Beech Leaf Disease in Maine

There is a lot that is still unclear about BLD. We do know:

• BLD can kill American beech, European beech, and many other beech cultivars of various origins;
• BLD symptoms are associated with the presence of a non-native foliar nematode (microscopic roundworm), Litylenchus crenatae mccannii;
• Disease symptoms tend to begin in smaller beech trees in the understory, however larger trees are also vulnerable to attack and decline;
• BLD may weaken beech trees over time, making them more vulnerable to secondary pests.

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All beech trees (Fagus spp.) are susceptible to BLD, including European and Asian species (this includes beech cultivars available in the nursery trade).

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American beech in Maine is already greatly impacted by a different invasive forest pest. The beech scale, introduced near the turn of the last century, makes beech vulnerable to attack from native fungi and the two together result in beech bark disease. This disease has made beech undesirable in the eyes of many who own and tend forests because thickets of diseased beech develop after some types of harvests. Even in this compromised state, beech plays an important role in Maine’s forests.

Beech is well known for producing beech nuts, the most nutritious of any of our hard mast species (‘hard mast’ is a general term describing nuts produced by forest trees, for example, oak acorns). Beech nuts are an important food source for many wildlife species. The early hairstreak, a rare butterfly; and the black bear, one of Maine’s iconic large mammals are two species that prefer beechnuts. Beech is also particularly important as a species that provides cavities for wildlife nesting or denning.

Even compromised by disease, beech is still used for wood products such as furniture and flooring among others.

More about American beech and wildlife (PDF | 3.61 MB) from the University of Maine and Maine Department of Inland Fisheries and Wildlife.

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• How the disease spreads;
• Details of how the disease develops and impacts forest ecosystems and how that development is different in stands already ravaged by beech bark disease;
• Whether other organisms (bacteria or fungi for instance) are needed for disease development or if the nematode can cause disease without other agents;
• If there are other plants that could be impacted by the nematode or organisms associated with them;
• If there are effective ways to manage the disease (several methods are being tested for use in ornamental trees);
• If there are any beech trees with tolerance of or resistance to BLD.

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Symptoms of BLD include:

• Dark bands between the veins of leaves;
• Leaves are cupped, deformed, shriveled and may be smaller than usual;
• Leaves can be thick and have a leathery texture, unlike the papery texture of healthy leaves;
• Premature leaf drop;
• Aborted buds leading to sparse leaf cover;
• Thinning canopy.

Images: (top left) Dark interveinal bands indicating BLD infection as seen looking up into the canopy from the understory; (top right) The full range of upper leaf surface symptoms from undersized leathery leaves to banding at various severity levels to asymptomatic leaves, all on the same branch; (bottom left) Dark banding and leaf deformation associated with BLD infection; (bottom middle) BLD symptoms as seen on winter leaves persisting on beech trees; (bottom right) European copper beech impacted by BLD, showing green discoloration, leaf deformation and interveinal banding.

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There are several other organisms that can change how beech leaves look. Below are the most commonly encountered and reported BLD look-alikes in Maine.

Woolly beech aphid

Woolly beech aphid (Phyllaphis fagi) causes leaf deformation and discoloration like BLD, however the leaves are often rolled or curled and do not show banding in between leaf veins like leaves impacted by BLD. Further, the discoloration from woolly beech leaf aphid is usually characterized by yellowing of the affected area and leaf margins. A clear sign of woolly beech aphid activity is the waxy filaments shed from the insects on the leaf underside. The damage caused by this pest is not serious and in most cases impacts the tree very little.

Erineum gall

Erineum gall damage occurs interveinally (between veins) like BLD (left picture), however the banding is not dark, and white to red velvety patches (erineum) are typically seen on leaf undersides (right picture). The symptom is caused by the activity of microscopic eriophyid mites (family Eriophyidae) that alters leaf growth. The damage caused by this pest is not serious and impacts the tree very little.

Beech anthracnose

Beech anthracnose (Discula umbrinella) causes various degrees of discoloration and deformation of leaves like BLD, but the fungus causes lesions (often round) leading to dead brown leaf tissue surrounded by yellow discoloration of leaf tissue (a halo) and no banding is associated with beech anthracnose. When symptoms are much more severe than those pictured, beech anthracnose can cause significant defoliation.

When in doubt, please report it!

Symptoms and signs of BLD can vary, and unusual leaf symptoms could indicate other tree health issues. Also, BLD symptoms have been seen on the beech leaves impacted by other organisms, including these look-alikes!

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Maine Forest Service will continue to monitor developments as more is learned about this disease. The following management recommendations are still experimental but show promising results.

A method for BLD management has been trialed in Ohio since 2015 and has shown positive results. The treatment involves a specific soil-drench-applied polyphosphite fertilizer. While the treatment does not eliminate the BLD causal agent or symptoms, it has been shown to decrease nematode abundance and limit symptom severity and thus maintain more of treated trees’ functional leaf area, presumably limiting the stress caused by severe BLD infestation. Multiple treatments may be needed before clear results are seen. The following PDF link outlines this treatment: Treatment of Beech Trees with Beech Leaf Disease in Maine (PDF)

The fungicide Arbotect 20-S with the active ingredient Thiabenzadole has an approved Special Local Needs label in Maine for the treatment of BLD. This product is also used to prevent Dutch elm disease in elms. It is not certain exactly how Thiabenzadole prevents BLD symptoms, but trials in other states have been promising when applied correctly via macroinjection. Two years of protection have been observed in those trials. A summer application of Arbotect 20-S will not cure symptoms in the year of application but should limit symptom development for the following two years. Those interested in treating their beech via this method are encouraged to contact a licensed pesticide applicator to see if they have the equipment and know-how to apply this treatment.

Pruning infected landscape trees may decrease the duration of foliar surface moisture and thus reduce BLD symptom severity since the causal agent nematode requires water for movement and other life processes. Management recommendations for horticultural settings are developing as more is learned about the disease. Trials of additional chemical applications are ongoing.

To slow the spread of BLD, consider:

• Avoid moving beech tree material including branches, twigs, leaves and seedlings and nursery stock from affected areas.
• Closely inspect any beech nursery stock for symptoms of BLD before purchase and outplanting.
• Avoid moving soil or other organic matter from affected areas.

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At this time we are not exactly sure how BLD-caused decline and mortality will advance in Maine. The disease will likely be fatal to smaller trees in the shorter term, but the longevity of more mature trees is unclear. Since BLD was found in Lincolnville in 2021, the Maine Forest Service has been closely following disease progression and impacts on nine long-term monitoring plots using annual tree-health measurements. Monitoring efforts in 2023 did not record any mortality of mature trees in dominant or co-dominant positions in the forest canopy. Based on observations from other states that have had BLD longer, mature beech will also eventually die. The timeframe of decline sometimes leading to mortality for BLD-infested trees in the Lake States, for example, is about 5-7 years. Observations there have indicated that chronic stress caused by BLD has often led to attack by secondary agents of decline and mortality like root rots, canker fungi and insects. In these cases BLD has not directly killed beech trees but has been a major contributor to mortality. The presence of beech bark disease in Maine’s forests may significantly influence the rate of decline of beech trees also infected with BLD.

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Tree removal is left to the discretion of the landowner. Trees infected with BLD can persist for several years depending on their general vigor, with lower-vigor trees typically declining more quickly. The impacts of BLD often reduce tree services like shade and aesthetics such that removal and replacement are practical decisions. Also, as trees decline and eventually die, they may develop rot that makes them less structurally stable. Removal should be considered for trees near structures or in high-use areas.

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Pre-emptive harvest of beech is not recommended. Oftentimes genetic diversity within tree populations leads to varying impacts from pests and diseases. A great example of this is seen in American beech either tolerant of or resistant to beech bark disease. Reactive harvesting can reduce or eliminate genotypes that show disease tolerance or resistance.

Research efforts on BLD management strategies, including silvicultural trials, are underway and could provide useful information towards addressing BLD-impacted stands. Landowners and land managers should carefully consider their beech resources and have a plan in mind in the event that practical measures to manage BLD are not found and trees continue to decline. If beech eventually die due to the impacts of BLD infection, some trees could be left for cavity-nesting wildlife, as they provide excellent habitat for several species.

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In the yard setting, the best thing to do with all plant material that is infested with BLD is to keep it on site and not transport it to new areas. Composting or burning leaves on site is recommended (make sure to acquire a Maine Forest Service Open Burn Permit). Since the threat of spreading BLD on wood and branches is unclear, precaution should be exercised. Chipping or burning branches is recommended. Wood chips from BLD-infested trees are safe to use on site. Use the firewood from harvesting BLD-infested beech locally, preferably on site.

In the forest setting, beech branches and other plant material should be left on site. There are currently no regulations concerning the movement or processing of BLD-infected wood. The most cautious approach would be to limit movement of beech material harvested from infected stands to areas already known to have BLD.

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While more is being learned about BLD’s mode of spread, there are still many unanswered questions. Natural resource organizations in some states where BLD is found have encouraged people to clean their boots in between moving from sites where BLD is present to other areas, as BLD nematodes and their eggs could be transported in plant material, for example, stuck in muddy boots or on tools and equipment. Cleaning boots, tools and equipment before heading to new sites, trails, etc., is typically a good practice to prevent the transport of invasive plant seeds and other unwanted organisms like jumping/snake (Amynthas) worms.

Moving any plant material from the genus Fagus (the beech family), especially living trees, seedlings or saplings, is strongly discouraged due to the potential to spread BLD to new areas.

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The Maine Forest Service is asking for the public’s help in identifying additional areas impacted by BLD. Photos of suspected symptomatic leaves can be submitted via our online form; reports can also be made to foresthealth@maine.gov or by calling (207) 287-2431. If possible, photos should include a clear shot of the underside of an affected leaf or leaves, however, please report concerns even if those photos cannot be provided.

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The USDA Forest Service has developed a Pest Alert on the disease (PDF | 2.19 MB), Cleveland Metroparks, Don’t Move Firewood and NY DEC have informative pages. 

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