WithTheGrain: Canola diseases on the watch-out list for 2018
Author: Alistair Lawson | Date: 13 Apr 2018
GRDC-funded research is helping growers and advisers to better manage important diseases of canola such as blackleg and sclerotinia stem rot.
In recent years, blackleg has been causing problems for some growers in areas of the high-rainfall zone where there is intensive canola production in two forms.
The traditional form, blackleg crown canker, results from infection of canola plants while they are still in the early seedling stage, usually during May and June.
Another form, upper canopy infection of blackleg (UCI), is the infection of flowers, peduncles, pods, stems and branches of canola plants. This form of blackleg was first observed in the southern region in 2010 and continued for the following three seasons, but generally not severe.
However, in 2014 and 2015, symptoms became more widespread, appearing to cause substantial yield loss.
In 2016, the GRDC invested in a Fast track project which investigated the causes and management of UCI.
According to Marcroft Grains Pathology principal Dr Steve Marcroft, data generated from this investment clearly showed that canola crops flowering during the winter period where conditions for blackleg infection are optimal consistently resulted in increased UCI.
“Trials in southern New South Wales in the 2016 and 2017 seasons where UCI was left uncontrolled resulted in a one tonne per hectare yield loss compared to where it was fully controlled,” he says.
“In both seasons, delaying the onset of flowering after mid-August reduced yield loss. In 2016, crops starting to flower in early August had minimal yield loss compared to those in 2017 which had 0.7t/ha yield loss.
“This tells us that although flowering time is one important factor in the development of UCI, seasonal conditions will determine the prevalence and severity of the infection.”
The instance of UCI has shown that blackleg is able to infect all parts of the canola plant (figure 1).
Genetic resistance is growers’ first line of defence against ‘normal’ blackleg crown canker, according to Dr Marcroft.
He says it is important for grower to know the resistance group as well las the blackleg rating when selecting what canola varieties they will grow in any given season.
“Australian canola varieties have two types of blackleg resistance genes – major and minor,” he says.
“Major resistance genes stop the fungus from infecting the plant which results in complete protection against the blackleg pathogen. We see this in the field through a lack of leaf lesions and crown canker.
“All canola varieties grown in Australia have one or multiple major resistance genes and these are classified into resistance groups which the describe the major genes present in the cultivar.”
Dr Marcroft says since blackleg is rapidly able to change, it can overcome cultivar resistance quickly in the field. Therefore, varieties dependent on major resistance genes tend to become more susceptible over time.
“Minor gene resistance, also known as quantitative resistance, adult plant resistance or crown canker resistance, reduces the severity pf crown canker but does not inhibit leaf lesions and upper canopy infection.
“The blackleg rating classifies cultivars according to their overall level of resistance and includes both major and minor gene resistance.”
New Blackleg CM app
A new app, Blackleg CM, has been developed to help growers and advisers effectively manage canola crops against blackleg infection.
The app allows the user to input information such as paddock selection, variety choice, seed dressing, banded fungicide and sprayed fungicide and takes into account costs, yield benefits and grain prices to give the best case, worst case and most likely estimates of economic return.
“The user has the option to change each parameter to tailor the output to their own individual circumstances,” Dr Marcroft says.
“Consequently, the user can explore their options for disease control and understand the relative importance of each factor.
“For example, distance to one year-old canola stubble has a large influence on disease severity, while two year-old stubble has a minor influence.
“The strength of the Blackleg CM app is that it allows the user to make as many comparisons as they wish in order to determine the best and most profitable way for them to reduce disease and increase profits.”
Sclerotinia stem rot
One of the key messages to come out of the five-year GRDC investment into the National Canola Pathology Program with regard to sclerotinia stem rot is the timing of fungicide application is critical in controlling the disease.
New South Wales Department of Primary Industries plant pathologist Dr Kurt Lindbeck says a foliar fungicide application is most effective at controlling sclerotinia stem rot when applied at 20-30 per cent bloom stage, or 14-16 flowers off the main stem (20 per cent) to 20 flowers off the main stem (30 per cent).
He says these fungicides are best applied as protectants and have limited curative activity against the disease.
“The greatest yield loss from sclerotinia stem rot occurs when the main stem becomes infected, especially early,” Dr Lindbeck says.
“Lateral branch infection does cause yield loss, but at a much-reduced level.”
Dr Lindbeck says weather conditions during flowering play a major role in the development of sclerotinia stem rot.
“The presence of moisture during flowering and petal fall will determine if the disease develops, whereas dry conditions during this time can quickly prevent development of the disease,” he says.
For the past three years, Dr Lindbeck has been conducting petal testing in southern NSW and northern Victoria to determine the relationships between petals infested with Sclerotinia ascospores, environmental conditions and development of stem rot.
In 2017, these tests found the highest levels of petal infestation – above 90 per cent – were detected in canola crops grown in higher-rainfall areas with a high frequency of canola and where the disease is frequently seen within crops.
“Given the large area of canola grown in Victoria’s Western District and the region’s high-rainfall status, I am often asked why we don’t see more sclerotinia stem rot in that area,” Dr Lindbeck says.
“I have looked at some of the weather data from the Western District and compared it to areas of northern Victoria, which frequently develops the disease. Large differences were detected in the level of in-crop relative humidity between Streatham in the Western District (lower relative humidity) and Dookie (higher relative humidity) in northern Victoria. Average daily wind speeds were also found to be higher at Westmere compared to Benalla.
“Leaf wetness within the crop canopy is a driving factor behind the development of sclerotinia stem rot. It is likely that higher wind speeds in the Western District result in reduced in-crop humidity and the crop is getting a greater chance to dry out.”
GRDC research codes: UM00051, MGP004, CSP00187, DAW0028
Dr Steve Marcroft,
03 5381 2294,
Dr Kurt Lindbeck,
02 6938 1608,
BlacklegCM can be downloaded on the App Store for Apple devices or Google Play for Android devices
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