Ascochyta blight (AB), caused by Ascochyta rabiei is a major disease of chickpea (Cicer arietinum L.), especially in areas where cool, cloudy, and humid weather persists during the crop season. Several epidemics of AB causing complete yield loss have been reported. The fungus mainly survives between seasons through infected seed and in infected crop debris. Despite extensive pathological and molecular studies, the nature and extent of pathogenic variability in A. rabiei have not been clearly established. Accumulation of phenols, phytoalexins (medicarpin and maackiain), and hydrolytic enzymes has been associated with host-plant resistance (HPR). Seed treatment and foliar application of fungicides are commonly recommended for AB management, but further information on biology and survival of A. rabiei is needed to devise more effective management strategies. Recent studies on inheritance of AB resistance indicate that several quantitative trait loci (QTLs) control resistance. In this paper we review the biology of A. rabiei, HPR, and management options, with an emphasis on future research priorities.

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Ascochyta blight of chickpea (Cicer arietinum L.): a review of biology, pathogenicity, and disease management

Chickpea (Cicer arietinum L.) is an important grain legume crop in Asia, Africa and Central and South America. Ascochyta blight caused by Ascochyta rabiei (Pass.) Lab. is one of the most serious diseases of the crop and severe epidemics have been reported. The available literature on Ascochyta blight is reviewed in order to ascertain the present status of knowledge of the disease and to identify the areas of research which need immediate attention. Distribution, yield losses, symptoms, taxonomy, reproduction, epidemiology and control of the disease are discussed. Further information on sexual reproduction, epidemiology and control is needed.

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A review of Ascochyta blight of chickpea

Coelomycetous fungi are an artificial taxonomic group which produce conidia inside a cavity i.e. conidiomata. Coelomycetes comprise about, 1000 genera and 7000 species, which can be endophytic, pathogenic or saprobic. Traditional classification of coelomycetes was previously based on morphology, such as the shape of conidiomata and mode of conidiogenesis, while it was treated as a distinct group i.e. Deuteromycotina. Sequence based taxonomic studies has been used to accommodate asexual fungi in a natural classification system, resolve generic boundaries of polyphyletic genera and species complexes, as well as establish asexual-sexual links. Nevertheless, most of genera lack sequence data, thus, morphology based identification is still important when introducing new genera or species. In this paper we illustrate, describe, and provide taxonomic notes for 235 dematiaceous coelomycetous genera, including five new genera viz. Apiculospora, Didymellocamarosporium, Melanocamarosporium, Melnikia and Paulkirkia. Phylogenetic analyses of combined sequence data are provided to show placements of dematiaceous coelomycetes in Dothideomycetes, Leotiomycetes and Sordariomycetes. One-hundred and fifty-two (65 %) of genera have sequence data, thus, their taxonomic placement in a natural classification system, is listed as an outline. However, 83 genera still lack sequence data, hence, they are treated as Ascomycota, genera incertae sedis. In addition, separate analyses are provided where better taxonomic resolution is needed.

Taxonomy and phylogeny of dematiaceous coelomycetes

The historical and contemporary population genetic structure of the chickpea Ascochyta blight pathogen, Ascochyta rabiei (teleomorph: Didymella rabiei), was determined in the US Pacific Northwest (PNW) using 17 putative AFLP loci, four genetically characterized, sequence-tagged microsatellite loci (STMS) and the mating type locus (MAT). A single multilocus genotype of A. rabiei (MAT1-1) was detected in 1983, which represented the first recorded appearance of Ascochyta blight of chickpea in the PNW. During the following year many additional alleles, including the other mating type allele (MAT1-2), were detected. By 1987, all alleles currently found in the PNW had been introduced. Highly significant genetic differentiation was detected among contemporary subpopulations from different hosts and geographical locations indicating restricted gene flow and/or genetic drift occurring within and among subpopulations and possible selection by host cultivar. Two distinct populations were inferred with high posterior probability which correlated to host of origin and date of sample using Bayesian model-based population structure analyses of multilocus genotypes. Allele frequencies, genotype distributions and population assignment probabilities were significantly different between the historical and contemporary samples of isolates and between isolates sampled from a resistance screening nursery and those sampled from commercial chickpea fields. A random mating model could not be rejected in any subpopulation, indicating the importance of the sexual stage of the fungus both as a source of primary inoculum for Ascochyta blight epidemics and potentially adaptive genotypic diversity.

Historical and contemporary multilocus population structure of Ascochyta rabiei (teleomorph: Didymella rabiei) in the Pacific Northwest of the United States

Ascochyta blight causes significant yield loss in pulse crops worldwide. Integrated disease management is essential to take advantage of cultivars with partial resistance to this disease. The most effective practices, established by decades of research, use a combination of disease-free seed, destruction or avoidance of inoculum sources, manipulation of sowing dates, seed and foliar fungicides, and cultivars with improved resistance. An understanding of the pathosystems and the inter-relationship between host, pathogen and the environment is essential to be able to make correct decisions for disease control without compromising the agronomic or economic ideal. For individual pathosystems, some components of the integrated management principles may need to be given greater consideration than others. For instance, destruction of infested residue may be incompatible with no or minimum tillage practices, or rotation intervals may need to be extended in environments that slow the speed of residue decomposition. For ascochyta-susceptible chickpeas the use of disease-free seed, or seed treatments, is crucial as seed-borne infection is highly effective as primary inoculum and epidemics develop rapidly from foci in favourable conditions. Implemented fungicide strategies differ according to cultivar resistance and the control efficacy of fungicides, and the effectiveness of genetic resistance varies according to seasonal conditions. Studies are being undertaken to develop advanced decision support tools to assist growers in making more informed decisions regarding fungicide and agronomic practices for disease control.

Integrated disease management of ascochyta blight in pulse crops

A. rabiei infected 70% of C. arietinum samples from C. Anatolia, Turkey. The standard blotter method, using 5 seeds/Petri dish and 12 h NUV 12 h darkness cycles at 22 deg C, proved most suitable for detecting the fungus. The inoculum was spore contamination and mycelium occurred in the seed coat alone or in seed coat and embryo. Pycnidia were observed only in the seed coat of seeds having deep lesions. Inter- and intracellular mycelium was localized to lesions. Pycnidia were subepidermal and contained mature spores. Pycnidiospores obtained from the seed surface and pycnidia from 14-month-old seed stored at 3 plus or minus 2 deg showed 33% germination. The light and temp. responses of the fungus on PDA revealed that opt. colony growth occurred at 19 deg . Pycnidial formation was max. under NUV but failed in darkness. Both superficial and deep infections were equally potent in the transmission of the disease. The fungus was highly pathogenic to seed and 40-day-old plants.This paper was presented at the 17th Int. Seed Testing Congress 1974.

S. Maden

Papers

Detection and location of seed-borne inoculum of Ascochyta rabiei and its transmission in chickpea (Cicer arietinum).