Further evidence that chickpea varieties differ in reaction of pods to Ascochyta

Author: Kevin Moore, Kristy Hobson, Steve Harden, Paul Nash, Gail Chiplin and Sean Bithell, Department of Primary Industries, NSW, Tamworth | Date: 27 Feb 2017

Take home message

The susceptibility of pods to Ascochyta blight is important as infection can cause pod abortion and blemish or kill seed; infected seed can lead to downgrading of grain, furthermore infected seed is a future inoculum source.

A 2016 field trial confirmed that chickpea varieties differ in pod reaction to Ascochyta; varieties with higher levels of resistance to vegetative plant tissues e.g. PBA Seamer had less disease on their pods.

However, as all varieties can get pod infection (albeit at different proportions), in Ascochyta conducive seasons pods need to be protected with fungicides.

Background

Knowing the susceptibility of chickpea pod tissue to Ascochyta blight (caused by Phoma rabiei formerly known as Ascochyta rabiei) is important because if pods get infected early in their development they will abort; if fully developed pods get infected near the peduncle (as many do because the calyx holds water), they will abort; or the seed will get infected and is killed or the seed becomes infected, but remains viable and is a potential source of inoculum to initiate an epidemic.  Further, if Ascochyta lesions are detected on desi or kabuli kernels and the incidence exceeds 1% by weight, the load will be downgraded or rejected.  http://www.pulseaus.com.au/storage/app/media/markets/20160801_Pulse-Standards.pdf

Current Australian chickpea varieties and advanced breeding lines differ in susceptibility of their vegetative plant tissues to Ascochyta Blight (Moore et al, 2016).  However, the chickpea industry believes that pods of all varieties are equally susceptible to Ascochyta (see this GRDC Factsheet on Chickpea Disease Management). The 2011 Tamworth chickpea Ascochyta management trial, VMP11 suggested that may not be the case - anecdotal evidence indicated varieties with higher levels of resistance to Ascochyta e.g. Genesis™ 425 had less disease on their pods and scientific evidence was obtained by the 2014 Tamworth chickpea Ascochyta yield loss trial, VMP14.  VMP14, which was inoculated before flowering, provided data on susceptibility of pods of ten genotypes consisting of released varieties and advanced breeding lines ie  CICA1007, (C1007), CICA1211 (C1211), GenesisTM425 (G425), GenesisTMKalkee (KAL), PBA Seamer (SEA, coded as CICA0912 in Figure 1), PBA HatTrick (HAT), PBA Monarch (MON), PBA Boundary (BOU), Kyabra (KYB), and Jimbour (JIM).  The genotypes fell into 4 susceptibility groups with no differences between entries within a group but significant differences between genotypes in different groups.  The four groups from least to most susceptible were (C1007, SEA, G425), (BOU, HAT, KAL, MON), (C1211) and (JIM, KYB) (Fig. 1). 

A criticism of VMP14 is that the data could be confounded, because the plots (JIM and KYB) with the most infected pods and the greater number of lesions were also those that had the highest levels of Ascochyta in the vegetative stage.  We reasoned (Moore at al., 2015) there was sufficient inoculum pressure in VMP14 that all pods of all genotypes in the trial would have been exposed to the same level of inoculum.  Nevertheless, the fact remains that plants had been exposed to at least five infection events during the vegetative phase and that could have affected resistance of genotypes and/or inoculum pressure within a plot.  The 2016 trial, reported here, addressed these concerns by protecting plants with foliar fungicide during the vegetative stage and by sampling pods after one post-inoculation infections event.

Figure 1. Predicted cumulative proportions of pods across four disease severity categories of Ascochyta lesions for the ten chickpea genotypes in the 2014, VMP14 trial

Figure 1. Predicted cumulative proportions of pods across four disease severity categories of Ascochyta lesions for the ten chickpea genotypes in the 2014, VMP14 trial

Methods

The trial (POD16) was sown into burnt barley stubble on 18 May 2016 using tyne openers on 50cm row spacing in plots 2m wide by 10m long. Granulock® Supreme Z (50kg/ha) and liquid Rhizobia were applied at sowing with Gesatop® 600 SC applied PSPE.  There were five varieties PBA Seamer PBA Boundary, PBA HatTrick, PBA Monarch and Kyabra coded as above and four replications. Plants were protected from Ascochyta infection with eight applications of 1.0L/ha chlorothalonil (720g/L active) applied before rainfall, the first application on 4th July and the last on 14 th October.  On 30th October when the desi varieties were at 50-60% podding and PBA Monarch was at 40-50% podding, the trial was inoculated during a rain event with a cocktail of 20 isolates of P. rabiei containing 550,000 conidia/mL of water and applied at 100 L/ha; there were four application passes giving a total of 2.2million conidia/mL.  From the start of inoculation (6pm) to when the rain stopped at 7am 31 October, 17mm fell.  On 10th and 14th November, 16.4mm and 12.8mm were recorded, respectively, this rainfall provided two potential post-inoculation secondary infection events.  On 18th November, from each plot in Reps 1-3, one branch was collected from 50 plants at random (Rep 4 was severely affected by Phytophthora root rot and was not sampled). 

Ascochyta assessment and statistics

Samples were air dried in glasshouse for 1 day, pods removed and sorted into four disease severity classes based on their Ascochyta status: clean = no Ascochyta lesions; 1 lesion = pods with a single lesion; 2-5 lesions and >5 lesions.  A lesion was not called Ascochyta unless pycnidia could be seen either with the naked eye or under a low power dissecting microscope.  For each variety the number of pods falling into each of the four Ascochyta classes was analysed using proportional odds logistic regression, implemented using the "polr" function of the r MASS package Venables & Ripley (2002). Differences between varieties were assessed by comparing the coefficients of the model for each variety. A 95% confidence interval (CI) for each coefficient was calculated. Two varieties differ in disease severity if their respective CI’s do not overlap.

Results

2016 results: To limit the potential effect of secondary infection via inoculum generated from susceptible vegetative tissue, the pod samples were harvested 18 days after inoculation.  At this early time point there were large differences in pod infection among the genotypes.  Only 1.3% of KYB pods were clean (no disease), whereas 22.8% of SEA pods had no Ascochyta (Table 1). Not only did KYB have a greater proportion of Ascochyta infected pods, but these pods were more severely diseased with most (63.7%) infected pods having more than five Ascochyta lesions; this compares with only 10.2% of infected SEA pods having more than five lesions (Table 1).

Analysis showed with the exception of BOU and HAT, varieties were significantly different (P = 0.05) with the ranking from least to most susceptible being SEA < MON < BOU = HAT < KYB (Fig. 2).

2014 and 2016 results: All varieties in POD16 had more pods infected with Ascochyta than the same varieties in 2014.  This is believed to reflect higher disease pressure in 2016 resulting from inoculation with 2.2million conidia/mL and very conducive conditions (total 46.2mm rain in 3 rain events).  Nevertheless, the ranking of varieties was the same as in 2014 (Fig. 1, Fig. 2, Table 1) and the conclusions reached were the same.

Table 1. Percentages of pods in four Ascochyta disease severity categories for five varieties in 2016 vs 2014

Genotype

% Clean

% 1 Lesion

% 2-5 Lesions

% >5 Lesions

SEA 2016

22.8

15.0

52.0

10.2

SEA 2014

98.5

1.0

0.3

0.3

MON 2016

16.5

12.9

51.4

19.3

MON 2014

86.2

7.8

3.3

2.8

BOU 2016

11.4

10.1

48.3

30.2

BOU 2014

84.3

5.5

6.3

4.0

HAT 2016

10.1

9.8

47.0

33.2

HAT 2014

86.2

9.3

4.0

0.5

KYB 2016

1.3

3.1

31.9

63.7

KYB 2014

33.8

15.5

30.5

20.3

Figure 2. Predicted cumulative proportions of pods across disease severity categories of Ascochyta lesions for the five chickpea varieties in the 2016 trial. (CICA0912 is PBA Seamer)

Figure 2. Predicted cumulative proportions of pods across disease severity categories of Ascochyta lesions for the five chickpea varieties in the 2016 trial. (CICA0912 is PBA Seamer)

Key pod infection findings of POD16 were:

  • Varieties differed in the relative proportion of severely infected pods 18 days after inoculation, with SEA < MON < BOU = HAT < KYB
  • This ranking agrees closely with current Ascochyta variety ratings for vegetative tissues
  • From a management perspective, all varieties can get pod infection (albeit at different proportions); therefore in Ascochyta-conducive seasons pods need to be protected with fungicides

References

Kevin Moore, Kristy Hobson, Steve Harden, Paul Nash, Gail Chiplin and Sean Bithell (2015).  Chickpea Ascochyta – evidence that varieties do differ in susceptibility of pods (2015). Proceedings GRDC Updates Coonabarabran 25-26 Feb 2015; Warren 27 Feb 2015; Goondiwindi 3-4 Mar 2015; Talwood 5 Mar 2015.

Kevin Moore, Kristy Hobson, Steve Harden, Paul Nash, Gail Chiplin and Sean Bithell (2016).  Effect of chickpea Ascochyta on yield of current varieties and advanced breeding lines – the 2015 Tamworth trial VMP15. Proceedings GRDC Updates Goondiwindi 1-2 March, North Star 3 March, Toowoomba Wellcamp Airport 21 June, Chinchilla 22 June, Gilgandra 20 July, Narrabri 22 July,  2016

Venables, W. N. & Ripley, B. D. (2002) Modern Applied Statistics with S. Fourth Edition. Springer, New York.   ISBN 0-387-95457-0

Acknowledgements

This research is made possible by the significant contributions of growers through both trial cooperation, field access and the support of the GRDC; the authors most gratefully thank them and the GRDC.  We also thank Woods Grains, Goondiwindi, and Glen Coughran “Beefwood”, Moree for seed and chemical companies who provided products for trial management.

Contact details

Kevin Moore
Department of Primary Industries, Tamworth, NSW
Ph: 02 6763 1133
Mob: 0488 251 866
Fax: 02 6763 1100
Email: kevin.moore@dpi.nsw.gov.au

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