Ascochyta blight of chickpeas is caused by the fungal pathogen Phoma rabiei (formerly known as Ascochyta rabiei ) which is specific to chickpeas. In recent years this disease has become the most important disease in chickpeas. In 1998 there was a serious outbreak of the disease in Victoria, South Australia and New South Wales which destroyed many crops. The extent of the chickpea cropping area fell considerably as a consequence of the varieties available being susceptible to ascochyta blight. Varieties now generally have much better foliar resistance to ascochyta blight. However, all varieties still need fungicides applied at flowering and podding stages as they are vulnerable to ascochyta blight during this growth stage.
Usually this disease becomes obvious in late winter when small patches of blighted plants appear throughout the paddock, (Figure 5.1). The disease spreads during cool, wet weather in spring from infected plants to surrounding plants by rain splash of spores. This forms large blighted patches within crops.
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, (Figure 5.2). Small black spots (pycnidia) can be seen in the affected areas. In severe cases the entire plant dries up quickly. Elongated lesions form on the stem and may girdle the stem which may then die and break off, (Figure 5.3). Regrowth may occur from this point. Affected areas on the pods tend to be round, sunken, with pale centres and dark margins, (Figure 5.4). The fungus can penetrate the pod and infect 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 (damping-off).
Figure 5.1 Ascochyta blighted patches occur within a crop as the disease spreads from infected plants to surrounding healthy plants
Figure 5.2 Typical leaf lesions caused by Ascochyta blight. These are seen as round spots with brown margins. Small dark specks (fruiting bodies) are often present
Figure 5.3 Typical stem lesions caused by Ascochyta infection. Stems break at lesions and wither
Figure 5.4 Ascochyta infected pod with a large lesion containing fruiting bodies
This fungal disease has an asexual (non-sexual) and sexual stage; the asexual stage is most common in Australia. In this stage the fungus survives mainly on infected seed and on crop residues. Spores of the fungus produced on crop residues are carried into new crops by wind. 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 than in dry conditions because spores of the fungus are carried onto neighbouring plants by wind and rain splash, (Figure 5.5).
Figure 5.5 Disease cycle of Phoma rabiei (formerly known as Ascochyta rabiei ). Illustration by Kylie Fowler
Management of ascochyta blight requires a combination of farm hygiene, resistant varieties, crop monitoring and the use of fungicides. When growing a new variety, obtain a copy of the variety management package for information on specific disease management. A key strategy is to consider growing a resistant variety such as PBA Slasher or Genesis™090. Note - a fungicide application is still required at early podding to protect seed quality.
For chickpea varieties (rated susceptible to moderately resistant) the following management strategies should be applied:
- use seed from a paddock where ascochyta was not detected or was well managed
- treat all seed with a seed dressing registered for ascochyta blight control
- choose a paddock at least 500 metres from last years chickpea crop
- generally the first fungicide spray will need to be applied 4 - 6 weeks after sowing. Moderately resistant varieties will require 2 - 4 strategic sprays throughout the growing season. The moderately susceptible varieties will require spraying every 2 - 3 weeks. Susceptible varieties will require spraying at least fortnightly throughout the growing season
- note: sprays only protect the plant parts contacted by the spray, subsequent plant growth will not be protected
- a fungicide spray at pod set is essential to ensure production of premium quality seed
- always disinfect machinery, vehicles and boots once they have been in an infected crop.
Breeding programs in Australia are developing chickpea varieties with improved ascochyta blight resistance. All current resistant varieties and future releases will still require spraying at podding time, as their pods are susceptible to infection. However, the greatly reduced number of spray applications required for resistant varieties make chickpea a valid crop option and profitable to grow.
As resistant varieties are grown over larger areas, there is likely to be a reduction in the amount of ascochyta blight inoculum present. This should reduce the risk of further serious disease outbreaks. Refer to Table 5.2. Chickpea variety disease reactions, at the end of this chapter.
DISEASES IN PULSES: CHICKPEAS
SCLEROTINIA OF CHICKPEAS