Confirmation of independent introductions of an exotic plant pathogen of Cornus species, Discula destructiva, on the east and west coasts of North America.

Cornus florida (flowering dogwood) and C. nuttallii (Pacific dogwood) are North American native tree species that belong to the big-bracted group of dogwoods. Cornus species are highly valued for their ornamental characteristics, and have fruits that contain high fat content for animals. Also, they are an important understory tree in natural forests. Dogwood anthracnose, caused by Discula destructiva, was observed in the late 1970s on the east and west coasts of the United States and by 1991 had quickly spread throughout most of the native ranges of C. florida and C. nuttalli. We investigated the genetic diversity and population structure of 93 D. destructiva isolates using 47 microsatellite loci developed from the sequenced genome of the type strain of D. destructiva. Clone-corrected data indicated low genetic diversity and the presence of four genetic clusters that corresponded to two major geographic areas, the eastern United States and the Pacific Northwest, and to the two collection time periods when the isolates were collected (pre- and post-1993). Linkage disequilibrium was present in five out of six subpopulations, suggesting that the fungus only reproduced asexually. Evidence of population bottlenecks was indicated across four identified genetic clusters, and was probably the result of the limited number of founding individuals on both coasts. These results support the hypothesis that D. destructiva is an exotic pathogen with independent introductions on the east and west coasts of North America. We also tested the cross-amplification of these microsatellite primers to other Discula species. Genomic DNA from 17 isolates of four other Discula species and two isolates of Juglanconis species (formerly Melanconis species) were amplified by 17 of 47 primer pairs. These primers may be useful for investigating the genetic diversity and population structure of these Discula species.

Development of microsatellites from Fothergilla ×intermedia (Hamamelidaceae) and cross transfer to four other genera within Hamamelidaceae.

PREMISE OF THE STUDY: We developed microsatellites from Fothergilla ×intermedia to establish loci capable of distinguishing species and cultivars, and to assess genetic diversity for use by ornamental breeders and to transfer within Hamamelidaceae. METHODS AND RESULTS: We sequenced a small insert genomic library enriched for microsatellites to develop 12 polymorphic microsatellite loci. The number of alleles detected ranged from four to 15 across five genera within Hamamelidaceae. Shannon's information index ranged from 0.07 to 0.14. CONCLUSIONS: These microsatellite loci provide a set of markers to evaluate genetic diversity of natural and cultivated collections and assist ornamental plant breeders for genetic studies of five popular genera of woody ornamental plants.