Canola

Diseases of the plant

By John David Converse

The Canola Plant

• Canola is a specific edible type of rapeseed, developed in the 1970s, which contains about 40 percent oil.

• North Dakota leads the U. S. in canola production with approximately 88 percent of domestic production.

The Canola Plant

• There are two types of canola varieties, the Argentine type of the species Brassica napus and the Polish type of the species Brassica rapa. Argentine varieties have a higher yield potential and are also taller and have a higher oil content than Polish varieties. Argentine varieties require about 95 days to reach maturity, while Polish varieties need approximately 80 days to reach maturity.

The Canola Plant

• Canola (Brassica napus L.) varieties have been developed as both spring and winter annuals. The spring type is best adapted to North Dakota conditions. The winter types have not survived in trials in North Dakota, northwestern Minnesota or in the prairie provinces of Canada.

Rotation

• Canola best follows cereal grains or fallow in rotation. A preferred crop rotation would have canola planted at least two cropping years between plantings.

• However, if planting canola after one cropping year, it is strongly recommended growing a variety that is moderately resistant or resistant to blackleg.

Blackleg

• Blackleg is a common disease that may cause lodging.

• Blackleg produces black lesions on the stem.

• The internal root tissues of blackleg-infected plants turn dark gray to black or have dark gray streaks in them.

• Blackleg infections near the soil surface may result in stem breakage at or near the soil surface.

Blackleg

• The first symptoms appear on the cotyledons or leaves as round-to-irregular (1-2 cm) white-to-buff lesions that contain numerous small black dots (pycnidia).

Blackleg

• As the season progresses, the fungus may spread to the stem and crown of the plant, producing a canker that can girdle the stem.

Blackleg

• Severely infected plants ripen prematurely and have a black-to-grey discolouration at the base of the stem or crown. In severe cases, infected plants will lodge. Seeds of severely infected plants are small and shrivelled and may be infected with the fungus.

Disease Cycle of Blackleg

• The blackleg fungus survives on canola residues (refuse) and on infected plants and seed. The fungus can be spread from field to field on canola refuse or diseased plants. The spores of the fungus are also spread by rain, wind and infected seed.

Rotation

• Canola is susceptible to sclerotinia stem rot. Infection risk increases if canola is planted close in rotation with other highly susceptible crops like sunflower, dry edible beans, or crambe.

• At least two years should separate canola and sugarbeet plantings. If planting canola within three years of susceptible crops, a fungicide application may be needed.

Rotation

• Less susceptible crops that could be planted sucessfully in a close rotation with canola are rowed soybeans, flax ,semi-leafless field pea or lentil.

• In years when ideal environmental conditions favor air-borne spore movement, all canola plantings without fungicide applied, regardless of rotation intervals, may have economic losses due to sclerotinia.

Sclerotinia

• Sclerotinia stem rot develops late in the season, with the first visual symptoms appearing by the end of flowering. Dead and lodging plants occur singly or in patches in infected fields.

Sclerotinia

• Infections of individual plants usually develop around cast petals. The infections may produce a target pattern of light brown, mushy tissues. Infections may spread from infected leaf petioles or branches to larger stems. Infected areas eventually become bleached or white and the tissues become shredded.

Sclerotinia

• If the main stem is infected, plants may die early, reducing seed production, and plants may lodge. Hard black bodies which resemble rat droppings may be produced in infected stems. These are known as sclerotia

Sclerotinia

• Sclerotinia produces shredded white stem tissues with stem bending or breakage at a height of 6 to 18 inches above the soil but no symptoms in the roots

Disease Cycle of Sclerotinia

• The Sclerotinia fungus, Sclerotinia sclerotiorum, produces sclerotia in the stems.

• If the soil is at or above field capacity for 10 to 14 days, the sclerotia may germinate to produce tiny mushroom-like bodies that resemble golf tees. These fruiting bodies, called apothecia, produce millions of airborne spores

Disease Cycle of Sclerotinia

• As canola petals die and fall onto lower portions of the plant, any spores on the dead petals may germinate and begin to grow if the canopy stays wet for long periods of time. Once growth is established on the cast petals the fungus invades the surrounding tissues. Infections in canola may continue to spread as long as the canopy remains wet.

Other Diseases

• Diseases of canola less often reported are white rust or staghead, and downy mildew.

Other Diseases

• Alternaria blackspot, and aster yellows are two other not as common diseases in Canola.

Final Thought

• Plant diseases can be a serious problem in canola production. Rotations must be planned carefully to keep disease incidence and levels low.