Generally, blackleg is more severe in areas that receive high rainfall and have intensive canola production. However, it can be present if unseasonably high rainfall occurs in lower rainfall regions in paddocks sown in close proximity to the previous year’s canola stubble. Fungal spores are released from canola stubble and spread extensively via wind and rain splash.
Blackleg is caused by the fungus Leptosphaeria maculans and is the most serious disease of canola in the medium to high rainfall production areas of Australia. Blackleg is normally successfully controlled, but yield losses of 50 per cent and greater have been recorded in worst affected cases. Where cultivar blackleg resistance has been overcome up to 90 per cent yield loss has been documented.
There is a very strong relationship between the intensity of canola production within a region and the level of blackleg development within commercial crops. The blackleg pathogen survives and reproduces on the previous season’s canola stubble. Therefore a 500,000ha canola crop will result in 500,000ha of blackleg-infested stubble the following season, releasing windblown spores every time it rains.
Blackleg survives on canola stubble and produces fruiting bodies that contain large quantities of airborne spores (capable of travelling several kilometres). Autumn and winter rainfall triggers spore release from the fungal fruiting bodies. Within two weeks of spores landing on canola cotyledons and young leaves, visible off-white coloured lesions develop. These lesions produce pycnidial fruiting bodies (dark coloured dots), which release rain-splashed spores.
Once the lesion has formed, the fungus grows within the plant’s vascular system to the crown. The fungus causes the plant’s crown to rot, resulting in a canker. Less severe infection can still result in the restriction of water and nutrient flow within the plant. Blackleg symptoms can also been found in plant roots. In severe cases the fungus will cause the entire plant to die prematurely.
Management practices to control crown rot blackleg are the same for the root rot form of the disease.