Virulence and impact of brazilian strains of Puccinia psidii on hawaiian ‘Ōhi‘a (Metrosideros polymorpha)

Abstract

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 predominantn ative forest tree, ‘öhi‘a ( etrosideros polymorpha). Multiple other strains of Puc­ Mcinia 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 ‘öhi‘a were investigated with two inoculation experiments conducted in Brazil. The first, a split-plot experiment, was conducted to deter- mine pathological impact of five Brazilian P. psidii strains on ‘öhi‘a seedlings and to assess variation in susceptibility of seedlings from six different open-­ ollinatedp ‘öhi‘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 ‘öhi‘a seedlings. Three of the five P. psidii strains were highly virulent on most of the inoculated ‘öhi‘a seedlings (93% 00% infection rates), and none of the ‘öhi‘a–1 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 ‘öhi‘a seedlings at 6 months postinfection. These results have immediate implications for designing Hawai‘i’s quarantine barriers.