Exploring Bacterial and Fungal Biodiversity in Eight Mediterranean Olive Orchards (Olea europaea L.) in Tunisia.

A wide array of bacteria and fungi are known for their association with pests that impact the health of the olive tree. The latter presents the most economically important cultivation in Tunisia. The microbial diversity associated with olive orchards in Tunisia remains unknown and undetermined. This study investigated microbial diversity to elucidate the microbial interactions that lead to olive disease, and the bio-prospects for potential microbial biocontrol agents associated with insect pests of economic relevance for olive cultivation in the Mediterranean area. Bacterial and fungal isolation was made from soil and olive tree pests. A total of 215 bacterial and fungal strains were randomly isolated from eight different biotopes situated in Sfax (Tunisia), with different management practices. 16S rRNA and ITS gene sequencing were used to identify the microbial community. The majority of the isolated bacteria, Staphylococcus, Bacillus, Alcaligenes, and Providencia, are typical of the olive ecosystem and the most common fungi are Penicillium, Aspergillus, and Cladosporium. The different olive orchards depicted distinct communities, and exhibited dissimilar amounts of bacteria and fungi with distinct ecological functions that could be considered as promising resources in biological control.

Identification of fungi in Tunisian olive orchards: characterization and biological control potential.

BACKGROUND: Olive production is the main agricultural activity in Tunisia. The diversity of fungi was explored in two different olive groves located in two distant geographical zones in Sfax (Tunisia) with different management practices. RESULTS: Fungal isolation was made from soil and the major olive tree pests, namely the Olive fly, Bactrocera oleae Gmelin (Diptera: Tephritidae), and the Olive psyllid, Euphyllura olivina Costa (Homoptera: Psyllidae). A total of 34 fungal isolates were identified according to their phenotypic, genotypic, biochemical and biological activities. Twenty fungal species were identified belonging to six different genera (Alternaria, Aspergillus, Cladosporium, Fusarium, Lecanicillium and Penicillium) by the analysis of their ITS1-5.8S-ITS2 ribosomal DNA region. Different bioassays performed in this work revealed that 25/34 (73.5%) of the identified fungal isolates showed an entomopathogenic and/or antagonistic activity, 9/34 (26.5%) of them displayed phytopathogenic features. CONCLUSIONS: Fungal species that showed entomopathogenic and/or antagonistic potentialities and that are non-phytopathogenic, (17/34; 50%) of our fungal isolates, could be explored for olive protection against fungal diseases and pests, and might have a future application as biocontrol agents.

Providencia entomophila sp. nov., a new bacterial species associated with major olive pests in Tunisia.

Bioprospection for potential microbial biocontrol agents associated with three major insect pests of economic relevance for olive cultivation in the Mediterranean area, namely the olive fly, Bactrocera oleae, the olive moth, Prays oleae, and the olive psyllid, Euphyllura olivina, led to the isolation of several strains of readily cultivable Gram-negative, rod-shaped bacteria from Tunisian olive orchards. Determination of 16S ribosomal RNA encoding sequences identified the bacteria as members of the taxonomic genus Providencia (Enterobacterales; Morganellaceae). A more detailed molecular taxonomic analysis based on a previously established set of protein-encoding marker genes together with DNA-DNA hybridization and metabolic profiling studies led to the conclusion that the new isolates should be organized in a new species within this genus. With reference to their original insect association, the designation "Providencia entomophila" is proposed here for this hypothetical new taxon.

Effect of olive fruit fly infestation on the quality of olive oil from Chemlali cultivar during ripening.

Olive fruits are seriously deteriorated by pre and post harvest damage due to the attack of insects, such as Bactrocera olaea, which strongly alters the quality of olives. Olives from Chemlali cultivar were collected and divided into different groups according to the presence or absence of infestation (0%, 5%, 10%, 15% and 100%) by the olive fruit fly (B. olaea). The infestation of this pest has been studied to evaluate its influence on virgin olive oil quality (free acidity, K(232) and K(270), fatty acids composition, pigment concentration, organoleptic characteristics, phenolic content, and antioxidant power). Results showed that both attacks by B. oleae and maturity process affected the quantitative and qualitative composition of the oil. These analyses demonstrated that the degree of fly attack was positively correlated with free acidity, the values of this parameter increase from 0.6 to 1.5 and 3.4, at the infestation levels 10%, 15% and 100%, respectively (R(2)=0.7418, P<0.05), and negatively related to the phenolic content which was always lower at the 100% infestation level (R(2)=0.9155, P<0.05), and consequently, the organoleptic characteristics. In addition, the infestation by olive fly did not cause an important change in the fatty acid composition while it is clear that fatty acids levels change in relation to the fruit repining stage. The antioxidant activity decreased during maturation, it was correlated to the total phenol content and the fruit infestation level.