<p><i>Erwinia psidii</i> is an emerging bacterial phytopathogen of increasing concern in South America due to its ability to infect economically important hosts such as guava (<i>Psidium guajava</i>) and eucalyptus (<i>Eucalyptus</i> spp.). First reported in Brazil in the 1980s, affecting guava trees, the pathogen later emerged in commercial eucalyptus plantations, suggesting a recent host shift within the Myrtaceae family. This emergence may be attributed to the expansion of acreage into warmer and more humid regions, as these conditions favor novel pathogen-host interactions. Studies present contrasting perspectives regarding the genetic diversity of <i>E. psidii</i> and the hypotheses about how this expansion in host range occurred. Nevertheless, there is consensus that <i>E. psidii</i> exhibits cross-pathogenicity between guava and eucalyptus, with no evidence of host specialization or differentiation among isolates from different geographic origins. The pathogen’s ability to survive on plant surfaces and debris, along with its dissemination through asymptomatic propagative material, highlights the importance of understanding disease epidemiology for effective monitoring and management. Advances in molecular diagnostics, including qPCR assays, have significantly improved early detection and surveillance. Currently, management relies on exclusion practices and cultural measures, although their effectiveness remains limited in large-scale forestry plantations. The identification of resistant genotypes, such as clones of <i>Eucalyptus urophylla</i> × <i>E. maidenii</i> and resistant guava varieties, offers potential strategies for control. The emergence of <i>E. psidii</i> as a multi-host pathogen highlights the importance of integrated strategies that combine surveillance, resistant germplasm, and diagnostic tools. Understanding the ecological and evolutionary mechanisms underlying host shifts is essential for anticipating disease dynamics and safeguarding guava and eucalyptus production systems.</p>