A single strain of Puccinia psidii, the causal agent of rust disease on Myrtaceae, was recently reported on multiple myrtaceous hosts in Hawai'i, but this strain has caused only mild levels of damage to the state's predominant native forest tree, ‘ōhi‘a ( Metrosideros polymorpha). Multiple other strains of Puccinia psidii have been identified from Brazil and characterized via extensive sampling and microsatellite analyses. Potential effects of other Brazilian P. psidii strains on Hawai'i's ‘ōhi‘a were investigated with two inoculation experiments conducted in Brazil. The first, a split-plot experiment, was conducted to determine pathological impact of five Brazilian P. psidii strains on ‘ōhi‘a seedlings and to assess variation in susceptibility of seedlings from six different open-pollinated ‘ōhi‘a parent trees to each P. psidii strain. The second experiment was conducted to determine influence of the rust disease on growth and survival of ‘ōhi‘a seedlings. Three of the five P. psidii strains were highly virulent on most of the inoculated ‘ōhi‘a seedlings (93%–100% infection rates), and none of the ‘ōhi'a families used in this test showed significant resistance. The other two strains tested were much less virulent. Infection by the highly virulent strains of P. psidii resulted, on average, in a 69% reduction in height growth and 27% increase in mortality of ‘ōhi‘a seedlings at 6 months postinfection. These results have immediate implications for designing Hawai'i's quarantine barriers.