Date of Award

Spring 5-8-2020

Level of Access

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Botany and Plant Pathology

Advisor

Seanna Annis

Second Committee Member

Jianjun Hao

Third Committee Member

Francis Drummond

Additional Committee Members

YongJiang Zhang

Abstract

Lowbush blueberry, Vaccinium angustifolium Aiton, is both an economically and culturally important crop in Maine, being one of the few endemic crops to North America. The fungus Monilinia vaccinii-corymbosi (Mvc) causes mummy berry disease and is a significant pathogen of both highbush (Vaccinium corymbosum) and lowbush blueberries. While impacts of this disease are not regularly documented, it is estimated that 30-50% of the yield in an unmanaged field can be lost because of Mvc. This disease is typically managed with fungicides or burning of the field during years when the field is pruned, however, the impacts to the population structure of this fungus by these management practices is not known. Management could have an effect on genetic diversity due to selection or the reduction of population size. Higher genetic diversity and rates of recombination of Mvc could potentially increase the risk of developing resistance to fungicides or other management strategies, and it could allow for the infection of genetically diverse lowbush blueberry plants. Using microsatellites developed for Mvc of highbush blueberry and specimens collected from three stages in its lifecycle, ascospores, conidia, and pseudosclerotia, the population structure of Mvc in lowbush blueberry fields was explored to better understand the genetic structure and reproductive system of Mvc.

The present study compared differences within and among Mvc populations obtained from various field types. Fields are selected and categorized as either entirely in prune or crop (which produce fruit every other year) or split between prune and crop (which produce fruit every year in separate portions of the field) which may affect genetic diversity of Mvc based on availability of susceptible lowbush blueberry tissue. Mvc populations were also compared among fields that are organic, transitioning to organic, managed with fungicides, or are completely unmanaged populations of Mvc harvested from the forest which may not be affected by genetic selection events associated with fungicide application or field burning.

A large degree of genetic diversity was observed from the twelve different fields, with 199 unique multilocus haplotypes (MLH) occurring in an original sample of 232 isolates, which was clone corrected to 203 isolates. Twelve private alleles, including six private alleles with frequencies above 0.05 which indicated gene flow, were observed in six out of twelve fields with the majority occurring in the Down East region. Fields were analyzed to determine differences within and among populations and regions, and the majority of variation was seen within fields. However, there were no significant differences among fields when comparing region, management strategy, or split to entire fields. There may have been a minor effect of manager on MVC populations. Factors that may affect gene flow are the close proximity of fields and sharing of equipment between fields. The optimum number of genetic clusters was identified to be two, and while no fields were fully assigned to one cluster or another, similar patterns were seen among some fields. The population of Mvc in Maine as a whole is mostly a sexual, outcrossing population as was seen in the diversity of MLH and validated by low amounts of linkage disequilibrium. Homothallic and heterothallic apothecia were both found in fields identified as mostly sexual and mostly clonal. Three fields were found to be clonal but are hypothesized to be displaying pseudohomothallism in which outcrossing individuals are closely related and their offspring appear to be clonal. Finally, the majority of private alleles were seen in pseudosclerotia isolates and the presence of two unique MLHs within a single pseudosclerotium could indicate that gene flow is occurring via conidia which result in the production of pseudosclerotia.

This study found that Mvc within Maine is a highly diverse sexual population, although homothallism is present. Some fields appear to have slightly higher amounts of clonal reproduction, but are not strictly reproducing clonally, as diverse MLHs were noted in these fields. It was also observed that while no fields fall neatly into one of the two genetic clusters identified, there are some fields with similar patterns of clusters which could indicate similarities between fields which share equipment and common managers. Management does not appear to greatly effect genetic diversity and Mvc may be one large statewide population in Maine.

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