The native honeysuckle leaf blight increases leaf loss in invasive Amur honeysuckle

Background/Question/Methods The invasive deciduous shrub Amur honeysuckle ( Lonicera maackii  (Rupr.) Herder) is affected by the native fungus honeysuckle leaf blight ( Insolibasidium deformans  (C.J. Gould) Oberw. & Bandoni). In Northern Kentucky, blight symptoms were first noticed in 2012 and have been present every year since. Honeysuckle leaf blight causes young leaves to yellow and eventually become necrotic, leading to premature leaf loss. Previous research indicates that open-grown honeysuckle stands have declined since the 1980s, and it has also shown that leaf blight causes growth decline in honeysuckle seedlings in a greenhouse setting. This study focused on the impact of leaf blight on growth in the field. Twenty shrubs at NKU REFS (Research & Education Field Station) in Melbourne, KY, were chosen, and two shoots from each were labeled, one with a low incidence of blight and the other with high incidence. Shoot lengths were measured every week, starting in early June into early August, with occasional measurements thereafter, and the number of blighted as well as total leaves were recorded. Changes in the numbers and proportions of blighted leaves, the number of leaves per shoot, and leaf density were compared between high- and low-blighted shoots with repeated-measures ANOVAs. Results/Conclusions High-blighted shoots had a significant decline in the number and proportion of blighted leaves over the course of the summer, whereas low-blight shoots remained relatively stable. Thus, shoots with high amounts of blight had increased rates of leaf loss relative to low-blighted shoots. High-blighted shoots exhibited lower leaf densities than low-blighted shoots, consistent with previous work showing that faster-growing honeysuckle are more likely to be infected. Based on the proportions of leaves retained, we estimate that the leaf area integrated over the study period for high-blighted shoots was 14% lower than for low-blighted shoots. We predict this reduced leaf area would translate into a similar amount of reduced growth, consistent with previous work in a greenhouse setting. These findings are also consistent with leaf blight being a cause of the observed dieback of Amur honeysuckle in our region.