Characterization of Streptomyces lydicus WYEC108 as a potential biocontrol agent against fungal root and seed rots

The actinomycete Streptomyces lydicus WYEC108 showed strong in vitro antagonism against various fungal plant pathogens in plate assays by producing extracellular antifungal metabolites. When Pythium ultimum or Rhizoctonia solani was grown in liquid medium with S. lydicus WYEC108, inhibition of growth of the fungi was observed. When WYEC108 spores or mycelia were used to coat pea seeds, the seeds were protected from invasion by P. ultimum in an oospore-enriched soil. While 100% of uncoated control seeds were infected by P. ultimum within 48 h after planting, less than 40% of coated seeds were infected. When the coated seeds were planted in soil 24 h prior to introduction of the pathogen, 96 h later, less than 30% of the germinating seeds were infected. Plant growth chamber studies were also carried out to test for plant growth effects and for suppression by S. lydicus WYEC108 of Pythium seed rot and root rot. When WYEC108 was applied as a spore-peat moss-sand formulation (108 CFU/g) to P. ultimum-infested sterile or nonsterile soil planted with pea and cotton seeds, significant increases in average plant stand, plant length, and plant weight were observed in both cases compared with untreated control plants grown in similar soils. WYEC108 hyphae colonized and were able to migrate downward with the root as it elongated. Over a period of 30 days, the population of WYEC108 colonized emerging roots of germinating seeds and remained stable (105 CFU/g) in the rhizosphere, whereas the nonrhizosphere population of WYEC108 declined at least 100-fold (from 105 to 103 or fewer CFU/g). The stability of the WYEC108 population incubated at 25 degrees C in the formulation, in sterile soil, and in nonsterile soil was also evaluated. In all three environments, the population of WYEC108 maintained its size for 90 days or more. When pea, cotton, and sweet corn seeds were placed into sterile and nonsterile soils containing 106 or more CFU of WYEC108 per g, it colonized the emerging roots. After a 1-week growing period, WYEC108 populations of 105 CFU/g (wet weight) of root were found on pea roots in the amended sterile soil environment versus 104 CFU/g in amended nonsterile soil. To further study the in vitro interaction between the streptomycete and P. ultimum, mycelia of WYEC108 were mixed with oospores of P. ultimum in agar, which was then used as a film to coat slide coverslips. After 6 h of incubation of these preparations in staining jars at 25 degrees C, direct interactions between the microorganisms were visualized by scanning electron microscopy. Results showed that WYEC108 was capable not only of destroying germinating oospores of P. ultimum but also of damaging the cell walls of the fungal hyphae. These results show that S. lydicus WYEC108 is potentially a potent biocontrol agent for use in controlling Pythium seed and root rot.