Enhanced pathogenicity of Leptosphaeria maculans Pycnidiospores from paired co-inoculation of Brassica napus cotyledons with ascospores.
OBJECTIVE
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.


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.
School of Earth and Geographical Sciences, Faculty of Natural and Agricultural Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. hli@cyllene.uwa.edu.au
Hua
H
Li
Nicole
N
Tapper
Neree
N
Dean
Martin
M
Barbetti
Krishnapillai
K
Sivasithamparam
D001203:Ascomycota / Q000472:pathogenicity*; D029688:Brassica napus / Q000382:microbiology*; D018548:Cotyledon / Q000382:microbiology*; D010935:Plant Diseases; D036226:Seedlings / Q000382:microbiology; D013172:Spores, Fungal / Q000472:pathogenicity*