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

Thirty-four isolates of Colletotrichum spp. were isolated from banana, ginger, Euphatorium thymifolia, soybean, longan, mango and Draceana sanderiana. They included endophytes from healthy plants and probable pathogens from disease lesions. Isolates were identified and grouped based on colony morphology, and size and shape of appressoria and conidia. Molecular analysis based on sequences of the rDNA internal transcribed spacers (ITS1 and ITS2), indicated that the Colletotrichum isolates comprised four clades that paralleled the morphological groupings. Most isolates clustered within three distinct clades which potentially represented different species. Endophytes isolated from different hosts are more likely to be the same species. Colletotrichum musae was positioned close to the C. gloeosporioides clades. Morphological and phylogenetic analysis of Colletotrichum pathogens and endophytes showed that endophytic isolates were most similar to C. gloeosporioides however, no pathogenic isolates clustered with endophytic isolates. The correlation between morphological and molecular-based clustering demonstrated the genetic relationships among the isolates and species of Colletotrichum and indicated that ITS rDNA sequence data were potentially useful in taxonomic species determination.

Morphological and molecular characterization of Colletotrichum species from herbaceous plants in Thailand

Fungal disease detection in plants: Traditional assays, novel diagnostic techniques and biosensors.

A collection of 114 Cicer accessions including 95 wild Cicer accessions from four different species (C. bijugum K. H. Rech., C. echinospermum P. H. Davis, C. pinnatifidum Jaub. & Sp. and C. reticulatum Ladiz.) was screened in glasshouse trials for resistance to ascochyta blight (caused by Ascochyta rabiei (Pass.) Lab.). Resistance was identified in accessions from all four wild Cicer species. There was variation for resistance within accessions of C. echinospermum, C. pinnatifidum and C. reticulatum, with only C. bijugum containing accessions in which all plants were completely resistant. Resistant accessions from C. echinospermum and C. reticulatum may offer accessible sources of resistance because fully fertile hybrids with cultivated chickpea can be readily obtained.

Prospecting for sources of resistance to ascochyta blight in wild Cicer species

Early diagnosis of seedborne fungal pathogens is particularly important, as often, infected seeds appear symptomless; seed diagnosis can avoid uncontrolled propagation of pathogens through long-distance exchange of such material. This will prevent economic losses and unnecessary use of fungicides, so reducing costs and the introduction of toxic substances into the environment. Traditional techniques for detection of seedborne fungi are based on incubation and grow-out methods. Although these are frequently used because of their simplicity of application, they are time-consuming, require mycological skills, and are sometimes not sensitive enough to low levels of seed infection. Recently, new identification techniques, based on DNA analysis, have been applied and are very efficient due to high sensitivity and specificity. The most common technique is conventional PCR, while other recent techniques include nested PCR, to obviate low levels of target pathogens, multiplex PCR, to detect several pathogens simultaneously, real-time PCR, to quantify fungi on seeds, and magnetic-capture hybridization PCR. The main drawbacks of molecular methods are the inability to distinguish between vital and non-vital inocula, and the difficulty in obtaining quality DNA template, due to PCR inhibitors in seeds. To reduce inhibitory effects, several modified PCR protocols, such as loop-mediated isothermal amplification, and non-destructive testing methods have been developed. Loop-mediated isothermal amplification and next-generation sequencing have been widely applied in nucleic acid analysis and, given the numerous advantages provided, their application can be substantially extended in the future for detection of fungal pathogens in seeds.

https://bsppjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/ppa.12515

Diagnostic methods for detecting fungal pathogens on vegetable seeds.

Ascochyta blight, caused by Ascochyta rabiei, is a major constraint to chickpea production in Australia. A diagnostic test based on restriction fragment length polymorphism (RFLP) analysis of polymerase chain reaction (PCR) amplified DNA was developed to detect A. rabiei in infected leaves and seeds of chickpea. The test was developed by using primers designed to conserved sequences of the 18–25S ribosomal genes to amplify the internal transcribed spacer (ITS) regions of A. rabiei and other closely related Ascochyta species commonly found in pulses (A. lentis, A. pinodes and A. fabae) and identifying restriction enzymes that specifically cut the A. rabiei ITS region. The test was so sensitive that it detected DNA of A. rabiei at a quantity of 0.1 pg. The test also detected DNA extracted from infected leaf or seed in dilutions to 1×10−4 and 1 × 10−5, respectively, thus indicating that fungal genomic DNA can be specifically amplified by PCR from a plant genomic DNA background. This study also showed that the best method to detect chickpea seeds infected with A. rabiei was based on incubating surfacesterilised chickpea seeds in a liquid, fungal-growth medium (Czapek-Dox) for 18 h prior to the PCR and RFLP methods. This test was sensitive enough to detect a 1% infection.

A rapid and sensitive polymerase chain reaction (PCR) assay for detection of Ascochyta rabiei, the cause of ascochyta blight of chickpea

The ascochyta blight resistance of 14 chickpea cultivars, 29 imported chickpea lines and 38 local breeding lines to four Australian isolates of Ascochyta rabiei was investigated. Each isolate was tested individually on each chickpea genotype. There were no differences between the four isolates in their pathogenicity. However, there were large differences between chickpea genotypes in the expression of disease symptoms. All of the Australian chickpea cultivars tested were susceptible to A. rabiei. Seven imported lines and three local breeding lines were found to be resistant to A. rabiei. The eight isolates of A. rabiei tested in these studies were all the same mating type, i.e. MAT1-1.

Screening chickpea germplasm for ascochyta blight resistance

Botrytis grey mould, caused by Botrytis cinerea and B fabae, is a serious disease affecting lentil production in Australia The causal pathogens are known to survive on infected lentil trash and infected seed but the significance of these inoculum sources and the effectiveness of management recommendations to growers have been unclear Studies at Horsham in Victoria and Wagga Wagga in New South Wales, in 2005 and 2006, determined the length of survival of B cinerea and B fabae on infected lentil and chickpea trash under field conditions on the soil surface and at 10 cm soil depth The location of Botrytis spp infection on lentil seed was also determined Both B cinerea and B fabae survived for ~15 months on infected trash left on the soil surface and buried at 10 cm The majority of Botrytis infection on lentil seed was externally borne and surface sterilisation of seed significantly reduced the levels of Botrytis spp infection Recommendations to growers to manage old pulse crop trash and treat seed with a fungicide seed dressing will be effective in reducing the disease pressure for the Botrytis spp pathogen on lentil

T. W. Bretag

Papers

A rapid and sensitive polymerase chain reaction (PCR) assay for detection of Ascochyta rabiei, the cause of ascochyta blight of chickpea

Screening chickpea germplasm for ascochyta blight resistance

Survival of Botrytis spp. on infected lentil and chickpea trash in Australia