Screening of soil bacteria from cotton cultivated fields reveals actinobacteria as the main group of isolates with antagonistic activity against the oomycete Pythium ultimum in vitro

The genus Pythium comprises approximately 120 species of oomycetes that inhabit diverse niches, ranging from terrestrial ecosystems to saltwater estuaries. While many species are strict soil saprophytes, others, such as P. ultimum, act as plant pathogens, causing diseases in a wide range of economically important crops. Current control measures for P. ultimum primarily rely on chemical applications to seeds and crops, as well as the use of resistant cultivars. However, the increasing emphasis on environmental conservation has led to the exploration of more sustainable agricultural alternatives, including biological control practices. In this study, we screened soil bacteria from cotton fields to identify isolates with controlling activity against this oomycete. Out of 100 bacterial isolates recovered, no effective antagonistic activity was observed among strains belonging to the genera Bacillus or Pseudomonas. The majority of controlling isolates were identified as various strains of actinobacteria, exhibiting distinct macroscopic characteristics and strong inhibition of P. ultimum growth. These actinobacterial strains caused significant macroscopic alterations in the oomycete mycelium, resulting in reduced density of its aerial structures. These promising findings highlight the potential of actinobacterial strains as biocontrol agents against one of the most problematic soil-borne plant pathogens, offering a viable alternative to chemical interventions in agriculture.