Nursery-linked plantation outbreaks and evidence for multiple introductions of the pitch canker pathogen Fusarium circinatum into South Africa

Abstract

In recent years, Pinus plantation forestry has been significantly hampered by outbreaks of pitch canker caused by the fungus Fusarium circinatum. In this study we investigated the role of Pinus host, geographic origin and reproductive mode in structuring the F. circinatum populations in plantations. For this purpose, 159 isolates originating from diseased plantation trees in the Western and Eastern Cape Provinces of South Africa, were genotyped using 10 microsatellite markers. Analyses of these data revealed 30 multilocus haplotypes and that the populations were distinct based on geographic origin as well as host. However, shared haplotypes were observed between populations, showing that these populations are connected possibly through the movement of haplotypes. A second aim was to determine whether the genetic variation found in these populations of the fungus could be attributed to outbreaks of the seedling disease caused by this pathogen in Pinus nurseries. To achieve this goal, an additional set of 43 isolates originating from pine seedling nurseries were genotyped and analysed. The results showed that the populations of F. circinatum in plantations most likely originated from the nursery outbreaks that occurred prior to the plantation outbreak. Inferences regarding reproductive mode further showed that sexual reproduction has little impact on the genetic makeup of the F. circinatum populations and that they primarily reproduce asexually. Overall the results of this study showed that the F. circinatum diversity in South Africa has arisen due to multiple introductions of the pathogen and is not due to sexual reproduction.