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Dead but alive –crop and weed residues aid survival of new pathogenic Diaporthe/Phomopsis species on soybean, sunflower, other hosts in Australia.
S. M. THOMPSON (1), Y. P. Tan (2), R. G. Shivas (2), S. M. Neate (3), E. A. Aitken (4). (1) University of Southern Queensland, Australia, Toowoomba Qld , Australia; (2) Department of Agriculture, Fisheries and Forestry, Brisbane, Australia; (3) University of Southern Queensland, Toowoomba Qld, Australia; (4) University of Queensland, Brisban

<i>Diaporthe/Phomopsis  </i>species cause damaging cankers on sunflower and soybeans. This study reveals that many<i> Diaporthe </i>species found in Australian broadacre cropping systems survive on both live and dead crop and weed hosts  Unincorporated crop and weed stubble is commonly found in zero and low till cropping systems where herbicides are often used for weed control. Although living plants are well recognised  as a ‘green bridge’ facilitating transmission of pathogens to crop hosts between seasons, the role of weed residues as a ‘brown bridge’ in aiding the survival of some groups of pathogens has not been previously studied. Of 27 novel  <i>Diaporthe</i> species identified, 11  species have been newly described to date, including numerous from sunflower, soybean and other crop hosts. Early  pathogenicity testing results indicate that some species are virulent on multiple hosts. Molecular analyses show that cankers on a single host may result from a complex of species.These findings have significant implications as even with effective rotational strategies, the overlooked weed residues on the surface are an effective ‘brown bridge’ and inoculum reservoir between pathogenic <i>Diaporthe</i> species and crop hosts.  A number of species have also been isolated from asymptomatic live hosts such as maize. The role of endophytic infection of crops and weeds by <i>Diaporthe</i> species as a survival aid needs further investigation and suggests a case for the redefinition of the word ‘host’.

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