Genetic Diversity of Calonectria reteaudii Isolates From Infected Eucalyptus Leaves and Associated Soils Indicates a Phyllosphere Origin of the Pathogen

Calonectria leaf blight is among the most prevalent diseases affecting planted Eucalyptus in Southeast Asia. Calonectria reteaudii was recently identified as the main causal agent of a localised epidemic in Malaysia, and a number of isolates were collected from diseased leaves and soils associated with symptomatic trees. This provided an opportunity to gain a better understanding of the pathogen ecology, particularly the relationship between soil and phyllosphere inoculum sources. A set of eight polymorphic SSR markers was used to genotype 21 isolates from leaves and 23 isolates from soils collected beneath infected trees. In addition, mating types of all isolates were identified using PCR-based diagnostic mating-type primers. Low genotypic diversity was found in both the leaf and soil populations of C. reteaudii. However, the isolates from diseased leaves exhibited higher levels of genotypic and genetic diversity compared to those from associated soils. All multilocus genotypes identified in the soil isolates were also represented in the leaf population. While all isolates from the soil were of the MAT1-1 mating type, both MAT1-1 and MAT1-2 were identified in isolates from the diseased leaves, but the MAT1-1/MAT1-2 ratio deviated significantly from a 1:1 ratio. The high level of clonality observed for the isolates suggests that conidia are most likely the primary source of pathogen dispersal. The results also suggest that isolates of C. reteaudii from the soils collected beneath heavily infected trees likely originated from the phyllosphere and not vice versa.