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Document Type

Campus-Only Access for One (1) Year

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Organismic and Evolutionary Biology

Year Degree Awarded

2019

Month Degree Awarded

February

First Advisor

Lynn S. Adler

Second Advisor

Anne Averill

Third Advisor

Robert Wick

Fourth Advisor

Joan Milam

Subject Categories

Agricultural Science | Agronomy and Crop Sciences | Apiculture | Entomology | Fruit Science | Other Ecology and Evolutionary Biology | Plant Biology | Plant Pathology

Abstract

Background: Many plants must balance the need for pollination services with mediating the risk of pollinator-vectored pathogens. Vaccinium corymbosum, highbush blueberry, is negatively affected by an insect-vectored, fungal plant pathogen, Monilinia vaccinii-corymosi (MVC), the cause of mummy berry disease, in which the asexual spore mimics pollen grains and is transferred from blighted tissue to flowers via pollinators, resulting in inedible, hardened fruits. Highbush blueberry plants require outcrossed pollen for maximum yield and fecundity. Therefore, yield of blueberry plants rely on a balance between adequate pollination service and disease avoidance.

Approach: To explore the relationship between pollinator community and infection we used field observations and infection assessments to determine if differences in floral visitors can help to explain variation in infection between cultivars. To better understand the key vectors involved in transmission of MVC we used molecular quantification techniques to assess pathogen load on insect bodies and used a cage trial to determine how much of the pathogen is deposited by two common pollinators in a single visit. Finally, we used inoculation trials followed by fluorescence microscopy to determine if plants must balance pathogen inhibition with fertilization success, as well as assessing whether pathogen germination contributes to disease resistance.

Results and Conclusions: When investigating community composition we found that Apis, Bombus and Andrena visitation varied with cultivar and that there was also a positive relationship between the proportion of floral visits by honeybees to individual plants and the percentage of infected fruits. This is the first study to our knowledge comparing fruit infection with visits by different bee species.

In our investigation of pathogen load on vectors and single-visit transmission success we found that bees, flies, and wasps were all common visitors and that all the bee species and several species of flies and wasps carried the pathogen. We found no differences between A. mellifera or B. impatiens in pathogen load or transfer efficiency in cages, suggesting that both of these species are equally capable of vectoring MVC during a single visit to a blighted stem and then a flower. Taken together, this research emphasizes the wide variety of floral visitors capable of carrying the MVC pathogen and demonstrates that two common pollinator species have similar potential to vector MVC to blueberry flowers during a single visit.

Finally, we found no tradeoff between pollen and fungal spore germination on floral reproductive parts, suggesting that disease resistance traits mediated by stigma traits may not come at a cost of reduced pollination. We also did not find a relationship between spore germination and published disease resistance. This study adds to our understanding of disease resistance in natural and agricultural systems, which is especially important due to mounting concerns over the use and cost of fungicides, including negative effects on non-target organisms. Our findings also increase our understanding of the potential for both wild and managed pollinator species to contribute to the vectoring of a highly damaging blueberry pathogen, and plant pathogens in other systems as well.

Available for download on Saturday, February 01, 2020

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