The causal agent of Ascochyta blight disease of chickpea (Cicer arietinum L.) is highly variable because of the presence of a sexual phase (Didymella rabiei). There is also selection pressure on the pathogen due to wide adoption of improved resistant chickpea cultivars in some countries. The pathogen is able to produce pathotypes with specific virulence on particular cultivars. Three pathotypes, I, II, and III, have been reported (3). In this study, we confirmed the presence of a new and highly virulent pathotype that we designate as pathotype IV. To test the pathogenicity of the isolates collected and maintained at ICARDA, 10 isolates representing a wide spectrum of pathogenic variation, including those classified by S. M. Udupa et al. (3) and a putatively identified more virulent type, which was collected from a chickpea production field in the Kaljebrine area, Syria, were inoculated onto a set of differential chickpea genotypes. The differential genotypes, ILC 1929, ILC 482, ILC 3279, and ICC 12004, were sown in individual 10-cm-diameter pots containing potting mix and arranged in a randomized block design with three replications in a plastic house maintained at 18 to 20°C. Each differential genotype was inoculated individually with the 10 isolates following the methodology of S. M. Udupa et al. (3). DNA was extracted from single-spored isolates to compare the genotypes of the isolates using three simple sequence repeat (SSR) markers (ArA03T, ArH05T, and ArH06T) (2) and to determine the frequency of mating types (MAT) through the use of MAT-specific PCR primers for MAT1-1 and MAT1-2 (1). Host genotype reactions were measured on a 1 to 9 rating scale (1 = resistant and 9 = plant death). On the basis of the pathogenicity tests, the isolates were classified into four pathotypes: I (least virulent, killed ILC 1929 but not ILC 482, ILC 3279, or ICC12004); II (virulent, killed ILC 1929 and ILC 482 but not ILC 3279 or ICC12004); III (more virulent, killed ILC 1929, ILC 482, and ILC 3279 but not ICC12004); and IV (highly virulent, killed all four host differentials). Of 10 single-spore isolates tested, four showed similar disease reactions unique to pathotype I, four revealed pathotype II reactions, and one isolate each behaved like pathotype III or pathotype IV. SSR fingerprinting of these isolates provided evidence for genetic diversity since SSR ArH05T was highly polymorphic and amplified five bands, including pathotypes III- and IV-specific bands, which need further investigation to discern if this locus has any role to play in the virulence. MAT-type analysis showed that seven isolates were MAT1-1 while the remaining three isolates were MAT1-2. Only pathotype I showed the profile of MAT1-2 and the other three pathotypes were MAT1-1. Initially, a number of chickpea wild relatives were screened to identify sources of resistance to pathotype IV, but none of the accessions tested showed resistance. However, efforts are underway to combine minor and major gene(s) available in the breeding program in addition to a further search of the wild gene pools to control pathotype IV.
References: (1) M. P. Barve et al. Fungal Genet. Biol. 39:151, 2003. (2) J. Geistlinger et al. Mol. Ecol. 9:1939, 2000. (3) S.M. Udupa et al. Theor. Appl. Genet. 97:299, 1998.