Faba bean (Vicia faba L.) agronomy update — chocolate spot (Botrytis fabae) management in faba beans at Kojonup in 2021

Key messages

• Foliar fungicide application on Western Australian faba bean crops can be safely delayed until flowering begins, even if chocolate spot is present at low levels earlier in the season.
• An alert system using in-crop monitors is being developed that might provide increased confidence in making fungicide application decisions – by supporting the decision not to spray.
• In high rainfall/risk areas the ‘alert’ system could need modification to account for extended periods of very high humidity and ‘discounting’ temperature.
• PBA Bendoc is rated ‘S’ for chocolate spot and might not be the best variety in high rainfall/high humidity regions. In these circumstances PBA Amberley would be a better choice.

Aims

Demonstrate the effectiveness of fungicide timing options on the management of chocolate spot and evaluate the effectiveness of a new decision support tool for spray management of the disease.

Introduction

The causal agent of chocolate spot, Botrytis fabae, needs temperatures between 15–25°C and humidity >75% for spores to be released and infect plants successfully. Using this information, the South Australian Research and Development Institute (SARDI) is developing a warning system for the disease based on canopy temperature and humidity. When temperature and humidity conditions are met for a period of 12 consecutive hours, a ‘red alert’ is sent to highlight that a 48-hour window now exists to apply a foliar fungicide on the crop to prevent infection. If this warning system is found to be reliable, it could enable farmers to make informed spray decisions, including the confidence to leave a crop unsprayed.

The current recommended practice in Western Australia is for farmers to monitor crops for signs of chocolate spot with a view to begin spraying fungicide as the canopy closes and flowering begins through to early pod fill. In the eastern states, it is common practice for farmers to spray faba bean crops with at least four foliar fungicides, from mid-winter onwards. The early sprays are often applied to control diseases other than chocolate spot, such as ascochyta. Although ascochyta is less common in WA than other states, some farmers here have duplicated this practice, at times resulting in unnecessary applications of ‘elite’ fungicides early in the growing season.

We aim to compare the current recommended WA fungicide spray strategy with the more regular fungicide applications common in other states and with a spray timing based on canopy humidity and temperature to determine if this could be a useful decision support tool for WA growers.

Method

In 2021 four fungicide strategies were compared (Table 4) at three WA sites – Kojonup/Muradup, Frankland River and Wittenoom Hills, using a randomised experimental block design with six replicates. Muradup was the site with the highest disease load, so this paper presents data only from this site. Information from the other sites will be available at https://www.farmtrials.com.au/. Fungicide products were chosen with the aim of rotating chemicals and adhering to label restrictions. Inevitably different strategies end up with different products – for example our ‘Alert’ treatment ended up having a ‘cheaper’ fungicide applied than a grower might select if they were doing a one-off spray – therefore we have decided not to present gross margins from this experiment.

Results

In this trial chocolate spot was rated repeatedly throughout the 2021 season using the scale in Table 6, with the final two ratings presented in Figure 2. The first signs of disease were seen in in June. Disease levels were very low at that time, with small specks seen on some leaves. Chocolate spot remained at very low levels until late-August when an increase in disease incidence and severity was noted to have occurred over a two-week period. By that time, the ‘regular spray’ treatment had received three foliar fungicides, the ‘recommended procedure’ (flowering onwards) had received one foliar fungicide and the other treatments had not been sprayed. The disease levels at this time are presented in Figure 2 and show that the ‘regular spray’ plots had significantly less disease than all other treatments at that time, although overall disease levels were still low.

Red alerts were received on the 10 and 11 September, indicating that the threshold temperature and humidity levels were exceeded for a 12-hour period. Following the alerts, the treatment was sprayed with a fungicide on 14 September. Four weeks later, the final disease rating of the season was performed on 18 October (Figure 2, see Table 3 for full details). By this time, disease levels had increased markedly across all treatments. The ‘regular spray’ plots had significantly less disease than the ‘alert’ treatment, while the ‘recommended procedure’ (flowering onwards) plots were in between for disease rating. The ‘regular spray’ plots showed discrete lesions on leaves while the nil plots had larger lesions and defoliation of some plants.

Yields across the trial averaged 1.6t/ha, which was quite good considering disease severity was fairly significant by the end of the season. There was a clear yield advantage when using any of the foliar fungicide strategies compared to nil fungicide. The ‘alert’ treatments yielded almost double the nil fungicide treatment. This shows the impact that one, strategically timed, fungicide application can have on a faba bean crop.

Conclusion

From early-July until the end of August long periods of relative humidity higher than 100% were measured within the crop canopy (Figure 1). During periods of such high humidity the Botrytis fungus can release spores at low temperatures, although this will occur more slowly than it would at the ideal temperature of 15-25°C. Thus, weather conditions in 2021 were ideal for the development of chocolate spot even without reaching the 15-25° temperature range to trigger the alert system. This helps explain why more alerts from the system being tested were not received and, hence, why disease in the alert plots was not as well controlled and yield not as high as the plots that received regular foliar fungicides in 2021. The alert system is still being developed and appropriate parameters refined. Based on our results in WA in 2021 and from other states where very high humidity over long periods of time has led to high disease levels at lower temperatures than expected, modifications to the alert system will be made by SARDI. We are confident that these modifications will enable alerts to successfully guide spray decisions even in seasons of unusually high rainfall and humidity such as 2021.

It is also reassuring to note that, although the regular spray treatment had lower disease severity than the WA recommended practice treatment did early in the season, by the end of the season the disease severity and seed yield between these two treatments were the same. This reaffirms our recommended practice for WA faba bean crops to begin receiving foliar fungicide applications from flowering onwards and that fungicides applied earlier than this may be unnecessary in our environment.

Interestingly the NVT faba bean experiment was in the same paddock, and it received two early fungicide sprays in June and a well-timed Aviator Xpro spray in late August. In the NVT experiment PBA Bendoc produced a yield of 1.9t/ha while PBA Amberley with a MSMR resistance rating to chocolate spot produced 3.1t/ha, indicating in this high-risk area, PBA Amberley would be a wise choice.

Acknowledgments

DPIRD staff Chris Matthews, Pam Burgess, Salzar Rahman, Michelle Sampson and TrialsCo helped manage the DPIRD field experiment. Living Farm managed the Kojonup Faba NVT trial.

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 authors would like to thank them for their continued support.

Contact details

Mark Seymour
DPIRD
Melijinup Road, Esperance
Ph: 90831 143
Email: mark.seymour@dpird.wa.gov.au