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AOBPreview published online on March 13, 2006
Annals of Botany, doi:10.1093/aob/mcl062
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© The Author 2006. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org
Received November 15, 2005
Revised January 1, 2006
Accepted January 6, 2006

Short Communication

Enhanced Pathogenicity of Leptosphaeria maculans Pycnidiospores from Paired Co-inoculation of Brassica napus Cotyledons with Ascospores

HUA LI 1 *, NICOLE TAPPER 2, NEREE DEAN 1, MARTIN BARBETTI 2, and KRISHNAPILLAI SIVASITHAMPARAM 1

1 School of Earth and Geographical Sciences, Faculty of Natural and Agricultural Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
2 School of Plant Biology, Faculty of Natural and Agricultural Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia

* To whom correspondence should be addressed.
HUA LI, E-mail: hli{at}cyllene.uwa.edu.au


  Abstract

• Background and Aims Blackleg, caused by Leptosphaeria maculans, is a major disease of oilseed rape (Brassica napus) worldwide, including Australia. In most cases, the severity of the disease in the field is related to infections caused by airborne ascospores. In contrast, pycnidiospores originating from leaf and stem lesions and stubble are widely assumed to play only a relatively minor role in the epidemiology of blackleg. It is not clear whether, under certain conditions, pycnidiospores can cause severe disease in the field. The aim of the work reported was to determine if the pathogenicity of pycnidiospores is enhanced by paired co-inoculation of B. napus cotyledons with ascospores.

• Methods Three investigations were carried out under controlled-environment conditions using various L. maculans isolates and B. napus cultivars with different levels of host resistance to blackleg.

• Key Results In all three experiments, co-inoculation with ascospores increased the ability of pycnidiospores to cause more disease on B. napus than when inoculations consisted of pycnidiospores alone. This effect was significantly influenced by the host resistance of the cultivar, but overall was independent of the L. maculans isolate used in the different experiments. This effect was also independent of timing of inoculation with the ascospores, with increased disease from pycnidiospores occurring on the cotyledon of the seedling in situations where inoculations with ascospores were carried out 0, 1 or 2 d after pycnidiospore inoculation. This enhanced pathogenicity of pycnidiospores was evident even when low concentrations of pycnidiospores were applied to the other cotyledon of the same seedling.

• Conclusions These results may explain continuing severe blackleg disease cycles throughout the cropping season even when ascospore fallout was low or constrained only to a brief period or phase of the cropping season, and suggest that disease epidemics may be polycyclic rather than monocyclic.

Keywords: Brassica napus, Leptosphaeria maculans, pycnidiospores, ascospores, co-inoculation, pathogenicity, blackleg disease.
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