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.
