By Dr. Matthew Borden
From the Turf October 2024 Issue | Tree Services Special Section
Beech are iconic trees known for their smooth gray bark, muscular roots, spreading canopy of vibrant green leaves, and use in many historic landscapes. Beech is also under severe threat from several important diseases. The result? There is a great need to care for them and to research how to address the stresses they face.
In eastern North American forests, we find American beech, Fagus grandifolia, sometimes called the “aristocrat of the forest!” Like oak, it’s considered a keystone species for its role in local ecosystems, providing vast crops of beechnuts in masting years, nesting habitat, and deep shade throughout the season. European beech cultivars we know as weeping beech, copper beech, tricolor beech, and fern leaf beech that are often prized for their form, leaf color, and stately nature.
As with all landscape trees, beech can have pests and diseases. Minor problems like aphids, caterpillars, and mites, cause little to no harm. A few diseases causing root rot and bleeding trunk canker are more challenging but can often be mitigated with good cultural care and minor intervention.
The first complex problem for beech began around 1890 with the introduction of beech scale, Cryptococcus fagisuga. Alone, the insect causes relatively little damage. Over the past century or so, the pest has moved at a slow pace of a few miles averaged per season, infesting new areas of American beech forests in what is called the “Advance front”.
Unfortunately, several closely related fungi (including Neonectria faginata and Neonectria ditissima) are considered associates of the scale insect. After 3-6 years of scale infestation, these fungi become established and cause full blown beech bark disease (BBD). While European beech show some resistance to the disease, mature American beech suffer high mortality. As the trees weaken from BBD, a swath of other organisms (e.g., boring beetles, diseases, decay fungi) may move in and finish them off, called the “Aftermath”.
BBD and the scale insect that facilitates the disease have spread south and west through much of the Northeastern U.S. and adjacent regions in Canada. They cause mortality in beech forests, although some trees naturally survive, and young trees are less susceptible. Unfortunately, another beech problem, beech leaf disease (BLD), began around 2012 and has spread at a more rapid pace. In fact, it’s already affecting beech in at least 15 states and Ontario. BLD is caused by a nematode, a type of microscopic roundworm. Unlike most nematode pests which infest plant roots (especially in turf), this species develops buds on beech twigs and causes unusual gall-like symptoms in the leaves.
While there are many unknowns about the organism and how exactly it spreads, evidence points to the species originating in Japan where it causes a non-lethal, minor form of the disease on the native beech. In North America, however, it causes a progressive disease that deprives beech leaves of their ability to produce food and energy for themselves. Over the course of several seasons and as the nematode populations build up, we see defoliation, bud abortion, and eventually branch dieback. As with BBD, other pests may take hold once the tree is severely weakened.
Addressing Problems In Beech Trees
Are you confident that you can recognize these beech problems? With beech scale, you will see cottony, white specks directly on the trunk, often running along vertical cracks that form as the insects’ feeding causes damage. The BBD-causing fungi that invade those feeding wounds transform smooth, gray beech bark with further cracking, mottling, crater-like scars, and cankers that can eventually girdle the trunk. In the late summer and fall, cankers may produce small reddish fruiting bodies. BLD can also be accurately diagnosed in the field once you know how to spot symptoms: dark green bands on leaves, and in later years severe leaf crinkling, stunting, and bare twigs as leaves and buds fail. There are some lookalikes, particularly a native aphid species that feeds on American beech and causes a chlorotic, yellow banded leaf symptom and leaf rolling. If in doubt, contact an arborist and a state plant diagnostic lab to confirm a diagnosis!
All this begs the question: should we avoid recommending and planting beech trees? For now, I say yes – at least in parts of the country where BLD is particularly likely to spread. Thankfully, there are good alternative shade trees. To fill large spaces over time, consider white oak, hickory, and katsursa tree. The weeping katsura is especially stunning and may help fill a gap left by a declining weeping beech.
Alternatives for beech hedges and smaller cultivars may include hornbeam, black tupelo, and Parrotia. Parrotia in particular are becoming known for their autumn color and few pest or disease concerns. If you are going to plant a beech tree, make sure it comes from a region unaffected by BLD. As with other nematode pests, there is a risk that a low number may be moved around prior to symptom development, and even if treatments are possible, you may want to avoid having a severe disease show up a season or two after planting.
BBD can best be avoided by treating the infestation of beech scale. The insect produces only one brood of eggs each season, around mid-summer. Therefore, insecticide treatments in late summer which target the young “crawler” stage can be effective. Once trees are ridden with fungi that follow, little can be done to slow progression. In contrast to the American beech in forests, European beech in landscapes is rarely severely affected. BLD has proven challenging to manage for both species – and this is where the team at the Bartlett Tree Research Laboratories has stepped up.
BLD poses unique management challenges and is unlike any other nematode pest we currently deal with in forestry, agriculture, or turf and ornamental industry. With only a handful of options to test, scientists at Bartlett began the process of lab and field trials to investigate possible treatments.
The first exciting development was the discovery of a treatment option that directly targets invasive nematode with a foliar application program, preventing them from infesting beech buds where the bulk of damage occurs. The key to this program is fluopyram. Luckily, the BLD-causing nematode, although unrelated to turf nematode pests, is extremely sensitive to this next-generation nematicide.
Trials have shown dramatic improvement in beech canopy health, provided that application timing is correct. Dr. Loyd and the team published this in the first ever collaborative BLD management research paper. Much more research has since been done to fine-tune the foliar program, and a second publication is forthcoming. Their latest discovery was introduced this summer: a different treatment using thiabendazole applied directly into the tree via injection systems.
This chemistry, formulated as Arbotect 20-S, is typically used for Dutch Elm Disease management, but also has powerful nematicidal properties with a history in anthelmintic medicine. The injection treatment shows excellent potential to suppress BLD in larger, mature beech trees for up to several seasons. This research is already in the publication process.
While these discoveries are not a solution to BLD at the forest ecosystem level, its immensely encouraging to know that some beech can be protected in places such as landscapes and arboreta, where they are valued as shade sources, historic specimens, and represent genetically diverse collections. These should be preserved for future work and future generations to enjoy. As with most invasive species, the scene will change over time and new tools may become available, such as resistant or tolerant varieties.
Dr. Andrew Loyd, plant pathologist and mycologist leading Bartlett’s BLD research, recently summarized the work saying, “At times in the past seven years while researching beech leaf disease, I’ve felt as if there would be no hope for management. But through painstaking, tedious work we have now been able to find two successful treatment options: a root flare injection suitable for large trees, and a foliar application program designed for smaller trees. And although frustrating at times, it has been a very rewarding process.”
Dr. Matthew Borden, DPM, is a plant pathology research scientist at Bartlett Tree Research Laboratories, a division of the F. A. Bartlett Tree Experts Company. Prior to joining Bartlett, Matt spent five years at the University of Florida in Dr. Adam Dale’s Turf and Ornamental Entomology Laboratory.
