Graduate Thesis Or Dissertation
 

Phenotyping Ornamental Plants: Case Studies in Fruit Quality Attributes from Vaccinium ovatum and Disease Resistance of Cotoneaster spp.

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  • Landscape plants are highly diverse and nursery producers often grow thousands of taxa to satisfy the varied needs and aesthetic tastes of consumers. Related to this diversity, ornamental plant breeders must be nimble and diverse in their approach. Two seemingly disparate studies were conducted to investigate improving ornamental shrubs – specifically, fruit quality and disease resistance. Vaccinium ovatum Pursh. (evergreen huckleberry) is an evergreen shrub native to the Pacific Northwest. With the notable exception of cultivated blueberry (Vaccinium corymbosum) species in Vaccinium are primarily diploids (2n = 2x = 24) but polyploids have been reported from experiments with chemical mitotic inhibitors as well as naturally occurring tetraploids in cultivated blueberry (Vaccinium corymbosum). There is interest in this species, but it requires improvement of the fruit and plant qualities for an eventual cultivar release. To obtain variation in plant qualities polyploidy was induced in a collection of plants in 2013. The purpose of this study was to assess the impacts of polyploidy on the fruit and plant qualities of Vaccinium ovatum. This fruit and plant quality study provides a contribution to the scientific knowledge base that is currently lacking on evergreen huckleberries. Plant qualities were determined by measuring plant height and width, obtained in fall 2017. The fruit volume (mm3) and brix (°Bx), for soluble solids content, were measured using a digital caliper and a digital refractometer respectively. Measurements were taken on diploid, mixoploid, and tetraploid (2x, 2x + 4x, 4x) cytotypes, once in 2017, five times over nine weeks in 2018, and three times over nine weeks in 2019. Fruit volume in 2017 increased from diploids to tetraploids (P < 0.0001), brix also increased in these cytotypes. Data from 2017 suggested there was a “gigas” effect from of polyploidy in evergreen huckleberries. However, 2018 and 2019 fruit volume of tetraploid fruit was smaller than that of diploid and mixoploid. Differences were observed in diploid fruit volume among all years (P < 0.0001). In tetraploids, brix was statistically significant across all years (P = 0.0002). The variation in data from the various years suggests that using ploidy as a way to produce larger sweeter fruit is not a plausible method. The genus Cotoneaster Medik. is composed of around 400 species with a wide variety of growth habits and form. These hardy landscape shrubs use to be commonplace because of their low maintenance and landscape functionality. However, the interest in and sales of Cotoneaster have declined for a variety of reasons, the largest being its susceptibility to a bacterial disease fire blight, caused by Erwinia amylovora. The resistance of 15 different genotypes of Cotoneaster was tested by inoculating leaves with a wildtype (Ea153) and an avrRpt2 strain (LA635) of Erwinia amylovora. Four studies took place in climate-controlled growth chambers and one study took place in a greenhouse in Corvallis, OR. Fire blight resistance was assessed by calculating the percent shoot necrosis (PSN = 100*(lesion length/total branch length)) once a week for six to eight weeks after inoculation. Across all studies, genotypes H2011-01-002 and H2011-02-001 consistently had the lowest levels of percent shoot necrosis. Plants inoculated with different isolates were directly compared from growth chamber studies done in 2019. Genotype H2011-02-005 was significantly more resistant to EA153 than to LA635 while C. splendens was significantly less resistant to EA153 than to LA635. Genotype H2011-02-001 has already been released as a new ornamental cultivar that has high fire blight resistance. Several other genotypes could be released as highly fire blight resistant for areas where the disease is widespread.
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