Species diversity of Lecanosticta and population genetics of Dothistroma species : important needle pathogens of Pinus

Abstract

Dothistroma needle blight (DNB), caused by Dothistroma pini and D. septosporum, and Brown spot needle blight (BSNB), caused by Lecanosticta species, are two pine needle diseases of Pinus species that have become diseases of emerging importance in the Northern Hemisphere in the past twenty years. This thesis aimed to address three research gaps pertaining to the species diversity of the genus, Lecanosticta, as a whole, as well as the population structure and genetic diversity of D. pini in Europe, and D. septosporum in a recent outbreak of DNB in Spain. Research Chapter 1 aimed to identify a large collection of Lecanosticta isolates from infected pine needles from Guatemala, Honduras and Nicaragua based on sequence comparisons of five gene regions and to determine if L. acicola is present in the region. Six species of Lecanosticta, of which four are novel taxa, were identified. Lecanosticta jani, L. pharomachri, L. tecunumanii and L. variabilis were described and new hosts and geographical records were reported for previously described L. brevispora and L. guatemalensis. Lecanosticta acicola was not found in any of the samples considered and this species was redefined as a Northern Hemisphere taxon with three distinct lineages based on the translation elongation 1-α gene region. It was concluded that due to the high species diversity of Lecanosticta in Mesoamerica, that this region is considered a centre of diversity for the genus. The second research chapter aimed to investigate the population diversity, structure and mode of reproduction and likely means of spread for a large collection of D. pini obtained from twelve countries in Europe over a 12-year period. Based on population structure analyses, the D. pini populations considered grouped in four main geographic clusters and variable population diversity was observed between countries. Both mating types were detected in most countries but evidence for sexual recombination was only supported in the population from Spain. The observed population structure as well as several shared multilocus haplotypes between non-bordering countries provides evidence that the occurrence and spread of D. pini on the continent has been strongly influenced by human mediated activities in Europe. The last research chapter of the thesis was to address the unprecedented DNB outbreaks observed in Cantabria in Spain, in 2015, by determining the causal agent, and to determine the population structure, genetic diversity and mode of reproduction of the pathogen involved, from three infected sites of planted Pinus nigra subspecies. Dothistroma septosporum was confirmed as the causal agent. Structure analyses revealed two genetic clusters in the populations examined. Both mating types were detected amongst the isolates, however, sexual recombination was not statistically supported for any of the sites. The high genetic diversity observed in isolates obtained from Cantabria suggested that the pathogen was not recently introduced into the region. In conclusion, the findings of this thesis emphasize the importance of quarantine and the need for caution when pine germ plasm and other pine material is moved between continents, countries and different regions. Of the nine species of Lecanosticta, for which four novel taxa were described, eight species occur only in Mesoamerica. Lecanosticta acicola, that has become an increasingly important pathogen of Pinus species, is the only species that occurs outside of this region. The other Lecanosticta species are of unknown importance but could pose a threat to Pinus species if introduced into new environments in the future especially considering that Mesoamerican pines are being used for plantation development in the Southern Hemisphere. Furthermore, through the first large scale population study of Dothistroma pini, it was shown that this pathogen is not new to the European continent, however, human activities have contributed to the spread of this pathogen. The further spread of genetically diverse individuals could lead to future unprecedented outbreaks of D. pini as well as the possible introduction of this pathogen into the Southern Hemisphere where it is currently still not present. The unprecedented outbreaks of D. septosporum in Cantabria was not due to a recent introduction into the region. It was shown that genetically diverse individuals were present in the infected sites and it is likely that these outbreaks were due to increased precipitation that prompted the high infection levels observed.