Identification and population genetic studies on Ceratocystis spp. infecting Eucalyptus and Acacia plantations in South Africa and Indonesia

Abstract

Ceratocystis infections pose significant threats to the sustainability of plantation-grown trees propagated for commercial purposes. Ceratocystis infections result in wilt, canker, vascular discolourations and eventual mortality of the host species. Symptoms typical of Ceratocystis infections have recently emerged on Eucalyptus spp. in Indonesia and South Africa. The aims of this study were to characterize Ceratocystis isolates collected from diseased Eucalyptus spp. in South Africa and those from Eucalyptus spp. and Acacia spp. in Indonesia, and also determine their population genetic diversity, population structure and their level of aggressiveness towards healthy Eucalyptus spp. For the South African population, isolates previously collected from heathy, but wounded Eucalyptus, were included for comparative purposes. Results of sequence data and multi-gene phylogenetic analyses revealed that C. manginecans and C. eucalypticola are the pathogens infecting Eucalyptus spp. in Indonesia and South Africa, respectively. In Indonesia, a greater genetic diversity was found in isolates from Acacia than those from Eucalyptus. This, together with the presence of the same multilocus haplotypes on both species corresponds with a longer history of infection on Acacia spp. and a recent host expansion onto Eucalyptus. On the contrary, in South Africa, isolates from wounds on healthy Eucalyptus trees and those from the diseased trees, as well as soil had low levels of diversity and lacked population structure. Interestingly, the same multi-locus haplotypes were found in both the non-diseased and diseased populations which shows that their isolates represent the same species. Therefore, the recent emergence of Ceratocystis disease outbreak on Eucalyptus spp. in South Africa seem to be associated with the susceptibility of the recently propagated Eucalyptus spp. Field inoculation tests showed that Acacia and Eucalyptus isolates collected in Indonesia comprise of a mixture of aggressive and non-aggressive individuals. On the contrary, inoculations tests done using isolates of C. eucalypticola described in the absence of a disease and those that recently emerged as causal agents of Eucalyptus wilt in South Africa resulted in similar aggressiveness levels. Results of this study have expanded current knowledge on Ceratocystis species by adding substantial information regarding their distribution and genetic diversity. They also highlight the need for the selection of resistant host species to help reduce plantation-grown tree mortality.