Arthropod dissemination of Discula destructiva : conidia on Cornus florida

In 1978, a fungal disease, identified as Discula destructiva Redlin sp. Nov., was discovered killing native flowering dogwoods, Comus florida Link., in the northeastern United States. Since then, the pathogen has spread southward along the Appalachian mountain range to Georgia and Alabama, infecting dogwoods on more than 2.3 million hectares.

Known methods of pathogen dissemination include wind and rain. Other possible mechanisms for dispersing fungal conidia involve animal vectors, such as arthropods which may carry conidia externally and/or internally. The objective of this research was to determine if arthropods could disseminate viable D. destructiva conidia. The adult convergent lady beetle (CLB), Hippodamia convergens Guerin- Ménfèville (Coleoptera: Coccinellidae), was selected as the model insect for this research.

In experiments testing for viability of conidia carried externally and internally, CLBs exposed to TN 8 carried viable conidia externally and internally, 100% of the time. All beetles infested with VA 17b carried viable conidia externally and 95% internally. These results were supported by data from time trials in which infested CLBs carried and deposited viable conidia as many as 16 d after exposure to the inoculum.

Scanning electron microscopy of CLB body surface showed significantly more conidia were attached to the ventral surface. No difference in conidia densities were observed among body regions on the ventral surface; however, regions within the dorsal surface did have significantly different amounts of conidia.

Greenhouse studies to determine if infested CLBs could initiate infection of healthy C. florida trees produced significant differences when infection by noninoculated CLBs, no CLBs, and inoculated CLBs were compared. Ninety percent of trees treated with inoculated CLBs displayed symptoms of D. destructiva infection. Scanning electron micrographs of leaves exposed to infested CLBs were observed for locations of deposited conidia. Conidia were deposited around trichomes on the adaxial C. florida leaf surface.

Insects are abundant and active organisms that inhabit the forest/urban ecosystems and that, indirectly, may play a role in epidemiology of dogwood anthracnose. Results from this research have indicated that insects can disseminate viable D. destructiva conidia and may be involved in the rapid movement of the pathogen along the eastern United States throughout C. florida's habitat range.