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Top 10 canola diseases to watch for in August

Aug. 1, 2013 – The time between now and harvest is an excellent time to look for diseases in canola. Fungicide is not effective at this stage (for most diseases), but scouting and identifying diseases can help with management in following years.

10. White rust. Look for white pustules and a bend in the pod area — a telltale look that gives the disease the “staghead” nickname. This disease was common 15 years ago but has been virtually eliminated in napus canola through advancement in genetic resistance. You can still find it in juncea and rapa canola, and on stinkweed. Read more in the Canola Encyclopedia.

9. Fusarium wilt. This can cause bleached stems with a slight pinkish tinge typical of fusarium fungus. Discoloration can occur up only one side of a stem, a typical sign of fusarium wilt. Like staghead, this has been virtually eliminated through genetic resistance.

8. Grey stem. Grey stem, as the name suggests, can discolour a large portion of the stem. It can look like blackleg, but won’t have picnidia. Clipped stems may be hollow. Grey stem is less common than blackleg. It comes on later in the season and tends to have very little effect on yield. Read more in the Canola Encyclopedia.

7. Downy mildew. This was fairly common early this year, much worse than usual. Lesions tend to concentrate on lower leaves, with little impact on yield. Look for white powdery fungal growth on the underside of leaves, as shown in the photo above from Holly Derksen, MAFRI. Downy mildew can also cause scars on the topside with chlorosis around them. Read more in the Canola Encyclopedia.

6. Alternaria black spot. This tends to be more common in juncea and rapa canola. Black spots start lower in the canopy and move up the plant, eventually moving to pods. The disease can continue advancing after swathing. Lesions on leaves can look like blackleg, but do not have picnidia and seem to move out from the centre in concentric circles. Fungicide can prevent migration up the plant, but fungicide is rarely economical. At the end of the season, it acts a decomposer, producing football-shaped spots. Read more in the Canola Encyclopedia.

5. Root rot and foot rot. Root rot is caused by rhizoctonia, the same pathogen that causes wirestem in seedlings, and it tends to occur more often in fields with tight canola rotations. In severe cases, root rot will pinch off stems at ground level. See the photo above. Look for a conical shape at the break point. With foot rot, a fusarium disease, the lesions tend to be at the top of the tap root and move to the base of the stem. Foot rot lesions are tan brown to tan greyish and relatively smooth. They may be on one side of the plant or girdle the entire stem base. Often you see concentric rings or markings on the lesions that likely correspond to periods of fungal growth/sporulation. Both root and foot rots tend to occur more often in fields with tight canola rotations. Clip stems above the break point: Clipped stems with root or foot rot will not have blackened tissue. Blackleg is a little more woody, rough and always black. A lab test will confirm ID and help you determine future management. Read more in the Canola Encyclopedia.

4. Aster yellows. Bladder-like pods are typical of aster yellows damage. Purpling of lower leaves and pods, and empty pods are also signs of aster yellows. The photo above, from MAFRI, shows an aster yellows infected plant with these symptoms. Incidence of aster yellows has dropped off this year after very high levels in 2012. Read more from the Canola Watch archive.

3. Clubroot. Pre-mature ripening can result from root rot, blackleg, sclerotinia stem rot and clubroot, but only clubroot will have the galls growing on the roots. Dig up plants to check. The disease has been confirmed in each Prairie province. Even plants without above-ground symptoms can have small galls. Dig up a few random plants near field entrances to survey potential low-level clubroot presence in your area. Soil tests can also determine if the pathogen is present in field entryways. Read more in the Canola Encyclopedia.

2. Sclerotinia stem rot. Infection can cause lesions on leaves and stems. As stem infection progresses, stem lesions will move up and down the stem. Stems will become bleached and will shred easily when twisted. Only sclerotinia stem rot will have black sclerotia bodies inside infected stems. Read more in the Canola Encyclopedia.

1. Blackleg. Blackleg is not the worst canola disease on the Prairies (that honour goes to sclerotinia stem rot), but it ranks No.1 on this top 10 list to emphasize the need for growers to re-familiarize themselves with what could become the No.1 disease without close management. Look for lesions on leaves and stems with tiny black picnidia spots inside. Grey stem, sclerotinia and alternaria lesions will not have the black spots. Eventually infection will move to the base of the stem, at ground level. The surest way to identify blackleg is to cut stems with garden clippers to look for blackening and see the extent of damage. R-rated varieties should have minimal damage to stems. Extensive stem damage could indicate a shift in blackleg races in your fields. Read more in the Canola Encyclopedia.


August 1, 2013
By Canola Council of Canada

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