The Spread of Plant Diseases by Long-Range Atmospheric Transport and Deposition of Pathogen Propagules A New Application for an Old Precipitation Sampling (NADP) Network

Sagar Krupa
University of Minnesota,
St. Paul, MN USA

For more than four decades, scientists have been reporting long-range airborne transport of fungal spores, resulting in the spread of plant diseases across continents (Rayner and Hopkins, 1962) and even across oceans (Stover, 1962). A case in point is a sugarcane rust epidemic that erupted in the Dominican Republic in 1978, after more than 20 years of rust-free cane cultivation. Careful evaluation of potential causes for this outbreak led researchers to conclude that fungal spores originating from rust-infested sugarcane fields in Cameroon were the mostly likely source of inoculum (Purdy, Krupa, and Dean, 1985). Satellite imagery of Saharan dust transport, combined with air trajectory analysis, demonstrated that over a 9-day period, streamlines from Cameroon had followed a westward path across the Atlantic Ocean to the Caribbean island of Hispaniola, site of the Dominican Republic. A recent review article (Brown and Hovemiller, 2002) cites this as one example of the aerial dispersal and long-range transport of pathogens that resulted in a new plant disease being established thousands of kilometers from the nearest infection or source area.

The NADP network offers a spatial array of some 250 precipitation sampling stations that can be used to track pathogen deposition and the potential spread of airborne plant diseases to U.S. crop and forest ecosystems. For example, residues from NADP rain sample filters have tested positive for the DNA fingerprint of wheat stem (Puccinia graminis) and Asian soybean (Phakopsora pachyrhizi) rusts. The use of Polymerase Chain Reactions (PCR) and confirmation of the results by targeted rust-DNA sequencing have made the tests very sensitive and specific for these diseases. Application of a source-receptor model to several positive cases helped researchers identify probable P. pachyrhizi source areas (Krupa, et al, 2006). PCR test results for pathogens in NADP samples along with models to locate pathogen source areas can form the basis of a crop management decision support system. Combining disease risk predictions with data and information on air temperature, surface moisture, and crop growth conditions could help agricultural entrepreneurs make decisions in minimizing fungicide applications and improving cropping practices, thus resulting in environmental conservation and economic benefits. Such a system offers the avenue for an important new application of the NADP precipitation-sampling system.

Brown and Hovemiller. 2002. Science. 297:537-541.
Krupa, Bowersox, Claybrooke, Barnes, Szabo, Harlin, and Kurle. 2006. Plant Disease. 90(9):1254-1259.
Purdy, Krupa, and Dean. 1985. Plant Disease. 69(8):689-693.
Rayner and Hopkins. 1962. Comm. Mycol. Inst. Misc. Publ. 16:1.
Stover. 1962. Trop. Agric. Trinidad. 39:327-338.