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A pioneering perspective on the aerial dissemination of a plant pathogen: posthumous publication of the doctoral thesis of Gary Franc


(Text by Tom Hill, Colorado State University, USA, posted by Cindy Morris)

Gary Franc, who died in October 2012, was a Professor of Plant Pathology at The University of Wyoming. Starting with his PhD studies at Colorado State University, Gary pursued a unique research theme involving the interaction of bacteria with atmospheric processes and their long distance transport within the water cycle. Here we are making available his PhD thesis, which contains much original, painstaking and unpublished work (thesis-part_1thesis-part_2).

Gary’s PhD was interdisciplinary and truly pioneering. He investigated the role of atmospheric transport in the distribution of two Erwinia species that cause blackleg disease of potato. His quest was to identify the original source of infections occurring in the San Luis Valley, Colorado, and he pieced together a path for the bacteria that began with their release from seawater, their activation into cloud droplets and, finally, their incorporation into precipitation falling on the Rockies. This was proposed as a mechanism to transfer these plant pathogenic bacteria from their vast, natural oceanic reservoir to terrestrial plant hosts far inland.


Gary Franc during a sampling campaign at Storm Peak Laboratory, which he co-established. Now directed by Gannet Hallar, it is a permanent research and educational facility, operated by the Desert Research Institute, and is a member of the NOAA Collaborative Aerosol Network.


Potatoes are susceptible to infection by the bacterium Erwinia (now Pectobacteria), which causes potato blackleg disease. Two subspecies, E. carotovora subsp. carotovora (now Pectobacterium carotovorum subsp. carotovorum) and E. carotovora subsp. atroseptica (now Pectobacterium atrosepticum) are the primary agents. They are present in rivers, and the use of contaminated water for irrigation is an efficient means of re-infesting potato crops. In the Rocky Mountains, thawing snowpacks contribute significantly to spring runoff, and the accumulated snow is deposited by storms usually originating over the Pacific Ocean.

To test if Erwinia in river water may have originated from thawing snowpack, Gary undertook a survey to determine if viable E. carotovora cells could be recovered from ocean water, rainwater, and aerosols on the west coast of the United States, and from snow collected at inland sites. Erwinia carotovora was recovered from at least 80% of ocean and rainwater samples and was also present in aerosols. In seawater, collected at sites ranging from Alaska to the west coast of central Mexico and from the northern coast of the Dominican Republic, it occurred at an average concentration of 1.4 cells per 100 ml. Approximately 5% of the snow samples collected at remote sites in the Rocky Mountains also yielded E. carotovora. Recovery of viable cells from snow was more likely when the transport time to inland sites was less and the average relative humidity along the route was higher.

In order to survive the long journey aloft, Gary inferred that aerosolized bacteria must also participate in cloud microphysical processes. That is, the probability of bacterial survival would increase with a higher relative humidity and if aerosolized cells could act as cloud condensation nuclei (CCN), which would afford them protection from both desiccation and radiation. CCN activity would also increase the likelihood of cell deposition in precipitation. Their presence in precipitation provided some indirect evidence of the potential of aerosolized cells to activate cloud droplets.

Subsequent work with Paul DeMott at CSU confirmed the CCN characteristics of Erwinia (Franc and DeMott 1998). This was the first full characterization of its kind for airborne cells. Several strains of E. carotovora subsp. carotovora and E. carotovora subsp. atroseptica were shown to be active as CCN. Approximately 25%–30% of the aerosolized bacterial cells activated droplets at 1% water supersaturation compared to 80% activation of the ammonium sulfate aerosol. Within winter storms, bacteria could, therefore, be accreted onto precipitating ice particles following their collection within cloud droplets.

Franc, G. D. 1988. Long distance transport of Erwinia carotovora in the atmosphere and surface water. PhD thesis, Department of Plant Pathology, Colorado State University: Fort Collins, 131pp. (thesis-part_1; thesis-part_2)

Franc, G. D. and DeMott, P. J. 1998. Cloud activation characteristics of airborne Erwinia carotovora cells. Journal of Applied Meteorology 37, 1293–1300.

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