Abstract: Amur honeysuckle (Lonicera maackii) is an invasive shrub with major impacts on native ecosystems in eastern North America. Over the past decade, major outbreaks of the native honeysuckle leaf blight (Insolibasidium deformans) have been observed on Amur honeysuckle in the Ohio River valley on an annual basis. While honeysuckle leaf blight was first described in the 1940s, widespread outbreaks were not reported recently; thus, honeysuckle leaf blight may be an example of an emerging pathogen with the potential to curb the spread and impacts of Amur honeysuckle. Honeysuckle stands in the Ohio River valley region have recently been reported to be declining, and other work shows that the leaf death caused by leaf blight has the largest impact on small shrubs and rapidly growing, small shoots, particularly those growing in the open. The goal of the current study was to determine if populations of honeysuckle are being negatively impacted by the leaf blight.
I looked at both shrub and shoot diameter distributions of seven open-grown and three forest canopy stands in northern Kentucky and southwestern Ohio. Shrub diameters were fit to the two-factor Weibull function. All stands had reverse-J shaped shrub diameter distributions, and the Weibull shape parameters indicated that shrub abundance decreased with diameter in all stands. Stand densities and basal areas fell within the range found in previous studies. Shoot size-class distributions, when compared to young stands found in the region in the 1980s, generally showed a higher proportion of small shoots. This was contrary to expectations, as the older stands in the current study were expected to have a higher proportion of large shoots. Median shrub size and the number of shoots per shrub were also smaller, indicating many young, small shoots and shrubs compared to the 1980s. The size-class distribution of dead stems reported in another recent study also showed an unexpectedly large number of smaller dead stems. Thus, the current size-class distributions of both stems and shrubs are most consistent with the replacement of damaged shoots and shrubs. Young, rapidly growing shoots and stems have been shown to be most susceptible to leaf blight; the current shrub and shoot diameter distributions are consistent with damage induced by honeysuckle leaf blight. However, long-term monitoring of individual shrubs and stems will be needed to link leaf blight with honeysuckle shoot and stem dynamics.
