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Accueil du site → Doctorat → Norvège → Studies on the Collectrichum sublineolum (P. Henn)-Sorghum bicolor (L. Moench) pahosystems in Ethiopia

Norwegian University of Life Sciences (2010)

Studies on the Collectrichum sublineolum (P. Henn)-Sorghum bicolor (L. Moench) pahosystems in Ethiopia

Chala, Alemayehu

Titre : Studies on the Collectrichum sublineolum (P. Henn)-Sorghum bicolor (L. Moench) pahosystems in Ethiopia

Auteur : Chala, Alemayehu

Université de soutenance : Norwegian University of Life Sciences, Ås

Grade : Doctoral thesis 2010

Résumé partiel
This thesis covers the assessment of anthracnose incidence and severity in Ethiopia, characterisation of Colletotrichum sublineolum isolates from different sorghum regions of Ethiopia, studying the impact of four selected host genotypes and weather variables on the severity and temporal dynamics of anthracnose, and an evaluation of Ethiopian sorghum accessions for resistance to anthracnose. For these purposes, field surveys were carried out, C. sublineolum isolates were characterised using phenotypic and molecular markers, and two separate field experiments were conducted. The field surveys were conducted in the cropping seasons 2005 and 2007, in 49 districts of six geographic regions. Anthracnose was observed in 41 (84%) of the surveyed districts but both disease incidence and severity varied significantly among the survey districts, geographic regions, altitude groups and climatic zones. Anthracnose incidence ranged from 0 to 77% while severity varied between 0 and 59% across the survey areas. Disease incidence and severity were significantly higher in the lowland (<1500 masl (meters above sea level)) and intermediate altitude areas (1500-2000 masl) as compared to the highlands (>2000 masl). Anthracnose severity was the highest (ca. 40%) in areas characterised by high rainfall (>1200 mm/annum) and intermediate temperature (16-30°C). Areas with moderate rainfall (800-1200 mm/annum) and intermediate or high (>30°C) temperature had a much lower anthracnose severity (<20%). Correlation analysis revealed a significantly positive relationship between rainfall and anthracnose development while temperature did not have a significant effect. In order to study the diversity within C. sublineolum from Ethiopia, sorghum leaves showing anthracnose symptoms were collected from five sampling sites (North, East, South, Southwest 1 and Southwest 2) in four geographic regions, and single spore isolates were categorised into five groups based on their sampling origin. For phenotypic characterisation 50 randomly selected single spore isolates (10 per sampling site) were cultured on PDA plates at 25ºC with four replications. There was significant variation among the isolates in growth rate (1.7-5.8 mm/day, P = 0.0023). However, colony colour and margin showed little variations. For molecular characterisation, amplified fragment length polymorphism analysis (AFLP) was conducted on 102 isolates using six primer combinations. The isolates were highly diverse as revealed by Dice similarity coefficients between individual isolates (0.32-0.96, mean 0.78), average gene diversity for each isolate-group (0.14-0.23, mean 0.19) and proportion of polymorphic peaks per isolate-group (44-68%, mean 58%). Pair-wise genetic distances, and cluster and principal coordinate analyses suggested the genetic separation of the Southern and Eastern isolates from the other groups and also from each other. Overall there was high genetic variation (FST = 0.42) and limited gene flow (Nm = 0.34), and hence such variations should be given due considerations in future sorghum breeding programmes.

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