Pathogenic Variability and Identification of Resistance for Ascochyta Blight of Chickpea in Pakistan

Abstract

Chickpea (Cicer arietinum L.) belongs to the family Fabaceae is the most important pulse crop of Pakistan, which is grown as a post-monsoon cool-season crop. It is infected by many diseases but blight caused by Ascochyta rabiei (Pass.) Lab. is considered to be the most devastating disease. During field survey, it was observed that blight was a common disease in the major chickpea growing areas of Punjab and NorthWest Frontier Province (NWFP). The maximum disease prevalence, incidence and severity was found in the districts of Rawalpindi in Punjab whereas in the district of Karak in NWFP. The lowest disease prevalence was recorded in Jhang (Punjab), and in Dera Ismail Khan (NWFP). Disease prevalence was the highest in the northern submountainous region where rainfall and temperature were conducive for disease development during crop growing season. Forty two isolates of A. rabiei representing all areas of chickpea of Punjab and NWFP were collected for pathogenic variability. The isolates exhibited variation in morphological and cultural characteristics. Two clusters were observed using UPGMA (unweigthed pair group method average) that was able to separate A. rabiei isolates on the basis of aggressiveness. The virulent isolates gave same intensity of infection, whereas others were observed with varying degrees of infection. Multivariate analyses were able to distinguish isolates on the basis of virulence rather than origin or morphological/cultural characterization. The susceptible differentials (C727, ILC 263, C 44 and CM 72) were identified but no variety could be established as resistant that might be due to complex nature of A. rabiei. Clustering pattern indicated the exchange of breeding material and disease cultures among the researchers or high heterogeneity in the isolates. Two isolates of A. rabiei representing the most aggressive and the least aggressive revealed variations in their pathogenic reaction. Significant differences among chickpea genotypes were observed for their response to isolates regarding disease development. Five varieties, C 727, C 44, Noor 91, Punjab 91 and lLC 263 exhibited high degree of susceptibi lity. Two genotypes. Dasht and Balkasar showed resistance to both the isolates when applied either alone or mixed in 1: I ratio. The aggresSiveness of the mixture of these two isolates was reduced to the level of the least aggressive isolate. The relationship of disease factors such as inoculum potential, phmt age. effect of leaf wetness and latent period with blight pathogenesis, revealed a linear relationship between disease severity and inoculum concentration. Inoculation of chickpea cultivars; Punjab 91 and C 727 at different physiological stages from 2 to 12 weeks (the seedling to the reproductive stage) revca led that 2-wccks old sccdli ngs were more suscept ible to disease than the adult plants. This suggests to screen large nurseries at seedl ing stage and then only tolerant lines to screen under field conditions. Effect of leaf wetness and incubation period on the disease development revealed tlml 2-3 days incubation period coupled with 1-2 sprays a day had a pronounced eITect on disease development. In order to identify the sources of genetic resistance to blight, 824 chickpea gemlplasm accessions were obtained during 1994 to 1996 from national and intemational research sources and were evaluated under greenhouse and field conditions for three years. None of the genotypes was resistant at pod fonnation stage during all the three years except thirty seven du.ring 1994, seventy two during 1995 and twenty one during 1996, which were moderately resistant. Chickpea genotypes, ILC 72, fLC 3279, ICC 3996 and NlFA 88 were found highly resistant both under field as well as greenhouse conditions. Disease at seedling and pod fonnation stage exhibited high association although level of infection was higher at pod fonnation stage. A relationship of morphological traits viz; number of hairs on dorsal and ventral sides of leaves, number and size of stomata, guard cells and stomatal aperture with blight resistance of six chickpea cultivars having different level of blight tolerance was studied. No relationship oflhese morphological traits with resistance was found. An experiment was conducted to detcnnine the relationship of chickpea genotypes towards blight disease reaction on the basis of biochemical markers (seed protein) of chickpea. Seed pmteins were analyzed through slab type SDS-PAGE using 11.25% Polyacrylamide gel and 6 III of sample quantity, Most of the genotypes were grouped 011 the basis of disease reaction and in-vitro fungus growth but no association between disease and SOS-PAGE was observed. Out of twelve 80S-PAGE markers. 6 were polymorphic. The genotypes with similar banding patterns can to be lested by 2-D electrophoresis and DNA markers. Cluster analysis revealed mixed grouping of susceptible and tolerant genotypes that indicated no response for classi fying chickpea for disease reaction on the basis of SOS-PAGE. A low level of genetic diversity was observed among 57 genotypes although those originated from diverse sources. As 80S· PAGE alone did not exhibit high level of variation and disease rating was more reliable than protein peptides, but simultaneous study for both aspects (d isease and biochemical analysis) suggested. Seven eultivars ofehiekpea (C 44, C 727, CM-72, Dasht, Parbat, NlFA 88 and Punjab 91) were tested to study the genotypic response to Ascochyta blight and yield losses. Disease severity index of the cultivars ranged from 44 to 82%. Maximum disease at vegetat ive stage was recorded on C 727 followed by C 44 and Punjab 91. Minimum disease at vegetative slage was observed on Dasht. Pod infection varied from 17 to 90% and was the highest on C 727 and least on Dasht. Minimum (2%) and maximum (42%) seed infection was in NlFA 88 and C 727, respectively. Comparison of data on pods per plant, seeps per plant, 1 OO~ seeds weight, yield per plant and yield per ha. from healthy and diseased conditions revealed that the disease caused more losses to C 727, C 44 and Punjab 91, whereas Dash! and NIFA 88 were moderately resistant to blight with minimum loss of yield and yield components.