A Population Genetics Study of \u3ci\u3eDiscula Destructiva\u3c/i\u3e, the Causal Agent of Dogwood Anthracnose

Flowering dogwood (Cornus florida L.) is prized as an ornamental and contributes millions of dollars to the economy through tourism and sales each year. Dogwood anthracnose, caused by Discula destructiva Redlin, was observed in the late 1970s on the east and west coasts of the United States and by 1991 had quickly spread throughout much of the native ranges of C. florida and C. nuttalli. The objectives of this study were to investigate the genetic diversity and population structure of D. destructiva using simple sequence repeats (SSRs), and to test the cross-transferability of these markers to other Discula species. Fifty SSRs were developed from the sequenced genome of a D. destructiva isolate and used to evaluate 93 fungal isolates from 14 locations collected between 1989 and 2000. The data set was clone corrected (n = 69) and analyzed using polymorphic only (n = 40) and polymorphic and monomorphic SSRs (n = 47). Haploid genetic diversity was very low for both data sets (Hexp = 0.18 and 0.21). Bayesian clustering and distance analyses identified four and five genetic clusters that correspond to two major geographic areas, the eastern United States and the Pacific Northwest, and two time periods when the isolates were collected, Pre-1993 and Post-1993. Linkage disequilibrium was present in all subpopulations, which strongly suggested that the fungus was only reproducing asexually. A population bottleneck was statistically indicated in all populations, 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 separate introductions on the east and west coasts of North America. The cross-transferability test revealed that genomic DNA from 19 isolates of five other Discula species and two isolates of Melanconis spp. amplified with 17 of 47 primer pairs. These primers may be useful for investigating the genetic diversity and population structure of these Discula species