Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/19/2004
Publication Date: 7/1/2004
Citation: Atallah, Z.K., Larget, B., Chen, X., Johnson, D.A. 2004. High genetic diversity, phenotypic uniformity, and evidence of outcrossing in sclerotinia sclerotitorum in the columbia basin of washington state. Phytopathology. 94(7):737-742. Interpretive Summary: Stem rot, caused by the fungal pathogen Sclerotinia sclerotiorum, is an important disease on potatoes in the Columbia Basin of Washington. Because of the lack of knowledge on population structures of the pathogen, the disease is poorly managed. In this study, samples of the fungus were collected from the major potato-growing region and used to determine the population structure. Using molecular markers, we found that the major variance of the pathogen was within subpopulations. Limited but statistically significant variance was found among different collection dates. Significant difference was not found in different years and locations. The data also suggested a potential for outcrossing, which also supported by the discovery of recombinant sexual spores of the fungus. The molecular groups were not correlated with the mycelial groups. Aggressiveness was not different among the fungal isolates in disease testing in the greenhouse. Testing with fungicides and temperatures did not yield significant difference among isolates. The findings in this study will be helpful in improving the disease management strategies.
Technical Abstract: Sclerotinia sclerotiorum, the causal agent of stem rot, is prevalent and poorly managed on potatoes in the Columbia Basin of Washington. Because of the ubiquitous nature of the fungus and high crop diversity within the Columbia Basin, understanding the population structure and the potential for outcrossing of the pathogen would be helpful in developing disease management strategies. The population structure of S. sclerotiorum in the Columbia Basin from potato was examined using microsatellite markers and mycelial compatibility. Analyses of molecular variance revealed that 92% of the variability among 167 isolates was found within subpopulations, with limited, yet statistically significant impact of the collection date, but not the year or location of collection. Linkage disequilibrium and index of association analyses noted a potential for outcrossing in two locations, which was substantiated by the discovery of recombinant ascospores in three field-generated apothecia from the 12 apothecia examined. Microsatellite haplotypes were not correlated with mycelial compatibility groups. This high haplotypic diversity did not seem to impact pathologically important phenotypes. Greenhouse inoculations of potato plants exhibited no significant differences in aggressiveness on potato stems. Moreover, in vitro studies of response to fungicides and temperature stimuli yielded no significant differences among studied isolates. These findings illustrate the potential for outcrossing in warm temperature regions of North America, where a diversity of crops are planted simultaneously and in neighboring fields. This study also indicates that the unsatisfactory management of potato stem rot is likely not directly attributable to genetic factors, but to gaps in agricultural practices.