Efficacy of Bacillus subtilis native strains for biocontrol of Phytophthora crown and root rot of pistachio in Iran

In Iran, Phytophthora crown and root rot of pistachio trees (also known as gummosis) destroys a significant number of fertile and non-fertile trees each year. To identify potential biocontrol agents effective in controlling pistachio gummosis, 200 soil samples were collected from 19 pistachio growing regions in Kerman province. Out of the 321 strains tested for antagonistic activity against Phytophthora pistaciae, 13 were selected as potential inhibitors of the disease. The tested strains were able to inhibit Phytophthora growth in dual culture, volatile and non-volatile assays by 14–72%, 12–68% and 27–85%, respectively. The highest inhibition was achieved by three strains identified as Bacillus subtilis using phenotypic characteristics, biochemical and physiological tests as well as sequencing the 16S rRNA genomic region for each strain. Co-inoculation experiments of six months old pistachio seedlings with P. pistaciae and the three selected B. subtilis strains reduced mortality rates by up to 80%. B. subtilis strain BSIPR35 was identified as the most promising biocontrol agent in greenhouse experiments. The ability of the selected strains to withstand drought, high temperature and salinity stresses was further tested. The growth of the strains was reduced under abiotic stresses ranging from 22 to 94%. All strains had the same growth under drought stress, while in salinity and under high temperature strain BSIPR35 acted superiorly compared to the other two strains. The bacterial strains may be effective in inhibiting gummosis in vivo, which requires further investigations.