Suit Up, Citizen Science Hero, and Help Scout for Beech Leaf Disease in Pennsylvania's Forests

This new disease was not like any beech issue we knew as managers, and its cause remained elusive for years. Only recently has its consistent association with a new subspecies of nematode (Litylenchus crenatae mccannii) been strongly supported through both experimentation and observation, though pinpointing a precise causal agent and mode of transmission will still require continued research.¹ BLD has been observed in the U.S. on the American beech (Fagus grandifolia) we commonly see in our Pennsylvania forests as well as on European (F. sylvatica) and Oriental (F. orientalis) beech specimens.

American beech itself is both loved and loathed by landowners and resource managers. This high-value wildlife species produces favored hard mast in the form of beech nuts and regenerates in low-light conditions to grow and provide complex vertical canopy structure. However, its prolific root suckering behavior and tendency to produce a dense understory canopy of "beech brush," especially in stressed stands where other forest health issues impact beech, has earned this species a notorious reputation as a sometimes-aggressive source of vegetative interference that can shade out desirable regeneration and is often quite costly to control.

Forest managers and scientists are very nervous about the increasingly concerning, detrimental effects BLD may have on this tree species on a large scale. Because of the uncertainty around the precise conditions and causes of this new disease, definitive control methods, management approaches, spread predictions, and large-scale disease progression patterns are not readily available. However, BLD is easily recognizable where it does appear.

Early symptoms of BLD are unmistakable, especially in the early summer on young leaves, with dark bands apparent between leaf veins. The best way to visualize this interveinal banding is to look up at the underside of a leaf held against a light source or with the sky as a backdrop. These interveinal spaces can appear dark and puckered from above as well. As symptoms progress, leaves take on a thickened, leathery texture, with shriveled or curled leaf edges. Canopies can look quite sparse, with few leaves, since BLD also affects bud development. BLD is easy to identify not just because of the distinctive symptoms but also because early symptoms in newly-affected stands tend to begin where we most readily notice them: in the understory. Affected saplings often die within two to five years but can succumb in as little as one year. The disease progresses more slowly in mature canopy beech from lower branches upwards; some stems die, and surviving trees are more susceptible to stress. Observations in areas where early infestations occurred show that almost 100% of beech are now symptomatic.²

The distinctive appearance of BLD, especially in the early part of the summer before advanced cases might cause leaf loss, makes it quite easy to identify and differentiate from other forest health issues affecting beech. Stems with only BLD lack the obvious, numerous, white scale insects and bark lesions characteristic of beech bark disease (BBD), the result of an insect-fungus complex. Mite damage and anthracnose can cause a similar dried-out leaf appearance, but not the darkening pattern between leaf veins that is so characteristic of BLD. However, as is often the case with forest health issues, multiple factors can affect a single tree at one time and cause compounded stress.

With so many unanswered questions, it can often be frustrating for landowners and managers to encounter new diseases, but enhancing forest health is not just about management; it also depends on robust monitoring efforts, and you - yes, you, can have an essential role to play. BLD has now been identified in 40 counties in Ohio, New York, Pennsylvania, Connecticut, and Ontario.³ We are likely still learning the extent of BLD's current distribution on the landscape. The more we understand about where it is, the better we can watch and study it, and the more likely we are to advance research that yields strategies and solutions.

Here's what to do to participate in the multi-state, multi-agency monitoring effort called the Beech Tree Health Survey: Visit Cleveland metroparks website and scroll down to the "Beech Leaf Disease" section (about halfway down the page). There, you'll find a pest alert with BLD information, a report describing and showing symptoms to watch, and detailed instructions for participating in the monitoring survey. You will be prompted to download an app (iOS or Android). Review the instructions, complete the tutorial, familiarize yourself with the symptoms of BLD, and search the forests you love for signs of BLD! Forest health experts at the PA Bureau of Forestry are particularly concerned with potential outbreaks along the known front of the current range, in Clinton, Tioga, Lycoming, Susquehanna, Bradford, Potter, Elk, and Cameron Counties, but statewide monitoring efforts are needed. Questions about the app and survey processes can be directed to the contact address provided in survey instructions. If you think you have found BLD in PA, please also notify the PA Bureau of Forestry at jirose@pa.gov.

The long-term effects of BLD are yet unknown in the landscape. Will it offer a biological control on troublesome beech brush? Will it have a detrimental effect on beech mast production and regeneration potential? How extensively will we observe beech decline and death across the landscape? Will we lose complex native understory habitat structure in a matter of years in affected stands? Will this open a window for the establishment of invasive plants in those areas? How will BLD affect the survival of BBD-resistant trees? Managers and researchers are working hard to find the answers to these questions, but to do so more effectively, we need the help of your citizen scientist's eyes and observations.

References

¹ Carta, L.K, et al. Beech leaf disease symptoms caused by newly recognized nematode subspecies Litylenchus crenatae mccannii (Anguinata) described from Fagus grandifolia in North America. For Path. 2020;00:e12580.

² Pogacnik, J.; and T. Macy. July 2016. Forest Health Pest Alert: Beech Leaf Disease.

³ Martin, D.; D. Volk; T. Macy; and J. Pogacnik. November 2019. Forest Health Pest Alert: Beech Leaf Disease. USFS Publication.