title: Suppression of Rhizoctonia solani diseases of sugar beet by antagonistic and plant growth-promoting yeasts.
abstract: OBJECTIVE
  Isolates of Candida valida, Rhodotorula glutinis and Trichosporon asahii from the rhizosphere of sugar beet in Egypt were examined for their ability to colonize roots, to promote plant growth and to protect sugar beet from Rhizoctonia solani AG-2-2 diseases, under glasshouse conditions.
  
  
  RESULTS
  Root colonization abilities of the three yeast species were tested using the root colonization plate assay and the sand-tube method. In the root colonization plate assay, C. valida and T. asahii colonized 95% of roots after 6 days, whilst Rhod. glutinis colonized 90% of roots after 8 days. Root-colonization abilities of the three yeast species tested by the sand-tube method showed that roots and soils attached to roots of sugar beet seedlings were colonized to different degrees. Population densities showed that the three yeast species were found at all depths of the rhizosphere soil adhering to taproots up to 10 cm, but population densities were significantly (P < 0.05) greater in the first 4 cm of the root system compared with other root depths. The three yeast species, applied individually or in combination, significantly (P < 0.05) promoted plant growth and reduced damping off, crown and root rots of sugar beet in glasshouse trials. The combination of the three yeasts (which were not inhibitory to each other) resulted in significantly (P < 0.05) better biocontrol of diseases and plant growth promotion than plants exposed to individual species.
  
  
  CONCLUSIONS
  Isolates of C. valida, Rhod. glutinis and T. asahii were capable of colonizing sugar beet roots, promoting growth of sugar beet and protecting the seedlings and mature plants from R. solani diseases. This is the first successful attempt to use yeasts as biocontrol agents against R. solani which causes root diseases.
  
  
  CONCLUSIONS
  Yeasts were shown to provide significant protection to sugar beet roots against R. solani, a serious soil-borne root pathogen. Yeasts also have the potential to be used as biological fertilizers.
authors.affiliation: Department of Biology, Faculty of Science, United Arab Emirates University, Al-Ain, 17551, United Arab Emirates. ktarabily@uaeu.ac.ae
authors.forename: K A
authors.initials: KA
authors.lastname: El-Tarabily
mesh_terms: D000898:Antibiosis; D027461:Beta vulgaris / Q000254:growth & development / Q000382:microbiology*; D002175:Candida / Q000502:physiology; D010935:Plant Diseases / Q000382:microbiology*; D018517:Plant Roots / Q000382:microbiology; D012232:Rhizoctonia / Q000254:growth & development* / Q000302:isolation & purification / Q000472:pathogenicity; D012248:Rhodotorula / Q000502:physiology; D012988:Soil Microbiology; D014250:Trichosporon / Q000502:physiology