Genetic variation in Spilocaea oleagina populations from New Zealand olive groves

Olive leaf spot caused by the fungus, Spilocaea oleagina, is the most important leaf disease of olives in many olive-growing regions worldwide with yield losses of up to 20%. The genetic structure of S. oleagina populations was investigated with universally primed-polymerase chain reaction (UP-PCR) techniques. Ninety-eight S. oleagina isolates were collected from 12 known and 4 unknown cultivars from olive groves in five New Zealand regions. UP-PCR profiles based on 159 markers were used to compute genetic distances between pairs of individuals. Low levels of gene and genotypic diversity were detected in all populations, with 76% of the loci being polymorphic and with Nei's diversity indices ranging from 0.0234 to 0.1393. Analysis of molecular variance showed small but significant (P = 0.001) variations among regions, although most of the molecular variability (87%) was found within populations. Clustered analysis showed no evidence of grouping according to geographic origin of the isolates. The low level of genetic diversity found within and among populations indicates that reproduction for this fungus is predominantly by asexual means and that any effective control strategies are likely to be useful in all or most New Zealand olive groves.