Genetic variation in native populations of the laurel wilt pathogen, Raffaelea lauricola, in Taiwan and Japan and the introduced population in the USA
Laurel wilt is a true vascular wilt disease caused by Raffaelea lauricola, which is a mycangial symbiont of Xyleborus glabratus, an ambrosia beetle. The fungus and vector are both native to Asia, but it is believed that both were introduced to the Savannah, Georgia area about 15 years ago. Laurel wilt has caused widespread mortality on redbay (Persea borbonia) and other members of the Lauraceae in the southeastern USA, and both pathogen and vector have spread as far as Texas. It is thought that there was a single introduction of R. lauricola to the USA, but there are no extensive studies on the genetic variation of R. lauricola populations that would suggest a genetic bottleneck in the USA. Ten isolates of R. lauricola from Japan, 55 from Taiwan, and 125 from the USA that were collected from X. glabratus adults or infected trees were analyzed with microsatellite and 28S rDNA markers, and with primers developed for two mating type genes. The new primers identified isolates as either MAT1 or MAT2 mating types in roughly equal proportions in Taiwan and Japan, where there was also high genetic diversity within populations based on all the markers, indicating these that individuals within these populations may have cryptic sex. Aside from a local population near Savannah and a single isolate in Alabama that had unique microsatellite alleles, the USA population was genetically uniform and included only the MAT2 mating type, indicating that the population in the USA has undergone a severe genetic bottleneck. This study suggests the importance of preventing a second introduction of R. lauricola to the USA, which could introduce the opposite mating type and allow for genetic recombination and a more aggressive strain of R. lauricola.