ARS | Publication request: Patterns of Phakopsora pachyrhizi spore deposition detected in North America rain and their use to calibrate IAMS soybean rust forecasts in 2007

Submitted to: Phytopathology
Publication Type: Abstract
Publication Acceptance Date: June 30, 2008
Publication Date: July 26, 2008
Citation: Barnes, C.W., Szabo, L.J., Isard, S.A., Ariatti, A., Tenuta, A.U., Hambleton, S., Tropiano, R., Bowersox, V.C., Claybrooke, R., Lehmann, C. 2008. Patterns of Phakopsora pachyrhizi Spore Deposition Detected in North America Rain and Their Use to Calibrate IAMS Soybean Rust Forecasts in 2007 [abstract]. Phytopathology. 98:518.

Technical Abstract: In 2007, rain was assayed for Phakopsora pachyrhizi spores (the causal agent of Asian soybean rust) at 75 National Atmospheric Deposition Program sites in the US and at 12 sites in Canada using two types of rain collectors. A nested real-time PCR assay was used to detect P. pachyrhizi DNA in the filtered residue. This year, P. pachyrhizi spores were detected 90 times in the US and 29 times in Canada. The average detection percentage from US sites was 8.5% of rain samples, with a peak of 27% in mid August. The number of spores per collection was estimated using models generated by assaying a known number of P. pachyrhizi spores. Estimates ranged from a single spore/m2 to over 500. P. pachyrhizi spores were generally detected 3-5 weeks prior to first disease reports in a state, matching IAMS disease forecasting model estimates. Spore deposition results from the US and Canada were used to further calibrate the IAMS model. Two time intervals were analyzed because they showed the clearest indication of long-distance transport of spores; July 9-18 and Aug 13-28, 2007. During the mid July interval, 9 rain samples were positive for P. pachyrhizi at 9 different US sites, while there were 11 positive rain samples at 8 Canadian sites. The positive sites tended to cluster around Lake Erie, with estimates of 186 spores/m2 in central Ohio. During the August transport event, 29 US and 9 Canadian sites were positive for P. pachyrhizi. However, the focus of deposition was further west in the central plains north to Manitoba and Saskatchewan, with the highest level of spore deposition in Iowa at 243 spores/m2. This second deposition event correlates with the finding of Asian soybean rust disease in this region in late September and October.