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United States Department of Agriculture

Agricultural Research Service

Research Project: SOYBEAN DISEASE AND PEST MANAGEMENT Title: Physiological specialization of Phakopsora pachyrhizi isolates on soybean in the United States from 2006 to 2009

Authors
item Twizeyimana, Mathias -
item Hartman, Glen

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 12, 2011
Publication Date: January 1, 2012
Citation: Twizeyimana, M., Hartman, G.L. 2012. Physiological specialization of Phakopsora pachyrhizi isolates on soybean in the United States from 2006 to 2009. Plant Disease. 96:75-81.

Interpretive Summary: Phakopsora pachyrhizi, cause of soybean rust, represents a classic case of a pathogen introduction that became established to a new geographical region (U.S.) on a perennial overwintering host (kudzu). The objective of our study was to classify the pathogenic variation in P. pachyrhizi isolates collected from the U.S., and to determine the spatial and temporal associations related to this pathogenic variation. A total of 72 isolates of P. pachyrhizi collected from infected kudzu and soybean leaves in the U.S. were purified, established, and increased on detached leaf cultures. These isolates were tested for virulence and aggressiveness on a differential set of soybean genotypes that included six genotypes with known resistance genes (Rpp), one resistant genotype without any known characterized resistance gene, and a susceptible genotype. Three pathotypes were identified among the 72 U.S. P. pachyrhizi isolates based on the virulence of these isolates on the differential set. Six aggressiveness groups were established based on sporulating uredinia production recorded for each isolate on each soybean genotype. All three pathotypes and all six aggressiveness groups were found in isolates collected from the southern region, from collections made in 2008, and from both hosts (kudzu or soybean). This study establishes a baseline of pathogenic variation of P. pachyrhizi in the U.S. that can then be compared to variation reported in other regions of the world. This information will be useful to plant pathologist and breeders interested in knowing more about pathogenic variability of the soybean rust pathogen.

Technical Abstract: Phakopsora pachyrhizi, cause of soybean rust, represents a classic case of a pathogen introduction that became established to a new geographical region on a perennial overwintering host (kudzu). The objective of our study was to classify the pathogenic variation in P. pachyrhizi isolates collected from the U.S., and to determine the spatial and temporal associations related to this pathogenic variation. A total of 72 isolates of P. pachyrhizi collected from infected kudzu and soybean leaves in the U.S. were purified, established, and increased on detached leaf cultures. These isolates were tested for virulence and aggressiveness on a differential set of soybean genotypes that included six genotypes with known resistance genes (Rpp), one resistant genotype without any known characterized resistance gene, and a susceptible genotype. Three pathotypes were identified among the 72 U.S. P. pachyrhizi isolates based on the virulence of these isolates on the differential set. Six aggressiveness groups were established based on sporulating uredinia production recorded for each isolate on each soybean genotype. All three pathotypes and all six aggressiveness groups were found in isolates collected from the southern region, from collections made in 2008, and from both hosts (kudzu or soybean). Shannon’s index based on the number of pathotypes, indicated that isolates from the southern region were more diverse (H values of 0.83) compared to the isolates collected in other regions. This study establishes a baseline of pathogenic variation of P. pachyrhizi in the U.S. that can then be compared to variation reported in other regions of the world and in future studies that monitor P. pachyrhizi virulence in association to deployment of rust resistance genes.

Last Modified: 12/21/2014
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