Date of Award

Fall 12-18-2015

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Ecology and Environmental Sciences

Advisor

Seanna Annis

Second Committee Member

Jianjun Hao

Third Committee Member

Paul Rawson

Abstract

Valdensia heterodoxa, a fungal pathogen that attacks multiple plant species in the Ericaceae family, was found in Nova Scotia in 1997 and more recently in Maine in 2009 on lowbush blueberry hosts (Vaccinium angustifolium and Vaccinium myrtilloides). The fungus leads to leaf abscission in blueberry crops and causes substantial yield declines when outbreaks in fields are severe. Spatial proximity of field sites to geographic features was assessed to indicate whether certain site conditions may promote pathogen establishment and whether pathogen sites were closer to road networks than would occur at random. Genetic diversity and population structure of isolates of V. heterodoxa, collected from 2003 to 2014 in Maine, Nova Scotia, and Prince Edward Island, were also evaluated to determine if there was clonality and indicate whether the fungus has spread between fields in the recent past. The aforementioned analyses can provide insight into how V. heterodoxa has been dispersed and whether dispersal was human-mediated.

Clonal susceptibility of lowbush blueberry was examined to determine whether genetic variation among hosts influences disease severity. Single and multiple spore inoculation trials were designed to evaluate differences in lesion size, conidial production, and sclerotial development among multiple pathogen isolate and host clone pairings. Inoculation experiments may provide a foundation for determining the level of susceptibility and potential resistance in lowbush blueberry fields and may suggest whether outbreaks of the fungus in the future have the potential to become more severe if all clones of lowbush blueberry are equally susceptible.

Proximity and genetic analyses provided evidence for recent spread of V. heterodoxa in Maine. Pathogen sites were closer to roads and general paths than random sites with in a field, and routes following road networks were shorter between nearby pathogen sites compared to random sites. Field edges and shaded areas were closer on average to infected compared to random sites and may be important in determining where V. heterodoxa establishes. Microsatellite analyses revealed many shared haplotypes among fields in both Maine and Maritime Canada. Clonal reproduction was evident throughout the two regions, and many fields consisted of a single multi-locus haplotype of the fungus. Field management described more of the partitioning of genetic diversity than sub-region.

Trials examining susceptibility and resistance in lowbush blueberry clones to V. heterodoxa were inconclusive. Inoculations of leaves with the fungus in trials rarely had an infection success rate of more than 50%. Successfully infected leaves of different clones did show variation in lesion size, as did inoculations with spores of different isolates. There may be genetic diversity in the host that affects establishment of the fungus; however, the failure rate of inoculations prevents any conclusions being drawn without further research.

Genetic and spatial analyses indicate that V. heterodoxa has been spread around Maine, most likely by humans, and has propagated by asexual reproduction to establish new infections in fields. Landscape features of the field and clonal diversity of the host may affect this establishment.

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