Masters Theses

Date of Award

12-1997

Degree Type

Thesis

Degree Name

Master of Science

Major

Entomology and Plant Pathology

Major Professor

Mark T. Windham

Committee Members

Bonnie Ownley, Jerome Grant, Kristine Johnson

Abstract

Beech trees (Fagus grandifolia Ehrhart) in North America are threatened by beech bark disease (BED), a destructive insect-pathogen complex thought to have originated in Europe. The disease complex is initiated when boles of American beech become infested with colonies of the beech scale, Cryptococcus fagisuga Lindinger, and culminates when one of two species of the fungus Nectria Fries invade the tree. The disease results in dieback of the crown and general tree decline, and often results in tree mortality. Ten long term study plots were established in the Great Smoky Mountains National Park (GSMNP) in 1994 to monitor the spread and impact of BED in the park.

Monitoring efforts were continued in 1996 and 1997. Beech trees within the plots were evaluated for mortality and sampled on the north and south tree-side with a 1089-cm2 sample square. Sampled areas were rated for abundance of fungal sexual fruiting structures, and trees were also given an overall rating. Signs of the fungi were observed within 9 of the 10 plots and just outside the border of the other. Total prevalence of the fungi in the combined plots was 9% in 1996 and 11.4% in 1997. Total mortality due to disease rose from nearly 16% in 1996 to nearly 18% in 1997. BED is both firmly established and on the rise in GSMNP.

The role of a common beech bark fungus, Ascodichaena rugosa Butin, in the occurrence and spread of the beech scale and BED was evaluated in the 10 study plots. Beech trees were rated for abundance of signs of A. rugosa in the same manner as they were rated for abundance of Nectria species. These ratings were tested for association with abundance ratings of the beech scale (Wiggins 1997) and Nectria spp. using Fisher's Exact Test and analyzed for direction with Kendall's tau-b statistic (P=0.05). Significant negative associations between abundances of A. rugosa and the beech scale were observed in north, south, and overall ratings during 1996 and 1997. Significant negative associations between abundance of A. rugosa and the incidence of BED were observed in north and overall ratings during 1996 and 1997. These results indicate that A. rugosa may limit the spread and development of infestations of the insect, in effect also reducing the occurrence and severity of BED.

Several biotic and abiotic variables were measured and tested for relationship to incidence of disease and mortality due to disease. Linear models were constructed accounting for significant portions of the variation in incidence and abundance of Nectria spp. and the mortality due to disease (P=0.05). R-square values for the models exceeded 0.30 only for mortality due to disease. Crown class and evergreen basal area were negatively correlated and elevation and number of stems positively correlated with disease mortality. It was concluded that a larger sample size was required to make biologically significant conclusions concerning the impact of these variables on disease.

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