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Fungicide timing essential for chocolate spot

SARDI’s Sara Blake talks pulse disease management at the Hart Field Day in South Australia in 2021.
Photo: SARDI

Fungicide timing essential for chocolate spot

SARDI’s Sara Blake talks pulse disease management at the Hart Field Day in South Australia in 2021.
Photo: SARDI

Key points

  • Surveys have shown that chocolate spot is reducing yield in faba bean crops in the south-east of South Australia and in Victoria
  • The highest profitability will be achieved by combining varietal resistance with optimal fungicide timings

Targeted disease management is vital to improve faba bean gross margins.

Recent broadacre crop disease surveys have found that chocolate spot is all too common in faba bean crops in South Australia and Victoria.

The surveys were conducted by the South Australian Research and Development Institute (SARDI) and Agriculture Victoria as part of GRDC’s national surveillance program.

The incidence of other pulse foliar diseases was generally low in the 2020 and 2021 surveys, demonstrating that growers are effectively managing most diseases. Disease was more prevalent in high-rainfall regions, in paddocks with high plant densities, and where the frequency of pulse crops was high.

Survey findings

Chocolate spot is of particular concern because it is often found at severities that can limit yield.

In SA, chocolate spot was observed in nine of the 14 faba bean paddocks surveyed in the south-east. In infected paddocks in 2020, an average of 81 per cent of plants were diseased. In 2021 the figure was 22 per cent.

In Victoria, chocolate spot was observed in 38 of the 40 faba bean paddocks surveyed. In infected paddocks in 2020, an average of 63 per cent of plants were diseased. In 2021, this figure was an average of 50 per cent of plants in the Western District and 28 per cent in the Wimmera.

The levels of disease detected in the south-east of SA and the Victorian Western District were high enough to cause yield loss in many of the paddocks.

Fungicide timing

The best integrated disease management strategies rely on selecting varieties with improved disease resistance, avoiding close bean rotations and use of foliar fungicides.

When fungicides are required, timing is critical. An application at canopy closure, before symptoms develop, is the most effective. Agriculture Victoria and SARDI have evaluated the effectiveness and profitability of fungicide management strategies for chocolate spot management.

However, varietal resistance delivers the clearest benefit. In a trial at Lake Linlithgow in the Victorian high-rainfall zone in 2020, varietal resistance produced the strongest yield protection under high disease pressure.

In the trial, PBA Amberley, which has the best-available chocolate spot resistance, outyielded the more-susceptible PBA Bendoc in every treatment regardless of fungicide control. No fungicide treatment was able to limit the severity of chocolate spot in the susceptible variety, PBA Bendoc at this site. The yield benefit from this varietal resistance was estimated at 2.8 tonnes per hectare, valued at $1120/ha.

Regardless of the fungicide used, all treatments increased gross margins for all varieties, under high disease pressure, despite the initial outlay.

The highest profitability will be achieved by combining varietal resistance with optimal fungicide timings. Applications at canopy closure, before symptoms develop, are most effective.

In PBA Amberley, gross margins increased by between $274/ha and $1051/ha depending on the fungicide treatment. In contrast, PBA Bendoc gross margins increased by between $300/ha and $785/ha.

In the medium-rainfall zone, similar experiments highlighted the importance of monitoring for disease and tailoring the number of applications to the environmental conditions. At Gymbowen, Victoria, only one fungicide application was required in both PBA Amberley and PBA Bendoc during 2020, to reduce chocolate spot severity and improve gross margins by between $182/ha and $367/ha.

In high-rainfall zones, multiple fungicide applications may be required in most seasons, product choice(s) should be made as part of a spray program that includes differing modes of action for resistance management and that does not exceed the maximum number of sprays as stated on the product label. Industry best practice is to not exceed a maximum of two sprays from the same mode of action in any one season.

More information: Sara Blake, 08 8429 2248, sara.blake@sa.gov.au; Dr Joshua Fanning, 03 4344 3111, joshua.fanning@agriculture.vic.gov.au

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