Blackleg is an ever-present and evolving risk to canola production, and it requires a national system of surveillance to detect any changes to the disease.

A severe disease risk such as blackleg of canola needs a vigilant industry-wide monitoring and warning system to keep on top of any changes in the plant-pathogen dynamics and to hone management practices.

Caused by the fungus Leptosphaeria maculans, blackleg is a stubble-borne disease that leads to seedling, crown canker and upper canopy infection. It is challenging to manage due to the dynamic nature of seasonal conditions together with changing farming systems and changing climate, as the fungus is able to evolve and readily adapt to these changes.

With GRDC investment, the University of Melbourne together with Marcroft Grains Pathology, CSIRO and an experienced team on the ground are monitoring changes in virulence of blackleg fungal populations nationally, alerting growers when canola varieties with specific resistance genes are at threat of high disease levels.

Field surveillance

From 2011 onwards, all canola varieties and National Variety Trials (NVT) lines have been classified for their type of blackleg resistance: either major gene resistance (often referred to as seedling resistance, although it is expressed throughout the life of the plant) or quantitative resistance (often referred to as adult plant resistance and involving the combined contribution of a number of minor genes).

Seven resistance genes – A, B, C, D, F, H and S – have been identified and every canola variety is described by one or several of these letters according to what type of resistance they contain.

The level of blackleg disease for each resistance group is monitored at 34 NVT sites across Australian canola growing regions. Each season the variety responses are upgraded and recommendations released to growers.

Blackleg resistance groups AD, ABDF and H were effective at almost all sites in 2021. Conversely, resistance groups A, B, C and BF were overcome or partially ineffective at most sites.

Where Group H varieties had been grown for several years as a grain-and-graze option – such as at Hamilton, Victoria – unusually high levels of disease were observed in the Group H cultivar in 2020 and 2021. Further molecular investigation revealed that the fungal population had evolved virulence to this resistance. A warning was released for growers.

These observations emphasise the importance of constantly monitoring the level of disease on your own property as the local fungal population will greatly influence the effectiveness of the resistance genes in your cultivars.

Previous work showed upper canopy infection (UCI) can result in up to 20 per cent yield loss through infection of upper stems, branches and flowers.

With the move to earlier sowing, canola crops are flowering in late July and early August when conditions are conducive for blackleg infection and spores can land directly on the upper canopy. While UCI was detected at all sites in 2021, disease levels were less severe than previous years due to the late break to the season resulting in later sowing times.

Fungicide applications reliably reduced UCI disease symptoms; however, yield responses to fungicides were inconsistent (from zero to 50 per cent yield increase).

Environmental stress, thermal time, timing of infection and plant genetic resistance were identified as potential factors involved in determining whether UCI causes yield loss.

In 2021, experiments were conducted to try to further tease apart the contributions of these factors. Initial analysis of the data shows genetic resistance to UCI is present in some cultivars and this can be detected using glasshouse and field-based experiments. However, it is still unclear whether resistance to UCI is the same as resistance to stem canker – in other words, quantitative resistance – and further work is underway.

More information: Alex Idnurm, 03 8344 2221, alexander.idnurm@unimelb.edu.au; Angela Van de Wouw, 0439 900 919, apvdw2@unimelb.edu.au; Steve Marcroft, 0409 978 941, steve@grainspathology.com.au