Blackleg

Blackleg is thought to be a canola disease of the past, but is still present in low levels inmany fields in Manitoba.

The development of blackleg tolerant varieties have provided canola growers in western Canada a honeymoon from the disease, but tight canolarotations and the development of new pathogroups that our ‘Resistant’ rated varieties donot have resistance to, are again leading to reduced yields andincomes for canola growers.

You may think that you don’t have blackleg problems, but can you explain reduced yields in some fields and premature ripening when the fields lookgreat all year long and you did everything ‘right’? Blackleg is the second most prevalent disease in Manitoba canola fields based on canola disease surveys from the past few years. In 2009, blackleg was foundin 56% of Manitoba fields surveyed in the canola disease survey (140 fields total), with a 4%incidence.

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“Infection is not always obvious and can be surprising where it is found. It is not unusual to observe blackleg symptoms in canola crops.”

Infection is not always obvious or can be surprising where it is found. It is not unusual to observe blackleg symptoms in canola crops, evenwhen resistant varieties are being grown. Typically, lesions on stems are usually white or grey with a dark broder and dotted with numeroussmall black pepper-like specks (pycnidia). Stem lesions can also be much lower on the stem at the base looking like general and severeinfections can cause the stem to girdle at the base, plants to ripen prematurely and the crop to lodge.

Weather events within the season can also provide opportunities for symptoms and infection to develop such as hail, providing stem damagefor the rain-splashed blackleg spores to enter the plant and frost to provide cankers for the same entry by rain-splash spores.

Scouting, identification and management are still the tools to keep blackleg in check. If you are seeing areas of premature ripening, investigate thecause – is it environmental (gravel ridge that dried up, drowned out spot) or a disease?

If it looks like disease, is it blackleg or something else (sclerotinia, root rot, fusarium wilt, etc.) If it is blackleg, this is a good indication that your field hassome level of infection. Genetics and crop rotation are the best tools to control and/or reduce the level infection once identified. Using an ‘R’ ratedvariety will provide tolerance to the most prevalent pathogroup of blackleg, but not immunity. Lengthening rotations to a 1 in 3 year or 1 in 4 year canolarotation will allow canola crop residues to break down and reduce inoculum levels in the field. The combined approach will hopefully help reduce theaddition of new innoculum as well as reduce the existing blackleg levels already present.