Black shank of tobacco, caused by the oomycete Phytophthora nicotianae, is an important disease of tobacco. Emergence of race 1 has led to loss of major gene resistance and to questions on the genetic diversity in the pathogen in North Carolina. Race 3 is reported for the first time in NC. Race 3 is virulent on plants with the Phl gene and not the Php gene and causes severe disease symptoms on mature plants. Isolates of race 3 were recovered from locations where the Phl gene was deployed and in fields characterized as the wild-type race, race 0, with no history of single-gene resistance. In order to determine whether races 1 and 3 can develop as natural variants from race 0, and to track loss of Php and Phl virulence in races 1 and race 3, soil was infested with one race of P. nicotianae and planted with tobacco varieties with multigenic resistance. Isolates were recovered after five months and screened for race. Additionally, zoospore isolates were derived from progenitor zoospore isolates representing the three races. Zoospore progeny were screened to identify changes in virulence during asexual sporulation. A subset of zoospore progeny was subjected to Fluorescent Amplified Fragment Length Polymorphism analysis to investigate genetic diversity generated through clonal sporulation. Results showed a gain and/or loss of virulence within all race progeny in soil and single-zoospore isolates. Race 1 was the most stable phenotype, with 91% in infested soil and 99.7% of the zoospore progeny retaining the virulence phenotype. The race structure in soil infested with races 0 and 3 were similar after five months. Races were recovered in a 2:1 ratio (race 0: race 3) with a small percentage of race 1. Races 0 and 3 zoospore progeny also segregated. Race 0 progeny were 67% race 0 and 33% gained virulence to the Phl gene (race 3). Similarly, 68% of the race 3-derived progeny retained the parental virulent phenotype, 31% lost the virulent phenotype (race 0), and 1% gained virulence to the Php gene (race 1). Estimates of genetic diversity within each group of related zoospores ranged from 0.17013 to 0.44196. Phenotypic and genotypic investigations revealed that asexual sporulation may be a major source of variation in natural populations. A state-wide survey of P. nicotianae populations was conducted in NC tobacco-producing regions. Isolates were obtained from 76 tobacco fields in 23 counties and screened for race and mating type. Race 1 was predominant in most regions, with 59% of fields consisting of 90 to 100% race 1. The occurrence of race 1 within fields was positively correlated with the history of monogenic resistance deployment. Race 3 was identified in low frequency throughout the state, primarily in wild-type populations where no monogenic resistance was deployed. The A1 and A2 mating types were found throughout NC and were recovered concurrently from multiple fields. Pairings of isolates from within fields yielded viable oospores, indicating for the first time, the potential for sexual reproduction by P. nicotianae. A subset of the survey isolates were screened for sensitivity to the fungicide mefenoxam. All isolates were sensitive, with a mean EC50 value of 0.4 μg/ml mefenoxam, indicating fungicide applications are still a reliable method of black shank management. Results reveal a rapid state-wide shift toward race 1, correlating with the deployment of monogenic resistance and indicate that sexual recombination may be important in generating variation within the pathogen population.
| Identifer | oai:union.ndltd.org:NCSU/oai:NCSU:etd-04282009-154531 |
| Date | 29 July 2009 |
| Creators | Gallup, Courtney Anne |
| Contributors | Dr. Kelly Ivors, Dr. Marc Cubeta, Dr. H. David Shew, Dr. Ignazio Carbone, Dr. Jennifer Nicholson |
| Publisher | NCSU |
| Source Sets | North Carolina State University |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://www.lib.ncsu.edu/theses/available/etd-04282009-154531/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dis sertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to NC State University or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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