Kirtland, Ohio — A new study led by scientists at the Holden Arboretum has found evidence that beech leaf disease (BLD) symptom development is genetically controlled, providing the first evidence that some American beech trees may have genetic resistance. This finding supports further investigation and development of breeding populations for BLD resistance. Nearly a third of beech trees near BLD's epicenter in northeast Ohio, the majority of which were smaller trees, have died since the disease it was discovered in 2012.
The new research, published in the journal Forest Pathology , was led by David Burke , Vice President for Science and Conservation at Holden Forests & Gardens, along with Mary Mason and Jennifer Koch of the USDA Forest Service's Northern Research Station.
For years, forest professionals in the region have noticed that in BLD-affected areas, not every beech shows the same severity of symptoms. They've been documenting and monitoring trees that appear healthier in otherwise heavily symptomatic areas, building a roster of these special trees. Such trees will be conserved for use in further experiments to determine if genetic resistance to BLD is a genetic trait that can be passed from parent trees to their progeny, an essential step required in the development of a breeding program to create resistant trees.
A Serendipitous Experiment
An existing research planting of American beech (Fagus grandifolia) at the Holden Arboretum provided a rare opportunity to test beech for genetic resistance to BLD. The trees were established years before BLD arrived, in 2005, and had been selected, bred, and tested by the USDA Forest Service for resistance to a different disease, beech bark disease. Once BLD arrived, Burke and his colleagues noticed that not all the trees were showing symptoms.
"We noticed that some trees were very healthy, even though many others were looking really sick like we'd seen in the adjoining forests," Burke says. "These trees still looked really good despite being 20 feet away from trees that were very clearly ill and unhealthy."
Because these trees were already growing in a research planting that provided a relatively controlled, homogenous environment — same soil, same light, same overall conditions — they suspected the differences in disease severity could be due to their underlying genetics.
Testing Trees for Genetic Resistance
To test for genetic resistance, the team propagated both sick and healthier trees using cut branch tips, grafting them onto other untested seedlings (rootstock), and grew them in the greenhouse under controlled conditions. Then the team tested the trees by transferring BLD-causing nematodes, Litylenchus crenatae, into the buds.
Researchers collected nematodes by gathering infected beech leaves in the forest, cutting them open in the lab, and floating them in water to extract the wormlike organisms. They counted them under a microscope to make sure every beech bud would receive the same "dose" of 100 live nematodes. The nematodes were transferred in water to beech buds in the fall. Infested trees were kept in the greenhouse overwinter and symptoms of BLD were measured when trees leafed out in the spring. When spring arrived, Burke says, the differences in symptoms on the different American beech were subtle but were clearly significant after the researchers measured the affected leaf area on each tree.
"We recorded the percentage of the leaf that actually displays the dark interveinal banding that's characteristic of beech leaf disease," explains Burke. Different beech genotypes showed different degrees of symptom development, indicating genetics plays a role in symptom severity. Since the trees were known to have some BBD resistance, this result gives hope that trees with good resistance to both diseases could be developed.
Nematodes at Work
This experimental setup also allowed the team to confirm that live nematodes alone, and not any bacteria or other microbes that might be associated with them, are what cause BLD symptoms. This rules out competing theories about the disease's cause.
The team also tested European and Japanese beech to see how they'd respond to the same levels of nematode exposure. European beech (Fagus sylvatica) fared similarly to American, developing symptoms on 40–50% of leaf area on average. But the beech from Japan (Fagus crenata) — where the BLD nematode is native — only showed symptoms on about 4% of overall leaf area. "It was very stark," says Burke. "It suggests that Japanese beech is resistant to or tolerant of the nematode, which is not surprising since they share a native range."
Next Steps
Next, the team will continue their search for low- and no-symptom American beech in forests where BLD has been established for at least a decade. Additional trees, and more propagated copies per tree, need to be tested to confirm the genetic control and better understand its variation and strength. Ideally the best trees might be control pollinated to each other to confirm genetic inheritance of control and possibly increase overall resistance to both diseases.
Holden researchers are also conducting a multi-year trial testing the effectiveness of treatment with a potassium fertilizer called PolyPhosphite 30® that may reduce disease severity, with results expected sometime next year.
"Beech is an important foundational forest tree species," Burke explains, particularly in beech-maple forests, where beech nuts are a critical food source for a wide range of wildlife. "The loss of beech from these forests could have really devastating impacts."
Want to help? Members of the public who spot healthy American beech trees in long-affected areas (e.g. Ohio, western Pennsylvania, western New York) can report them to researchers using a free app called TreeSnap
