The pathogen accumulation hypothesis proposes that invasive species may accumulate pathogens in their new range. Amur honeysuckle (Lonicera maackii) is a widespread invasive species in North America. Since 2012, there have been extensive outbreaks of the native leaf fungal pathogen, honeysuckle leaf blight (Insolibasidium deformans), a possible accumulating pathogen, on L. maackii. The blight infects leaves in the late spring/early summer and eventually kills them. The objective of our study was to describe patterns of blight occurrence and predict its effects on L. maackii growing in both the forest understory and in the open in northern Kentucky, USA. In 2018, pairs of high- and low-blight occurrence shoots were tagged on understory shrubs at the Northern Kentucky University Research & Education Field Station (NKU REFS) to follow the progression of blight through the summer. In 2020 and 2021, understory (NKU REFS) and open-grown (nearby NKU campus and southwest Ohio) shrubs were destructively sampled. After division into long and short shoots, leaves on each shoot type were separated into blighted and unblighted components. Allometric equations based on basal diameters were developed for each stand type and shrub component.
Blight progressed such that high-blight shoots showed a leaf-proportion-retained x time area that was 83% of low-blight shoots and was found primarily on long shoots. The fraction of long shoot leaf mass was highest on smaller shrubs, especially in the open. Long shoots had a much higher blighted mass fraction than short shoots, and so leaf blight was most extensive on small open-grown shrubs. Modeling using our allometric equations predicted that leaf blight had the greatest effect on open-grown shrubs with diameters < 5 cm, with little effect on larger diameters or on understory shrubs. Accumulation of this pathogen appears to have little to no effect on understory shrubs, but there may decreased density and extent of open-grown Amur honeysuckle shrubs, due to its effects on smaller shrubs; there is some evidence for this latter prediction from previous studies. However, future work is needed on open-grown stand structure to support these predictions.