Last spring, in 2022, many Islanders were troubled by the appearance of new beech leaves. Leaves were malformed, often discolored, and with distinct banding or blotch patterns. The Polly Hill Arboretum explains in a press release that it observed in mid-May these symptoms were not confined only to the American beeches (Fagus grandiflora), which are native to the Island, but also occurred on several cultivars of European beech (Fagus sylvatica). Upon further investigation, it was determined that beech leaf disease (BLD) and its associated nematode (Litylenchus crenatae ssp. mccannii) were the cause of the symptoms. First discovered in the U.S. near Cleveland, Ohio, in 2012, BLD has rapidly spread, and was first reported in Massachusetts in August 2020.
The nematodes that cause BLD originated in Japan; they are microscopic roundworms that infect trees in the bud stage. The infestation and feeding progresses over time, damaging the internal leaf tissues and leaving a tree with deformed leaves and a thin canopy. As the pest becomes more established on a tree, it ultimately results in mortality. Death can occur within six years for mature trees, and for younger trees, it takes as little as two years.
Nematodes cannot fly or disperse far and wide on their own. One theory about how these pests have spread so fast, the release relates, is that they are moved by migratory bird species that feed on beech buds. Scientists have also found that white-tailed deer can host nematode populations, adding another potential mode of transfer between trees separated by distance. Native trees in Island natural areas show signs of infestation; this is worrisome, as it will impact the ecology of local woodlands.
Many unresolved questions remain about beech leaf disease. In response to these questions, the arboretum hosted a workshop in September 2022 for Island arborists. Bartlett Tree Care scientists discussed the latest findings on BLD.
The arboretum is experimenting with various treatments focused on limiting nematode populations. One such treatment product, called Broadform, was applied in late summer to fall of 2022, when the adult nematodes leave the old leaf and infect the new buds for the following year.
Currently, all treatments are experimental, and there is no guarantee that an infected tree will survive. The arboretum believes most of its mature beeches were likely infected with the nematode before last spring, possibly as long ago as two to three years.