Blackleg: Scouting, identification and next steps

Around 60 per cent seed colour change (swath timing) is the best time to scout for blackleg as the basal cankers, which cause significant yield loss, are easy to see. Check fields – even if growing resistant (R) rated hybrids. An R rating is not immunity and does not tell much about if it will be effective against the races in a individual field.

To scout:

Pull up at least 50 plants in a W-pattern.
Clip with clippers at the base of stem/top of root (roughly half an inch into the root tissue) and look for blackened tissue inside the crown of the stem. The amount of infection present will help identify the level of risk and the best management practices for that field in following years.
Use the zero to five blackleg disease rating system to identify severity of the infection along with the incidence of the disease. Calculate an average for the field.

Blackleg rating from 0 (left) to 5 (right). Credit: Justine Cornelsen

Yield Loss. Updated yield loss models from the University of Alberta show that blackleg starts to rob yield at a disease severity rating of 2. Severity ratings over 1 start to see potential losses of 20 per cent per unit increase on the BL disease severity scale of 0-to-5 rating scale. Average severity ratings less than 0 to 1 have minimal losses. See the recent yield loss models study.

Blackleg that is severe enough to cause stem discolouration would have infected the plant very early in the season — cotyledon to two-leaf stages. This level of infection is not from disease that entered the plant recently from root maggot tunneling, for example. Root rot and foot rot might look similar to blackleg from the outside, but dry rot (base of stem turns woody) is more likely to be blackleg. Blackleg is an aerial disease which pushes up spores into the canopy to cause infection. This is the primary pathway for infection on new canola plants.

If not sure, get it tested. Suspected plant samples can be submitted to diagnostic labs to identify if blackleg species are present. Specific tests can also determine if Leptosphaeria maculans or Leptosphaeria biglobosa is the causal agent. L. maculans race identification testing is also available through private labs.

Quick comparison of blackleg, clubroot and sclerotinia stem rot

wdt_ID	Comparisons	Blackleg	Clubroot	Sclerotinia stem rot
1	Susceptible host plants	Canola and other brassicas	Canola and other brassicas, including some common weeds.	Broadleaf crops, including canola, soybeans, sunflower, pulses.
2	Key distinguishing features	Lesions with specks of picnidia inside form on leaves. Infection spreads to the base of the stem, where it forms cankers. Clipping open stems will reveal blackened tissue inside. Moderate cases will cause yield loss, even if the plant doesn’t die. Severe	Galls form on roots. In serious cases, large tuberous galls will restrict flow of nutrients and water up and down the plant, killing the plant.	Lesions form on leaves and stems. Eventually the stems rot, then turn white and brittle. Plants die prematurely and seed set is greatly reduced. Black sclerotia form inside (and sometimes outside) damaged stems.
3	How it rests waiting for the next host crop	On pieces of infected canola stubble	As each gall breaks down, it releases billions of spores into the soil.	As sclerotia bodies in the soil
4	How it spreads	Pseudothecia and pycnidia on infected canola residue release spores that continue the infection cycle. Pycnidiospores from pycnidia travel only a few metres. Smaller ascospores, shot into the air by pseudothecia, can travel farther.	Spores move with the soil. When soil moves, clubroot moves. Soil moving through fields and from field to field on machinery is the most common vector for spread of the disease. Spores infect host roots, continuing the cycle.	Sclerotia produce apothecia that release spores. Spores can carry for kilometres on the wind, but most will come from within fields or from connecting fields. When release coincides with canola flowering and moist/humid conditions, infection can occur.
5	Benefit of crop rotation	2- or 3-year break between canola can almost eliminate risk because host stubble deteriorates.	2- or 3-year break between canola can reduce viable spores by 90%, but this can still be enough spores for widespread damage if spore counts start off high. (1,000,000 per gram of soil.)	Rotation benefit for sclerotinia is less than it is for blackleg and clubroot because many crops are susceptible, sclerotia are widespread and released spores carry to nearby fields. However, higher severity will be observed with shorter rotations of susc
6	Genetic resistance available	Yes	Yes	Improving tolerance in selected varieties
7	Genetic resistance groups identified	Yes	No, but coming soon?	No
8	How to use R varieties as part of a disease management program.	Always use R or MR varieties, and if blackleg is getting worse, switch to a different R-gene.	Use R varieties in areas where clubroot is found or suspected. This will help keep soil spore loads low and limit yield loss for the long term.	Sclerotinia-tolerant varieties can reduce disease in high-risk situations, but fungicide may still be required.
9	Effective fungicides available	Yes, for early-season infection	No	Yes, for application at flowering

Canola stem diseases – a comparison

wdt_ID	Plant Disease	Sclerotinia	Blackleg	Blackleg	Verticillium Stripe	Grey Stem	Fusarium Wilt
1	Species	Sclerotinia sclerotiorum	Leptosphaeria maculans	Leptosphaeria biglobosa	Verticillium longisporum	Pseudocercosporella capsellae	Fusarium oxysporum
2	Stem Symptoms and distinguishing features	Bleached white appearance. Infected stems tend to shred and shatter very easily. White mouldy growth. Black sclerotia bodies will form inside the stem.	Stem lesions with pycnidia (black spots) forming inside the lesion. Base of stem (crown) becomes woody. Cross section cut reveals blackening.	Shallow stem lesions with pycnidia	Shredding of the stem tissue. Tiny black microsclerotia form beneath the peeling outer layer.	Large purple to grey-speckled stem. Pod lesions are also possible.	Discolouration of the stems; yellow or reddish-brown streaks on stems.
3	Pod	Withered pods if branch below is infected. Sometimes white mould, lesions.	No symptoms	No symptoms	No symptoms	Grey speckled pods.	No symptoms.
4	Crown (base of stem) exterior		Cankering
5	Crown cross-section	Clean, dried down	Solid black sections, often pie shaped. Fully black in extreme cases.	Typically does not reach stem in time	Greyish hue across entire cut. Gets darker as microsclerotia build up. Can extend many inches up the stem.
6	When to scout	Prior to swath timing	Prior to swath timing, 60% SCC.	Prior to swath timing, 60% SCC	Easier to ID post-harvest	Prior to swath timing, 60% SCC.	Prior to swath timing, 60% SCC.
7	Yield Loss	Potential yield loss in a field can be determined by: % Potential Yield Loss = % Infection x 0.5	For every unit of increase in disease severity, a 17% loss in plant seed yield can be expected.	Comes in too late in the season to cause a significant impact	Does occur but no system to measure at this time.	Not known.	Varieties are resistant, rare to find.

Next steps. If blackleg levels are rising, canola growers will want to make a plan to keep a lid on infection levels. Here are effective management steps:

Crop rotation. From a blackleg management standpoint, crop rotation allows for the decomposition of infected canola residue, reducing the spores available to infect the next canola crop. This could require a delay of up to three years between canola crops.
Resistant varieties. Grow B. napus varieties that carry at least a moderately resistant (MR) blackleg disease resistance rating, or better yet an ‘R’ rating. Resistance reduces blackleg infection to specific races of L. maculans, but does not mean the variety is immune to the disease.
Rotate varieties. Many genes are responsible for blackleg resistance in canola and at least 16 described virulent L. maculans races are currently found on the Prairies. Growing the same hybrid repeatedly on the same field will select for races of L. maculans that can overcome genetic resistance in that hybrid. Rotating varieties or resistance gene groups creates the opportunity to bring a mix of major resistance genes to the field over time, which can reduce selection pressure and improve durability. Quantitative resistance can provide effective support for major genes, but quantitative resistance is not standardized in the same way and we can’t know how much is in a cultivar.

Keep It Clean – Blackleg resources

Blackleg: Scouting, identification and next steps

Quick comparison of blackleg, clubroot and sclerotinia stem rot

Canola stem diseases – a comparison

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