The economics of managing Ascochyta in chickpea when disease occurs at different growth stages and implications for spray timing

Take home messages

• Impact of Ascochyta at different growth stages was investigated in a 2020 field experiment at Trangie with varieties Kyabra, PBA HatTrick and PBA Seamer
• Ascochyta caused yield losses from 100% to nil depending on when the disease occurred, and the variety grown
• Highest gross margin (GM)’s (> $800/ha) occurred with the lowest incidence of Ascochyta and also with the least susceptible variety, i.e. PBA Seamer
• Manage Ascochyta early and grow varieties with best resistance to minimise impact of Ascochyta
• Follow current Ascochyta advice to maximise enduring profitability.

Why did we do this research?

Current management of chickpea Ascochyta in north central/northern NSW and southern QLD is based on results of field trials conducted mostly at Tamworth, grower experiences and feedback from agronomists. The Tamworth experiments have tried to simulate what happens in most farmers crops, i.e. initial infection occurs during the first post emergence rainfall event – accordingly, all the Tamworth experiments have established Ascochyta during that event. But what is the impact if infection occurs at later stages of growth and how does that affect management? The Trangie Agricultural Research Centre provided an opportunity to address that question – the soil and climate are typical of the Macquarie valley, a major chickpea production region. Overhead irrigation (lateral) and inoculation with conidia and infected spreader plants optimised infection and disease development.

Experiment details

Treatments

Aim: to assess the impact of Ascochyta blight (AB) disease occurring at three different growth stages, on yield of three chickpea varieties with different levels of AB resistance.

Ascochyta treatments (5):

1. LOW (NIL): un-inoculated (NIL disease = CONTROL) plus foliar chlorothalonil fungicide (1.0 L/ha, chlorothalonil 720g/L) applied before rain or irrigation events
2. HIGH: inoculated with disease twice (at seedling (SDG, 3-4 nodes) and vegetative (VEG, 7-8 nodes) growth stages); NIL fungicide applied
3. SDG: inoculate with disease at seedling stage (3-4 nodes), allow disease to progress for 2-3 rain events to 7-8 nodes, then control disease for rest of season with chlorothalonil
4. VEG: protect plants from emergence to vegetative stage (7-8 nodes) with chlorothalonil; inoculate with disease and allow to progress for 2-3 rain events to first pods, then control disease with chlorothalonil
5. POD: protect plants from emergence to reproductive stage (first pods) with chlorothalonil, inoculate with disease and allow disease to progress through to harvest.

Variety treatments (3, and level of AB resistance):

1. Kyabra VS = Very susceptible
2. PBA HatTrick MS = Moderately susceptible
3. PBA Seamer MR = Moderately resistant

Replication:       4 reps

Method

The experiment was conducted at Trangie Agricultural Research Centre in central west NSW, on a grey vertosol soil with access to overhead (lateral) irrigation. The experiment was sown as a randomised block design using a small plot seeder, with each plot 2m x 10m. Buffer plots were sown (with PBA Seamer) at the same plot size between each treatment plot, to reduce the impact of inter-plot interference from Ascochyta inoculation and fungicide application. A back-pack sprayer with a 2m wide hand-held wand and 015 110 degree flat fan nozzles @ 50cm, was used to apply both Ascochyta inoculum and fungicide. Buffer plots received the full set of six fungicide applications.

Ascochyta disease was generated in treatment plots by a combination of two inoculation methods:

1. Ascochyta applied to whole plot as conidial suspension (600,000 conidia/mL), and
2. Ascochyta infected spreader plants transplanted to centre of plot.

Ascochyta treatments for each growth stage were applied just prior to either a forecast rain event, or irrigation.

Ascochyta treatment and fungicide were applied as follows:

up to 1 July        NIL fungicides applied prior to first disease treatment

1 July                   inoculation 1 - SDG & HGH treatments (pre-irrigation)

9 July                   fungicide 1 pre-rain - applied to LOW, VEG & POD (not SDG or HGH)

5 August             inoculation 2 - VEG & HGH treatments (pre-rain)

13 August           fungicide 2 pre-rain – applied to LOW, SDG & POD (not VEG or HGH)

9 September      fungicide 3 pre-rain – applied to LOW, SDG & POD (not VEG or HGH)

19 September   inoculation 3 - POD treatment (pre-rain)

29 September   fungicide 4 pre-rain – applied to LOW, SDG & VEG plots (not POD or HGH)

7 October           fungicide 5 pre-rain – applied to LOW, SDG & VEG plots (not POD or HGH)

21 October         fungicide 6 pre-rain – applied to LOW, SDG & VEG plots (not POD or HGH)

Site details & agronomy management

Sowing date: 26 May 2020
Harvest date: 27 November 2020

Seed treatment:
PBA Seamer pre-treated with thiram at purchase
Kyabra & PBA HatTrick treated with P-Pickel T® pre-sowing

Fertiliser at sowing: Granulock® Z @ 80 kg/ha

Inoculant: Group N, liquid inject at sowing

Target plant density: 30 plants/m²

Actual establishment achieved:

Kyabra & PBA Seamer = 31 plants/m² (new seed)
PBA HatTrick = 21 plants/m² (retained seed ex 2019 harvest, poor storage conditions)

Buffers between treatment plots: PBA Seamer at 30 plants/m²

Herbicide management:

pre-sow:             TriflurX® (trifluralin 480 g/L) @ 1.7 L/ha
PSPE:                   Terbyne® Xtreme® (terbuthylazine 875 g/kg) @ 0.86 kg/ha
in-crop:               haloxyfop 520 @ 100 mL/ha + clethodim 240 @ 250 mL/ha

Fungicide application:
up to 6 chlorothalonil 720g/L @ 1.0 L/ha for LOW (NIL disease) treatment; total number of applications varied for each growth stage treatment

Insecticide & mouse management:
17 Sept               Affirm® (emamectin) insecticide @ 300 mL/ha (by plane)
6 Oct                   Altacor® (chlorantraniliprole) insecticide @ 70 g/ha (by plane)
12 Oct                 Zinc phosphide mouse bait @ 1.0 kg/ha (by plane)

Harvest management:
desiccation not required due to heatwave conditions from 15 November on
experiment harvested at 9% moisture content.

Pre-sowing rainfall (1/01/20 to 25/05/20): 351 mm
In-crop rainfall (26/05/20 to 27/11/20): 201 mm in 49 events
Plus in-crop irrigations: 20 mm (as 2 x 10 mm events)

Table 1. Summary of dates of disease inoculations and fungicide applications for 2020 Trangie chickpea Ascochyta management trial, relative to dates of subsequent rain or irrigation events.

Table 2. Log of operations for 2020 Trangie experiment on managing chickpea Ascochyta when disease occurs at different growth stages: Inoc = inoculation, DAS = days after sowing, DAI = days after inoculation.

Results

The effects of Ascochyta treatment, Variety and Ascochyta x Variety were highly significant (P<0.001) for all variables measured.

Disease incidence

The seedling, (SDG) and HIGH Ascochyta treatments were inoculated on 1 Jul 20, 36 days after sowing, DAS (Table 2). The HIGH treatment was inoculated a second time on 5 Aug (71 DAS) to maximise disease development (Table 2). This resulted in severe early disease (assessed on 26 Aug) in Kyabra, moderate disease in PBA HatTrick and demonstrated the improved Ascochyta resistance in PBA Seamer (Table 3). The vegetative, VEG treatment was inoculated on 5 Aug (71 DAS), prior to which plants had been protected with foliar fungicide applied on 9 Jul. This regime produced low disease in Kyabra and PBA HatTrick at the 1^st and 2^nd assessments (26 Aug, 1 Oct) and nil in PBA Seamer (Table 3). The podding, POD treatment was inoculated on 19 Sep (116 DAS), prior to which plants had been protected with foliar fungicides applied on 9 Jul, 13 Aug and 9 Sep. This regime produced nil disease in all three varieties at the 1^st assessment (26 Aug) and low disease at the two subsequent assessments (1 Oct, 14 Oct) (Table 3). We assign the low incidence of Ascochyta with the POD treatment as a consequence of the three fungicide sprays applied before inoculation and less favourable seasonal conditions.

Table 3. Incidence of Ascochyta (% plot with disease) in Kyabra, PBA HatTrick and PBA Seamer chickpeas when disease was established at different growth stages but controlled before and after; l.s.d. (P=0.05) for Scores 1, 2 & 3 = 13.26%, 8.63% & 11.74%, respectively.

This approach, i.e. inoculating Ascochyta at different times and protecting before and after, allowed us to determine the impact of disease at those stages on yield.

Impact on yield

Table 4. Effect of Ascochyta on grain yield, gross margin (GM) and yield loss for three chickpea varieties when disease occurs at different growth stages; l.s.d. (P=0.05) yield 286 kg/ha. GM is based on chickpea price of $600/t, fungicide product $16/ha/application, fungicide ground rig application $5/ha, other production costs $300/ha.

Grain yield (Table 4) ranged from nil (Kyabra HIGH) to over 2 t/ha (all PBA Seamer treatments). For the very susceptible Kyabra, lowest yields occurred with the SDG and HIGH treatments with yield losses of 99% and 100% respectively. The moderately susceptible PBA HatTric k also had lowest yields for SDG and HIGH treatments, with losses of 48% and 87% respectively (Table 4). The least susceptible variety PBA Seamer only lost 3% and 11% yield from SDG and HIGH treatments.

Gross margins (GM)

The highest GM’s occurred with the lowest incidence of Ascochyta (LOW) and also with the least susceptible variety PBA Seamer (Table 4). All PBA Seamer treatments, including the one with most Ascochyta, (HIGH) had GM over $800/ha (Table 4). However, the experiment showed that controlling Ascochyta in the very susceptible Kyabra is profitable with a GM of $701/ha and $862/ha for the LOW and POD treatments respectively.

Conclusions

Generating chickpea Ascochyta at different stages of growth showed when and how disease affects yield. The impact of disease on a chickpea crop depends primarily on when the disease occurs, how it is managed, and the variety grown. Allowing Ascochyta to establish early in the life of your crop results in greatest impact on yield and lowest profitability, even if the disease is subsequently controlled with foliar fungicides. This is especially true for very susceptible and moderately susceptible varieties. Your best option for minimising impact of Ascochyta on chickpea production and maximising profitability is to follow current recommendations (Moore and Heuston, 2020) by controlling disease early and growing the least susceptible variety. This approach will also reduce the build-up and carryover of Ascochyta inoculum for your and your neighbour’s future chickpea crops.

References

Kevin Moore and Penny Heuston (2020) Managing Ascochyta blight in chickpeas in 2020. NSW DPI Fact Sheet, May 2020

Acknowledgements

This research is a component of the “Pulse Integrated Disease Management – Northern NSW/QLD” project (BLG209), as part of the Grains Agronomy & Pathology Partnership (GAPP) between GRDC and NSW DPI. The research undertaken in this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC. We would like to thank GRDC, growers and their agronomists for their continued support of this research.

Leigh Jenkins and Kevin Moore would also like to acknowledge and thank NSW DPI Technical support staff for their assistance with laboratory preparation and field work, including Scott Richards, Liz Jenkins and Joanna Wallace at Trangie ARC; and Paul Nash and Gail Chiplin at Tamworth AI.

Contact details

Leigh Jenkins & Kevin Moore
NSW DPI, Trangie Agricultural Research Centre & Tamworth Agricultural Institute
Ph: LJ 0419 277 480, KM 0488 251 866
Email: leigh.jenkins@dpi.nsw.gov.au, kevin.moore@dpi.nsw.gov.au

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