Note: Current varieties have limited resistance to Ascochyta blight
Ascochyta blight of chickpea is caused by the fungal pathogen Ascochyta rabiei (also known as, Didymella rabiei or Phoma rabiei) which is specific to chickpea. This is considered to be the most important fungal disease of chickpea worldwide. In 1998 a serious outbreak of the disease occurred in Victoria, South Australia and New South Wales which destroyed many crops. As a consequence, the area sown to chickpea fell significantly. Since then, resistant varieties had been released, and chickpea production was increasing. During 2016, a further breakdown of resistance occurred in Victoria and since then it has spread across Eastern Australia and resulted in all varieties being rated as Moderately Susceptible to Very Susceptible.
Breeding and pre-breeding lines currently under evaluation are showing a more resistant reaction to Ascochyta, however, until more resistance is available to growers, fungicides will play an important role in Ascochyta management.
What to Look For
All above ground parts of chickpea plants can be affected by Ascochyta blight. Symptoms can appear on plants at any growth stage, with infection looking more severe when plants are smaller. The disease appears as spot-like lesions which are initially light grey to pale yellow/white and then develop a tan to dark brown margin and small black dots in the lesion (pycnidia). If these lesions occur on the stem, the lesion constricts the stem, causing girdling, and eventually causes stem breakage. The disease spreads during rainfall events in spring from infected plants to surrounding healthy plants by rain splash of spores. If there is limited disease control, obvious symptoms will be visible, particularly in late winter when small patches of blighted plants will be visible throughout the paddock. In severe cases the entire plant dries up quickly. Regrowth may occur, but the spores and disease risk are still in the paddock.
All pods are susceptible to Ascochyta blight and if the disease is already present in the crop and rainfall occurs during podding, lesions are likely to occur on pods. The infection of pods often has similar symptoms to leaf lesions, although the lesions may be darker. Pycnidia will most likely still be visible. The fungus can penetrate through the pod and infect the seed causing lesions on the seed.
When infected seeds are sown, the emerging seedlings will develop dark brown lesions at the base of the stem. Affected seedlings may collapse and die.
Typical first signs of Ascochyta blight symptoms, prior to the development of small dark specks (pycnidia) (Photo: Joshua Fanning – Agriculture Victoria)
Typical leaf lesions caused by Ascochyta blight. These are seen as round spots with brown margins. Small dark specks (pycnidia) are often present. (Photo: Luise Fanning – Agriculture Victoria)
Typical stem lesions caused by Ascochyta infection, leading to stem breakages (Photo: Joshua Fanning – Agriculture Victoria)
Ascochyta blight patches occur within a crop as the disease spreads from infected plants to surrounding healthy plants through rainsplash. (Photo: Unknown)
Typical symptoms of pod infection caused by chickpea Ascochyta blight which often leads to seed infection. (Photo: Joshua Fanning – Agriculture Victoria)
Disease Cycle
This fungal disease has an asexual (non-sexual) and sexual stage; the asexual stage occurs in Australia, whilst the sexual stage has not been found in Australia. The fungus survives on infected seed and on old chickpea stubble. Sowing infected seed will give rise to infected seedlings that initiate infection within crops. The pathogen can also be carried into new season crops on wind-blown chickpea trash, which can spread the pathogen when it comes into contact with seedlings. Infection can occur at any stage of plant growth provided conditions are favourable. Moisture is essential for infection to occur. During wet weather, the disease can spread further as fungus spores are carried onto neighbouring plants by wind and rain splash.
Disease cycle of Phoma rabiei (formerly known as Ascochyta rabiei). Illustration by Kylie Fowler
Management
Management of Ascochyta blight requires a combination of farm hygiene, variety resistance, crop monitoring and the use of fungicides. An integrated disease management strategy is essential to reduce the grain yield losses caused by Ascochyta blight.
Paddock Selection
Infected crop residues harbour Ascochyta rabiei. Therefore, avoid planting this season’s crop on or near old chickpea stubble, with at least 500m between crops being preferable. A program of stubble reduction may also be undertaken by grazing or burying to reduce the carry-over of infected stubble into the following season. Allow a break of at least 3 years between chickpea crops.
Resistant Varieties
Sow varieties with the greatest resistance. Please note, the Ascochyta blight pathogen population has evolved in recent years. This has resulted in changes in pathogenicity and has decreased the level of resistance in current chickpea varieties. Please check current sowing guides for the latest disease resistance ratings in your region.
Clean Seed
It is essential to sow clean seeds. Sowing old or damaged seeds can reduce seedling vigour and increase susceptibility to infection. In addition to re-infection through seed, reduced plant establishment occurs by planting infected seed.
A comparison of normal (clean) chickpea seed and infected chickpea seed with showing typical Ascochyta blight lesions (Photo: Joshua Fanning – Agriculture Victoria)
Seed Treatment
Use a registered seed treatment for the control of Ascochyta blight of chickpea. Seed treatments reduce the chance of seed transmission and provide early seedling protection. Seed treatments can have a deleterious effect on rhizobia. Therefore, seed should be treated with fungicide and then inoculated with rhizobia in two separate operations. Rhizobia should be applied to seed immediately before sowing, especially on acid soils.
Sowing
Early sowing encourages increased levels of the disease. Three-years of field experimental results at the Horsham SmartFarm demonstrated double the level of disease severity in May sown chickpeas compared to July sown, in the absence of fungicides. In the no disease plots, there was no difference in yield between the two sowing times in 2 out of the 3 seasons. Therefore, follow the recommended sowing dates for your district and tailor the other factors in an integrated disease management strategy to the sowing time.
Agronomy
Sowing chickpeas between standing cereal stubble has shown to reduce disease severity and subsequent grain yield losses. Experimental results at Horsham showed a susceptible variety has a similar disease severity to a moderately susceptible variety when grown between standing stubble rows as compared to slashed stubble rows.
Foliar Fungicides
Varieties rated MRMS or more susceptible will require varying levels of fungicides to control Ascochyta blight depending on the environment and season. In some seasons, varieties rated Susceptible may need between 4 and 8 fungicide applications, depending on the number of rainfall events and the fungicide actives applied. In moderately susceptible varieties (the best currently available), the first fungicide is required at the 4-6 node stage if rain is predicted. Depending on subsequent rainfall and the fungicides applied, subsequent application may be required every 2-4 weeks after. More recently released dual active fungicides have been shown to be very effective and will require less applications in a season. See the latest data for recommendations.
All varieties are susceptible to pod infection and will require protection during podding prior to rainfall events to prevent seed staining and abortion. However, a resistant variety will only need a fungicide at podding to protect seed quality.
Note: Some fungicides only protect the plant parts which come in contact with the spray, subsequent plant growth will not be protected. Fungicides should be applied according to label directions for use, ensuring the key points of spray timing and frequency are observed as well as grazing and harvest withholding periods.
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