Beech Leaf Disease

Research supports that Litylenchus nematodes are associated with the disease but are unlikely to be the sole vector of infection. Further research is being conducted to determine how BLD spreads between host trees and if there are any other symbiotic relationships that are required for this transmission of this disease from tree to tree. It is possible that the nematode itself may produce a toxin or is associated with another pathogen that leads to the infections observed in BLD infected trees (Carta et al., 2020).

In Ontario populations, Litylenchus crenatae ssp. mccannii can be found on symptomatic leaves between June and August. The nematodes are mainly associated with symptomatic leaves but are also found in much smaller numbers on asymptomatic leaves. From November to March, nematodes were associated with buds and detached leaves more often than they were associated to attached leaves (Reed et al., 2020).

BLD appears to spread very quickly, characteristic of an invasive pathogen, and spread does not appear to be influenced by slope, aspect, or soil conditions. Other nematodes can be spread by mites, insects, animals and in infected plant material. Where BLD is established, nearly 100% of American beech trees show symptoms (Pogacnik & Macy, 2016).

Infected leaves my look similar to other pest and pathogen damage, such as beech blight aphid, European beech scale, erineum patches, and other leaf fungi like anthracnose (Carta et al., 2020).   The presents of BLD may increase susceptibility of beech trees to these other forest pests and pathogens causing greater mortality.

BLD is found mainly infecting American beech (Fagus grandifolia) in the forest which are native to North America but has been reported on European beech (Fagus sylvatica) and other ornamental beeches in nurseries.  This indicates that the risk of the disease may extend to other species in the genus Fagus spp.

Symptoms of BLD are only observed on leaves and buds.

In early stages:

• Striping appears on the leaves, caused by darkening between the leaf veins.
  • The darkened area is slightly raised and is thicker than the rest of the leaf tissue.
  • The striping only appears between the veins.
  • This is most apparent when viewing from below.
  • Striping is present upon leaf-out in the spring.
  • Striping is noticeable throughout the summer and winter months
• Very little premature leaf drop occurs during early stages

Disease progression varies based on tree size, with most mortality occurring in saplings. In saplings, progression from early stage symptoms to severe late stage symptoms is rapid and mortality can occur in as little as one year. In larger trees, the disease progresses more slowly, beginning in lower branches and moving upward. On a single tree, some branches may be unaffected where others are heavily infected. Young trees seem to be more susceptible to mortality than mature trees.

As the disease progresses:

• Leaves near branch tips will become shrivelled, discoloured, and deformed.
• Reduced bud and leaf production will occur

Late stage leaf deformation, photo by John Pogacnik, Ohio Department of Natural Resources

Erineum patch

Powdery Mildew

Anthracnose

BLD was first discovered in Lake County, Ohio in 2012. It is now found in Ohio, Pennsylvania, Connecticut, and New York in the United States. The first detection in Canada was in Elgin County, Ontario in 2017, and has now been detected in four additional counties. Counties where BLD is present in Ontario include Elgin, Chatham-Kent, Norfolk, Oxford, Lambton, Niagara, Haldimand, Middlesex, and City of Toronto. As this disease spread North and Eastward in Ontario new detections become difficult when beech trees are affected by other diseases such as, Beech Bark Disease. Currently BLD infects a small numbers of beech trees, making monitoring for this disease essential for early detection.

Beech leaf disease map showing the spread of the disease between 2012 and 2023. Source: www.clevelandmetroparks.com

Beech trees are already at risk from beech scale and beech bark disease. BLD not only causes juvenile tree mortality but also makes trees more susceptible to other pests and pathogens.

Economic Impacts

Researchers in Ohio have used an online tool to predict economic losses if half of the beech trees in the state were lost. This amounted to $225 million USD in ecosystem services such as water filtration, biodiversity maintenance, carbon sequestration, and aesthetic/recreational value (Ohio State University, 2019). Beech trees are distributed throughout the Great Lakes-St. Lawrence and deciduous forest regions of southern and central Ontario and reduced availability of healthy beech could have similar effects on Ontario’s economy.

Ecological Impacts

Beech trees are valuable nut-producing trees and provide food for many forest dwelling animals. Black bears, for example, rely heavily on beech nuts for food in northern hardwood forests. Loss of beech trees can also cause shifts in forest structure, including a loss of canopy structure, shift towards younger, smaller forests, and long-term changes to species composition (Wilke, 2019).

Social Impacts

Beech trees can grow to be beautiful, large hardwoods, and the loss of these can greatly affect aesthetic value. Canopies are large and provide dense shade. Nature enthusiasts admire the lush foliage that turns electric yellow in the fall, and the smooth gray bark that is often used as canvas (Popkin, 2018). Beech trees are commonly found in deciduous forests stands and urban greenspaces.

As of this time, there is little information on best management practices for BLD since the transmission of this disease in not currently known. The ISC will be updated the best management practices as research on the disease continues.

Prevent

Little is known on the origins or biology of BLD, no effective control or eradication measures have been developed.

Similar to emerald ash borer, it is believed that the human transport of wood products may have distributed this nematode. It is also possible that it has arrived in North America through an invertebrate vector (Carta & Wick, 2018).

Management should focus on preventing the introduction of BLD and containing spread where BLD has established. More research must be conducted on determining the cause of the disease so these types of preventative measures can be developed. In general, the spread of invasive species can be prevented by restricting movement of firewood and natural wood products, conducting beech tree inventories to identify resources at risk, monitoring trees closely for signs and symptoms, increase diversity of tree species, and implement management practices to keep forests healthy.

Respond & Control

As the cause of BLD is unknown, control measures have not yet been determined. Experts have commented that affected areas should be quarantined before spread continues and the disease becomes too widespread to manage. Quarantines will prevent further spread of the disease, allowing researchers time to determine cause and effective control measures. You can help to prevent further spread of this disease by ensuring that you are not moving beech seedlings, the leaf litter from an infected area, or firewood.