Canola disease update 2015 (Vic)

GRDC project codes: UM00051, MGP0003, DAN177

Keywords: canola, blackleg, disease management, resistance groups, branch infection, sclerotinia stem rot.

Take home messages

• Blackleg symptoms have occurred higher in the canopy than previously; these symptoms have caused some yield loss.
• Blackleg stem canker in 2014 was very low across Victoria except for the North East. This was due to low rainfall.
• Blackleg pod infection can cause yield loss in some years but was extremely low in 2014.
• Regional monitoring results for each blackleg resistance group are available on the NVT online website. Consult the Blackleg Management Guide for details of resistance groups.

In 2014, Group E had low levels of infection in all regions of Victoria. However, moderate infection occurred in Group E on the Eyre Peninsula, South Australia in both 2013 and 2014. This indicates that in future, Group E is likely to get disease similar to other resistance groups. The Group D cultivar in Victoria had moderate levels of stem canker in the North East and Western District. The levels are not high enough to cause yield loss but need to be monitored in 2015 to see if they increase. Low levels in 2014 may also have been due to environmental conditions not conducive to blackleg.

• Sclerotinia was almost non-existent in Victoria in 2014 (consistent with low rainfall in spring).
• Other fungal diseases did not cause any yield losses in 2014 (Alternaria, White Leaf Spot, Downy Mildew, Powdery Mildew, Damping off).
• See separate paper for viruses.

Blackleg - 2015 update

Branch symptoms of blackleg - Branch canker in 2014

Normal blackleg infection occurs as lesions, typically on the older leaves, but can develop on any plant part. The fungus grows from the lesion to the crown (junction between the stem and the roots) where it causes a necrosis, blocking the vascular tissue and can cause the plant to fall over and die. In recent years, we have observed infection inside the stem and necrotic lesions on the outside of the stem and branches. In 2014, these symptoms were more widespread and more severe. In some cases, these necrotic lesions have caused significant yield losses.

Figure 1. Images of stem/branch infection in 2014.

Causes of stem/branch cankers

We do not know why we are now seeing stem/branch cankers, however we have isolated the pathogen and confirmed that it is blackleg causing these symptoms.

Anecdotal observations

This new stem/branch canker may have always been present but has been exacerbated by particular environmental influences in 2014.

The most severe blackleg infection normally occurs in late May to mid July. This infection timing normally coincides with plants that are germinating to the 6th leaf growth stage, that is, when the plants are only in the vegetative leaf growth stage, therefore there is no stem or branches to infect. In 2014, crops were sown early and warm conditions through early winter meant that plants were more developed during July. Therefore, when blackleg severity was at its most intense, many canola crops had bolted and already had flowers and branches. This issue was also exacerbated by frost damage on the branches when they were still green and soft. This frost damage on the branches kills plant cells and thus allows an entry point for the blackleg fungus.

Management of stem/branch infection

• Try to ensure stem elongation occurs in the normal flowering window, not in winter when blackleg intensity is at its highest.
• Cultivars with effective major gene resistance did not develop stem/branch infection. Therefore, blackleg resistance groups D, E and F did not get stem/branch canker, even at sites where groups A, B, C and S did get branch cankers.
• Seed dressing fungicides and Prosaro® applied at the 4-6 leaf growth stage will not protect the stem/branches. It is unknown if later applications of fungicide will protect plants.

Victoria 2014 blackleg severity

Background

• The fungal disease blackleg can be minimised by a number of factors including sowing cultivars with high blackleg resistance, avoiding last year’s stubble and applying fungicides (see the current Blackleg Management Guide for details - www.grdc.com.au). An additional method for minimising disease is rotating cultivars with different resistance genes.
• All canola cultivars are classified into different resistance groups. Refer to the current Blackleg Management Guide (www.grdc.com.au) for individual cultivar groups.
• Cultivars representing each of the resistance groups are sown at 40 National Variety Trials (NVT) across Australia and monitored for levels of blackleg infection. These data indicate which resistance groups have higher levels of disease compared to the national average at each of the regionally based NVT canola yield sites (these data indicate if the blackleg population is skewed towards being able to attack one or more particular resistance groups).
• It is important to note that blackleg monitoring sites are sown without any fungicide protection on seed or fertiliser and do not receive any foliar fungicide applications.

2014 summary

In Victoria in 2014, eight sites were monitored for blackleg severity. Each site contained each of the seven blackleg resistance groups; A, B, C, D, E, F and S. Overall, blackleg severity has not increased in recent years.

In 2014, there were generally low levels of blackleg across Victoria which is consistent with low rainfall. Canola is also most susceptible to blackleg from germination to the 5^th leaf growth stage. In 2014, early sowing and warm autumn conditions meant that canola plants grew quickly through this seedling growth stage, thereby escaping the disease infection period.

In 2013 and 2014, the representative Group D resistant cultivar was observed to have moderate levels of blackleg infection in the North East and Western District. Group D still has low blackleg infection in all other sites and regions across Victoria. Although the level of disease in the Group D cultivar did not cause any yield loss in 2014, it should be monitored in 2015 so that growers know if it has increased to a dangerous level and can change cultivars prior to the 2016 season.

Summary of all Victorian blackleg monitoring sites

Cultivars representing each of the resistance groups were sown adjacent to canola NVT sites across Australia and monitored for levels of blackleg. These data indicate which resistance groups have high levels of disease compared to the national average at each site.

For more detail consult the individual site summaries and recommendations on the NVT online website.

Key

L - Low blackleg severity in this group – continue with current management techniques.
M - Moderate blackleg severity in this group – monitor crops for disease, see Blackleg Management Guide.
H - High blackleg severity in this group – high risk of yield loss, see Blackleg Management Guide.

Other diseases in 2014

In 2014, other fungal diseases were generally not an issue in canola, as warm conditions early in the growing season resulted in fast growing crops that escaped most leaf diseases. Initial disease symptoms occurred in most districts but disease progression stopped at the beginning of August as the season turned dry. At Hamilton, White Leaf Spot increased up the canopy as it also did in 2013, however yield loss was still unlikely. In the North East, Powdery Mildew occurred on mature plants but was not severe enough to cause any yield losses. There were no other diseases present. Sclerotinia did not occur in Victoria in 2014 which is consistent with the dry spring.

Sclerotinia stem rot – 2015 update

How does the disease develop?

Sclerotinia stem rot has a complex lifecycle compared to many other foliar diseases. There are several key stages that must be synchronised and completed in order for plant infection to occur. Weather conditions must also be suitable for the pathogen at each stage. These stages of development are:

1. Soil borne sclerotia are the main source of inoculum each year. Sclerotia soften and germinate in winter once soil has been wet at the surface. This requires continuous wet conditions for about 10 days and often not until full ground cover is reached by the developing crop. Most sclerotia will remain viable for up to 3–4 years then survival slowly declines.
2. Airborne spores of the fungus are released from apothecia (a small, golf tee shaped structure, 5–10mm in diameter) which germinate from sclerotia in the soil.
3. Spores of the sclerotinia pathogen cannot infect canola leaves and stems directly. They mainly utilise petals as a food source to germinate, grow and colonise. While petals are the most common food source, other plant parts such as old leaves under the canopy are also prone to infection and colonisation. When the infected petal undergoes senescence, it may become lodged onto a leaf, within a leaf axil or at branch junctions along the stem. If conditions are moist, the fungus grows out of the petal and invades healthy plant stem tissue which will result in a stem lesion and production of further sclerotia within the stem which will be returned to the soil after harvest. Sclerotinia is more prevalent in crops with heavy vegetative growth, where air circulation is likely to be limited.
4. Sclerotia also have the ability to germinate in the soil, produce mycelium and directly infect canola plants in close proximity, causing a basal infection.
5. Weather conditions during flowering play a major role in determining the development of the disease. The presence of moisture during flowering and petal fall will determine if sclerotinia develops. Dry conditions during this time can quickly prevent development of the disease, hence even if flower petals are infected, dry conditions during petal fall will prevent stem infection development.

Research findings in 2014

Commercial canola crops in southern NSW were monitored for the development of sclerotinia stem rot in high sclerotinia risk districts in 2014. These crops were located east of Cootamundra and south of Henty. Consistent with results from 2013, observations within these crops found a very strong relationship between prolonged periods of leaf wetness and stem rot development.

There was potential for high levels of stem rot to develop at several of the disease monitoring sites in 2014. Rapidly developing crops, the presence of apothecia, and high levels of petal infestation by sclerotinia, all indicated that epidemics of the disease were likely. However, drier than average conditions throughout August and spring kept potential stem rot levels low in many districts. Dry conditions within the crop canopy did not allow the pathogen to spread from petals into stems.

Where did the disease occur in Victoria in 2014?

A survey of 15 commercial canola crops in the Western District, south of Ararat was conducted in October 2014. Crop inspections revealed very low levels of infection, generally less than 1%. Data loggers measuring temperature and humidity were placed in the field at Streatham and indicated that environmental conditions within the crop canopy were not favourable for sclerotinia infection to occur, in particular extended periods of leaf wetness. Canola crop in the North East of Victoria, where the disease can occur, also reported lower levels of stem rot compared to 2013. Drier than average conditions in late winter and spring kept disease levels low compared to potential disease levels observed earlier in the season.

Ideal infection conditions for sclerotinia stem rot are periods of continuous leaf wetness for 36-48 hours during flowering. The leaf wetness can take the form of light rain, heavy dew or fog. This allows enough moisture for senescent petals to fall and stick to stems. Heavy rain can actually reduce disease potential by washing petals off plants.

What are the indicators that sclerotinia stem rot could be a problem in 2015?

• Spring rainfall. Epidemics of sclerotinia stem rot generally occur in districts with reliable spring rainfall and long flowering periods for canola.
• Frequency of sclerotinia outbreaks. Use the past frequency of sclerotinia stem rot outbreaks in the district as a guide to the likelihood of a sclerotinia outbreak. Paddocks with a recent history of sclerotinia are a good indicator of potential risk, as well as those paddocks that are adjacent. Also consider the frequency of canola in the paddock. Canola is a very good host for the disease and can quickly build up levels of soil-borne sclerotia.
• Commencement of flowering. The commencement of flowering can determine the severity of a sclerotinia outbreak. Spore release, petal infection and stem infection have a better chance of occurring when conditions are wet for extended periods, especially for more than 48 hours. Canola crops which flower earlier in winter, when conditions are cooler and wetter, are more prone to disease development.

If I had sclerotinia in my canola crop last year, what should I do this season?

The biggest challenge in managing sclerotinia stem rot is deciding whether or not there is a risk of disease development and what will be the potential yield loss. Research in Australia and Canada has shown that the relationship between the presence of the pathogen (as infected petals) and development of sclerotinia stem rot is not very clear due to the strong reliance on moisture for infection and disease development.

Important management options include:

1. Sowing canola seed that is free of sclerotia. This applies to growers retaining seed on-farm for sowing. Consider grading seed to remove sclerotia that would otherwise be sown with the seed and infect this season’s crop.
2. Rotate canola crops. Continual wheat/canola rotations are excellent for building up levels of viable sclerotia in the soil. A 12 month break from canola is not effective at reducing sclerotial survival. Consider other low risk crops such as cereals, field peas or faba beans.
3. Follow recommended sowing dates and rates for your district. DO NOT BE TEMPTED TO SOW CROPS EARLY IF YOU ARE LOCATED IN A SCLEROTINIA PRONE DISTRICT. Early flowering crops are more prone to developing sclerotinia stem rot by increasing opportunities for infected petals to lodge in a wet crop canopy. In addition, early sown crops will most likely develop bulky crop canopies which retain moisture and increase the likelihood of infection. Wider row spacings can also help by increasing air flow through the crop canopy to some degree and delaying the onset of canopy closure.
4. Consider the use of a foliar fungicide. Weigh up yield potential, disease risk and costs of fungicide application when deciding to apply a foliar fungicide.
5. Monitor crops for disease development and identify the type of stem infection. Main stem infections cause the most yield loss and indicate infection events early in the growing season. Lateral branch infections cause lower levels of yield loss and indicate infection events later in the growing season.

Use of foliar fungicides

At this time, there are no commercial canola cultivars available on the Australian market with resistance to sclerotinia stem rot. Management of the disease relies on the use of cultural and chemical methods of control. Foliar fungicides should be considered in those districts which are at a high risk of disease development (e.g. districts where the disease frequently occurs, long flowering period and reliable spring rainfall). There are several foliar fungicides currently registered for use in Australia to manage sclerotinia stem rot.

Points to consider when using a foliar fungicide to manage sclerotinia stem rot

1. The most yield loss from sclerotinia occurs from early infection events. Early infection is likely to result in premature ripening of plants and produce little or no yield.
2. Plants become susceptible to infection once flowering commences. Research in Australia and Canada has shown that an application of foliar fungicide around the 20%-30% flowering stage (20% flowering is 14–16 flowers on the main stem, 30% flowering is approx. 20 flowers on the main stem) can be effective in significantly reducing the level of sclerotinia stem infection. Most registered products can be applied up to the 50% flowering (full bloom) stage.
3. The objective of the fungicide application is to prevent early infection of petals while ensuring that fungicide also penetrates into the lower crop canopy to protect potential infection sites (such as lower leaves, leaf axils and stems). Timing of fungicide application is critical.
4. A foliar fungicide application is most effective when applied before an infection event (e.g. before a rain event during flowering). These fungicides are best applied as protectants and have no curative activity.
5. In general, foliar fungicides offer a period of protection of up to three weeks. After this time, the protectant activity of the fungicide is compromised.
6. Use high water rates and fine droplet sizes for good canopy penetration and coverage.

In 2014, some commercial crops which received an application of foliar fungicide still developed stem rot later in the season. This is not unexpected as the fungicide will have a limited period of protection during a time of rapid plant growth. The main aim of foliar fungicide applications is the prevention of main stem infections, which cause the greatest yield loss. Development of lateral branch infections later in the season is not uncommon and will result in lower yield loss.

Consult the Sclerotinia Stem Rot in Canola factsheet for further information. This publication is available from the GRDC website.

Contact details

Steve Marcroft
Marcroft Grains Pathology, Grains Innovation Park, Horsham
03 53 812294
steve@grainspathology.com.au

Kurt Lindbeck
NSW Department of Primary Industries, Wagga Wagga Agricultural Institute
02 69 381 608
kurt.lindbeck@dpi.nsw.gov.au