Myrtus communis in Europe threatened by the pandemic and South African strains of the myrtle rust pathogen Austropuccinia psidii

Austropuccinia psidii, the causal agent of myrtle rust, has emerged as a significant threat to Myrtaceaein planted and natural woody ecosystems. The first detection of A. puccinia in South Africa was fromseverely infected ornamental Myrtus communis. This raised concern that M. communis, the sole Myrtaceaespecies native to Europe and an important component of vegetation in Mediterranean regions, could bethreatened by the rust. In light of the potential threat to this unique species, seed was collected from 12Italian provenances of M. communis, including mainland and island (Sardinia and Sicily) populations.We assessed the susceptibility of these provenances to both the pandemic and South African strains ofA. psidii. In Colombia, where the pandemic strain of A. psidii is native, seedlings rapidly became infectedby natural inoculum. In South Africa, a preliminary screening of seedlings by artificial inoculation with asingle-uredinium isolate produced high levels of disease. Finally, plants of each of the 12 provenances wereplanted and monitored in Florence, Italy. To date, these showed no signs of disease, but will continue to bemonitored. This study highlights the significant threat that both the pandemic and South African strainsof A. puccinia pose to M. communis in Europe.