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Fungal diseases of canola in Western Australia
Based on Bulletin 4406a [Reviewed June 2005]
By Martin Barbetti, Ravjit Khangura
February 2000
This information is based on Bulletin 4406, Fungal diseases of canola in Western Australia, prepared by Martin Barbetti and Ravjit Khangura
Contents
- Identifying canola diseases by the symptoms in the crop
- Management of canola diseases
- Acknowledgements
There has been a dramatic growth in the canola industry in Western Australia, from 35 000 hectares in 1993 to 920 000 hectares in 1999. Along with this expansion in the area sown, the incidence and severity of various fungal diseases have also increased, especially blackleg. These pages describe the most important and most prevalent fungal diseases of canola in Western Australia.
Diseases may affect all parts of the canola plant, from the germinating seed to seeds developing in the pods. Diseases may be identified in the field by the symptoms on the affected plant parts.
Identifying canola diseases by the symptoms in the crop
Canola plants may show lesions (spots, dead areas, malformations) at all stages - click on the stage that shows the disease symptoms in your crop.
- Germination or emergence (emergence may be poor)
- Seedlings
- Leaves
- Stems
- Flowers, flower stalks, pods, developing seeds
Symptoms on germinating canola seeds or on seedlings
Damping-off can produce various types of symptoms, ranging from pre-emergence rot (failure of plants to emerge), post-emergence damping-off (plants emerge but then collapse at ground level), and the distinct Rhizoctonia hypocotyl rot.
Symptoms usually occur in patches and the area affected can increase rapidly, particularly under conditions less than ideal for seedling germination and establishment.
Management of canola diseases
Management measures are not available or warranted for some canola diseases in Western Australia.
Downy mildew rarely caused any yield loss until recently; management measures have not been warranted before 1998, but potential management measures will now be investigated.
Click here for further information.
Damping off: No management measures are available for damping off caused by Pythium or Fusarium.
Click here for further information.
Other common diseases are controlled by:
- sowing resistant varieties;
- using appropriate crop rotations and locations of crops;
- carrying out crop hygiene measures to reduce the carry over of the fungus from crop to crop.
For management of blackleg, white leaf spot, sclerotinia stem rot, alternaria black spot, and white rust (staghead), click on the disease of interest.
Seedling hypocotyl rot and damping-off - symptoms, causes and management
While seedling hypocotyl rot and damping-off occur sporadically in Western Australia, since 1994 there has been a significant increase in the extent of hypocotyl rot and damping-off associated with cases of poor establishment or complete stand failure of canola seedlings.
Rhizoctonia spp., common soil inhabiting fungi, are one of the main pathogens associated with these types of conditions. Rhizoctonia solani isolates of type ZG5 (AG2-1) and type ZG1-1 (AG8) are highly pathogenic on oilseed rape, delaying seedling emergence and causing severe hypocotyl or root rot, respctively. ZG5 also induces post-emergence damping-off.
Increasing depth of sowing from one to three centimetres can significantly delay and reduce seedling emergence and increase disease severity from ZG5 type isolates. ZG5 isolates can produce severe hypocotyl rot symptoms on mustard and mild symptoms on narrow-leafed lupin and clover, but generally do not infect any of the cereal crops. In contrast, the crops grown in Western Australia are highly susceptible to ZG1-1, except mustard, which is only moderately susceptible.
Damping-off is also caused by other species of fungi such as Pythium and Fusarium. These fungi are common soil inhabitants that cause damage particularly under conditions that are less than ideal for seed germination and seedling growth.
No control measures are currently available for Pythium or Fusarium but useful control of Rhizoctonia can be obtained by applying a fungicidal treatment to the seed.
Symptoms on canola seedlings - seedlings collapse
With post emergence damping-off, the seedlings emerge, but they collapse at ground level - see the illustration on the left.
Symptoms usually occur in patches and the area affected can increase rapidly, particularly under conditions less than ideal for seedling germination and establishment.
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No control measures are currently available for Pythium and Fusarium, but useful control of Rhizoctonia can be obtained from applying a fungicidal treatment to the seed.
Click here if seedlings have spots
Symptoms on seedlings - spots
Blackleg infection of seedlings shows up as spots on the seedling leaves. Lesions develop on seedlings as white grey circular spots on the cotyledons (seed leaves) - (see Figure 4).
Infected cotyledons die prematurely and the fungus can invade the stem by advancing through the stem of the leaf.
Conidia (spores) produced on the cotyledons can lead to secondary infections in the crop.
Tiny dark fungal fruiting bodies later develop in the spots.
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Click here for management methods.
Downy mildew may infect seedlings, producing yellowish-brown angular lesions on the upper surfaces of cotyledons. these lesions may correspond with patches of white fluffy fungal growth on the under surfaces.
Click here for management methods.
Blackleg
Blackleg is the most common and serious disease of canola in Western Australia. It is a major limitation to the sustainability of a large-scale canola industry. There has been an increasing incidence and severity of blackleg in Western Australian crops since 1993, such that severe crown cankering now occurs in most crops and occasional sporadic incidences of extensive seedling death occur in worst affected areas.
This disease causes significant production losses, despite adult plant resistance present in commercial varieties and the use of fungicide applied to fertiliser. There is an urgent need for better management of blackleg if the canola industry is to remain viable and avoid a contraction as occurred in the early 1970s, following blackleg epidemics.
The Mediterranean-type environment of Western Australia:
- favours disease carry-over on residues,
- encourages severe epidemics of residue-borne pathogens,
- aligns ascospore showers to emergence and
- maximises the disease impact on yield as the moisture supply dwindles at the end of the season.
There are now more than 1.5 million hectares of infected residues in Western Australian canola production areas and consequently it is no longer feasible to isolate new plantings from canola residues, particularly where large amounts of residues have accumulated.
Improved management is needed if canola is to be a large and sustainable component of long-term rotations. Improvements should include:
- development and use of varieties with improved seedling and adult plant resistance,
- refining fungicide use for seedling protection, and
- reducing inoculum pressure on the crop.
Reduced disease inoculum pressure may be achieved by avoiding proximity to residues and by restricting rotations to one year in four or greater, which goes against the current industry trend. Reduced disease inoculum pressure may also result from improved crop residue management, including residue management, including residue removal and treatments to enhance residue decomposition and reduce spore load.
Blackleg is caused by the fungus Leptosphaeria maculans. The fungus survives in undecomposed infected crop residues for several years, so sowing into old canola paddocks can result in heavy losses. Stems colonised by the fungus constitute a disease risk for next season's crops planted within five kilometres, particularly those planted downwind and those within one kilometre.
Figure 6. Life cycle of the blackleg fungus
Pseudothecia (spore bodies) develop on infected crop residues left on the soil surface (see illustration on the left), and persist on plant fragments re-exposed by cultivation, even after several years in the soil.
Figure 7. Blackleg - fungal fruiting bodies (pseudothecia) on infected crop residues
Ascospores are released from pseudothecia after rain in autumn and winter and are widely dispersed by wind to infect crops. Heavy ascopore discharge during the early seedling stage (the cotyledon to two-leaf stages) can cause a severe disease epidemic.
Conidia are produced in the lesions and washed down the stem to infect the stem base. They are also rain-splashed to nearby plants, causing secondary infections.
On seedlings, the fungus can grow down through the cotyledon and leaf petioles and directly penetrate the stem base, causing cankers - see Figure 8.
Figure 8. Typical severe blackleg crown canker
Prolonged moist weather favours rapid spread and development of the disease.
If seed infection is the only source of inoculum the disease incidence in the crop will normally be relatively low. The high incidence of infected residues has made seed infection of little importance in disease epidemics occurring in Western Australia.
Susceptible varieties can be a complete loss. Varieties with adult plant resistance will develop leaf spots but generally do not develop severe crown cankers (see Figure 9) unless they were heavily diseased as seedlings at the cotyledon to 2-leaf stages.
Figure 9. Blackleg crown rot at the rosette stage of the plant
Management of blackleg
All canola residues of any age are a source of inoculum and risk to subsequent crops. There are three primary ways of minimising the risk of blackleg:
- Choose a variety with the highest level of disease resistance;
- Maximise distance between last year's residue and this year's crop (upwind is better!); and
- Reduce existing canola residue, for example by raking and burning, or by burying it.
The document Managing blackleg in 2001 provides a full description of ways to manage blackleg in each rainfall and disease risk zone, supplementing the brief description below.
- Only sow varieties with very high levels of resistance, and practice crop rotation. All varieties recommended for sowing in Western Australia have good adult plant resistance to blackleg.
- However, since all varieties currently grown are susceptible at the seedling stage, there is still a need to maintain good management and hygiene practices to keep blackleg disease to a minimum, even when using resistant varieties with good adult plant resistance.
- It is recommended to leave canola out of the rotation for at least three years to allow diseased residues to decompose and to reduce the risks from ascospore infection. In recent years, some growers have successfully reduced this break period, but there is increased risk from this practice. If the seedling stage of crop development coincided with heavy airborne spore discharges from nearby residues, even adult plant resistance varieties can suffer substantial damage.
- Provided there is no erosion risk, it is recommended to destroy crop residues after harvest to reduce carry-over of the fungus on infected stems. Grazing stubbles heavily also helps reduce fungus carry-over.
- Canola should be grown as far as possible from previous canola crops to reduce the risk of infection by wind-borne ascospores. Planting in areas downwind from old residues, where the prevailing winds are likely to blow air-borne spores, should be avoided.
- Crop protection agents are an important tool for blackleg management. For details on use of fungicides for blackleg control, refer to the detailed information provided in the current brochure for managing blackleg.
In summary, the best way for Western Australian growers to manage blackleg is:
- to select the variety with the highest resistance for their specific district,
- to avoid paddocks with canola residues,
- to reduce existing residues by grazing, burying or raking and burning,
- to assess the level of risk from the disease, and to consider whether or not it is appropriate to use a fungicide treatment.
Symptoms on older leaves
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Blackleg lesions are clearly defined as white or grey areas Small dark fruiting bodies develop in the spores.with a dark purplish margin (see Figure 10).
Figure 10. Blackleg lesions on leaf. -
White leaf spot shows up as greyish-white to brown leaf lesions, often with a brown margin, especially as they mature (see Figure 11).
Leaf spots can be up to 1 cm in diameter, often joining up to form large dead areas.
White leaf spot can cause complete loss of leaves in highly susceptible varieties, when yield losses may be as high as 30% if the disease is severe, but losses are unlikely to exceed about 3% to 5% on current varieties.
Nitrogen deficient crops seem to be more severely affected by the disease.
Figure 11. White leaf spot lesions; early lesion development
Figure 12. Mature and sporulating white leaf spot lesions
Figure 13. A crop severely affected by white leaf spot. -
White rust or staghead shows up as white or whitish cream pustules on the underside of leaves. Pustules rupture the host epidermis and expose a white chalky dust. Losses from foliar infection alone are probably minimal.
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Alternaria black spot shows up as dark, target-like spots on leaves. The spots may be entirely brown, black or greyish-white with a dark border.
The leaf spots may coalesce to produce large dead areas and premature leaf fall.
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Downy mildew symptoms include yellowish-brown angular lesions on the upper surfaces of leaves (see Figure 14.
Figure 14. Downy mildew lesions on lower surface of leaf.These lesions may correspond with patches of white fluffy fungal growth on the under surfaces.
White leaf spot life cycle
White leaf spot disease can be found in most plantings, but is not usually a cause of large yield losses.
White leaf spot is caused by the fungus Pseudocercosporella capsellae. The fungus survives on residues of infected plants. Under favourable autumn and winter conditions it produces wind-borne conidia, which mainly cause leaf lesions (see Figure 15).
Figure 15. White spot lesions on leaf.
In turn, the conidia produced in these lesions are carried by wind and rain to cause secondary spread of the disease. Whiteleaf spot is also spread from infected seeds and from pieces of infected debris present with the seed.
Optimum temperatures for infection are 13 to 18°C, but high moisture levels are necessary for disease development. The disease usually develops after periods of high rainfall.
Nitrogen deficient crops seem to be more severely affected by the disease.
Click here for management of white leaf spot
Management of white leaf spot
Undertake crop rotation and good hygiene as for blackleg management.
- Leave canola out of the rotation for at least three years to allow disease residues to decompose and reduce the risk of infection by spores.
- In recent years, some growers have successfully reduced this break period, but there is increased risk from this practice. If the seedling stage of crop development (cotyledon to 2-leaf stage coincides with heavy airborne spore discharges from nearby stubbles, even adult plant resistant varieties can suffer substantial damage).
- If there is no erosion risk, destroy crop residues after harvest to reduce carry over of the fungus on infected stems. Graze stubbles heavily to reduce fungus carry over.
- Plant canola as far as possible from previous canola crops to reduce the risk of infection by wind-borne spores. Avoid planting in areas downwind from old stubbles where the prevailing winds are likely to blow air-borne spores.
White rust (staghead)
White rust normally affects only Brassica rapa (turnip rape) and Brassica juncea (Indian mustard) in Western Australia. The disease is not generally found in Brassica napus (canola).
Symptoms
White or whitish cream pustules form on the underside of leaves and on floral parts. Pustules rupture the host epidermis and expose a white chalky dust. During flowering, deformities of the inflorescence develop (stagheads) as a result of systemic infection.
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Losses from foliar infection alone are probably minimal. Yield losses from staghead are directly proportional to the number of stagheads. In Western Australia, individual crops losses have been estimated as high as 20 per cent but would rarely exceed 3 to 5 per cent.
Cause
White rust is caused by the fungus Albugo candida.
Click here for management of white rust (staghead).
Management of white rust (staghead)
- Obtain seed from disease-free or low disease crops.
- Destroy crop residues after harvest to help reduce carry-over of the fungus.
- Leave Brassica rapa and Brassica juncea out of the rotation as long as possible to allow diseased residues to decompose and reduce the risk of subsequent infections.
- If white rust becomes a persistent problem, grow Brassica napus rather than Brassica rapa and Brassica juncea varieties.
Alternaria black spot
Both Brassica napus and Brassica rapa canolas are susceptible to Alternaria black spot but the disease normally only damages Brassica rapa in Western Australia.
Black spot is common but seldom serious in Western Australia, but it has occasionally caused serious yield loss in wet or humid spring seasons in eastern Australia.
Alternaria black spot is caused by the fungus Alternaria brassicae. The fungus oversummers on canola residues. The main source of infections is windborne spores blown in from alternative weed hosts and from spores from infected stubbles spread by wind and rain. The fungus can also be carried on the seed. Plants can be infected at any stage of growth.
The fungus can also be carried on the seed.
Plants can be infected at any stage of growth (see Figure 17).
Figure 17. Alternaria black spot lesions on pods and stems.
Click here for alternaria black spot control.
Management of alternaria black spot
- If alternaria black spot has been a problem, obtain seed from disease-free or low disease crops.
- Destroy crop residues after harvest to reduce carryover of the fungus on infected stems.
- Leave canola out of the rotation as long as possible, to allow diseased residues to decompose and reduce the risk of subsequent infections.
- Plant canola as far away as possible from previous canola crops to reduce the risk of infection from infected trash.
- Control wild radish, turnip, mustard and other cruciferous weeds in and near to canola crops.
Alternaria black spot seems likely to only become a problem if Brassica rapa varieties are grown. Brassica napus varieties are not only more resistant but often will mature later during warmer and drier conditions less favourable for disease development.
Background information on downy mildew
Downy mildew is a sporadically occurring disease caused by the fungus Peronospora parasitica (see Figure 18).
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Downy mildew is a sporadically occurring disease that until recently only rarely caused any disease loss. However, from 1998 onwards, very severe downy mildew has been widespread on seedlings, and in some cases it appears to have severely retarded seedling growth and vigour.
This disease is usually most evident under cool moist winter conditions with the crop normally growing away from it as the weather warms up in spring.
Symptoms on stems
Blackleg lesions are white or grey circular spots with a dark, purplish margin.
Small dark fungal fruiting bodies (pycnidia) develop in the spots.
Badly affected plants are ringbarked at ground level by cankers and the crop can lodge (see illustrations).
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Click here for management of blackleg
White leaf spot shows up as greyish-white to brown lesions, often with a brown margin.
Click here for management of white leaf spot
Alternaria black spot shows up as dark, target-like spots on stems. The spots may be entirely brown, black or greyish-white with a dark border (see Figure 24).
Figure 24. Alternaria black spot lesions on pods and stems.
Click here for management of Alternaria black spot
Sclerotinia stem rot causes the stalks to rot above the soil level, causing the plants to wilt and ripen prematurely (see Figure 25).
Severely attacked plants can lodge.
Click here for management of Sclerotinia stem rotSclerotinia stem rot life cycle
In some countries sclerotinia stem rot is the most important disease of canola. It is widespread in Western Australia, but generally not very severe, except in the more coastal parts of the northern production region.
Sclerotinia stem rot is caused by the fungus Sclerotinia sclerotiorum. Sclerotinia can survive as sclerotia (hard, dark resting bodies) in the soil for many years. The fungus probably also survives in the soil by colonising the roots of other plant species.
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During cool moist weather sclerotia near the surface germinate and produce small, cream, mushroom-like bodies called apothecia, containing many ascospores. These are carried by the wind to nearby crops.
Figure 27. Mature Sclerotinia lesion showing the typical bleached appearance.
Normally the spore must first germinate on, then infect, dead or dying plant material such as dead leaves or fallen flower petals caught in between the stems and leaves before it invades healthy tissues. Cool, wet weather favours the disease and mists, dews and fogs provide enough moisture for infection.
Infections high on the stems result in contamination of the seed with sclerotia.
Click here for management of sclerotinia stem rot
Management of sclerotinia stem rot
- Leave canola out of the rotation for as long as possible (at least three years) to allow diseased residues and sclerotia to decompose, to reduce the risk of subsequent infections.
- Include species unaffected by Sclerotinia in rotations - for example, use cereals.
- Do not include species such as lupins and peas, which are very susceptible, in severely affected areas.
- Use canola seed that is free of sclerotial contamination.
Deep ploughing of infected stubbles may reduce carryover to subsequent crops, since deep burial, greater than 15 cm, hinders sclerotial germination and development of apothecia.
Symptoms on flowers, pods and developing seeds
Blackleg can infect any part of the plant at all growth stages. Lesions develop as white or grey circular spots with a dark, purplish margin (see Figure 28).
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Small dark fungal fruiting bodies (pycnidia) develop in the spots.
Infection of the flowering parts can cause blighting of the flowers.
Pods with fungal lesions usually produce infected seed, which is often shrivelled and may not germinate.
Invasion of the crown is the most damaging phase of this disease. It usually results from cotyledon and leaf infection at the seedling stage. Badly affected plants are ringbarked at ground level by the canker, and lodge. Less severely affected plants remain standing but because the sap flow is restricted, pods fail to fill and seed is pinched.Susceptible varieties can be a complete loss. Varieties with adult plant resistance will develop leaf spots but often do not develop high levels of severe crown cankers unless they were heavily diseased during the early seedling (cotyledon to two leaf) stage.
Click here for management of blackleg.
White leaf spot shows up as greyish-white to brown lesions often with a brown margin.
Click here for management of white leaf spot.
Alternaria black spot may cause flower abortion. Infected inflorescences and pods may be entirely brown, black or greyish white with a dark border. peduncles (flower stalks) may have lesions. Heavily infected pods dry off prematurely, contain shrivelled seeds and easily shatter.
Click here for management of Alternaria black spot.
Acknowledgements
The authors thank the Grains Research and Development Corporation for financial support for canola disease research. Mr Rod Lewis of Agriculture Western Australia for the illustration of the life cycle of the blackleg fungus and for adapting other illustrations for the web.