Ascochyta found in previously resistant chickpeas

Ascochyta chickpea Kevin MooreBy Mary Raynes, Pulse Australia. 9 Sept 2015.

There are several confirmed reports of Ascochyta blight (AB), caused by the pathogenic fungus Phoma rabiei (previously referred to as Ascochyta rabiei) in commercial crops of the previously resistant chickpeas in the Victorian Mallee.
*It has since been found on the Yorke Peninsula, SA in Genesis 090 chickpeas (Crop Watch SA Vol 12, 15 Sept 2015).

Resistant chickpea varieties may be demonstrating a breakdown in the disease resistance. All growers and advisers in Victoria and South Australia are encouraged to monitor chickpea crops in preparation for the application of preventative sprays on current and unprotected plant material.

Paddock symptoms

First symptoms are typical ascochyta blight leaf lesions with pale centres, dark edges and black pycnidia (which look like black dots) in the centre of the lesions. In more advanced lesions, concentric rings of black pycnidia may appear on the leaves and stems.

Generally this disease becomes obvious in late winter when small patches of blighted plants (blackened side branches, dead leaves, potentially broken stems) appear throughout the paddock.

Identification of Ascochyta blight (AB) on chickpea plants

The fungus attacks all above-ground plant parts including leaves, stems and pods at any stage of plant growth.

Initially ascochyta blight appears on the younger leaves as small water-soaked pale spots. These spots rapidly enlarge, under cool and wet conditions, joining with other spots on the leaves and blighting the leaves and buds. On these lesions small black spots (pycnidia) can be seen in the affected areas.

In severe cases the entire plant dries up suddenly. Elongated lesions form on the stem and may girdle the stem. The stem may die and break off. Regrowth may occur from this point. Affected areas on the pods tend to be round, sunken, with pale centres and dark margins. The fungus can penetrate the pod and infect the seed.

Severity of Ascochyta blight (AB) in crop

The disease spreads during cool, wet weather in winter and spring from isolated plants to surrounding plants by rain-splash of spores. This forms large blighted patches within crops. Plants are most susceptible in rainy humid conditions.

Pods on all varieties are susceptible to infection irrespective of foliar resistance. Infection of pods will lead to pod abortion and seed staining. Hence ascochyta blight causes low yields with poor seed quality and can result in up to 100% yield loss in the most susceptible varieties.

Where did the inoculum come from?

Ascochyta blight survives on chickpea residue, on seed and on volunteer chickpeas.

Has the Ascochyta pathogen changed?

Early data from research conducted by Australian chickpea researchers indicates that the pathogen has changed and is adapting to resistant varieties. Genetic resistance to ascochyta blight in chickpeas is highly complex which makes it difficult to monitor changes in pathogen virulence. Ongoing research is being conducted to understand these complex interactions.

At this stage, Australian chickpea researchers have detected a growing number of isolates that are highly aggressive on Genesis090 and PBA HatTrick under controlled environment trials. Several highly aggressive isolates appear to be broadly adapted to growing region and cultivar and are being monitored for. The most highly aggressive and commonly detected isolates are being used for resistance selection in the national chickpea breeding program.

You can hear more about the survey findings on isolate pathogencity in Ascochyta blight in chickpea in a video interview with Dr Rebecca Ford recorded in 2014.

Treatment

Currently the available products for use in chickpea include Unite and Barrack (chlorothalonil), Captan, Amistar Xtra (azoxystrobin plus cyproconazole) and Prosaro (prothioconazole plus tebuconazole).

Fungicides should be rotated between groups with no more than two applications of the same group applied during a single growing season (except for chlorothalonil which can be applied up to 3 times with no restrictions in terms of application order).

There is a high risk that the pathogen may develop resistance to fungicides with repeated use of the same fungicides.

Chlorothalonil is the most effective foliar fungicide against ascochyta, with Barrack and Unite products registered for ascochyta blight control on chickpea, plus under permit Captan, Amistar Xtra (azoxystrobin plus cyproconazole) and Prosaro (prothioconazole plus tebuconazole).

Mancozeb (various) or metiram (Polyram®) can also control ascochyta if timeliness of application is observed.

On susceptible cultivars fortnightly sprays with mancozeb are required compared to chlorothalonil sprays every 21 days. (Refer to Pulse Australia leaflet ‘Strategies for control of foliar diseases in chickpea’).

Sprays should be applied ahead of rain fronts to prevent fungal infection and spread.

In the meantime apparently the availability of macozeb is improving. The label rate for a 750 g/kg product for both chickpea 1.0 – 2.2 kg/ha. Please refer to local experience when deciding upon use rates but a typical rule of thumb is:

  • Prophylactic sprays for ascochyta blight, 1.0 kg/ha
  • Once Ascochyta blight has been confirmed within a crop, increase your rate to 2.0 kg/ha

Permits

The following permits have been issued for the control of ascochyta blight in chickpea crops:

  • Captan (800 & 900 g/kg Captan products) – APVMA Permit PER81406
  • Amistar Xtra Fungicide (200 g/L azoxystrobin & 80 g/L cyproconazole) – APVMA Permit PER81470
  • Prosaro 420 SC Foliar Fungicide (210 g/L prothioconazole & 210 g/L tebuconazole) – APVMA Permit PER81474

Optimize the number and sequence of foliar fungicide sprays

  • The number of fungicide sprays during the season depends on chickpea variety and disease severity.
  • Southern Pulse pathologists are encouraging growers & agronomist to apply fungicide to all chickpea crops at podding even on the resistant varieties, such as Genesis 509 and Genesis 090 for protection against ascochyta blight, since pods are not resistant on any variety.
  • Fungicide sprays are no longer necessary when the crop reaches the late-pod stage, regardless of disease severity at that time.
  • If applied after late-pod stage, fungicide may delay plant maturation.
  • If disease is prolific, multiple applications may be required.

State laboratory – for samples

Please send your samples of chickpea to:

Courier:
SARDI, Plant and Soil Health
Plant Research Centre
Gate 2B, Hartley Grove
Urrbrae SA 5064

Postal:
SARDI, Plant and Soil Health
Locked Bag 100
Glen Osmond SA 5064

or

CropSafe
DPI Victoria
110 Natimuk Road
Horsham. VIC 3400
03 53 622 111

For further information:

Acknowledgements:

Dr Jenny Davidson, Senior Plant Pathologist, SARDI
Helen Richardson, Plant Pathologist, DEDJTR VIC
Associate Professor, Rebecca Ford, Griffith University
Southern Pulse Agronomy Program
GRDC

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