Canola Pest and Disease Management

Canola Disease and Pest Management: Key Strategies to Protect Yield and Quality

Canola can be infected and infested by various pathogens and pests during all plant growth stages. The farmer should be aware of the crop, pest, and disease history of his/her field (and region), the environmental conditions that favor the “enemies”' development and spread, and the most susceptible growth stages of the plants. Knowing this information, he/she can monitor the crop’s health, adopt and apply prevention strategies, and integrate management strategies to reduce yield losses.

Which are the most important diseases of canola, and how to control them

The Stem rot disease (Sclerotinia sclerotiorum) is considered the most destructive disease of canola globally, causing yield losses of 10-80% (Wang et al., 2014). This fungal disease is favored by prolonged wet and humid weather conditions during late winter and spring, causing significant problems in canola crops with extended flowering and podding periods. Common symptoms of the disease are light brown to greyish lesions on canola’s stems, branches, and pods. White moldy growth may also cover the infected plant parts. The discoloration makes the infected plants stand out from the healthy green ones. Usually, in severe infections, the plants ripen earlier and produce fewer seeds, and the stems may split open-shred and die. Sclerotinia (small, roundish, black spores) can be seen inside the infected stems (1, 2).

Once the disease is established in a field, its control is tough due to the long viability (more than 8 years) of sclerotinia (the resting stage of the fungus) in the soil. The severity of annual infections depends on the weather conditions. In fields with a known disease history, the farmer should apply a 4-year rotation program and avoid plant canola (wheat, lupin, sunflower) this time. Successful weed management is necessary to reduce the population of host species like devil’s thorn, wild mustard, and other broad-leaf weeds (3). Furthermore, all crop residues should be removed from the field and destroyed. Since the success of infection depends on the soil moisture level, the farmer should be very careful when irrigating during flowering.

Additionally, a fungicide treatment when plants start flowering (20-30% flowering) can significantly reduce the infection level (4). To decide if chemical control is necessary, the farmer can fill in the sclerotinia stem rot checklist (sclerotinia treatment decision tool). A field score of more than 40 is considered the action threshold for canola (5, 6).

Another serious canola disease is the ‘Blackleg disease.’ Depending on the fungus species infecting canola (Leptosphaeria maculans or L. biglobosa), the severity of the symptoms may be high or mild, respectively. In most areas, the aggressive fungus species are more common, and as a result, the yield losses are more extensive. The disease is spread from crop residues and air and water splashes, putting canola at risk during all its growth stages. The new infections usually happen in autumn and winter. Usually, (whitish) lesions may appear on the stems that develop in vascular discoloration and can lead to the lodging of the plants, rot (canker of the roots), and premature ripening. The disease severity can be graded on a scale from 1 to 5 (with 5 as the most severe case). Based on research data, the pod number and seed production are linearly reduced as the severity of the infection (per plant) increases. More specifically, a 17.2% yield loss is expected every time the severity increases by 1 level (7). As for stem rot disease, the farmer can determine the risk of severe blackleg infection (in spring) (4).

Integrate management measures are needed to control this disease successfully. The farmer can apply crop rotation with non-host crop species, destroy crop residues, use resistant/tolerant canola varieties, seeds with a fungicide dressing, or fungicide-amended fertilizer (4, 8).

Other diseases that can periodically be an important problem for canola farmers are Clubroot, Downy mildew, Alternaria Black Spot, Clubroot, Seedling Damping-off (Fusarium spp, Pythium spp., Rhizoctonia solani), White leafspot, Aster yellow, and Verticillium Stripe.

Keep in mind that the canola’s roots during the seedling and rosette stages are at risk from the pathogens causing the ‘Damping-off’ disease, while all stages can be infected by fungal leaf diseases like Alternaria and White rust.

Attention

Crop rotation is an efficient way to decrease the disease pressure (or avoid epidemics) in canola. Farmers can choose and cultivate non-host (or susceptible) crop species to have better results and control important canola diseases. For example, it is advised to avoid rotating canola with sunflower, dry pea, and dry beans since these crops are susceptible to white mold and Sclerotinia stem rot, as canola is as well (here, here).

It is best to avoid applying a no-tillage system in fields with a known and severe problem from Sclerotinia and Blackleg disease.

Which are the most important pests of canola, and how to control them

Cutworms

The farmer should take all preventative measures to avoid any plant damage during emergence and early seedling stages (up to 3 weeks past emergence). More specifically, severe cutworm infestation early in the spring can lead to complete crop failure. The next period during which a high level of surveillance is needed to avoid plant damage and yield losses is during flowering and podding (9).

Diamondback moth

The Diamondback moth is considered the most destructive and widespread insect in canola crops globally. The problem is more severe if canola is cultivated near other cabbage crops (insect hosts). The insect can cause extensive damage during warm, dry seasons due to its ability to feed on almost all plant parts (leaves, buds, flowers, seed pods, the green outer layer of the stems, and the developing seeds) (14). It has multiple generations per season. The second one is the most destructive because it feeds from the flowers, causing flower abortion, delayed plant maturity, and reduced seed set.

The farmer can monitor the insect population using sex pheromone traps (wing or delta trap). The action threshold for diamondback moths is 100 per trap per week, or 2-3 larvae and 4-6 larvae per plant during the flowering and podding stages, respectively (14, 3, 15). However, since insects can develop resistance to pesticides quite easily, farmers should apply chemical control with great caution, alternating the active compounds and using an integrated management approach.

The action thresholds for chemical control

During the flowering stage of canola, there is a high risk of infestation by aphids, Diamondback moths, and bollworms.

The action threshold for chemical control is:

For cutworms: 4-5 cutworms per square meter (3-4 cutworms per square yard, or per 0.8 square meters) (10).
For Striped and Crucifer flea beetles: when 25% of the leaf surface has been damaged (in the cotyledons and first 1-3 true leaves) (11).
For mites: 10 mites per 100 cm² (15.5 in²) in the cotyledons stage, or 50% of cotyledons are bleached white (12).
For aphids: when 20% of the heads have been infested during flowering, or when 2 aphids are counted per plant in the seedling stage, or 5 per leaf in the rosette stage (5).
For the bollworm: when 4-5 larvae are found per square meter during flowering (3).
For Cabbage seedpod weevil: 25-40 adults collected in 10 sweeps (with a net) in the field during 10-20% flowering (13).

Finally, some natural enemies, such as parasitic wasps and predators (flies, lacewings, minute pirate bugs, spiders, and birds), can keep the pest under control.

References

Wang, Y.; Hou, Y.; Chen, C.; Zhou, M.G. Detection of resistance in Sclerotinia sclerotiorum to carbendazim and dimethachlon in Jiangsu Province of China. Australas. Plant Pathol. 2014, 43, 307–312.

Further reading