Movement of Phakopsora pachyrhizi (Soybean Rust) Spores by Non-Conventional Means

Authors: Hartman, Glen; HAUDENSHIELD, S - UNIV OF ILLINOIS

Submitted to: European Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/7/2008
Publication Date: 1/11/2009
Citation: Hartman, G.L., Haudenshield, S. 2009. Movement of Phakopsora pachyrhizi (Soybean Rust) Spores by Non-Conventional Means. European Journal of Plant Pathology. 123:225-228.

Interpretive Summary: There has been much effort to document the occurrence of Phakopsora pachyrhizi in sentinel and mobile locations throughout the continental United States. The movement of rust spores has been documented to occur by wind-blown spores. Currently the fungus appears to over winter in the warmer areas of the U.S. including kudzu patches in Florida, Louisiana, Mississippi, Texas, and possibly other states along the Gulf Coast. The first author returned from a research trip to the southern part of the U.S. during which several days (15-16 October 2007) were spent in soybean fields and/or plots where rust occurred. The trousers and shoes were used to detect soybean rust using a microscope and a quantitative molecular assay (Q-PCR). We observed 4 and 48 spores from the tongue and 7 and 9 spores per cm2 from the front toe of the left and right shoes, respectively. From the trousers, we observed 99 and 22 spores per cm2 from the left and right leg, respectively, and 3 spores per cm2 from the pocket. These results were further confirmed using Q-PCR. Although we did not demonstrate viability of hitchhiking spores, and there is no documented evidence to indicate that soybean rust spores have been spread by human garments, it seems plausible that this could occur considering the large number of spores that can be found on clothing. This information is useful for soybean pathologist, epidemiologists, and models that are interested in the movement of soybean rust spores.

Technical Abstract: There has been much effort to document the occurrence of Phakopsora pachyrhizi in sentinel and mobile locations throughout the continental United States. But before the introduction of soybean rust to the continental U.S., there were concerns about its entry in ways unrelated to air movement, including human-assisted introduction. Although there has been no apparent documentation of such transport, conveyance by humans remains a potential for spore movement. The first author returned from a research trip to the southern part of the U.S. during which several days (15-16 October 2007) were spent in soybean fields and/or plots where rust occurred. The trousers and shoes from those two days were isolated from other clothing in a plastic bag. Five days later, double-sided cellophane tape (1.9 cm wide) was adhered to microscope slides and then pressed to various parts of the trousers and shoes. We observed 4 and 48 spores from the tongue and 7 and 9 spores per cm2 from the front toe of the left and right shoes respectively. From the trousers, we observed 99 and 22 spores per cm2 from the left and right leg, respectively, and 3 spores per cm2 from the pocket. These results were further confirmed using Q-PCR. DNA detection crossed the quantitation threshold between 23 and 36 amplification cycles (the Ct value). Estimates of the number of P. pachyrhizi spores were made by comparing the Ct values determined with the Ct values of the reference DNA plotted as a standard curve. In the first experiment, an average of 250 spores per tape section was determined, and when repeated, an average of 190 spores was determined. Although we did not demonstrate viability of hitchhiking spores, and there is no documented evidence to indicate that soybean rust spores have been spread by human garments, it seems plausible that this could occur considering the large number of spores that can be found on clothing. It may be justified to raise traveler’s awareness that transporting rust spores to other locations via clothing is quite likely.