Blackleg in canola - outcomes from 2016 and update for 2017

Take home messages

• In 2016 canola emergence in central-west of NSW coincided with blackleg spore release and wet seasonal conditions resulted in extreme levels of blackleg leaf lesions in some areas, particularly in cultivars with Group A resistance. However, crown canker severity was much lower than expected given the severe leaf infection.
• Upper canopy blackleg infection was severe in early-flowering crops with significant yield loss. Risk of yield loss associated with upper canopy infection can be reduced by selecting the correct variety for the sowing date.
• In 2017 dry conditions post-sowing delayed blackleg infection. Lesions observed in unprotected crops in early-mid June resulting from infection following widespread rain in the third week of May could progress to form crown cankers. Cold weather has slowed crop development, so flowering is not likely to not occur as early as in 2016. Although dry conditions are forecasted to persist, rainfall events in late winter and early spring could expose crops to blackleg spores during flowering and may promote upper canopy infection.
• White leaf spot and downy mildew have been widespread on canola seedlings in 2017. Both diseases are sporadic and do not generally affect grain yield.

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 further within this paper.

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 while it infects the crop (Figure 1). Once the canola plant has begun to flower, infection of flowers, peduncles, pods, main stem and branches of the plant has collectively been termed ‘upper canopy infection’. 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.

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.

Blackleg in 2016

In recent years, dry summers in many regions resulted in delayed release of blackleg spores which, combined with earlier breaks to the season (with earlier sowing), resulted in low levels of blackleg crown canker. Earlier sown crops develop 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, commonly utilised by growers 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 has resulted in a large area of canola stubble that can release blackleg spores.

Leaf lesions

In 2016, the recommended 500m buffer distance between this year’s crop and last year’s canola stubble was difficult to achieve due to intensification of canola production in many regions. The blackleg rating of many Group A open-pollinated triazine tolerant cultivars was reduced from moderately resistant (MR) to moderately resistant – moderately susceptible (MR-MS), indicating their increased susceptibility to disease due to changes in blackleg populations. 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. Under these high disease situations, application of foliar fungicides provided excellent return of investment to growers with many reporting significant yield increases.

Blackleg crown canker

Despite extreme levels of leaf lesion infection seen across many regions early in the season, severe blackleg 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 blackleg monitoring sites (22 of 31) 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. 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 (greater than 10mm) reduced grain size by 11 to 14% and seeds/pod by up to 20%. 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.

Field experiments in 2016 showed the level of pod infection can be reduced by managing the crop to delay flowering outside the main blackleg infection period in winter. In 2016 a field experiment with cv 44Y89 (CL) (fast-mid phenology, blackleg group BC, rated MR) was conducted at Wagga Wagga with untreated and ‘full fungicide’ (fungicide applied to seed, fertiliser, early stem elongation Prosaro^® and two applications of Prosaro^® during flowering) treatments sown on 31 March , 13 and 29 April (Table 2). 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 (low severity) to 4 (high severity) scale) in the earliest sowing time which started flowering on 22 June, with many pods lost or prematurely shattered at crop maturity (1.78, Table 2). 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 31 March and 13 April sowing times, respectively. Although many pods were still infected at the 29 April sowing time (2.0, Table 2), few pods were lost (0.13, Table 2) and infections were generally not as severe and there was no significant yield gain from fungicide applications. The untreated yield was similar across all sowing dates (2.9 t/ha, Table 2) 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 slow developing cultivar so that flowering does not start in early winter 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, Victoria 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.

Figure 2. Effect of fungicide treatment and blackleg resistance on the severity of blackleg leaf lesions in an experiment sown on 29 April at Canowindra, NSW. Leaf lesions were assessed on 19 June, approximately seven weeks after sowing.

Other canola leaf diseases present in 2017

White leaf spot (Mycosphaerella capsellae)

White leaf spot is a sporadic fungal disease that has been widespread across southern NSW in 2017. Like blackleg, it is a stubble-borne pathogen that causes leaf lesions under cool conditions. Leaf lesions are greyish white with a defined margin which can become brown as the lesions mature. Lesions commonly coalesce to form large lesions. White leaf spot lesions can be mistaken for blackleg but lack the black pepper-like spots (fruiting bodies) that are present with blackleg infection. Infection is generally contained to the lower canopy and spread from spores within the canopy is reduced once conditions warm up in spring. Infection reduces green leaf area, however, as white leaf spot is a sporadic disease, the level of associated yield loss is unknown. Yield loss may result in situations with significant reductions in leaf area as plants can have reduced vigour and biomass accumulation. There are currently no registered fungicides for white leaf spot control. In areas with persistent outbreaks, white leaf spot can be managed by reducing canola volunteers and weedy cruciferous hosts, by isolating crops from the stubble of previous crops and by maintaining plant health.

Downy mildew (Peronospora parasitica)

Downy mildew is another fungal disease that has been widespread in 2017. The disease persists in the soil for long periods and can also be seed-borne. Disease occurs in cool conditions where leaves are in contact with the soil surface or other leaves. Symptoms first appear as chlorotic patches that can coalesce to form irregularly shaped lesions. Grey fungal growth on the underside of blotches is a defining characteristic of downy mildew. Although infection is generally not associated with yield loss, crop establishment can be reduced by the death of young seedlings due to defoliation. Crop rotation and control of cruciferous weeds can reduce soil inoculum.

Useful resources

NVT website

Blackleg Management Guide

Managing blackleg

Canola Disease Back Pocket Guide

Flowering time and blackleg

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC — the author would like to thank them for their continued support.

Contact details

Susie Sprague
CSIRO Agriculture and Food, PO Box 1700, Canberra, ACT 2601
(02) 6246 5387, 0466 643 227
Susan.Sprague@csiro.au