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 a ntagonism against various fungal plant pathogens in plate assays by pr oducing 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 o r mycelia were used to coat pea seeds, the seeds were protected from i nvasion by P. ultimum in an oospore-enriched soil. While 100% of uncoa ted control seeds were infected by P. ultimum within 48 h after planti ng, 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 gr owth chamber studies were also carried out to test for plant growth ef fects and for suppression by S. lydicus WYEC108 of Pythium seed rot an d root rot, When WYEC108 was applied as a spore-peat moss-sand formula tion (10(8) CFU/g) to P. ultimum-infested sterile or nonsterile soil p lanted with pea and cotton seeds, significant increases in average pla nt stand, plant length, and plant weight were observed in both cases c ompared 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 coloni zed emerging roots of germinating seeds and remained stable (10(5) CFU /g) in the rhizosphere, whereas the nonrhizosphere population of WYEC1 08 declined at least 100-fold (from 10(5) to 10(3) or fewer CFU/g). Th e stability of the WYEC108 population incubated at 25 degrees C in the formulation, in sterile soil, and in nonsterile soil was also evaluat ed, In all three environments, the population of WYEC108 maintained it s size for 90 days or more, When pea, cotton, and sweet corn seeds wer e placed into sterile and nonsterile soils containing 10(6) or more CF U of WYEC108 per g, it colonized the emerging roots, After a 1-week gr owing period, WYEC108 populations of 10(5) CFU/g (wet weight) of root were found on pea roots in the amended sterile soil environment versus 10(4) CFU/g in amended nonsterile soil, To further study the in vitro interaction between the streptomycete and P. ultimum, mycelia of WYEC 108 were mixed with oospores of P. ultimum in agar, which was then use d as a him to coat slide coverslips. After 6 h of incubation of these preparations in staining jars at 25 degrees C, direct interactions bet ween the microorganisms were visualized by scanning electron microscop y. Results showed that WYEC108 was capable not only of destroying germ inating oospores of P. ultimum but also of damaging the cell walls of the fungal hyphae, These results show that S. lydicus WYEC108 is poten tially a potent biocontrol agent for use in controlling Pythium seed a nd root rot.