Monitor blackleg severity to ensure return on fungicide application

During 2015 and 2016 the blackleg fungus overcame the blackleg resistance of some Group A cultivars. These cultivars dropped from Moderately Resistant (MR) to Moderately Susceptible (MS) and all updated ratings are available in the 2017 Autumn Blackleg Management guide. In the lead up to the 2017 season, fungicide application was recommended to protect MS cultivars sown in medium to higher rainfall regions – where blackleg risk was higher. However, seasonal conditions in 2017 have not been conducive for the blackleg fungus in many regions.

Blackleg likes continual wet conditions for spore release and germination, which is why blackleg severity on seedlings was so high in 2016 (Figures 1 and 2). In contrast, in 2017 a large area of southern Australia has received rainfall in only a couple of major events and drier conditions in between these events. Consequently blackleg lesions are only starting to occur now (mid June).

If your crop is already past the vulnerable seedling stage (1-4 leaf) and has no or few lesions it will most likely not develop severe crown canker and may not benefit from a foliar fungicide application (Figure 3). If your crop was sown later, has a MS or lower blackleg rating and still in the vulnerable seedling stage in mid June it may develop severe crown canker and therefore benefit from a foliar fungicide application. Monitor your crop for blackleg lesions on the 1st four leaves, estimate the potential crop yield and decide if it is economical to protect the crop. Foliar fungicide has highest efficacy against blackleg crown canker if applied at the 4-6 leaf stage, but is still very effective up to the 8-9 leaf stage. If you are unsure about the blackleg severity on your crop and the potential yield you can wait until the 8-9 leaf growth stage and then make your decision.

Case studies

  • ATR Bonito (Group A, MS rating) – 3 leaf growth stage, numerous blackleg lesions on the 1st and 2nd leaves with good yield potential. Growers should apply foliar fungicide to minimise yield loss.
  • ATR Mako (Group A, MR rating) – 3 leaf growth stage, blackleg lesions on 1st and 2nd leaves with good yield potential. Growers should monitor crops to determine whether it is economical to protect the crop with a fungicide application and can wait until the 8-9 leaf growth stage to make a decision whether to treat or not.
  • ATR Bonito (Group A, MS rating) – 6 leaf growth stage, few blackleg lesions on young leaves with good yield potential. In this situation growers are unlikely to get economic return from a foliar fungicide application.

Examples of severity of blackleg infection and suggested management

Click images below to view full size.

Figure 1. Very severe blackleg leaf lesions in 2016 - Growers are likely to get return from applying foliar fungicide even on MR cultivars.

Figure 1. Very severe blackleg leaf lesions in 2016 – Growers are likely to get return from applying foliar fungicide even on MR cultivars.

Figure 2. High blackleg leaf lesions in 2016 – Growers are likely to get return from applying foliar fungicide even on MR cultivars. Recommend to apply foliar fungicide if on MS cultivars.

Figure 2. High blackleg leaf lesions in 2016 – Growers are likely to get return from applying foliar fungicide even on MR cultivars. Recommend to apply foliar fungicide if on MS cultivars.

Figure 3. Normal blackleg severity – Growers are unlikely to get return from applying fungicide on MR cultivars and may get return on MS cultivar. Growers should make decision at 8-9 leaf stage whether to apply foliar fungicide.

Figure 3. Normal blackleg severity – Growers are unlikely to get return from applying fungicide on MR cultivars and may get return on MS cultivar. Growers should make decision at 8-9 leaf stage whether to apply foliar fungicide.

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