Isolation of bacterial strains from compost teas and screening of their PGPR properties on potato plants.

The beneficial effect of compost and compost tea on plant growth and protection is mainly associated with the microbial diversity and the presence of bacteria with plant growth-promoting effect. PGPR are considered as eco-friendly bio-fertilizers that may reduce the use of chemical pesticides and fertilizers. Three composts (AT, A10, and A30) were previously prepared from industrial wastes (olive mill wastewater, olive pomace, coffee ground, and phosphogypsum). In the present study, we isolated three bacterial strains from the compost teas. The phylogenetic identification of these bacterial strains (B.AT, B.A10, and B.A30) showed that they correspond to Serratia liquefaciens (B.AT and B.A10) and Achromobacter spanius (B.A30) species. A further characterization of the PGPR traits of these bacteria showed that they produce siderophore, exopolysaccharides, and IAA. Their effect on potato plant growth, yields, and tuber quality was performed under field culture conditions. Results showed that these strains can be characterized as PGPR, the best effect on potato plant growth was observed with Serratia liquefaciens (B.AT), the best yield and tuber quality was observed with Serratia liquefaciens (B.A10) while bacterial treatment with Achromobacter spanius (B.A30) is a Cd-tolerant PGPR.

Effect of phosphogypsum addition in the composting process on the physico-chemical proprieties and the microbial diversity of the resulting compost tea.

Phosphoric acid production and olive oil production are among the most important economical sectors in Tunisia. However, they generate huge amounts of wastes (phosphogypsum, olive mill waste water, and olive pomace). In a previous study, we used phosphogypsum (PG), in co-composting with organic wastes. Three composts were produced; their PG content was of 0 (AT), 10 (A10), and 30% (A30). In the present study, we focused on their derived compost teas. The physico-chemical characterization of the different compost teas showed that those from A10 and A30 composts presented higher P and Ca contents than that from control one (AT). The microbial characterization using DGGE showed a noticeable microbial diversity in the different compost teas and that the addition of 10% and 30% PG in the compost had different effects on the compost tea microbial diversity. The identification results showed that the addition of 10 and 30% of PG did not affect the presence of PGPR (plant growth-promoting rhizobacteria) and fungal soil antagonists in the compost teas. Two PGPRs were isolated from AT and A30 compost teas, and their effect on the growth of potato plants in vitro was evaluated.

Effect of compost tea containing phosphogypsum on potato plant growth and protection against Fusarium solani infection.

Three composts made of industrial wastes were prepared by mixing olive oil mill waste water (OMW), olive pomace, coffee grounds, and phosphogypsum (0, 10, and 30%). Potato plants (Solanum tuberosum) cultivated in a greenhouse were used to screen compost tea suppressive ability. All compost tea treatments inhibited Fusarium solani growth and improved plant growth and response to F. solani infection. The antagonistic effects of the different treatments were associated with a marked increase of the antioxidant enzymes and PR (pathogenesis related) protein expression and a decrease of disease severity. These results also showed that plant growth and disease suppression were improved by application of phosphogypsum-supplemented compost teas (A10 and A30). This enhancement can be attributed to the influence of phosphogypsum on nutrient elements and microbial diversity in the resulting compost teas.