Abstract:
Arabica coffee is the key cash crop and top mainstay of the Ethiopian economy and requires
sustainable production methods. Southwestern natural forests, the site of this study, are
believed to be the centre of origin and diversity for Coffea arabica and still harbour wild
Arabica coffee that may serve as an important gene pool for future breeding. Cost reductions,
sustainability and quality improvement are now the major priorities in coffee production
systems and require organic growing of coffee. Current developments in sustainability
involve rational exploitation of soil microbial activities that positively affect plant growth and
this study examines this possibility. The composition of coffee shade tree species and density
of arbuscular mycorrhizal fungi (AMF) spores and coffee-associated rhizobacteria in different
coffee production systems in southwestern Ethiopia were investigated. The main objectives
were to: 1) systematically identify the dominant coffee shade tree species; 2) quantify and
characterize AMF populations with respect to spatial distribution; 3) screen for beneficial
rhizobacteria (microbial biofertilizers and biocontrol agents), particularly in the rhizosphere of
coffee plants; and 4) characterize rhizobacterial isolates of particular interest using molecular
tools (polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP)
analysis and 16S rDNA gene sequencing). Sampling and determination of microbial
functional characteristics followed standard methods. Nineteen dominant shade tree species
belonging to 14 plant families were identified, with the tree legume (Millettia ferruginea)
dominating. All soil samples contained AMF spores and members of the Glomeromycota,
Glomus spp. dominating. AMF spore density was affected by sampling point, site, depth,
shade tree species and shade tree/coffee plant age. Coffee-associated rhizobacterial isolates
showed multiple beneficial traits (phosphate solubilization, production of organic acids,
siderophores, indoleacetic acid, hydrogen cyanide, lytic enzymes and degradation of an
ethylene precursor). Many isolates also revealed a potent inhibitory effect against emerging
fungal coffee pathogens such as Fusarium xylarioides, F. stilboides and F. oxysporum. According
to in vitro studies Bacillus, Erwinia, Ochrobactrum, Pseudomonas, and Serratia spp. were the most
important isolates to act as potential biofertilizers, biocontrol agents or both. Thus, these
indigenous isolates deserve particular attention and further greenhouse and field trials could
ascertain their future applicability for inoculum development.
