http://hdl.handle.net/1957/17954 2013-06-19T19:21:23Z http://hdl.handle.net/1957/38181 Fungal endophyte diversity in foliage of native and cultivated Rhododendron species determined by culturing, ITS sequencing, and pyrosequencing Raizen, Nathaniel L. Western Oregon is home to native Rhododendron species and is the center for cultivated Rhododendron production in the United States. These Rhododendron spp. are known to be infected with fungal endophytes. However, the community structure of these endophytes in native and cultivated Rhododendron is poorly understood. Our study targeted the foliar fungal endophyte communities of two native Rhododendron spp. and two non-native commercially cultivated Rhododendron varieties. Parallel culturedependent (fungal isolation and identification based on ITS sequencing) and cultureindependent sequencing approaches (metagenomic sequencing of the ITS region using 454 pyrosequencing) were employed, and results provided evidence of distinctly different community structure in each host species. Additionally, results indicated higher diversity among cultivated and nursery grown Rhododendron. This suggests that the close proximity of the nurseries sampled to the forest environment allows exposure to two distinct sources of endophyte infection. Together, our results show the importance of host identity and environment in structuring the associated endophyte communities. Graduation date: 2013 2013-03-21T00:00:00Z http://hdl.handle.net/1957/37709 Organic pesticide modification of the species interactions in an annual plant community Pfleeger, Thomas G. Graduation date: 1991 1990-10-12T00:00:00Z http://hdl.handle.net/1957/37609 Population dynamics, extraction, and response to nematicide of three plant parasitic nematodes on peppermint (mentha piperita L.) Merrifield, Kathryn J. The efficiencies of wet sieving/sucrose centrifugation (WS/SC) recovery of Pratylenchus penetrans (59 %), Paratylenchus sp., (80 %), and Criconemella xenoplax (66 %) were established. Baermann funnels (BF) recovered significantly more P. penetrans (p = 0.01) and significantly less (p = 0.01) C. xenoplax than did WS/SC. While densities of P. penetrans in stored soil remained constant over the three days following field sample collection, Paratylenchus sp. and C. xenoplax densities increased significantly on the second day and decreased to their original level on the third day. During mist chamber extraction, P. penetrans continued to emerge from peppermint root tissue for 38 days, but 90 % of the total was recovered after 10 days. The standard core, consisting of 500 g dry soil plus the roots and rhizomes in that soil, was developed to express endoparasitic and ectoparasitic nematode densities in peppermint field soil, roots, and rhizomes. Enumerating nematode densities within the different plantsoil components of a particular volume of soil more closely describes the total nematode population pressure on the plant growing in that volume of soil. Therefore, endoparasitic nematode population levels were expressed as numbers in standard core soil, roots, rhizomes, or total core (soil, root, and rhizome populations combined). P. penetrans populations in peppermint fields peaked in early May, decreased through the summer, peaked again in August, and decreased through the fall to a low winter level. Peaks in the P. penetrans population followed peppermint root weight peaks by 3 to 6 wks. Paratylenchus sp. populations remained at relatively low levels throughout the year except for a pronounced peak in August, which followed the root weight peak by 3 to 6 wks. The C. xenoplax population also peaked 3 to 6 wks after the August root weight peak but fluctuated markedly throughout the remainder of the year. From 70 to 90 % of the total P. penetrans population was in roots in early May, decreased to 40 to 50 % by late June and 20 to 40 % in August. Up to 20 % of the population was in rhizomes on some dates, but the rhizome percentage was usually less than 10. Fewer P. penetrans were recovered from rhizomes during the harsh winter of 1988-89 than during the mild winter of 1989-90. Analysis of point samples (pretreatment, posttreatment, and harvest samples) and area under nematode population curves (AUNPC) were used to compare nematode populations in oxamyl-treated (1.1 kg a.i./ha) and nontreated plots in two peppermint fields through the two growing seasons. Point sample analyses detected significant decreases in treated soil, root, and total standard core P. benetrans populations compared to nontreated populations in several pretreatment and harvest sample dates and in two rhizome harvest sample dates. No treatment differences were observed in Paratylenchus sp. or C. xenoplax populations using this analysis. AUNPC analysis detected significant decreases in several treated root and rhizome P. penetrans populations compared to nontreated populations and in total core populations in field 1 during one growing season and in field 2 during two growing seasons. Significant decreases in C. xenoplax populations were observed in one field during one growing season. Peppermint hay weight was significantly greater in treated than in nontreated plots in one of three fields in 1988 and in one of three fields during 1989. Oil in ml/kg fresh hay weight was significantly lower in treated than in nontreated plots in one of three fields during 1989. No treatment differences were detected in milliliters of oil distilled from 2m² field area. Peppermint oil production is the final measure of a treatment from a mint grower's perspective. Because oxamyl had no effect on mint oil production, AUNPC appears to be a better measure of parasitic nematode pressure on peppermint, since this method of analysis detected fewer significant differences between nematode populations between treated and non-treated plots. Graduation date: 1991 1990-06-04T00:00:00Z http://hdl.handle.net/1957/37339 Effect of wheat cultivar mixtures on populations of Puccinia striiformis races DiLeone, Julie A. This study quantified the frequency of simple versus complex races of Puccinia striiformis Westend. in mixtures of wheat cultivars possessing different race-specific resistance genes. A simple race of a pathogen can infect only one component, and a complex race of the pathogen can infect two or more components of an intraspecific plant mixture. The treatments were designed so that the race that was complex changed depending on the host mixture, thus enabling us to observe the influence of pathogen complexity in different host genetic backgrounds. Six cultivar mixtures and one pure stand of winter wheat were inoculated with three races of P. striiformis (CDL 27, CDL 29, and CDL 41) at two locations for two seasons. Potted plants of three winter wheat cultivars (Paha, Tres, and Tyee) that were each susceptible to one of the three races of the pathogen were used to sample the pathogen during the field epidemics. Disease incidence on the differential cultivars was used to calculate the proportion of the three races in each treatment. The specific cultivars included in the mixtures influenced the frequencies of the three races. Increasing the number of virulent races in a mixture reduced the frequency of the complex race relative to the other two races. When two of the races (races 29 and 41) were complex on the same mixture, location had an effect on which of the races was more frequent. When race 29 was the complex race in the mixture, it was more frequent than when race 41 was the complex race. The results suggest that environmental interactions, genetic background of the pathogen race, host composition, and interaction among pathogen races may be as important in determining race frequencies in mixtures as is stabilizing selection sensu Vanderplank (1968). Graduation date: 1993 1993-01-28T00:00:00Z