Title:

Experimental Evidence That Bark Responses to Infection Shape Insect-Fungus Interactions in A Widespread Tree Disease Complex

Beech bark disease (BBD) in North America arises from a complex of interactions between American beech (Fagus grandifolia), the beech scale (Cryptococcus fagisuga), and two species of pathogenic fungi in the genus Neonectria. On susceptible beech, the beech scale is known to facilitate initial Neonectria infection. However, as the disease progresses, the spatiotemporal relationship between insect and fungal components appears to break down. This indicates that more than a simple facilitative interaction may take place between the beech scale and Neonectria spp. in long affected stands (aftermath forests). We hypothesize both positive and negative interactions occur between scale insects and Neonectria fungi on host trees, and that the direction of these interactions varies predictably in space and time as a function of bark response, cankering, necrosis, and regrowth. To test this, in 2019 and 2020, we conducted field experiments involving experimental infestation of C. fagisuga and inoculation of Neonectria spp. on American beech. We experimented on rough bark (disease-altered) and smooth bark trees from two diameter classes. Small, smooth-barked trees were most susceptible to scale insect colonization while large, smooth-barked trees were the least susceptible. In contrast, trees with rough, disease-altered bark exhibited intermediate susceptibility irrespective of tree size. Across all tree stratifications, Neonectria-induced lesions were larger when colonies of scale insect were absent from within radius (1.5 cm) of the inoculation. Neonectria species and bark type also influenced whether the lesion was active or not active at the time of late summer sampling. Experimental results from scale insect infestation demonstrate that altered bark from prior BBD can reduce scale insect establishment rates. While Neonectria inoculation results demonstrate the potential for bark-mediated antagonisms between scale insects and fungi within the host. Inoculation results also demonstrate that host bark structure may affect the timing or duration of fungal lesion growth. Future studies of this forest disease should consider host-mediated antagonistic relationships in addition to facilitative relationships between insect and fungal components of BBD.

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