Masters Theses

Date of Award

5-1995

Degree Type

Thesis

Degree Name

Master of Science

Major

Entomology and Plant Pathology

Major Professor

Jerome F. Grant, Mark T. Windham

Committee Members

Charles Pless

Abstract

Discula destructiva Redlin, the causal agent of dogwood anthracnose, is responsible for the decline and mortality of the eastern flowering dogwood, Cornus florida Link. The pathogen is believed to be disseminated by wind, rain, and man. Until recently, little research had been conducted to identify other means of dissemination (birds, deer, insects, etc.). In an earlier study, convergent lady beetles, Hippodamia convergens Guédrin-Méneville, acquired and carried viable conidia of D. destructiva on their bodies for as many as 16 days under laboratory conditions. The objectives of this research, however, were expanded into the natural environment to determine: 1) if arthropods collected from diseased trees in forested areas had viable conidia of D. destructiva on their bodies and 2) the effects of light intensity (sunlight and shade) on viability of conidia of D. destructiva over selected periods of time on lady beetles in field cages.

In 1994, 7.2% of arthropods (n=375) collected from diseased trees were infested with viable conidia of D. destructiva. When conidia-infested arthropods were evaluated throughout the summer, conidial infestations were greater at each site during June than on other sampling dates. The level of infestation was greatest at Rich Mountain (22.0%) followed by Jakes Creek (11.1%) and Sugarlands (8.6%) during June, when epidemics of dogwood anthracnose are normally increasing. Frequency of conidia-infested arthropods followed changes in the rate of disease severity of dogwood anthracnose, based on disease ratings using the Horsfal-Barratt scale.

Infestation of viable conidia on convergent lady beetles collected from shaded cages 1 day after release were greater than those in sunlight (20% and 3.3%, respectively). Also, beetles in the shade carried viable conidia for longer periods of time than those in sunlight (16 days and 1 day, respectively). The greatest level of infestation (ca. 12%) occurred 1 day after release and dropped significantly during the remainder of the study.

The results of this study have shown that arthropods carry viable conidia of D. destructiva in the natural environment. Optimal conditions for potential dissemination of conidia by arthropods were identified as: 1) viable conidia carried by arthropods within 24 h after infestation, 2) conidia transported by arthropods under shaded conditions, which are more conducive to the survivability of conidia, and 3) conidia carried by arthropods when disease severity of dogwood anthracnose was at its greatest levels. This research has demonstrated that more than one species of arthropod carried viable conidia of this fungal pathogen and suggests that arthropods may play an important role in the epidemiology of dogwood anthracnose.

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