Blackleg pod infection, resistance group monitoring and sclerotinia

Stephen Marcroft¹, Angela Van de Wouw^{1, 5}, Vicki Elliott¹, Kurt Lindbeck² , Andrew Ware³, Ravjit Khangura⁴ and Barb Howlett⁵

¹Marcroft Grains Pathology P/L, Grains Innovation Park, Horsham; ²Department of Primary Industries, Wagga Wagga Agricultural Institute; ³SARDI, Port Lincoln; ⁴ DAFWA, South Perth, ⁵ The University of Melbourne

Take home messages

• Blackleg pod infection was severe in some locations in 2013.
• Pod infection can cause significant yield loss.
• 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.
• Group E cultivars have developed low levels of stem canker on the Eyre Peninsula.
• Group D cultivars have developed moderate levels of stem canker in the North East.
• Sclerotinia was prevalent in NSW, WA and North eastern Vic.
• Sclerotinia was more severe where extended wetness and warm weather coincided during flowering.
• When determining if to spray for sclerotinia weigh up yield potential, disease risk and costs of fungicide application.

Blackleg pod and seed infection

Blackleg pod infection is caused by lesions forming on the pods, the same process as what causes lesions on the leaves. In 2013 pod infection was observed in all monitored regions of Victoria. Pod infection will result in seed infection.  Infected seeds may die and shrivel and/or cause pod shatter, reducing yield. Seed retained from infected pod will have reduced germination and may result in seedling blight.

Pod infection was assessed on six cultivars which were present at each site and chosen as they represent the resistance groups used in blackleg management (groups A, B, C, D, E and G). Plants were assessed by counting all pods on randomly selected plants and then counting the number of pod lesions to determine the average number of lesions per plant (Table 1).

Cultivar and regional effects were recorded with groups D, E and G showing no or very low pod infection. Although pod infection was not shown to be correlated to stem canker infection it was clearly evident that in cultivars which have effective seedling resistance (such as group D, E and G) very little, if any pod infection was observed compared to cultivars reliant on adult plant resistance (groups A, B and C). These data suggest that seedling (major gene) resistance may play a role in controlling pod infection and further investigation is ongoing.

Table 1. Mean pod infection data for eight locations across Victorian canola growing regions (pod infection is measured as the percentage of pod with a blackleg lesion)

Key findings on pod infection research:

1. Pod infection varies between sites / regions and seasonal conditions. Moist conditions during flowering / pod development appear to result in pod lesions.
2. Pod infection does not correlate with stem canker severity, i.e. a cankered plant may have no pod infection or a plant with no canker may have severe pod infection.
3. Pod infection will result in seed infection; infected seeds may shrivel and cause yield loss. Pod lesions may also cause pod shatter causing significant yield loss. In 2013 some sites had more yield loss from pod lesions than traditional stem canker.
4. Retained seed from pods with lesions will have reduced germination and seedlings may die from seedling blight.
5. Spraying canola plants at the 4^th leaf stage to control stem canker does not reduce pod infections.
6. It is not known if later fungicide applications reduce pod infection.
7. Cultivars with effective major gene resistance (seedling resistance) do not get pod lesions.
   

Victoria 2013 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 32 National Variety Trial across Australia and monitored for levels of blackleg development. 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.

• It is important to note that blackleg monitoring sites are sown without any fungicide protection to seed or fertiliser and do not receive any foliar fungicide applications. 

2013 summary

In 2013, eight sites were monitored for blackleg severity. Each site contained each of the six blackleg resistance groups; Groups A, B, C, D, E and G. Overall blackleg severity has not increased in recent years.

In 2013 there were generally low levels of blackleg across Victoria. This is due to the very dry start to the season. Blackleg fruiting bodies on the stubble were not mature and did not release from the stubble until the end of July. This late onset of spore production reduced disease pressure.

In 2013 the Group D resistant cultivar Hyola®444TT was observed to have moderate levels of blackleg infection in one site in the North East. Group D still has low blackleg infection to blackleg in all other sites and regions across Victoria. This is the same situation as occurred with Group D cultivars in 2010 on the Eyre Peninsula. If the same pattern of increased infection occurs, the level of blackleg infection in Group D cultivars will increase in 2014 and then become severe in 2015. 

If you are in the North East and have grown Group D cultivars over the past 2 years, Group D cultivars may have increased disease severity in 2014.

For individual site results consult the NVTonline website.

Summary of all Australian blackleg monitoring sites.

Cultivars representing each of the resistance groups were sown adjacent to canola National Variety Trial 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 NVTonline website.

Table 2. Levels of disease for each resistance group at the canola NVT sites across Australia

Key

Sclerotinia Stem Rot – the new challenge

How does the disease develop?

The fungal pathogen that causes sclerotinia stem rot is called Sclerotinia sclerotiorum.  This fungus can infect over 300 plant species, mostly broadleaf plants, including many crop, pasture and weed species. This includes plants like canola, lupin, pulses, sunflower, lucerne, cape weed, and shepherds purse. The main features of the disease are:

1. Airborne spores of the fungus are released from apothecia (a small, golf tee shaped structure, 5 – 10 mm in diameter) which germinate from sclerotia in the soil. For this to occur prolonged moist soil conditions in combination with moderate temperatures of 15°C to 25°C are considered ideal. Most sclerotia will remain viable for up to 3 – 4 years then survival slowly declines.
2. Spores of the sclerotinia pathogen cannot infect canola leaves and stems directly. They require petals as a food source for spores to germinate, grow and colonise the petal. When the infected petal eventually drops, 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.
3. Sclerotia also have the ability to germinate in the soil, produce mycelium and directly infect canola plants in close proximity, causing a basal infection.
4. Weather conditions during flowering play a critical role in determining the development of the disease. Sclerotinia development requires both moisture, and warm temperatures during flowering and petal fall. Dry and or cool conditions during this time will prevent the development of the disease. Hence, even if flower petals are infected, dry conditions or cool wet conditions during petal fall will prevent stem infection development.
   

Research findings in 2013

In 2013 sclerotinia was observed in all canola producing states, however it was more severe in North Eastern Victoria, NSW and WA. This is because in the southern growing regions rainfall is normally associated with cold fronts which result in cooler conditions not conducive to sclerotinia development. 

In NSW a number of commercial canola crops were monitored for the development of sclerotinia stem rot in 2013. These crops were around Cootamundra and south of Henty, in traditionally high disease risk districts. Results from observations within these crops found a very strong relationship between leaf wetness and stem rot development.  While the level of stem rot development varied between the crops south of Henty and those at Cootamundra, it was found those extended periods of continual leaf wetness of at least 24 hours or longer were critical ‘trigger’ points for stem rot development in both regions. 

It was also found that petal infection is important in the initial establishment of stem rot.  But, once canopy closure occurred and a humid microclimate was established, the infection of plant tissue under the crop canopy can provide ready opportunities for continual disease development later in the season.  These tissues include lower leaves and senescent leaves that can become colonised and later adhere to stems, causing stem lesion development and yield loss.  This work will continue in 2014 to collect and collate data which will be used to develop a disease prediction model.

Where did the disease occur in 2013?

In 2013 epidemics of sclerotinia in southern NSW and north eastern Victoria were observed in traditionally high rainfall districts. These included districts east of Cootamundra, Young and Cowra, south of Henty, around Corowa and Howlong and districts along the Murray River. Infection levels observed in some crops were as high as 30 – 60%.  In other districts, crop infection levels were generally low.

Why did we observe higher levels of sclerotinia stem rot in 2013?

The weather conditions during the winter of 2013 could be considered ideal for the development of sclerotinia stem rot.  Mild winter temperatures resulted in many canola crops flowering 3 – 4 weeks earlier than would be considered ‘normal’ for southern NSW and northern Victoria.  Canola crops were observed to be flowering as early as the middle of July.  These flowering crops also coincided with good rainfall throughout late July and August, which provided ideal conditions for apothecia development and release of ascospores.  Frequent rainfall events throughout August provided long periods of leaf wetness and ideal conditions for infected petals to drop into wet crop canopies and allow infection to occur.

What are the indicators that sclerotinia stem rot could be a problem in 2014?

• Epidemics of sclerotinia stem rot generally occur in districts with reliable spring rainfall and long flowering periods for canola. 
• 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. 
• 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 24 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. Separate this season’s paddock away from last year’s canola stubble. Not only does this work for other diseases such as blackleg, but also for sclerotinia.
3. 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 pea or faba bean.
4. Follow recommended sowing dates and rates for your district.  Canola crops which flower early, with a bulky crop canopy are more prone to developing sclerotinia stem rot.  Bulky crop canopies retain moisture and increase the likelihood of infection.  Wider row spacing’s can also help by increasing air flow through the canopy to some degree until the canopy closes.
5. 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.
6. 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.

When is the best time to apply a foliar fungicide?

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 reducing the level of sclerotinia infection. 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. 

In 2013 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 and that 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 cause 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

steve@grainspathology.com.au