Variability in Fusarium oxysporum f. sp. ciceris for Chickpea Wilt Resistance in Pakistan

Abstract

Chickpea (eiCer arietinum L.) is the most important pulse crop among the major Rabi pulses of Pakistan. Fusarium wilt which is caused by Fusarium oxysporum f. sp. ciceris is considered to be the most devastating disease of this crop. Survey for the assessment of chickpea wilt disease was conducted in February - March 2005 and 2006 of major chickpea growing areas of Pakistan. During this survey, maximum disease prevalence (92.58 ± 3.69), incidence (82.52 ± 5.14) and severity (7.00 ± 0.48) was recorded in the districts Khushab, Bhakkar, Layyah, Mianwali and Sargodha. The lowest disease prevalence (29.70 ± 3.19), incidence (9.50 ± 1.72) and severity (2.1 ± 0.10) was recorded in districts Jafarabad, Dera Allah Yar, Attock, Peshawar, Rawalpindi / Islamabad. A strong correlation between disease incidence and climatic conditions in all districts was observed. Pathogenesis studies of these isolates were done in order to correlate epidemiological factors of chickpea wilt. Studies showed that seedling mortality was minimum at 3g inoculum concentration and maximum at 20g. Similarly, seedling mortality was maximum in sandy soil and minimum in clayey soil. Whereas, at low level of soil moisture disease severity was maximum and at high level it was minimum. The effect of disease development on the sap extract of leaves and roots showed maximum growth on root sap then leaves. In physiological studies the fungus grew best on Waksman Agar (WA) medium and least on Potato Dextrose Agar (PDA) medium. Glucose was found to be the best and mannitol least carbon source. Similarly potassium nitrate (KN03) was the best and ammonium sulphate (NH4)2S04 was least nitrogen source. Growth of F. oxysporum was maximun1 at 30°C after 7 days of incubation, which reduced drastically below and above this temperature. Similarly, the fungus growth was best at pH 7; below and above this pH it was reduced. In case of pathogenic variability of 10 chickpea genotypes the most virulent isolate was Foc-15 (BalU1U) and the least virulent was Foc-17 (Fateh-Jang) from 27 F. oxysporum f. sp. ciceris isolates. The remaining isolates showed intermediate response of variation in virulence. Cultivar ICCV2 (1±0.0) was found resistant and AUG-424 (9.0±0.0) susceptible xv against all the isolates. On morphological and cultural basis the isolate Foc-9 (90 mm) was found fast growing whereas, isolate Foc-22 (39 mm) was slow growing. The rest of isolates remained intermediate. The biggest size of the micro-conidia (3.7 x 4.5 ~m) was obtained from the isolate Foc-14 and the smallest size (2.7 x 4.7 ~m) from isolates Foc-2. Similarly the largest macro-conidia were obtained from the isolates Foc-6 (6.5 x 17.5 ~m) and the smallest from isolates Foc-25 (7.5 x 20.1 ~m) . Maximum dry mycelium weight was obtained from isolate Foc-21 (1.56 g) and minimum from Foc-22 (0.31 g). Sensitivity of F. oxysporum f. sp ciceris isolates to thirteen fungicides @ 100 ppm was observed. Benlate was found most effective fungicide against all the isolates in suppressing the fungal growth under test. The least effective fungicide were Captan and PCNB. The rest of fungicides showed intermediate response. In case of mycelial compatibility grouping out of 350 combinations only 91 combinations showed compatible reactions, whereas, 259 were identified as non-compatible. To identify genetic sources of resistance against wilt disease under artificial disease conditions in three consecutive years (2005, 2006 and 2007), 553 germplasm lines were screened. Disease observations were recorded from seedling to reproductive stages. At seedling stage, 233 genotypes were found resistant, 102 tolerant and 218 susceptible. Whereas, at reproductive stage 125 lines were resistant, 53 tolerant and 375 were observed susceptible. To evaluate the genetic relationship in the morphological traits and RAPD markers genetically diverse 38 chickpea genotypes were selected, which provided ample opportunity to create favorable gene combinations. 35 Random amplified polymorphic DNA (RAP D) markers were used to assess genetic diversity in comparison with agromorphological traits. Agro-morphological traits have given significant results among genotypes. Days to 50% flowering ranged from 92-112, plant height ranged from 54.2- 87cm, Primary and secondary branches per plant varied from 1.4-6 and 4-20, pods per plant varied from 8-34, whereas grain yield per plant ranged from 3.47-10.2g. Disease was scored against the test genotypes, which, was recorded on 1-9 rating scale. The cluster analysis for the phylogenetic relationship among genotypes on the basis of agromorphological traits revealed three distinct groups at 92% similarity level.