Beech are ecologically important trees in the eastern United States and are common feature trees in landscapes. Beech leaf disease (BLD) is a disease caused by a foliar-feeding nematode (a microscopic worm), Litylenchus crenatae subsp. mccannii. Beech leaf disease was first observed in Lake County, Ohio, in 2012 and has since been detected in Massachusetts, Connecticut, New Jersey, New York, Pennsylvania, Rhode Island, West Virginia, and the Canadian Province of Ontario. BLD is mainly known to affect American beech (Fagus grandifolia). However, symptoms have been observed in European beech (Fagus sylvatica) and Oriental beech (Fagus orientalis).
Symptoms
Symptoms of beech leaf disease include dark bands forming between the veins of leaves; leaves becoming curled, deformed, and shriveled; premature leaf drop; aborted buds; and thinning canopy. Early symptoms include dark-green striped bands between lateral leaf veins and reduced leaf size. Banded areas usually become ‘leathery,’ and leaf curling may be observed. Banding is most apparent when viewed from below, looking upward into the canopy. As symptoms progress, buds fail to develop, leaf production is reduced, and premature leaf drop leads to an overall reduction in canopy cover, ultimately resulting in the death of sapling-sized trees within two years and mature trees within seven years. In areas where the disease is established, the proportion of symptomatic trees can reach more than 90%. As BLD progressively worsens, the tree’s overall health weakens, exposing it to secondary insect pests and diseases.
Disease Life Cycle
The causal agent of BLD is the foliar-feeding nematode Litylenchus crenatae subsp. mccannii. This nematode has an egg stage, several juvenile stages, and an adult stage where both male and female nematodes are present. Late in the season, adults penetrate leaf buds, feed on them, and reproduce within overwintering buds, where they cause gall-like cell division between the leaf veins. BLD symptoms appear on the infested leaves when the buds open in spring. Numerous eggs are present, which may be shed via wind and rain splash. As the season progresses, remaining nematodes and hatching eggs produce large numbers of nematodes, with activity peaking from late summer to early fall. The nematodes can locally move on trunks in shallow water films due to rain, irrigation, or melting snow. Additionally, there have been reports of the nematodes “hitching” a ride on non-flying arthropods (e.g., mites). Although it is still unclear how the nematodes move long distances, birds have been implicated, and research is underway.
Management
No treatments are currently available for beech trees affected by BLD; however, several methods are being studied and are likely to be effective at reducing the incidence of BLD. A typical BLD program begins in late spring with a potassium phosphite-based soil application, as these products applied in early and late season have been shown to slow disease progression and severity. This is followed by a foliar spray program coinciding with peak nematode activity beginning in mid-to-late summer and continuing through the fall. It is also essential to keep trees as stress-free as possible. Providing a thick layer of mulch under the tree helps keep the root system cool, suppress weeds, and maintain moisture levels. Removing severely infected foliage from trees and raking and destroying fallen leaves in the fall is also beneficial, as they still contain the nematode.
