Invasive Amur honeysuckle and native honeysuckle leaf blight: The smallest shrubs are the most affected

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Caitlyn M. Helton and Richard L. Boyce, Biological Sciences, Northern Kentucky University, Highland Heights, KY

Background/Question/Methods
The deciduous shrub Amur honeysuckle (Lonicera maackii (Rupr.) Herder) is an important invasive plant species in the Ohio River Valley. Its leaves can be infected by the native fungus honeysuckle leaf blight (Insolibasidium deformans (C.J. Gould) Oberw. & Bandoni). In the Cincinnati, Ohio, region, blight symptoms were first noticed in 2012, and they have been seen every year since. Honeysuckle leaf blight causes young leaves to yellow and eventually become necrotic, leading to premature leaf loss. Previous work has found that some types of honeysuckle stands in the region have declined since the 1980s, and it has also shown that leaf blight causes growth decline in honeysuckle seedlings in a greenhouse setting. Aboveground portions of eighteen forest understory shrubs with basal diameters of 0.9-7.9 cm at the Northern Kentucky University Research & Education Field Station in Melbourne, KY, were destructively sampled in summer 2020. Shrubs were sectioned, and shoots were sorted into long (>5 cm internodal distance) and short shoots. Leaves were detached and sorted into blighted and unblighted leaves. The shrub components were regressed against basal diameters after log-transformation, including fractions of blighted leaf mass.
Results/Conclusions
Allometric equations developed from these shrubs were consistent with those developed in the region in the 1980s. Smaller shrubs had a larger fraction of leaf biomass on long shoots. In addition, long shoot leaves had significantly higher rates of leaf blight than short shoots (12.4 vs. 0.3% by weight), and so there were larger blighted leaf fractions on smaller shrubs. Thus, smaller shrubs are more affected by leaf blight than larger ones. Because blighted leaves die prematurely, we predict that reduced growth and/or increased mortality will be seen in smaller shrubs. If this is the case, honeysuckle leaf blight might be effective in controlling the establishment and spread of honeysuckle populations, because small seedlings would be most affected. This prediction would be supported if a reduction in smaller size classes was observed in Amur honeysuckle populations.