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