Ascochyta rabiei: A threat to global chickpea production

Abstract

The necrotrophic fungus Ascochyta rabiei causes Ascochyta blight (AB) disease in chickpea. A. rabiei infects all aerial parts of the plant, which results in severe yield loss. At present, AB disease occurs in most chickpea-growing countries. Globally increased incidences of A. rabiei infection and the emergence of new aggressive isolates directed the interest of researchers toward understanding the evolution of pathogenic determinants in this fungus. In this review, we summarize the molecular and genetic studies of the pathogen along with approaches that are helping in combating the disease. Possible areas of future research are also suggested.

Taxonomy: kingdom Mycota, phylum Ascomycota, class Dothideomycetes, subclass Coelomycetes, order Pleosporales, family Didymellaceae, genus Ascochyta, species rabiei.

Primary host: A. rabiei survives primarily on Cicer species.

Disease symptoms: A. rabiei infects aboveground parts of the plant including leaves, petioles, stems, pods, and seeds. The disease symptoms first appear as watersoaked lesions on the leaves and stems, which turn brown or dark brown. Early symptoms include small circular necrotic lesions visible on the leaves and oval brown lesions on the stem. At later stages of infection, the lesions may girdle the stem and the region above the girdle falls off. The disease severity increases at the reproductive stage and rounded lesions with concentric rings, due to asexual structures called pycnidia, appear on leaves, stems, and pods. The infected pod becomes blighted and often results in shrivelled and infected seeds.

Disease management strategies: Crop failures may be avoided by judicious practices of integrated disease management based on the use of resistant or tolerant cultivars and growing chickpea in areas where conditions are least favourable for AB disease development. Use of healthy seeds free of A. rabiei, seed treatments with fungicides, and proper destruction of diseased stubbles can also reduce the fungal inoculum load. Crop rotation with nonhost crops is critical for controlling the disease. Planting moderately resistant cultivars and prudent application of fungicides is also a way to combat AB disease. However, the scarcity of AB-resistant accessions and the continuous evolution of the pathogen challenges the disease management process.

Useful websites: https://www.ndsu.edu/pubweb/pulse-info/resourcespdf/Ascochyta%20blight%20of%20chickpea.pdf https://saskpulse.com/files/newsletters/180531_ascochyta_in_chickpeas-compressed.pdf http://www.pulseaus.com.au/growing-pulses/bmp/chickpea/ascochyta-blight http://agriculture.vic.gov.au/agriculture/pests-diseases-and-weeds/plant-diseases/grains-pulses-and-cereals/ascochyta-blight-of-chickpea http://www.croppro.com.au/crop_disease_manual/ch05s02.php https://www.northernpulse.com/uploads/resources/722/handout-chickpeaascochyta-nov13-2011.pdf http://oar.icrisat.org/184/1/24_2010_IB_no_82_Host_Plant https://www.crop.bayer.com.au/find-crop-solutions/by-pest/diseases/ascochyta-blight.

Keywords: Ascomycota; fungicide resistance; host resistance; necrotrophic fungus.

FIGURE 1

Disease progression of Ascochyta rabiei: sexual and asexual stages in the life cycle. (i) The airborne fungal conidia infect a chickpea plant. (ii) Conidia land on the leaf surface of the chickpea plant and (iii) start germinating through the formation of germ tubes. Later on, the germ tubes form an appressorium‐like structure at the tip of hyphae. (iv) The appressorium punctures the epidermal layer and (v) invades the subepidermal tissues. (vi) At a later stage pycnidia are formed, which contain asexual conidial spores that are dispersed by rain splash. (vii) During the sexual cycle, a specialized structure called the pseudothecium is formed on the infected plant in moist and cool conditions (winter). The pseudothecium is a cup‐like structure with an inner fertile layer called the hymen and an outer narrow opening called the ostiole. (viii) The hymen gives rise to sac‐like structures called asci that each contain eight ascospores. (ix) In spring, the pseudothecium forcefully discharges ascospores, which are carried by wind over distances of up to 10 km.

FIGURE 2

Morphology of Ascochyta rabiei. (a–d) Scanning electron and (e–i) light microscopy images. (a) Fungal spores on a susceptible chickpea leaf; bar = 10 μm. (b) Germinating spore (GS) with germ tube; bar = 10 μm. (c) Penetration peg (PP) formation; bar = 20 μm. (d) Fully grown fungal hyphae on chickpea leaves; bar = 10 μm. (e,f) Early stages of spore germination and germ tube formation observed on a glass slide. (g,h) Fungus showing hyphal branching after 2 and 3 days of growth. (i) A. rabiei growing on potato dextrose agar supplemented with chickpea meal.

FIGURE 3

Symptoms of Ascochyta blight on chickpea. (a) Circular brown lesions on leaves and pods and elongated/oval‐shaped necrotic spots on the stem. (b) Enlarged view of lesions on the leaf. The circular lesions formed on leaves bear concentric rings of pycnidia, which is the most distinctive diagnostic feature of the disease. (c) Infected plant in the field showing severe symptoms on the stem. (d) At later stages, lesions girdle the stem and the area above the girdle falls off. (e) Debris having both pycnidia and pseudothecia. (f) Blighted patch of chickpea in the field.