Submitted to: Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/23/2007
Publication Date: N/A
Citation: Interpretive Summary: Phytophthora blight caused by the fungus Phytophthora capsici is a common, serious disease of numerous crops in the U.S., China, and many parts of the world. The disease is one of the most serious threats to processing peppers, causing up to 100% yield losses in China. Recent surveys show that the aggressiveness of strains of the fungus varies considerably, but that some strains of this fungus do not attack at all. We conducted molecular studies on strains from diseased plants and from soil, in order to compare and distinguish those strains that attack crops from those that are harmless. The molecular patterns in two different kinds of tests successfully distinguished the two groups of fungi. This information could lead to the rapid detection of specific types of fungi present in soils and crops, and to the development of new disease control strategies. This research will be useful to breeders, seed companies, APHIS, and research scientists concerned with controlling diseases caused by different strains of a fungus.
Technical Abstract: Thirty-six Phytophthora capsici strains and one Phytophthora parasitica strain were evaluated for pathogenicity and disease severity on pepper (Capsicum annuum) plants. The strains represent a range of geographic locations and were collected primarily from pepper stems or roots of plants with symptoms of stem and fruit blight of peppers, and from soil. Strains were designated pathogenic or nonpathogenic (NP) based on disease severity (DS). Among the 36 P. capsici strains, 13 were not pathogenic to pepper, and 23 were pathogenic. Genetic diversity of the strains was assessed by sequence analysis of the rDNA internal transcribed spacers (ITS1 and ITS2) and the 5.8S rDNA gene, and by RAPD analysis. Using ITS region analysis, the strains grouped into two clusters. While all pathogenic strains were placed in group I, they separated into four subgroups (subgroups I to IV). The NP P. capsici strains resolved into two groups, in which 84.6% of NP strains were in ITS group II, and 15.4% of the NP strains were in ITS group I. Pathogenic and nonpathogenic strains also separated into different clusters based on RAPD data, although two of the NP strains grouped with pathogenic strains. The population of pathogenic strains was less diverse than that of the NP strains, suggesting that the pathogenic strains were possibly of monophyletic origin. For both pathogenic and NP P. capsici strains, no relationship was observed between the genetic profiles and geographic or host plant part origin. This may indicate that pathogens do not originate independently at each locality.