More than 10 species of Phytophthora have been implicated as causes of root rot, crown rot, and trunk cankers on almond and walnut trees worldwide. In California, many isolates of the causal species were collected over the last several decades and identified according to their morphology, with some support from DNA sequencing. In this report, species identities were reexamined among 103 of the walnut and almond isolates according to sequences from the internal transcribed spacer 1 and 2 regions of the rRNA genes (ITS) and the cytochrome oxidase subunit 1 gene (COI). The reexaminations, supported by BLAST search comparisons with NCBI sequences from the Center for Plant Health Science and Technology (CPHST) or from first reports of a new species, resolved what had originally been identified as 14 species into 16 species of Phytophthora. Isolates originally identified as P. citricola sensu lato were resolved into P. acerina and P. pini; the former species was only found on almond, whereas the latter was found on both hosts, but most frequently from walnut. Also, some isolates originally identified as P. megasperma were resolved here as P. crassamura, and P. parasitica isolates were resolved into P. nicotianae and P. iranica. Almond isolates originally identified as P. cinnamomi had only 98% and 94% sequence identity with type ITS and COI sequences, respectively, and were closest in sequence identity to P. parasitica, (96% and 95% sequence identity for ITS and COI, respectively), suggesting the need for further examinations to identify the species. The results of this study will facilitate precise communications regarding etiology and management of Phytophthora diseases of almond and walnut. Specifically, the findings will aid in appropriate selection of isolates for testing genetic resistance of rootstocks, fungicide efficacy, and additional approaches to management of Phytophthora diseases.
|Advisor:||Rizzo, David M|
|Commitee:||Browne, Greg T, Trouillas, Florent P|
|School:||University of California, Davis|
|School Location:||United States -- California|
|Source:||MAI 81/11(E), Masters Abstracts International|
|Subjects:||Plant Pathology, Agriculture|
|Keywords:||Almond, Phylogenetics, Phytophthora, Walnut|
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