Screening, cultural and biochemical characterization of coffee bacterial blight (Pseudomonas syringae pv. garcae van Hall) isolates from Sidama and Gedeo zones, SNNP Regional State, Ethiopia

Abstract

Coffee (Coffea arabica L.) is one of the most important commodity crops and plays a vital role in cultural and socio economic life in Ethiopia. Bacterial blight of coffee (BBC) caused by the phytopathogenic bacterium, Pseudomonas syringae pv. garcae van Hall is an important disease of Arabica coffee in Ethiopia due to its increasing incidence and severity. There is no information on the current status and the characterization of the pathogen in this study area in Ethiopia. Therefore, this study was carried out to assess the disease intensity, cultural and biochemical characterization of BBC isolates from Sidama and Gedeo zones, SNNP Regional State, Ethiopia and the variations within P. syringae pv garcae isolates from six different districts viz. Wensho, Aletawondo, Aletachuko, Dara, Dilla and Wonago of the study zones were carried out. A total 204 coffee trees, 96 coffee fields or peasant associations in 6 districts and in 2 coffee producing zones in SNNP Region was surveyed during the study time. The distribution map was done by JPEG map created using Arc GIS 10.0 map data of GPS reading. The frequency and intensity of BBC disease was varied between zones and the districts of coffee producing surveyed areas. Bacterial blight of coffee disease found in all assessed coffee localities that have been being posing considerable coffee tree losses. The percentage of disease incidence (70.0, 56.1, 44.6, 39.3%) and the severity (29.9, 15.7, 13.7, 12.5%) were recorded in Wensho, Dara, Aletachuko, Aletawondo districts of Sidama zone and the incidence (72.2 and 47%) and severity (21.6 and 13.1 %) were recorded in Dilla and Wonago areas of Gedeo zone, respectively. The prevalence of BBC disease was recorded in both Sidama and Gedeo zones, 87.5 and 93.8%, respectively while the mean BBC disease prevalence was 90.7%. The intensity of BBC disease also varied between different altitudinal categories; as altitude increases both incidence and severity were increased and at the lowest altitude  ranges between 1625 and1725 m. a. s. l, both disease incidence and severity were recorded lowest. Symptomatic of 204 diseased samples were initially isolated and purified on nutrient agar (NA) and nutrient broth (NB) slants. The growth of bacterial colonies showed round colonies with 2-3mm in diameter and light cream with entire margins, dome shaped, shiny, smooth and mucoid on 5% NAS. A yellow supernatant fluid was observed when grown on 5% NAS media. The gram staining method revealed that the test isolates showed rod shaped and gram negative bacteria. Among 204 isolates, 37 BBC isolates were selected for biochemical tests based on similar morphological and in growth characteristics on selective media. All the BBC isolates were consistently gave similar positive results of catalase, KOH solubility, gelatin liquefaction, tween 80 hydrolysis and Levan production and negative in Arginine dihydrolase. The biochemical test results indicated that, 16 isolates showed negative reaction or failed to produce the dark colour to the hydrolysis of β-glucosidase activity. Hypersensitive and pathogenicity test response was conducted on tobacco leaves in which all the isolates induced the hypersensitive reaction and confirmed. The morphological, cultural and microscopic characters, gram stain, β-glucosidase activity and biochemical nature of the test isolates confirmed the organism identity as Pseudomonas syringae pv. garcae. The present study provided first information on in Gedeo and Sidama zones is on threatening at present, unless otherwise taking appropriate measurement which solves this problem, otherwise, the country will be lost foreign currency as a result, the life standard of farmers will be collapsed and leading to lost foreign income.