EVALUATION OF BACTERIAL ISOLATES FROM SALTY SOILS AND BACILLUS THURINGIENSIS STRAINS FOR THE BIOCONTROL OF FUSARIUM DRY ROT OF POTATO TUBERS
N. Sadfi, M. Chérif, I. Fliss, A. Boudabbous, H. Antoun
A total of 83 spore-forming bacteria belonging to the genus Bacillus was isolated from Tunisian salty soils. These isolates as well as five additional strains of Bacillus thuringiensis, previously selected for their efficiency against insects, were tested in vitro and in vivo against Fusarium roseum var. sambucinum, the causal agent of dry rot of potato tubers. Results of the in vitro dual culture screening revealed that more than 50% of Bacillus spp. isolated from salty soils inhibited the growth of the pathogen in vitro. By contrast, all five B. thuringiensis strains failed to inhibit the growth of the pathogen in vitro. On wounded potato tubers, the most effective isolates obtained from salty soils were X7, X9, X16, I32 and G7, with a percentage of dry rot reduction ranging from 66 to 89%. These effective Bacillus isolates were identified as belonging to one of the species B. cereus (X9, X16 and G7), B. lentimorbus (X7) or B. licheniformis (I32). Although ineffective in vitro, B. thuringiensis strains inhibited dry rot development in vivo, with percentage inhibition scores ranging from 41 to 52%. While Bacillus isolates selected from salty soils best inhibited dry rot development when applied as young cultures (24 h), B. thuringiensis strains generally performed better as older cultures (48-72 h). The cell-filtrates of Bacillus spp. were unable to inhibit the growth of Fusarium. By contrast, volatiles liberated by the antagonists seem to contribute to the inhibition of the pathogen. The two isolates X16 of B. cereus and I32 of B. licheniformis as well as all 3 tested strains of B. thuringiensis (1T, 10T and 55T) were able to degrade colloidal chitin. Our experiments with the chromogenic chito-oligosaccharides indicated also that B. cereus (X16) and B. thuringiensis (55T) are able to produce N-acetyl-b-D-glucosaminidases, chitobiosidases and endochitinases. The isolate X16 of B. cereus consistently showed a chitinase activity 2 to 3 fold higher than that of strain 55T of B. thuringiensis. The hydrolysis of chromogenic chito-oligosaccharide analogs correlated well with the release of reducing sugars from chitin or the formation of clearing zones on chitin agar. The diversity and complexity of chitinases produced by our selected strains may contribute significantly to their antagonistic activity towards F. roseum var. sambucinum.