Blackleg in the farming system and understanding disease risks Northern

¹CSIRO Agriculture & Food, Canberra, ACT
²Marcroft Grains Pathology, Horsham, VIC
³School of BioSciences, University of Melbourne, VIC
⁴SARDI, Port Lincoln, SA
⁵NSWDPI, Wagga Wagga/Orange, NSW

Take home messages

• Crop emergence coincided with blackleg spore release and wet seasonal conditions resulting in extreme levels of blackleg leaf lesions in some areas in 2016. However, crown canker severity was much lower than expected given the severe leaf infection.
• Upper canopy infection is the collective term for flower, peduncle, pod, main stem and branch infection but does not include crown canker. In 2016, severe flower and pod infection was present but stem/branch infection was very limited compared to 2014/2015. 2016 data strongly indicates that the risk can be reduced by flowering in August (not flowering in June / July). Cool moist spring (2016) compared to hot dry spring (2014-2015) may delay branch infection to post-harvest.
• In 2016, infection of pods by blackleg caused significant yield loss by premature loss of pods or reduced seed size. Early flowering crops are at greater risk of pod infection because they have prolonged exposure to blackleg spores and periods of wet weather during pod development.
• Risk of yield loss associated with Upper Canopy Infection can be reduced by selecting the correct variety for the sowing date.
• Treatment of retained seed for 2017: If you had high levels of blackleg or alternaria pod infection do not retain seed. If retaining 2016 seed, grade it for larger seed which is less likely to be infected with blackleg and ensure even and adequate treatment with an appropriate fungicide to control seedling blight.
• Remember that 2016 was a highly unusual season – plan for 2017 assuming more average seasonal conditions, keeping in mind the increased risk associated with cultivars that have reduced resistance and high inoculum loads from 2016 crop residue. Also consider blackleg risk for the paddock – rotation history, distance to last year’s canola stubble and environment.

Periods of infection by blackleg for different plant parts

Blackleg is able to infect all parts of the canola plant. Figure 1 shows the relationship between the period of blackleg spore release and symptom development on different plant parts. Lesions form on leaves throughout the growing season however severe crown canker is most likely to develop when plants are infected during the early seedling stage. The fungus grows from the cotyledons and leaves asymptomatically through the vascular tissues to the crown where it causes necrosis resulting in a crown canker at the base of the plant. Yield loss results from restricted water and nutrient uptake by the plant. Protection during the seedling stage is critical to reduce crown canker severity. Lesions can also develop on all other plant parts and these infections may go on to develop cankers as described below.

Winter is the main period in which conditions are generally most conducive for infection as rainfall triggers release of mature spores from crop residue and provides ideal conditions for the fungus to survive whilst it infects the crop (Figure 1). Once the plant has begun to flower, infection of flowers, peduncles, pods, main stem and branches of the plant has collectively been termed “Upper Canopy Infection” (Figure 2). Any plant parts exposed to spores during the winter period are likely to become infected and potentially cause yield loss. Upper Canopy Infection has become increasingly prevalent over recent years and is hypothesised to be associated with the shift to earlier flowering as a result of earlier sowing of cultivars and more rapid phenological development during warmer autumns and winter. While the cost to yield and control of leaf lesions leading to crown canker is well understood, the factors contributing to Upper Canopy Infection and possible control strategies are currently under investigation with preliminary data presented in the current paper.

Figure 1 - Periods of infection by blackleg for different parts of the canola plant in relation to the period of spore release and start of flowering in medium and high rainfall zones. Solid lines indicate main periods of infection and dashed lines indicate reduced risk from infection. For start of flowering, solid line indicates period in which disease risk is reduced while dashed line indicates period of increased disease risk.

Figure 2. Upper canopy infection includes blackleg infection of flowers, peduncles, pods, main stems and branches.

Factors contributing to severe blackleg in 2016 – leaf and pod lesions

In recent years, dry summers in many regions resulted in delayed release of blackleg spores which, combined with earlier breaks to the season, resulted in low levels of blackleg crown canker. Earlier sown crops grow rapidly, quickly passing the most vulnerable seedling stage when leaf infection can lead to severe crown canker. Despite the low levels of disease, the blackleg pathogen was still evolving to overcome resistance genes in host cultivars. Predominance of Group A resistance in open pollinated triazine tolerant cultivars in many regions over a number of years resulted in blackleg populations with a high frequency of isolates virulent towards Group A in 2016. In addition, increased canola plantings in the past few years have resulted in a large area of canola stubble that can release blackleg spores.

In 2016, the recommended 500m distance between this years’ crop and last years’ canola stubble was difficult to achieve due to increased intensification of canola production. The Blackleg Rating of many Group A open-pollinated triazine tolerant cultivars fell from Moderately Resistant (MR) to Moderately Resistant – Moderately Susceptible (MR-MS), indicating their increased susceptibility to disease. This, in combination with the very wet winter, resulted in conditions that were highly conducive for blackleg infection. Researchers and agronomists reported very severe leaf lesion infection and seedling death.

Seedling leaf lesions

The result of this severe leaf infection meant that agronomists and growers had to determine if foliar fungicide application was warranted in regions that do not normally apply foliar fungicides. Ideally Prosaro® is applied at the 4-6 leaf growth stage. Research shows that combined seed and fertiliser treatment protect the seedling and Prosaro extends the length of protection during the stage when leaf infection can result in serious stem canker. Foliar applications at the seedling stage are generally most effective under very high disease situations, such as those present in 2016.   Many crops were past the 4-6 leaf growth stage by the time the severe infection was discovered, with crops typically sprayed between the 4-6 leaf and 10-leaf growth stages. This seedling spray successfully controlled leaf lesions and generally gave an excellent return to growers with many reporting significant yield increases. Research findings indicate that application to older seedlings has limited control of crown canker but yield gains were possibly related to increased photosynthetic capacity of the crop by maintaining green leaf area.

Crown canker severity in 2016

Despite extreme levels of leaf lesion infection seen across many regions early in the season, severe crown cankers were not observed at maturity as expected. This unexpected result is thought to be due to the extremely mild spring whereby plants were not heat or water stressed and therefore crown cankers were not expressed as per normal seasons. Despite this, Group A cultivars showed higher levels of crown canker severity at maturity compared to all other resistance groups at most (22 of 31) blackleg monitoring sites across south-eastern Australia. For more information on disease severity of the blackleg resistance groups in all areas of the cropping region, please see the NVT website for the 2016 results.

Upper canopy infection – flowers, peduncles, pods, stems and branches

Warm autumn and winter conditions promoted rapid plant development and early flowering. As the 2016 season progressed, blackleg remained severe with infection of flowers and pods. Although infected flowers are aborted, this type of infection is unlikely to result in significant yield loss as canola produces many more flowers than are fertilised to produce seed. However, infected flowers can act as an entry point for stem/branch infection (as seen in Figure 2). With the increased threat of sclerotinia from wet conditions, many growers sprayed Prosaro at 20-30% bloom as a dual control for upper canopy blackleg and sclerotinia.

Pod infection

In 2016, the main symptom of upper canopy infection causing yield loss was infected pods. Infection of pods by blackleg can cause complete loss of pods as they break off the plant or shatter prematurely. Grain inside infected pods retained on the plant can also be affected (Table 1). Pods with increasing severity of blackleg lesions have reduced grain size and seeds/pod. Severe blackleg lesions (>10mm) reduced grain size by 11-14% and seeds/pod by up to 4.6. In addition, seed contained within infected pods and retained for future use is infected with blackleg. Plants growing from infected seed can have seedling blight resulting in poor crop establishment. Given the high level of pod infection by blackleg and alternaria (both of which can cause seedling blight) observed in many crops in 2016, it is recommended that seed is not retained from crops. If retaining 2016 seed, grade it for larger seed which is less likely to be infected with blackleg and ensure even and adequate treatment with an appropriate fungicide to control seedling blight.

Table 1. Effect of blackleg pod infection on the seed yield of individual pods collected from cv 44Y89 at Canowindra and Wagga Wagga, NSW in 2016.  TGW = thousand grain weight. Values followed by the same letter within each column are not significantly different (P<0.05).

Field experiments in 2016 showed the level of pod infection can be significantly reduced by delaying flowering outside the main blackleg infection period in winter. In 2016, a field experiment with cv 44Y89 was conducted at Wagga Wagga with untreated and full fungicide (seed, fertiliser, late seedling Prosaro and 2 Prosaro applications during flowering) treatments sown on 1, 14 and 28 April (Table 2) (It should be noted that these treatments were applied for experimental purposes only as Prosaro use is limited to a maximum of 2 sprays per crop (label requirement)).. Crown canker, sclerotinia and frost were absent or at low levels with blackleg pod infection the primary cause of yield loss. Pod lesions were severe (3.9 on a 0 to 4 scale) in the earliest sowing time which started flowering on 22 June, with many pods lost or prematurely shattered at crop maturity (1.8). Infected and lost pods were reduced in later sowing times as flowering was delayed. The earliest formed pods were the most severely affected as they had the longest exposure to blackleg inoculum and also time for infection to progress within the plant. Fungicide applications during flowering controlled blackleg pod infection with yield increases of 1t/ha and 0.6t/ha in the 1 and 14 April sowing times, respectively. Although many pods were still infected at the 28 April sowing time (2.0), few pods were lost (0.13) and infections were generally not as severe and there was no significant yield gain achieved by fungicide applications. The untreated yield was similar across all sowing dates (2.9 t/ha) but overall yield potential declined with later sowing. This work highlights the risk of blackleg-related yield loss associated with early flowering but also the potential yield gains that can be achieved by earlier sowing. Growers considering sowing canola in early-mid April should select a cultivar with an appropriate flowering window and must consider strategic application of fungicides for sclerotinia and blackleg control to protect higher yield potentials if flowering starts in the main winter period.

Blackleg infection studies under controlled conditions at Horsham, VIC and Canberra, ACT support the findings from field experiments that disease risk for pod infection is reduced when plants flower outside the main winter infection period (Table 3). Genetic resistance effective against leaf infection by blackleg was also effective against pod infection. 

Table 2. Effect of sowing date and fungicide control on pod blackleg at maturity and grain yield of cv 44Y89 at Wagga Wagga, NSW in 2016. Treatments compare a completely bare untreated control with a “full fungicide” treatment (fungicide applied to seed, fertiliser, late seedling Prosaro and 2 applications of Prosaro during flowering ). Blackleg stem canker and sclerotinia infection were at low levels. 0 = no infection, 1 = symptoms present on <10% of pods, 2= symptoms present on 11-30%, 3=symptoms present on 31-50% with some yield loss, 4 = symptoms present on >50% with yield loss.  It should be noted that these treatments were applied for experimental purposes only as Prosaro use is limited to a maximum of 2 sprays per crop (label requirement).

Table 3. Effect of sowing date and cultivar on the percentage of pods with a blackleg lesion at Horsham, VIC. Shaded scores were those that commenced flowering before mid-August, non-shaded scores commenced flowering after mid-August. Cultivars are listed in order of maturity.

Stem and branch infection

In 2010, cankers on the upper stems and branches were observed in commercial paddocks (Figure 1). These cankers appeared to cause yield loss as the pods on affected branches senesced prematurely leading to early pod shatter.  Stem/branch cankers are not correlated with the presence of crown cankers. In 2011, 2012 and 2013 stem/branch cankers were observed each year but symptoms were not generally severe and were not present in all regions. In 2014 and 2015, the symptoms were widespread and appeared to cause significant yield loss. In contrast to previous years, severe stem/branch infection was not present at most sites in 2016 (Table 4).

As with pod infection, experiments in 2016 clearly show that flowering during the winter period where conditions for blackleg infection are optimal consistently result in increased stem and branch infection (Table 5). Similarly to pod infection, genetic resistance appears to be effective against stem/branch infection.

Table 4. Stem and branch infection on cultivar ATR Gem at blackleg monitoring sites across eastern Australia in 2016. 0 = no infection, 1 = <10% plant part affected, 2= 11-30% plant part affected, 3 = 31-50% plant part affected and some yield loss, 4 = >50% of plant part affected and yield loss.

Table 5. Branch infection (0-4 scale), shaded scores were plants that flowered before mid-August, non-shaded flowered after mid-August. 0 = no infection, 1 = <10% plant part affected, 2= 11-30% plant part affected, 3 = 31-50% plant part affected and some yield loss, 4 = >50% of plant part affected and yield loss.

Why did 2016 have less branch/stem infection than 2014 & 2015?

The 2016 season was very different to 2014 and 2015 with prolonged wet periods providing excellent conditions for infection by blackleg and a cool, wet spring providing ideal conditions for plant growth. In contrast, 2014 and 2015 both had failed springs with plants very stressed prior to harvest. It is speculated that to get stem/branch infection you may require 2 factors to occur:

1. Elongation and flowering in winter to enable infection of the stems/branches to occur.
2. Stressed plants during the spring for the lesions/cankers to develop.

Preliminary experiments undertaken in 2016 where plants were sown in pots on canola stubble, stem and branch infections were not present at seed maturity but have occurred post-harvest. The upper canopy infection has still occurred but the cool, moist spring may have resulted in the symptoms occurring later in the season, in fact post-harvest (Figure 4). Further research is required to verify this hypothesis. However, it may explain why early sown crops in the past have not had upper canopy infection symptoms, given that UCI may require both early flowering and a hot dry spring. 

Figure 4. Symptoms one month post-harvest. Left image were plants in which disease was controlled by fungicides for the duration of the growing season. Right image is plants that received no fungicide. There were limited differences between the treatments at windrowing.

Conclusions and recommendations

The blackleg risk for 2017 is dependent on seasonal conditions leading into sowing as these dictate the timing of spore release. However, two key factors increase the likelihood of increased blackleg severity. Firstly, despite the lower than expected severity of crown, stem and branch cankers in 2016, the high level of blackleg infected crops will create a large inoculum load across canola growing regions in 2017. Poor cereal prices will keep rotations tight and therefore adequate separation of new crops from previous stubble is problematic.

Research undertaken in 2016 shows that disease risk is related to crop growth stage during the winter period when spores are released and conditions are most conducive for infection. Risk of severe seedling infection is reduced when sowing in April and upper canopy infection is significantly reduced when the flowering window is predominantly outside the winter period. Adequate control options are available to protect crops through the early seedling stage but further research is required to establish the correct timing of application for control of upper canopy infection. Fungicide tolerance in blackleg is still an issue, however recent studies indicate that blackleg isolates with tolerance to fluquinconazole do not as yet have tolerance to flutriafol or to Prosaro® (tebuconazole and prothioconazole).

In 2017, isolates able to attack Group A cultivars will still be over represented in the blackleg population, however growers are now well warned so they can change to a Group A cultivar with a higher blackleg rating, protect Group A cultivars with fungicide or change resistance groups.  Growers are advised to consult the Blackleg Management Guide on the GRDC website. Retaining seed from 2016 crops with high levels of pods infected with blackleg or alternaria is not recommended. If retaining 2016 seed, grade it for larger seed which is less likely to be infected with blackleg and ensure even and adequate treatment with an appropriate fungicide to control seedling blight.

Useful resources

GRDC Blackleg Management Guide Factsheet

NVT Variety Trials online

Acknowledgements

The authors gratefully acknowledge the GRDC and their organisations for project funding. We also acknowledge the many grower collaborators who generously hosted field experiments.

Contact details

Susie Sprague
GPO Box 1700, Canberra, ACT 2601
Ph: 02 6246 5387
Mb: 0466 643 227
Email: Susan.Sprague@csiro.au

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