By Canola Council of Canada
Aug. 12, 2013 - The week or two before swathing or during swathing are the best times to survey the disease situation in your canola fields. Large lesions — the ones that will lead to yield loss — are easiest to spot at this stage and when plants are still standing. And clubroot galls, if present, are larger and easier to find.
"Get off the swather a few times in each field and take a look," says Greg Sekulic, agronomy specialist with the Canola Council of Canada. "To identify diseases present and their severity are keys to management. A close look now gives you the best hope of early detection of clubroot or of a rising blackleg threat, for example."
Growers can use this scouting information to plan rotations, choose varieties, and update fungicide decision-making for next year. In most cases, fungicide is not effective at this stage of the season. In cases where fungicide use is still possible for the target disease, use the tool at www.spraytoswath.ca to check on pre-harvest intervals.
Here's how to identify the major diseases of canola as fields near the swathing stage:
Sclerotinia stem rot. Look for areas with lodged or prematurely ripened plants. Examine the lower to middle areas of the stem looking for large bleached or tan lesions. In serious cases, white, downy growth appears on green stems. As the infected stem dries, it will be bleached or brown (like a bone) and may start to crack open or shred. Hard, black sclerotia bodies — similar in appearance of mouse droppings — found inside these bleached stems are a sure sign of sclerotinia stem rot.
Blackleg. Look for areas with lodged or prematurely ripened plants. Examine the bottom to middle areas of the stem for lesions. Black pepper specks (picnidia) may appear within the lesions. When blackleg is severe enough to cause yield loss, the plant will have irregular, knotty, woody cankers at the base of the stem. This infection will eventually grow through the stem, cutting off nutrient flow. If you see plants drying up, use garden clippers and slice through the stem at the base. If more than half the area of the stem is blackened, blackleg has likely reduced the yield of that plant.
Clubroot. Above-ground symptoms, including wilting and pre-mature ripening, should be evident in severely-infected plants. Even if you don't see above-ground symptoms, pull plants and look for galls. Light or severe infection has essentially the same risk for spreading the disease with equipment. If galls are present, the best management at this stage is to prevent spread within the field, and certainly from field to field.
Alternaria black spot. This is rarely a problem in napus canola when it is alive, but some superficial infection may turn swaths black. Alternaria tends to be more damaging in juncea or rapa canola. In these crops, small black spots will move up the plant, eventually reaching the pods. When infected areas make up 50 per cent or more of the crop, swathing early may be the best way to salvage the yield in those infected plants.
Aster yellows. Common symptoms include the malformed bladder-like pods, which produce little to no yield. The disease can also result in normal looking pods that contain only a few misshapen seeds. Other symptoms include sprouting in the pod, and purplish plants and pods, although this purpling can result from many potential causes.
Foot rot and brown girdling root rot. Canola plants exhibiting brown superficial symptoms at ground level likely have fusarium foot rot, which produces tan brown lesions with concentric markings. Another possibility affecting canola roots at adult stages is brown girdling root rot (BGRR), which is more of an issue in Polish canola. Symptoms of BGRR are rusty brown lesions on the canola tap root, which may girdle and pinch off the root if severe. These diseases can be much higher in tight canola rotations.
Grey stem. Grey silvery to purplish patches develop on stem. These can cover whole stems and continue to spread in stubble as plants decompose. Grey stem usually infects too late to cause significant yield loss. To differentiate blackleg and grey stem at the end of the season, cut the lower stem and look for dead blackened tissue in the crown — a characteristic of blackleg, not grey stem.
Identifying diseases present in a field is important for early detection and management. So do check those sickly damaged looking plants. But for an overall assessment of disease severity, the survey has to be random.
"Disease assessment that concentrates only on damaged plants or only in a particularly bad part of the field can give a skewed overall impression of the disease threat," Sekulic says. "Following a few disease survey protocols can give a better impression of the true threat."
Stop at five random spots in the field before swathing or while swathing and check 20 standing plants at each spot. Count how many have disease, identify the disease, and then rate the diseases severity if you want. "That's what provincial disease surveyors do," Sekulic says. "By following these steps, growers will have results they can use to compare their situation with the provincial disease reports."