Low genetic diversity and clonal spread of Fusarium circinatum in Colombian pine populations.

Ceriani, C.*, Wingfield, M. J., Fru, F., Van Wyk, S., Rodas, C., Wingfield, B. D., Steenkamp, E. T.

Forestry and Agricultural Biotechnology Institute, University of Pretoria

Fusarium circinatum, a significant fungal pathogen affecting Pinus species, poses a substantial threat to commercial forestry worldwide. In Colombia, the pathogen was initially detected in nursery plants and later identified in plantations, where it was linked to basal cankers on young trees. This study investigates the genetic diversity of F. circinatum populations in Colombia by analyzing 136 isolates collected from diseased nursery plants (2005–2007) and plantation trees (2017 and 2020) across different geographical regions, Pinus species and collection dates. Genotyping with 10 microsatellite markers revealed 33 distinct multilocus haplotypes. Genetic diversity analysis indicated low diversity levels both across the entire collection and within specific groupings. A subset of isolates appeared to have unique origins within Colombia (p < .05), and population differentiation between nursery and plantation collections was minimal (i.e., high gene flow). Notably, all isolates possessed the MAT1-1 mating type, and no evidence for random association among microsatellite alleles was observed (p = .0001). These findings suggest that F. circinatum was introduced into Colombia through a limited number of events, likely via seed or plant material used in nursery production. The pathogen appears to have spread from nurseries to plantations primarily through asexual reproduction, likely with the help of asymptomatic plants, leading to a predominantly clonal population structure in Colombia. This highlights the critical role of nursery practices in pathogen dissemination and the necessity for strict biosecurity measures to limit further spread.

Keywords: pitch canker, genetic diversity, microsatellite markers, plantation forestry, South America