Molecular characterization of selected Pythium species

Pythium includes several plant pathogenic species responsible for damping off of seedlings and root rot diseases. These pathogens are among the most important in greenhouse crops due to the great economic losses they cause. Despite the efforts made to control Pythium diseases, greenhouse crops are affected year after year.; To develop effective management strategies it is important to understand the population dynamics of these organisms. The objectives of this investigation were to (1) develop AFLP fingerprints to identify the plant pathogenic Pythium species; (2) analyze the population genetics of two of the most frequently encountered species in Pennsylvania greenhouses, P. irregulare and P. aphanidermatum; and (3) describe the population structure of theses species and factors correlated with that structure.; Amplified fragment length polymorphisms (AFLP) were generated with two primer combinations for P. aphanidermatum and three for P. irregulare. These markers were analyzed using phylogenetics as well as population genetics tools. Sequence analyses of the internal transcribed spacers (ITS) region of the ribosomal DNA and the cytochrome c oxidase two (COX II) genes were also performed to support the results obtained from fragment data.; The fingerprints for nine Pythium species were obtained that can be used as a reference for identification of isolates to species level, and to develop probes for species identification. Two distinct populations were identified within P. irregulare that could not be correlated with host, geographic origin or resistance to mefenoxam. The differences among these populations were large and justify considering P. irregulare as a complex that includes at least two cryptic species. P. aphanidermatum has little genetic diversity, but 9.5% of its diversity correlates with host association and 9.3% to mefenoxam resistance. Sexual reproduction and outcrossing may contribute to the genetic homogeneity of this species.