Browsing by Author "Turner B. Sutton, Committee Chair"

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    The Common Barberry: The Past and Present Situation in Minnesota and the Risk of Wheat Stem Rust Epidemics
    (2003-06-04) Peterson, Paul David Jr.; Kurt J. Leonard, Committee Member; Steven Leath, Committee Member; William C. Kimler, Committee Member; Turner B. Sutton, Committee Chair
    One of the classic host-pathogen relationships in plant pathology is between the common barberry, Berberis vulgaris, and Puccinia graminis, the cause of stem rust, a destructive disease of small grains. As the alternate host of P. graminis, the barberry is the key in the sexual stage of the pathogen's life cycle. The combination of extensive small grains production and widespread cultivation of the common barberry in the north central United States resulted in major stem rust epidemics by the late 1800s and early 1900s. In 1918, the Barberry Eradication Program was initiated by the U. S. Department of Agriculture in cooperation with important small grain producing states. Before the termination of the program in the late 1970s, more than 500 million barberry bushes were destroyed. Since the end of the program, however, scientists have voiced concerns about the potential for barberry to reemerge as a source of inoculum and of new genetic forms. The objectives of this research were to examine the history of barberry eradication in Minnesota, to determine whether barberry has reemerged on sites in Minnesota and what effect this reemergence may have on future stem rust epidemics. An evaluation of archival records revealed that the origin of the Barberry Eradication Program was in reaction to successive stem rust epidemics and concerns over production shortages during World War I. The program that developed after 1918 was an unprecedented collaborative effort among federal-state agencies, land-grant colleges, and agricultural industry. Initial survey work was focused on the removal of barberry bushes in cities, towns, and rural planted sites; priorities shifted and procedures changed with the discovery of large numbers of escaped bushes, particularly in southeastern Minnesota. More than one million barberry bushes were destroyed in Minnesota between 1918-1990. A field survey of 72 of the approximately 1200 active sites in Minnesota was conducted. Active sites were defined as those sites that remained to be inspected for barberry regrowth at the end of the eradication program in 1980. Barberry had reemerged on 32 of the 72 sites. More than 90% of the barberry bushes were found in counties with less than 400 ha of wheat per county, mostly in southeastern Minnesota, but one bush was found in a major wheat-producing county in northwestern Minnesota. Reemergence of barberry may play a role in future epidemics of stem rust, particularly with regard to sexual reproduction in the pathogen population. Aecial isolates of P. graminis collected from common barberry in Minnesota between 1912 and 2002 were obtained and used to evaluate changes in forma speciales over time. Uredinial isolates collected in Minnesota during the same time period were compared to the aecial isolates to evaluate changes in race structure of P. graminis f.sp. tritici. Forma speciales in aecial populations changed over the 20th century and coincided with changes in barberry populations. P. graminis f. sp. tritici has become the predominant forma speciales identified in collections since 1990, resembling the pathogen population structure before eradication. Removal of barberry from areas around wheat fields contributed to a reduction in race diversity in uredinial populations; however, diversity in aecial populations was unchanged in relation to barberry removal. With the lessons learned historically during barberry eradication, the knowledge that barberry has reemerged on many sites and the recent changes in the P. graminis population, there is reason to be concerned over the possibility of increasing stem rust epidemics in Minnesota.
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    Pierce's Disease of Grapevines: Indentifying The Primary Vectors In The Southeastern United States
    (2007-04-17) Myers, Ashley Laurel; Turner B. Sutton, Committee Chair; David F. Ritchie, Committee Member; George G. Kennedy, Committee Member
    In the past 10 years the winegrape industry in the Southeastern United States has experienced rapid growth. However, further expansion may be inhibited by Pierce's disease (PD), caused by the bacterium Xylella fastidiosa that is transmitted from reservoir hosts to grapevines by sharpshooters and spittlebugs. Epidemiological studies were conducted to identify the primary vectors of X. fastidiosa to grapes in the Southeast by surveying sharpshooter populations in the eastern Piedmont and Coastal Plain of North Carolina where PD is most threatening, identifying potential sharpshooter vectors by PCR assays, conducting greenhouse experiments with potential vectors to determine transmission ability, and performing phylogenetic analyses of X. fastidiosa PCR products to provide information on what populations of X. fastidiosa sharpshooters in NC are carrying. In 2004 and 2005, leafhoppers were trapped in three vineyards in the eastern Piedmont and one vineyard in the northeastern Coastal Plain. Four insects have been identified as most abundant, Oncometopia orbona, Graphocephala versuta, Paraphlepsius irroratus, and Agalliota constricta. Specimens of O. orbona, G. versuta, and P. irroratus were tested for the presence of X. fastidiosa using a vacuum extraction method and nested PCR. Over the two seasons 27% of the O. orbona, 24% of the G. versuta, and 33% of the P. irroratus trapped were positive for X. fastidiosa. Transmission experiments were conducted with field-caught O. orbona and G. versuta. One hundred sixty-six vines used in transmission experiments were assayed for the presence of X. fastidiosa by ELISA. Bacterial DNA from an additional sample (n = 6) of symptomatic plants was subjected to two-step PCR to confirm ELISA results. Data indicate both G.versuta and O.orbona transmit X. fastidiosa to grape. Phylogenetic analysis of X. fastidiosa DNA from insects and sequences obtained in silico using Neighbor-Joining of 1000 bootstraps resulted in one most parsimonious tree with three populations grouping by host. SNAP workbench analyses collapsed sequences into to 12 haplotypes and Hudson's ranked Z statistic showed no population subdivision between insect hosts.