Currently, an invasive myrtle rust disease pathogen is spreading around the globe, which threatens native and planted trees and shrubs in the Myrtaceae family, such as eucalypt, guava, and allspice. The established belief was that this rust disease originated on guava in Brazil, then the rust pathogen shifted its host to eucalypt after non-native eucalypts were planted in Brazil, and subsequently shifted to other hosts around the world. Collaborative studies among the Universidade Federal de Viçosa (UFV) – Brazil, Washington State University (WSU), and the Forest Service conducted DNA analyses on myrtle rust pathogen (Puccinia psidii) samples collected from diverse tree hosts and geographic locations in Brazil. These studies determined that the myrtle rust pathogen infecting eucalypt plantations in Brazil did not originate from guava. The scientists also determined that the rust disease was composed of different strainsassociated with different tree hosts, thereby demonstrating that all members of this rust pathogen species do not behave the same. This work has important regulatory and management implications for areas that are threatened by the myrtle rust pathogen, because it shows that different strains threaten different host tree species. For example, the myrtle rust pathogen has been recently introduced to Hawaii where it threatens the ohi’a tree, a major component of Hawaii’s native forest. This work indicates that other myrtle rust pathogen strains likely pose additional threats to ohi’a and other native trees in Hawaii, if they invade Hawaii.