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3.Studies about Plant Disease Controlling Pseudomonas fluorescens HP72 and Development of Plant Disease Controlling Materials

Hiroshi Oyaizu(Graduate School of Agricultural and Life Sciences, The University of Tokyo)

Plant disease control by microorganisms will be very important in the 21st century because agricultural chemicals are causing serious environmental problems. Several years ago we isolated fluorescent Pseudomonas which has strong antifungal activity. We isolated approximately one thousands of fluorescent pseudomonads, and selected strains which showed the most highest affinity to the root of bent grass. Finally, we got Pseudomonas fluorescens strain HP72. When this strain was applied on the bent grass, the brown patch disease was completely controlled. By the RAPD-PCR we could isolate DNA fragment very specific to this strain, and we could enumerate the strain in the fields by the FIA-PCR method. The strain was very effective to control fungal diseases of gulf course turf. The strain can survive very long periods in the soil of golf course and ratio to the total fluorescent Pseudomonas was very high. However, the strain do not grow at higher temperature than 33℃, and therefore, in summer the population and ratio to the total fluorescent Pseudomonas came down. However, the population recovers automatically in autumn. In the field experiment, when the population of HP72 was high, the fungal diseases were almost completely controlled. An antifungal chemical was purified from the liquid culture and identified as 2,4-diacetylphloroglucinol (Phl). Then, we tried to reveal what causes disease control. We made four mutants which do not produce Phl on any kinds of agar media. Disease controlling activity was examined for the four mutants. In spite of inability of producing antifungal chemical, the four mutants could control the brown patch disease. Then, plant growth promoting activity was examined for the four strains. The four mutants showed plant growth promoting activity. Therefore, the plant growth promoting activity is more important for disease control than the antifungal chemical production. The survival of the strain HP72 was tested for other plants. The strain disappeared very quickly on the roots of egg plant and tomato. The strain survived longer on the roots of barley and wheat, but finally disappeared after four to five weeks. The affinity of plant roots of HP72 is considered to be restricted to bent grass.

Keywords: Pseudomonas fluorescens, plant root association, plant disease controlling bacteria