Fungal and bacterial diseases of chickpea
Ascochyta rabiei only infects chickpeas and is the most serious disease of commercial chickpea crops in Western Australia. The disease has caused significant yield losses in all the chickpea growing areas, except the Ord River Irrigation Area where it has not yet been detected.
The overall appearance of Ascochyta in a chickpea crop depends on susceptibility of the variety grown and the level of fungicide protection that has been applied. In susceptible and unprotected crops, patches of plants that appear stunted become evident in the crop. The stunted appearance results from branches and leaves that have broken due to Ascochyta infection. New branches that have grown during periods of dry weather, and are thus have not yet been infected, may also be evident on these plants.
Stem and leaf breakage is less common in varieties with moderate resistance to the disease or in crops that have received an early fungicide application. In these crops the patches of stunted looking plants do not develop. Lighter coloured areas resulting from dead leaves and leaf lesions can, however, still be seen.
Closer examination of leaf, stem and pod will reveal spots with brown margins and grey centres, it is these lesions on stems that cause them to break, wilt and die. This stem damage causes the majority of the yield loss associated with this disease. Tiny black specks (called pycnidia) will develop in the grey centre of spots (lesions) as they enlarge.
The fungus is carried to new areas in infested seed batches. The seeds themselves may be infected, or small pieces of infected trash may be carried with the seed. Seed infection levels as low as 0.1 percent can cause significant disease outbreaks in new crops. Spores from pycnidia that develop in lesions formed on seedlings grown from infested seeds are moved by rain-splash to adjacent plants. The predominantly local spread from these initial infection points produces a patchy appearance from low levels of seed infection. After flowering, infection of pods can lead to infected seed.
After harvest, the infected stubble also carries the infection into the next season. The fungus can survive as pycnidia and fungal growth (mycelium) on chickpea stubble for up to 20 months in Western Australia. After the break of the season, wetting of the infected stubble will cause the release of rain splashed spores (conidia) from the pycnidia. The rain-splash spores are spread, usually only a few meters, to new plants where they produce new lesions that form pycnidia in 6 to 12 days, depending on temperature. The cycle then proceeds as described for seed borne infection. Small pieces of stubble, such as pods and leaves, can be blown several hundred metres in a crop. If this stubble contains pycnidia, then new infections can start long distances away from the original source of infection.
The sexual stage (Didymella rabiei) of the Ascochyta fungus was first identified in Western Australia in 2002. It develops on the previous season's stubble after exposure to cold, wet conditions in winter and produces ascospores (dry spores) that are spread by wind in mid-to-late winter. Ascospores may infect crops at more than a kilometre from the previous season's stubble.
The most effective ways to manage this disease are to: minimise introduction of the disease on seed; isolate the new crop from the previous season's stubble to minimise infection of the crop by winds-blown trash and spores, and; to maintain a wide rotation to stop the disease carryover within a paddock. After the crop is established, fungicides are required early in the season to reduce early infection and disease spread, additional fungicide sprays will be required depending on the resistance of the variety and the frequency of rain during the growing season.
Botrytis Grey Mould
Botrytis cinerea causes two diseases in chickpea: the most important of these is a stem disease known as Botrytis Grey Mould; the other is a damping-off disease of seedlings called Botrytis seed-borne root rot has not been observed in Western Australia. Botrytis Grey Mould has caused serious crop losses in the northern cropping areas of Western Australia. The fungus that causes Botrytis Grey Mould has a wide host range and can infect other pulse crops including lentil, vetch, faba bean and field pea.
Kabuli type chickpeas are more susceptible than desi types to both Botrytis Grey Mould and Botrytis seed-borne root rot. Chickpeas with vigorous seedling growth, early canopy closure and early flowering are more likely to develop Botrytis Grey Mould than other varieties.
The most obvious symptom of Botrytis seed-borne root rot is a reduction in plant establishment. Seedlings that develop from infected seed, sown without a fungicide seed dressing, may emerge but die within a few weeks. This disease is a major concern in south-eastern Australia.
The symptoms of Botrytis Grey Mould become apparent in late winter and spring. Dead plants or dead branches, scattered throughout the crop are the first obvious symptoms. The disease usually first appears around flowering time, when the crop canopy is fully developed and the weather is warm and humid. These conditions favour infection and disease development.
Close inspection of plants will reveal the presence of a fluffy grey or grey-brown mould on the outside of the dead branches, particularly under a closed canopy or lodged plants. Infection will spread out, from the plants first infected by the disease, to form patches of dead plants. Infections usually first take hold in areas of the crop that are densely sown such as headlands that have been double sown, or in areas damaged by vehicular traffic.
A less conspicuous symptom of Botrytis Grey Mould is reduction in the number of pods as a result of flower infection. During favourable (humid and warm) weather flowers can become infected more easily than other plant parts. The infected flowers abort and do not set pods. Infected flowers are easily dislodged leaving no conspicuous symptom of Botrytis Grey Mould.
Botrytis can also infect pods after flowering and be carried into the next season through infected seed. Spores produced on dead seedlings can be blown onto nearby plants and produce secondary infections in warm and humid weather.
Botrytis survives over summer on the previous season's stubble as fungal growth (mycelium). It also produces sclerotia (fungal survival structures) in response to cold, wet winter conditions. The fungal growth and sclerotia produce spores in spring once daytime temperatures are greater than 20C. Spores produced on stubble can be blown several hundred metres to initiate infection in new crops.
The best ways to manage this disease are to avoid the use of Botrytis Grey Mould infected seed. Isolate this year's crop from the previous season's stubble of chickpea, vetch and lentil, to minimise infection by trash and spores blown into the crop by the wind, and to maintain a wide rotation both of chickpea and other potential host crops. Observe the recommended sowing times and seed rates, don't sow too early or with seed rates that are too high. Fungicides are usually not a cost-effective means of controlling this disease; however, Botrytis Grey Mould reduction is a side benefit of the fungicide application for Ascochyta Blight control.
More information on chickpea diseases at 'Chickpea pest and disease management'
Page reviewed: 30 November 2005
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