Canola Disease Guide

Identification, Characteristics, Diagnosis

Alternaria black spot is caused by several species of fungal pathogens in the Alternaria genus. The fungus overwinters in infected crop residue and on host weeds. Spores from the fungus spread aerially to new plants where they will sit until enough moisture induces spore germination. As such, Alternaria disease development is favored by humid, wet conditions with enough wind to spread spores through the crop canopy. This disease tends to feed on dead or mature tissue, resulting in infections that tend to be superficial with little to no impact on yield. Typically, ripened plants are more prone to infection than younger plants because as canola plants age, their cuticle becomes thinner and more susceptible to infection. As a result, canola hybrids that are planted earlier or mature earlier may show symptoms prior to later planted or later maturing hybrids.

Brown-colored spots can appear early in the season on cotyledons and lower leaves. The spots will eventually turn black as the fungal spores grow. At maturity, the canola stems and pods will develop black dots that can continue to grow into larger sunken lesions with a dark border and a greyish center. When disease is severe, it can cause the upper part of the plant to wither and may cause seeds to be small and shriveled. Alternaria infected canola can also result in uneven drying of the disease-infected pods.¹⁰ If infection is heavy on a canola crop, growers may notice a black dust that settles onto their equipment during swathing or harvest. The maximum infection of Alternaria is observed at maturity. Because the infection becomes more severe as the plant matures, it may cause the pods to shatter, depending on the pod integrity of the particular hybrid.¹⁰

All canola cultivars are susceptible to Alternaria, but the canola currently being grown (Brassica napus) is overall less susceptible than other cultivars due to its thick wax coating on the stem and pods.

Management

Alternaria is hard to predict, but the following good management practices can help reduce the potential for disease development:

• Increasing crop rotation.
• Balanced fertility to reduce plant stress.
• Lower plant populations to reduce lodging.
• Choosing B. napus cultivars over B. rapa cultivars, as B. napus is less susceptible.
• Fungicide application to protect the crop from sclerotinia (spores) .
• If canola is infected with Alternaria, swathing the crop and harvesting as soon as possible can reduce yield loss due to shatter. A lighter swath with good airflow for rapid drydown can help to reduce shatter loss.

Identification, Characteristics, Diagnosis

Aster yellows is a plant disease that affects canola in Canada. It is transmitted by aster leafhopper infected with the aster yellows phytoplasma (insect-transmitted bacteria). These insects are windblown into Canada from the southern United States.

Symptoms of aster yellows in canola usually start appearing around flowering. The affected plants are typically taller and stand out above the plant canopy. They are darker in color and have green, bladder-like structures that replace the flowers and pods.

Generally, aster yellows is a cosmetic disease and doesn't cause much yield loss. However, there can be years with outbreaks. In 2023, there were higher than normal instances of aster yellows, particularly in northeast Saskatchewan. Outbreaks seem to occur every 7 to 10 years.

There are no known treatments for aster yellows in canola and no known sources of resistance. It is important to note that aster yellow infection is not hybrid specific. Infection simply depends on the population level of aster leafhoppers in an area. Aster leafhoppers tend to target canola when it is flowering. Controlling weed hosts of the disease may help reduce incidence.

Management

There are no known treatments and no known sources of resistance for aster yellows in canola. Fortunately, aster yellows does not typically cause significant yield loss.

Identification, Characteristics, Diagnosis

Blackleg is a disease of canola caused by the fungus Leptosphaeria maculans, which overwinters on infected canola residue. The fungus grows spore-producing structures called pseudothecia in the spring, which then produce ascospores that are transported by wind and splashing raindrops to infect new plants. The pycnidia (black, pepper-looking spots) that form and are present in the lesions after infection can also produce spores which then continue to infect the plant tissue around them. Once the plant is infected, the fungus grows inside the lower stem of the plant and restricts moisture and nutrient movement, which can eventually lead to lodging and complete girdling of the stem.

Blackleg is easiest to scout for at swathing or at plant maturity. Earlier in the season, symptoms of the infection can be seen on leaves, stems and pods as whitish lesions that are dotted with pycnidia. At maturity, the lesions are normally found at the base of the stem. Pulling those plants and using clippers to snip them at the soil surface (the crown of the stem) to get a look at the cross-section of the stem can reveal blackened tissue inside. Keep in mind that the symptoms of blackleg and Verticillium wilt are similar in appearance.

Blackleg can become quite serious and cause significant yield loss. Drier conditions tend to decrease disease development, but canola fields with early flea beetle damage, hail damage or early season moisture may cause the disease to be more prevalent.

Management

Managing blackleg can be done through the following series of integrated management practices:

• Longer crop rotations.
• Growing hybrids with strong major and minor gene resistance (DEKALB^® brand Canola lists the BL genetics of their hybrids), and selecting hybrids with multiple resistance genes in areas with tighter rotations.
• Rotate canola hybrids with different sources of major gene resistance.

• Testing plant samples to identify which blackleg species is infecting the field.
• Keep your canola field CLEAN by growing certified seed, using seed treatments (early protection and decrease insect damage) and controlling host plants and volunteer canola.

Identification, Characteristics, Diagnosis

Clubroot is a soilborne disease of Brassicaceae plants, particularly canola. In Canada, the Canadian Clubroot Differential (CCD) pathotyping system has identified at least 43 clubroot pathotypes, with most being rare or identified from limited samples. The pathotypes 3A, 3D, and 3H are the most frequently identified in field samples. Infected canola will show symptoms of gall development (club-like tumours) or swelling of the roots that cause premature death of the plant. Clubroot was first discovered in canola near Edmonton, Alberta in 2003 and has since spread to many fields across the western Prairies.

Plasmodiophora brassicae, which causes clubroot, is a fungus-like organism that cannot grow or multiply without a living host. It is a hardy pathogen that overwinters in soil. In the spring as plants begin to grow, secretions from the roots stimulate P. brassicae spore germination. P. brassicae zoospores can travel short distances through water in the soil and infect root hairs. Long-living resting spores can survive in the soil for 15 to 20 years.

Warm (20 to 24 °C), acidic soils (pH < 6.5) and high soil moisture favor P. brassicae infection and clubroot disease development. Soils with a pH > 7.2 inhibit spore germination and disease development.

Clubroot galls tie up nutrients in severely infected roots so the plant cannot effectively transport water and nutrients. Symptoms of clubroot may vary, depending on the growth stage of the crop when infection occurs. Wilting, stunting and yellowing of canola plants are aboveground symptoms that may indicate clubroot infection but may also be confused with other diseases such as Sclerotinia, blackleg and Fusarium wilt. Later-stage clubroot infection will result in premature ripening and shriveled canola seeds, reducing yield and oil content. Digging up the distressed plants and examining the roots is useful for determining if plants are infected with clubroot, as root gall formation is obvious.

Hybridization nodules are rare, but may be confused with clubroot galls. They are small, round nodules located on the root nodes. Hybridization nodules are uniformly dense like healthy roots. Clubroot galls are spongy, marbled and will have a peaty appearance.²

Figure 9. Clubroot infected roots on a canola plant. The galls are large and round on these particular roots.

A majority of new canola hybrids are bred to contain resistance to the predominant clubroot strains and pathotypes across Western Canada. If a hybrid is labelled with an ‘R’, it is considered resistant. In field tests, resistant hybrids were found to experience only 30% as many clubroot infections as susceptible hybrids.¹ Though resistant hybrids are not immune to the disease, they highly restrict the development of clubroot symptoms in fields with low to moderate disease pressure.

Management

The following tactics can help manage clubroot in canola:

• Growing clubroot resistant canola hybrids.
• When growing canola in areas known to be high risk for clubroot, choosing a hybrid known to have multigenic resistance.
•  Minimizing soil movement through strict sanitation of equipment and vehicles that travel in and out of the field.
• Direct seeding, reducing tillage in contaminated fields to help prevent movement of contaminated soil.
• Controlling host weeds to minimize spore production. Weeds that will carry and increase the disease include volunteer canola, shepherd’s purse, stinkweed, flixweed and wild mustard.
• Diligent and thorough disease scouting.
• Effective crop rotations with a minimum two-year break between canola crops (and other hosts) will help reduce clubroot spore level in the soil. Over 90% of the spores are not viable after a two-year break.
• There are currently no fungicides registered for suppression or control of clubroot in canola.

Figure 10. Fields with visual clubroot galls in the survey done from 2013 to 2021 for the prevalence of clubroot in Cavalier Country, North Dakota, US.¹² (Data courtesy of U.S. Canola Association)

Identification, Characteristics, Diagnosis

Powdery mildew in canola is caused by the fungus Erysiphe crucirferarum. Visual symptoms include powdery white fungal growth on canola stems, leaves and pods. Severe infections are most noticeable at swathing or combining, when white dust is released which settles on equipment.

The pathogen overwinters as small, black spheres known as cleistothescia which germinate in spring to produce ascospores and conidia, two different infectious structures that can be blown to new fields via the wind. The disease cycle begins when the ascospores or conidia land on the upper surfaces of canola leaves or stems, air temperature is 68 to 80 °F and relative humidity (RH) is anywhere from 50 to 95%.

Plant symptoms are first noticed on the underside of the older leaves. Over time, white spots will appear on upper and lower leaf surfaces, stems, and pods and may end up covering the entire plant. A single infection site can quickly produce conidia and begin covering nearby plants. Infected leaves may wither and die, ultimately being the main cause of any yield loss. 

Management

Fortunately, powdery mildew is not considered a serious canola problem in Canada. The disease is fairly uncommon and often the infection occurs too late in the season to cause significant yield loss. If there is infection potential, an excessively dense, wet crop canopy combined with warm temperatures allows the disease to establish. Avoiding excessive nitrogen fertilizer, over-irrigation, and managing volunteer hosts and weeds can help prevent the disease.

Identification, Characteristics, Diagnosis

Root rot has the potential to severely affect yields. Shorter canola cropping rotations on the Prairies have increased the intensity and incidence of root rot. The primary pathogens involved in the root rot disease complex are soilborne and include Pythium species, Fusarium species, and Rhizoctonia solani, as outlined in Table 2.

Canola is most affected by root rot when soil conditions are excessively wet post-seeding or at early flowering, followed by hot, dry, windy conditions.

Identifying root rot can be difficult, as the disease complex manifests itself in different ways. Seedling diseases are exhibited at the germination and emergence phase, while adult root rots may show symptoms at the two-leaf stage and later. If emergence is poor, patchy or uneven, digging up the seedlings can allow you to take a closer look at the roots and hypocotyl (young stem).

Symptoms of Root Rots

• Seeds fail to germinate at all and turn soft and pulpy in texture.
• Germinated seeds decay and do not emerge from the soil.
• Normal, healthy looking seedlings wilt, turn purple or chlorotic, topple over and die. The hypocotyl shows signs of girdling and decay.
• Normal, healthy-looking seedlings have a constricted, reddish-brown hypocotyl.
• Seedlings that have emerged and look healthy at the two- to four-leaf stage seem to stop growing. 

Management

Increased canola crop intensity can increase the risk of root diseases in canola. Including pulse and cereal crops into rotation can help reduce the severity of root rots, as these crops are infected by different pathogens.

Other management tips for root rot include:

• Use seed fungicide treatment to control the root rot seedling disease complex.
• Controlling volunteer canola and other cruciferous weeds such as shepherd’s purse, stinkweed, volunteer canola, and flixweed.⁷
• Ensure adequate and balanced fertility levels in the soil. Nitrogen, in particular, can decrease disease severity.
• Avoid deep-seeding canola. The added stress of having to grow through more soil will decrease the seedlings' ability to resist infection.
• Avoid too cold, too wet, too dry, or too crusty of a seed bed. These factors slow germination and growth, extending the period of time when root rot disease can infect. 

Identification, Characteristics, Diagnosis

Sclerotinia stem rot, also known as white mold, is one of the most prevalent diseases of canola on the prairies. When environmental conditions are favorable for disease development, yield losses can exceed 50% in individual fields and 10 to 15% across large areas.³ Sclerotinia is highly favored by warm and moist environmental conditions and dense, high-yielding canopies. Crops such as sunflowers, beans, lentils, peas, and mustard are all susceptible to white mold. Common broadleaf weeds including shepherd’s purse, thistle, stinkweed, chickweed, hemp-nettle, false ragweed and narrow-leafed hawk’s beard are all host plants to the disease as well.

White mold is caused by the fungal pathogen Sclerotinia sclerotiorum, which lives both on and in infected canola plants and other broadleaf plants as white fluffy mycelium that later forms dense black masses of fungal cell structures known as sclerotia. The sclerotia fall to the ground during harvest, allowing the fungus to overwinter and remain viable in the soil for five years or more in the absence of a host plant.³

From the soil, sclerotia infect canola plants in the summer. Typically, sclerotia germinate to produce mushroom-like fruiting structures called apothecia that can extend to the soil surface. During periods of prolonged moist soil conditions (10 days above the wilting point or greater) and moderate to warm temperatures (15 to 25 °C), apothecia develop and produce millions of ascospores that are then windblown to infect canola plants.³

Ascospores do not infect leaves and stems directly. They first infect flowers, fallen petals or even pollen on the stems and leaves. Fallen flower petals that are lodged in leaf axils on stems are often sites of initial infection. These plant parts provide the sources of nutrition required for the fungal spores to germinate, grow and eventually invade the plant. In almost all cases, Sclerotinia infections in canola occur from ascospores. Once infection takes place, plant tissues die and form necrotic lesions. Infection can spread from leaves and stems as the fungal mycelium grows within the plant. Toward the end of the growing season, new sclerotia are formed inside dead stems, fall to the ground during harvest and repeat the disease cycle.

Primary infection occurs during flowering, although in Western Canada the infectious apothecia can appear as early as June and continue to develop until late September.³ The critical stage for damaging disease development is from early to full bloom. Rarely do apothecia appear in fields before plants are at the bud stage and seldom does infection develop before the mid-flowering stage. Good weather for canola growth is also good weather for initial pathogen infection and disease development. Environmental conditions conducive to sclerotia germination, ascospore production and pathogen growth, include:

• One to two inches of rain or irrigation within one to two weeks of flowering.
• Prolonged soil moisture (10 days or more above the wilting point).
• Air temperatures of 59 to 77 °F.³

These conditions occur as the crop canopy closes, shading the soil surface. Humid conditions and temperatures between 68 to 77 °F are ideal for lesion development after initial infection. Infection and lesion development will slow or stop with dry, warmer conditions that occur during and after flowering (infection does not occur when temperatures are above 86 °F).³ Disease development can resume with the return of favorable conditions.

Within two to three weeks after initial infection on dead flower petals, lesions appear as water-soaked spots or areas of brown to grey discoloration on leaves and stems, particularly around leaf axils (Figure 20). As infection develops on the stems, lesions enlarge and turn grey-white.

Premature death of the plant may occur, while severely infected plants often lodge and shatter at swathing. White mold has some characteristic and identifiable signs associated with disease development, including:

• Brown or bleached necrotic lesions appear around penetration sites and spread to other parts of the plant.
• White cottony mycelium grow on both the inside and outside of infected stems.
• Hard, black sclerotia appear on the outer surface of diseased tissue but can be observed inside infected tissue as well (Figure 21). Sclerotia can be small and oval, similar to canola seed, or larger and irregularly shaped (up to two centimeters or 0.8 inch long).

Figure 17. Sclerotinia lesions on canola stems.

Most canola hybrids are susceptible to white mold when environmental conditions are favorable. The following management strategies may help control the disease:

Fungicides – an effective management tool for controlling Sclerotinia stem rot when the risk of infection is high. Because disease incidence and development is highly dependent on conditions around flowering, it is important to scout canola just prior to flower and early flowering to assess conditions. The optimal timing for a foliar fungicide application is at 20 to 50% bloom. Additionally, a fungicide application needs to cover as many flower petals as possible and penetrate the canopy to protect leaf axils and bases from possible Sclerotinia infection.

Crop Rotation – rotations with non-host crops including wheat, corn, barley or grass will help to reduce the number of sclerotia in the soil.²

Seeding and Row Spacing – seeding rate, row spacing, nitrogen fertilizers and location selection to minimize humidity within the canopy can help lessen white mold incidence and severity.

Weed Control – controlling the weeds in your field that act as hosts to white mold will help to minimize the disease.

Tillage – deep tillage can prevent germination of apothecia, but deeply buried sclerotia survive well and may return to the soil surface with subsequent tillage.

Harvest – growers can lose up to one third of their crop to Sclerotinia stem rot during swathing and suffer a downgrading of canola due to sclerotia in seed samples.³ In addition, the increases in sclerotia development during swathing can allow the pathogen to remain in the field after harvest. Therefore, avoid swathing when heavy rain is forecasted and the disease is present in the field.³

Irrigation – avoid heavy irrigation that encourages prolonged wetness on the leaves beyond 12 hours during periods of high humidity, particularly during flowering. Keep the top of the soil as dry as possible as the crop matures.⁴

Identification, Characteristics, Diagnosis

Verticillium stripe was first reported in 2014, making it a new disease across the Prairies. Little is currently known about its severity and potential impact on yield. The disease is caused by the fungus Verticillium longisporum, which overwinters in the soil as microsclerotia that can survive for many years. Root exudates from growing plants trigger germination of the microsclerotia, which then infect plants through their roots – either through natural openings or through wounds caused by root-feeding insects. Once inside the plant, the fungus moves into the vascular system, preventing the movement of nutrients and water.

The symptoms of Verticillium stripe become more apparent as the infected canola plant matures. First, infected plants usually experience stunting and premature ripening. Then darker, vertical striping usually appears on the outside of the plant which may follow one side of the plant all the way to the top. The outer layer of the stem may look ‘shredded’, and peeling back the stem will reveal microsclerotia developing inside. Dark tissue can also be seen in the interior of the stem in cross section, similar in appearance to blackleg.

This disease is most active in hot and dry conditions, which likely contributed to its increased presence in the Prairies in the 2022 and 2023 growing seasons. An infected canola plant can exhibit reductions in both seed size and seed quality. The earlier the infection of the crop, the more severe the yield loss can be.

Management

Currently, there are no foliar fungicides or seed treatment fungicides registered for control of this disease and there are no identified resistance genes in commercial canola hybrids. However, differences in hybrid susceptibility have been observed. The plant pathology group at the Crop Science division of Bayer is working to build a rating scale and conducting research to better understand the disease and any resistance DEKALB^® brand cultivars may have. Other currently known management practices include extending crop rotation and using good biosecurity practices similar to those for managing clubroot. Equipment should be sanitized and in-field traffic should be controlled.