Isolement de microorganismes à partir du sol des régions arides et sélection d’isolats à effet antagoniste sur l’agent de l’Alternariose

Abstract

Early blight is one of the most important diseases of tomato in eastern Algeria. Tomato samples exploration (leaves, stems and fruits) with symptoms of early blight, grown under glass in Hamma Bouziane locality (Constantine), allowed obtaining a filamentous fungal isolate (Alt 1). The morphological and molecular identification (DNA-ITS analysis) of this isolate revealed that it is Alternaria alternata (HQ846574.1). Pathogenicity testing of seedlings by isolate resulted in the challenge of the typical early blight disease with a severity of ≥ 76%. Samples exploration from various environments in the Algerian Sahara (Biskra and Tougourt) resulted in the obtaining of 45 microbial isolates capable of developing an effect on early blight agent. The selection test showed that; E1B3, S5 (isolated from Biskra) and E2X1, S1.1 (isolated from Tougourt) possess a considerable antifungal effect against A. alternata (inhibition rates vary between 60% and 75%), however, the E1B3 isolate developed the best effect. The morphological and molecular identification of the selected isolates by analysis of the 16S DNA and the "gyrase-A" gene showed that the bacterial isolates; E1B3, S5, and S1.1, are identified as Bacillus mojavensis (KC977492), Bacillus amyloliquefaciens (KC977493), and Bacillus Subtilis (KP699114.1) respectively, whereas the bacterial isolate E2X1 is identified as Providencia vermicola (KP059130.1). To the best of our knowledge, the isolation of P. vermicola from the palmerais rhizospheric soil and the demonstration of its antifungal potential are established for the first time in this work. The in vivo tests that carried out in pots and field conditions on tomato showed different effects according to the bacterial species, and B. mojavensis developed the best effect considering the most studied parameters (disease severity, Seedling size, leaves number and flowers number per seedling). The P. vermicola strain showed the greatest result on the severity reduction of early blight disease, indicating that the present work is the first one which revealed this effect. In contrast, B. mojavensis developed the best protective effect, considering the majority of the parameters studied in vitro and in vivo. This impact is due to the ability of this strain to produce antifungal substances in this case; the enzymes (lipase and protease) and the three families of lipopeptides (iturin, surfactins, and fengycins). The evaluation of the antagonistic effect developed by B. mojavenssvis versus A. alternata through flow cytometer (FCM) showed a considerable decrease in the rate of A. Alternata living cells, confirming the inhibitory effect of B. mojavensis against the pathogenic agent of early blight.