Chickpea Ascochyta research: what if I miss a spray – are there salvage options with new chemistry; how long do fungicides persist?

Take home messages

• Follow latest advice for managing Ascochyta - applying fungicides before rain is a key component of this advice
• Chlorothalonil and mancozeb fungicides are persistent and rain fast (up to 50mm rain in 10 minutes)
• If you miss an Ascochyta fungicide spray, research indicates salvage sprays with new chemistry may be an option within tight timeframes, but this requires field confirmation.
• Do not rely on salvage fungicide sprays as part of your 2019 Ascochyta management plan – aim to spray crops prior to rainfall events.

Background

Traditional chickpea Ascochyta blight fungicides e.g. chlorothalonil and mancozeb need to be applied before rain because they are protectants only. However, new chemistry and formulations offer the possibility of limited salvage fungicide options for Ascochyta if applied soon after infection (rainfall) events.  Efficacy of these fungicides is far more reliable when applied prior to an infection event.  Prophylactic use of efficacious fungicides applied prior to infection events, is and should remain the bedrock of Ascochyta management plans.

Likely scenarios when a preventative i.e. pre-rainfall, fungicide application is missed are:

• rain occurs when it was not predicted
• un-availability of spray contractors
• machinery breakdowns before or during application
• insufficient time to spray entire chickpea crop prior to rain.

This paper summarises recent research that shows chickpea Ascochyta blight may be able to be controlled if a pre-rainfall fungicide application is missed.

2017 Tamworth chickpea Ascochyta salvage spray field trial (FUN17)

The aims of this trial were to (i) evaluate efficacy of applying fungicides to a crop in which Ascochyta had established and (ii) to determine if newer fungicides had any ‘kickback’ activity when applied after a rain event.

Treatments

1. Nil (tap water)
2. Aviator Xpro® @ 400 mL/ha in 100 L/ha water backpack (registered but restrictions on number of applications per season and stage of crop development); no claim for kickback activity. Actives: 75g/L bixafen + 150g/L prothioconazole
3. Unite 720® @ 1000 mL/ha in 100 L/ha water backpack (registered); no claim for kickback activity.  Active: 720g/L chlorothalonil
4. Veritas® @ 1000 mL/ha in 100 L/ha water backpack; no claim for kickback activity. Active 200g/L tebuconazole + 120 g/L azoxystrobin (registered, but restrictions on number of applications per season)

Operations

Kyabra sown 30 May 2017, seed treated P-Pickel T, plots 4 m x 11 m; 4 reps as a randomised complete block (RCB)

Inoculated 14 Jul in rain (15.2mm)

Post-inoculation rain and spray applications

14 Jul               15.2 mm
20 Jul               2.0 mm
4 Aug               20.0 mm
3 Sep               3.0 mm
5 Sep               1^st sprays 41hr post rain
14 Sep             8.0 mm
18 Sep             1^st Ascochyta scores

Methodology

Ascochyta was deliberately allowed to establish in the trial to provide high disease pressure under which to test the aims of the trial.  Ascochyta was established by spraying the trial during a rain event with a suspension of Ascochyta inoculum containing 483,333 conidia/mL at a water rate of 100L/ha.  This resulted in uniform infection ie every plant in the trial developed Ascochyta. High disease pressure was favoured further by waiting for three more infection cycles (rain events on 20 Jul, 4 Aug and 3 Sep) before applying the first fungicide sprays on 5 Sep. Ascochyta was scored on 18 Sep by assessing each plot on a scale of 1-5 where 1 = least disease and 5 = most disease.

Key findings

Aviator Xpro and Veritas reduced Ascochyta compared with Unite 720 and the nil control (Table 1). There was no difference in control of Ascochyta between Aviator Xpro and Veritas. Unite had no post-infection efficacy on Ascochyta.  The lower Ascochyta score compared with the control in Table 1 reflects the prophylactic activity from the application on the 5^th of September for the 14th of September infection event.

The best management recommendation for Ascochyta control remains that fungicide should be applied prior to forecast rain to provide the greatest level of protection. Post-infection sprays should not be a planned part of your standard Ascochyta management plan.

Table 1. Ascochyta (AB) severity score (1-5) on Kyabra chickpeas sprayed with Aviator Xpro, Veritas, Unite or Water (Nil) 41 hours after rain started in 2017 Tamworth field trial (FUN17)  F pr AB<0.001; l.s.d AB Score = 0.884; AB score 1 = least disease, 5 = most disease

2018 Tamworth chickpea Ascochyta salvage glasshouse experiments

Two glasshouse experiments were conducted in 2018 to provide additional evidence to support the 2017 field trial, FUN17.

Experiment 1 (FUN18GH)

In the first replicated experiment, chickpea plants (cv Kyabra) with 4-5 nodes were inoculated with Ascochyta twice to optimise infection. Plants were allowed to dry for 30- 60 minutes between 1st & 2nd inoculations.  The rate was 8.3 x 10⁵ conidia/mL applied to run-off and incubated in a rainfall simulator.

Twenty-four (24 hr) or 48 hours after inoculation, treatments of Unite 720 @ 1000 mL/ha, Aviator Xpro @ 400mL/ha, Veritas @ 1.0L/ha were applied with a water-only treatment as the nil control.  The plants were placed under glasshouse conditions conducive to Ascochyta development (23C, 80% RH).

Only Aviator Xpro and Veritas stopped Ascochyta development and they did so at both application times. Unite 720 (chlorothalonil) had no post inoculation activity - there was just as much Ascochyta with the Unite 720 treatments as with the Nil water control. All reps of the Unite 720 and Nil treatments had maximum Ascochyta disease score of 5 (on 1-5 scale); the other treatments all scored 1 (no disease).

Trial results indicate Aviator Xpro and Veritas provided control of Ascochyta blight infections when applied 24 and 48 hours post- infection.

Experiment 2 (FUN18GH)

In the second experiment, Aviator Xpro @ 400 mL/ha and Veritas @1000 mL/ha were applied to Kyabra chickpeas with 14 nodes at four times: 24, 48, 72 and 96 hours after inoculation with a water-only treatment as the Nil control.

Inoculation, incubation and experimental conditions were as for experiment 1.  The number of petioles, leaves and stems with at least one Ascochyta lesion were counted 14 days after inoculation.  Data was analysed using the glmer function from the R package lme4; means were compared by the Tukey method.

Ascochyta developed on all tissues at all times in the Nil water control (Tables 2-4).  No Ascochyta developed on any tissue when Aviator Xpro or Veritas were applied 24 or 48 hrs after inoculation and very little or none developed when Aviator Xpro or Veritas were applied 72 hrs after inoculation (Tables 2-4).  However, applying Aviator Xpro or Veritas 96 hrs after inoculation did not stop Ascochyta with no significant difference in numbers of petioles, leaves or stems with Ascochyta between these fungicides applied at 96 hrs after inoculation and the Nil control (Tables 2-4).

Table 2. Number of petioles with Ascochyta at 14 days after inoculation on chickpeas sprayed with Aviator Xpro, Veritas or water (Nil) 24, 48, 72 and 96 hours after inoculation

numbers followed by the same letter are not significantly different within timings, P =0.05

Table 3. Number of leaves with Ascochyta at 14 days after inoculation on chickpeas sprayed with Aviator Xpro, Veritas or water (Nil) 24, 48, 72 and 96 hours after inoculation

numbers followed by the same letter are not significantly different within timings, P =0.05

Table 4. Number of stems with Ascochyta at 14 days after inoculation on chickpeas sprayed with Aviator Xpro, Veritas or water (Nil) 24, 48, 72 and 96 hours after inoculation

numbers followed by the same letter are not significantly different within timings, P =0.05

The results of the 2018 glasshouse experiments looking at the impact of Ascochyta infection period prior to application of Aviator Xpro and Veritas need to be validated in field trials.

However, the results indicate if growers wait 72 hours after rain starts before spraying, Ascochyta may still develop and if they wait to 96 hours they will not stop the disease.

If a pre-rainfall spray is missed, one management option available to growers may be to apply Aviator Xpro (before late flowering (BBCH 69) with a maximum of two sprays during the season) or Veritas (may be applied twice in a season, with no restriction on use at flowering).

However, growers are encouraged to implement the current recommended management practice of applying before rain .

2007 Tamworth chickpea Ascochyta fungicide rain fastness experiment

This replicated experiment was designed to answer a question many agronomists and growers have asked i.e. “Yesterday, I sprayed my chickpeas with an Ascochyta fungicide and today it rained - is my crop still protected?”

Chickpeas cv Jimbour with 3-5 nodes were sprayed with Bravo® (720g/L chlorothalonil), Dithane^TM Rainshield^TM (750g/kg mancozeb) or water (Nil fungicide). Fungicides were applied with a backpack at standard rates ie Bravo @ 1000 mL/ha or Dithane Rainshield @ 2kg/ha in 100L/ha water by placing pots on ground and walking at 6.0 km/h. As soon as the fungicides had dried, the plants were placed in a rainfall simulator and exposed to 50mm, 100mm or 150mm of ‘rain’ at a rate of 50mm per 10 minutes; plants not exposed to rain were the Nil rain (Dry) control.  After exposure, plants were inoculated with Ascochyta, placed in a humid chamber for 48 hours and assessed 10 days later.

Bravo was very rain fast – 150mm rain in 30 min did not appear to reduce efficacy compared with 50mm in 10 min.

Dithane Rainshield was not as rain fast with 150 mm having significantly more stem lesions than 50mm. However, it’s highly unlikely a chickpea crop in southern Australia would ever be exposed to 50mm rain in 10 min (the lowest intensity we could get with this simulator).

So, the answer to the growers’ questions is “Yes – your crop is still protected, although new growth emerging post-fungicide is not”.

2018 Tamworth chickpea Ascochyta fungicide persistence glasshouse experiment

A glasshouse experiment was conducted to determine how long a fungicide protects tissue to which it has been applied. Fungicides (Unite 720 @ 1000 mL/ha, Aviator Xpro @ 400 mL) were applied once to Kyabra or PBA Seamer with 4-5 nodes and inoculated with Ascochyta 1, 2 or 4 weeks later; water was the Nil control. New growth was removed every 2-3 days to remove tissue that had not been sprayed from the experiment.  Ascochyta was assessed on petioles, leaves and stems on a 1-9 scale where 1= nil disease and 9 = tissue dead.  The data was analysed using the lme function of the R package nlme.

The findings were similar for petioles, leaves and stems.  No Ascochyta developed on either variety with Aviator Xpro at any time of inoculation.   There was little or no disease with Unite 720.  For the Nil control, Kyabra had more Ascochyta than PBA Seamer and disease scores tended to be lower for later inoculation.

This experiment supports previous research that showed chickpea Ascochyta fungicides provide lasting protection for the tissues to which they have been applied.

Chickpea Ascochyta management in 2019

The current recommendation for cost-effective management of chickpea Ascochyta includes:

• Treat all planting seed with a registered fungicide, applied properly. Seed treatment protects against seed transmitted Ascochyta, Botrytis and a range of opportunistic soil fungi that can attack seedlings if seed has lower vigour, is planted deep or if conditions don’t favour rapid emergence e.g. cold, wet soil, herbicide residues.
• Paddock selection – avoid planting chickpeas in the same paddock for at least 3 years. Avoid planting chickpea immediately next to last year’s chickpea crop.
• Grow varieties with the highest level of Ascochyta resistance suitable for your area
• For NSW and southern QLD, in high risk Ascochyta situations i.e. paddocks that had chickpeas in 2016, 2017 or 2018, apply a preventative fungicide before the first post emergent rain event
• In central QLD, where the Ascochyta risk is lower compared to southern regions, grow the highest yielding varieties but have in place an Ascochyta plan.  In most seasons in CQ, there will be no cost benefit of applying a fungicide before Ascochyta is detected. When conditions do favour Ascochyta, a reactive foliar fungicide program and protective pod sprays are warranted. Monitor the crop 10-14 days after each rain event.

References

Bates D, Maechler M, Bolker B, Walker S (2015). Fitting Linear Mixed-Effects Models.   Using lme4. Journal of Statistical Software, 67(1), 1-48. doi:10.18637/jss.v067.i01.

Pinheiro J, Bates D, DebRoy S, Sarkar D, R Core Team (2018). nlme: Linear and Nonlinear Mixed Effects Models. R package version 3.1-137, <URL: https://CRAN.R-project.org/package=nlme>.

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 authors would like to thank them for their continued support.   We thank Paul Nash and Gail Chiplin for technical assistance and chemical companies for product.

Contact details

Kevin Moore
NSW DPI
Tamworth Ag Institute
4 Marsden Park Rd, Calala, NSW 2340
Ph: 0488 251 866
Fx: 02 6763 1222
Email: kevin.moore@dpi.nsw.gov.au

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