The effect of standing canola stubble on the ascospore release pattern of Blackleg

Much of the existing research of the behavior of Blackleg (Leptosphaeria maculans) in canola relates to more ‘traditional’ production systems where tillage was widely used to manage previous crop residues. Production systems have changed markedly in the past 20 years – no-till practices, wider row spacings, auto-guidance systems allowing inter-row sowing and disc seeders have all reduced soil disturbance and promote crop residue retention and longevity. These changing production systems are creating a different micro-climate for developing crops and altering pest and disease behavior.

Implications on these changing production systems on blackleg have been investigated during the 2014 and 2015 growing seasons. Spore release patterns were evaluated by placing pots containing canola at cotyledon stage at two week intervals from May through to mid-August. These young plants act as trap plants enabling spore loads to be estimated. In this study the pots were placed in two paddocks in the Wagga region that had canola in the previous season. In total there were 8 batches of pots placed out in the fields each season. Blackleg spores in the environment would infect these plants which were rated for internal infection at maturity to provide an indication of key spore release periods.

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Figure1. 2014 Internal ratings/batch (click image for closer look)

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Figure 2. 2015 Internal ratings/batch (click image for closer look)

Figures 1 and 2 show the internal infection ratings (scored on a 1-9 scale, where 9=no infection and 1=dead) for each batch for 2014 and 2015 seasons respectively. These ratings show that a higher number of spores were being released during the later batches in 2015 compared to the drier 2014 indicating that moisture is a major determining factor.

Horizontal and vertical stubble samples were also collected at the same time pots were placed in the fields. To determine the spore production two segments at 0-5cm and 15-20cm were placed in a spore liberation chamber. Spores liberated from the respective stubble segments are captured on slides which can then be assessed to identify potential differences. Slides were scored on a 0-4 scale (0 = no spores and 4 = high numbers of spores) to identify differences. More accurate quantification of spore numbers is in progress.

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Figure 3. 2015 Spore liberation data (click image for closer look)

The spore liberation data indicates that the release pattern is very different from the two different stubble orientations. The spore release is more consistent for the horizontal stubble and poses the greatest risk to emerging seedlings. In contrast, it appears that fewer spores are released from vertical stubble during the critical infection periods, with the majority initially being released from the lower section. Late winter rainfall and the development of the surrounding crop, create a micro-environment that keeps the vertical stubble moist resulting in more spores being released from the 15-20cm segments later in the season. The results from this study imply that the changing farming practices are also altering the behavior of blackleg with the peak spore release period much later in the season and, while potentially reducing the exposure to emerging seedlings, may result in further implications as the crop develops – emphasizing the importance of a combination of major gene resistance and durable adult plant resistance found in Pioneer’s Y-series canola hybrids.

Acknowledgements:

Dr Raymond Cowley, Dr Steve Marcroft, Dr Angela De Wouw, Dr Phil Salsbury, Dr Kurt Lindbeck, DuPont Pioneer