Isolation and characterization of Gibberella xylarioides (coffee wilt disease) and antagonistic effects of rhizobacteria on the pathogen

Abstract

Although Arabica coffee is the backbone for Ethiopian economy, its production is being challenged by several biotic factors. Coffee Wilt Disease (CWD) caused by Gibberella xylarioides is among those biotic factors that reduce coffee yield in small scale farmers’ fields. Thus, this study was designed to isolate, characterize G. xylarioides, and evaluate antagonistic effects of rhizobacterial antagonists against the pathogen under in vitro and in vivo conditions. The study was conducted in the field, laboratory and greenhouse condition. The disease assessment and sample collection were conducted in three locations of small scale farmers’ coffee fields in Gera, Limu kossa and Gimbie localities of southwestern Ethiopia. The isolation of pathogen, its response to rhizobacterial antagonists was using suitable culture media. The greenhouse antagonism study was conducted with four antagonistic bacteria of one Bacillus (JU544) and three Pseudomonas spp.(JU941, JU23and JU13) which performed best under in vitro with three inoculation periods in Complete Randomized Block Design (RCBD) replicated three times. The CWD was prevalent in all assessed small scale farmers’ coffee fields posing a significant yield loss with its highest incidence at Gera (9.9 to 37.4%) and the least CWD incidence at Gimbie (5.2 to 18.4 %). In Limu kossa, the CWD incidence was ranging from 5.1 to 22.2%. Based on cultural and morphological characterization of G. xylarioides populations, most isolates from Gera, Limu Kossa and Gimbie had different appearances in colony pigmentation, colony density and aerial mycelium growth. Both Gera and Limu Kossa isolates were slightly different in their radial growth compared to Gimbie isolates. Morphologically, most of the isolates of G. xylarioides had nearly similar conidial size of both micro and macro conidia. Out of 81 rhizobacterial antagonists tested on Half Strength King’s B (HSKB) medium against G. xylarioides,13.6% of them significantly (p<0.0001) reduced the radial mycelial growth of the pathogen. From 11 rhizobacterial isolates tested for their phytobeneficial traits, eight of them produced protease. While five of them produced Hydrogen cyanide (HCN) and chitinase enzymes; seven of them produced lipase enzyme. The bio-control agents, time of applications and the interaction of the two were significantly (p<0.0001) reduced the CWD severity and incidence. The CWD control efficiency was significant (p<0.0001) and the highest bio-control efficiency was 72.64% when the coffee seedlings were treated with Bacillus spp. (JU544) seven days before the pathogen. The bacterial antagonists, time of applications and interaction of the two significantly (p<0.001) reduced the progression of CWD incidence (AUDPCi). The rhizobacterial antagonists especially Bacillus spp. (JU544) effectively reduced CWD severity and incidence under greenhouse condition. Therefore, field trials may ascertain their future applicability for inoculum development. Hence, the efficient antagonistic bacterial isolates should be re-evaluated under field conditions across different environmental regions