinnamomi that plagues North Carolina growers doesn’t overwinter in Michigan soils, although there are other species of Phytophthora that do. The cold-hardiness of the Turkish fir mother trees from which seed has so-far been collected is also a consideration in Michigan. “We have had two very cold winters since we planted the Turkish fir trials. We have seen winter injury in both our trial locations, one in the central and the other in the northwestern part of the Lower Peninsula,” added O’Donnell.