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Date: 18 December 2008
Source: Rural Solutions, Government of South Australia [edited] <http://www.ruralsolutions.sa.gov.au/news/newspaper_articles/2008_-_12_december/blackleg_robbing_yields_in_canola_crops>


Blackleg robbing yields in canola crops

Blackleg disease is a major problem in canola crops on Lower Eyre Peninsula, and so the Lower Eyre Agricultural Development Association
(LEADA) program this year [2008] aimed to gather data about the incidence and severity of blackleg across the region.

A survey of canola paddocks was undertaken with 100 plants sampled and scored for blackleg internal infection at the base of the stem in each of 82 paddocks. Average blackleg internal infection scores in individual paddocks ranged from less than 1 percent up to 38 percent.
But even at the highest levels of internal infection, no external signs of blackleg, such as cankers and collapse of plants, were evident.

All varieties showed a wide range of internal infection levels. The majority of canola varieties sampled had high levels of blackleg disease resistance rating.

More than half the paddocks surveyed (44 of the 82 total) had average internal infection levels of less than 10 percent, which is not likely to have any effect on yield and oil content. But 12 paddocks had average infection levels between 20 and 40 percent. Work by Dr Steve Marcroft showed that plants with this level of internal infection were about 7 percent lower yielding. Another study by Trent Potter put the yield loss at about 14 percent.

It appears that yield loss with "hidden" blackleg infection is significant in some paddocks therefore, and the impact of internal infection on yield may well vary depending on the growing conditions.
Full survey results are still being collated.

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[Blackleg (also called phoma stem canker) caused by the fungus _Leptosphaeria maculans_ is the most serious disease of oilseed rape with a possible yield loss of up to 50 percent or more. It can also affect other _Brassica_ hosts such as cabbage and turnip. Symptoms on rapeseed may include lesions on leaves, stems and pods, with stem cankers leading to premature ripening or collapse of the crop.
However, symptomless infections may also occur, as described in the report above.

The fungus is seed-borne and is also spread by wind, splash dispersal and infected crop litter. It may overwinter in stubble which is therefore often sprayed with fungicides. Disease management includes use of clean seed, cultural practices to minimise inoculum, fungicides, and planting of resistant cultivars. The pathogen is extremely diverse with new strains evolving frequently some of which have overcome major resistance genes within certain varieties in the past. For more information see ProMED-mail post no. 20071203.3890 and links below.

Canola is a group of rapeseed cultivars with modified oil components.

Maps
Australia:
<http://www.ga.gov.au/image_cache/GA4073.jpg>  and <http://healthmap.org/promed?v=-25.7,134.5,4>
South Australia:
<http://www.ozhorizons.com.au/sa/map.htm>

Pictures
Blackleg leaf symptoms on canola:
<http://www.apsnet.org/Education/LessonsPlantPath/BlacklegCanola/text/fig05.htm>
Blackleg stem cankers:
<http://www.ag.ndsu.edu/pubs/plantsci/crops/pp1201-4.jpg>
Rape planting with blackleg:
<http://www.apsnet.org/education/LessonsPlantPath/BlacklegCanola/images/fig07.jpg>

Links
Blackleg information:
<http://www.apsnet.org/education/lessonsPlantPath/BlacklegCanola/>,
<http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex147>  and <http://www.dpi.vic.gov.au/dpi/nreninf.nsf/v/4C130B7D3F20ED44CA25745D002341C2/$file/Blackleg_of_Canola.pdf>
Pathogen life cycle:
<http://www.botany.unimelb.edu.au/blackleg/template-assets-custom/images/L.maculans%20lifecycle/lifecycle.gif>
_L. maculans_ taxonomy:
<http://www.indexfungorum.org/Names/SynSpecies.asp?RecordID=230154>
- Mod.DHA]

[see also in the archive:
2008
----
Fungal diseases, wheat, oilseed rape - UK 20080930.3083
2007
----
Blackleg, canola - Canada: new strain, (SK) 20071203.3890 Foliar diseases, brassica crops - UK 20070913.3037
2006
----
Blackleg, canola, path.gr.3 - Hungary: 1st report 20060419.1156
2005
----
Blackleg, canola - USA (ND): 1st report 20050626.1807 Blackleg, canola - Argentina: 1st report 20050319.0807
2003
----
Blackleg, canola - Canada (Manitoba) 20030918.2367 Blackleg, canola - Brazil (Rio Grande do Sul) 20030225.0478
2001
----
Blackleg, root infection, canola - Australia (SA, NSW) 20010819.1959]