BLACKLEG

Introduction

Blackleg, caused by the fungus Leptosphaeria maculans, is the most serious disease of canola in Australia. Blackleg is most severe in regions of high canola intensity with moderate to high rainfall. Although not common, yield losses of 50 percent and greater have been recorded. Where cultivar blackleg resistance has been overcome, up to 90 percent yield loss has been recorded.

Blackleg is Challenging to Control

  • blackleg is a sexually reproducing pathogen that will overcome cultivar resistance genes
  • fungal spores are released from canola stubble and therefore the disease is more severe in areas of intensive canola production
  • spores are spread extensively and quickly via wind and rain splash.

What to Look For

In the autumn and winter, rainfall triggers spore release from the fruiting bodies on stubble. Within two weeks of spores landing on canola cotyledons and young leaves, clearly visible off-white coloured lesions develop. Within the lesion pycnidial fruiting bodies (dark coloured dots) release rain-splashed spores, (see Figures 11.1 and 11.2).

Once the lesion has formed, the fungus grows within the plant’s vascular system to the crown where it causes the crown of the plant to rot resulting in a canker, (Figure 11.3). Severe canker will sever the roots from the stem, whereas a less severe infection will result in internal infection of the crown restricting water and nutrient flow within the plant.

Blackleg symptoms have also been found in the plant roots, in severe cases this root infection appears to cause the entire plant to die prematurely, (Figure 11.4). The root rot form of the disease is caused by the same blackleg strains that cause the stem canker.

Figure 11.1 Blackleg fruiting bodies (pseudothecia) on stubble that produce windborne spores

Figure 11.2 Blackleg lesions containing pycnidial fruiting bodies (black dots)

Figure 11.3 Stem canker can kill the plant or cause partial restriction of the vascular tissue

Figure 11.4 Root rot form of blackleg (diseased plant on left

Disease Cycle

Blackleg survives on canola stubble producing fruiting bodies (pseudothecia) that contain large quantities of airborne spores; capable of travelling several kilometres, (Figure 11.5). Timing of spore release from the stubble is dependent on autumn rainfall. Higher rainfall results in earlier spore release and consequently may lead to increased disease severity. These dark coloured, raised fruiting bodies (pseudothecia) can easily be seen with the naked eye.

Figure 11.5 Disease Cycle of Blackleg on Canola. Illustration by Kylie Fowler

Management

Management practices to control crown rot blackleg are the same for the root rot form of the disease as follow:

  • consult the Blackleg Management Guide on the GRDC website ( www.grdc.com.au ) for up to date ratings, resistance groups and management practices
  • monitor your crop to determine yield losses, (Figure 11.6)
  • choose a cultivar with adequate blackleg resistance for your region
  • never sow your canola crop into last year’s canola stubble
  • use fungicides strategically to control blackleg
  • if your monitoring identifies yield loss and you have grown the same cultivar for three years or more, choose a cultivar from a different resistance group.

Figure 11.6 Cutting a plant at the crown to assess internal infection

DISEASES OF CANOLA AND THEIR MANAGEMENT

SCLEROTINA STEM ROT