1Hyogo Prefectural Agricultural Institute, Befu, Kasai 679-01, Japan; 2Taki Chemical Company, Befu, Kakogawa 675-01, Japan; 3Faculty of Agriculture, Kobe University, Nada, Kobe 657, Japan; 4Faculty of Agriculture, Kinki University, Nara 631, Japan.

Background and objectives
Recent agricultural technology is concerned with safety in human health and sustainability in natural ecosystems. It is desirable to use agricultural techniques that are harmonized with ecosystems in nature. It is a necessity for us to apply some measures to prevent soil-borne diseases since the presently used methods for control are often very hazardous to the environment. There are no agricultural chemicals that are effective for preventing such soil-borne diseases without polluting the environment. A new and effective method of biological control of bacterial wilt of tomato, caused by Pseudomonas solanacearum, was established by using two endophytic pseudomonads, Pseudomonas fluorescens FPT-9601 (FPT) and Pseudomonas sp. FPH-9601 (FPH).

Materials and methods
Two pseudomonads strains, FPT and FPH, were used as biocontrol agents for bacterial wilt of tomato. The two strains were selected by bioassay from 30,000 strains of fluorescent pseudomonads which were isolated from the endorhizosphere of tomato plants grown in a crop field infested with the tomato bacterial wilt pathogen Pseudomonas solanacearum. To prepare a plug mixture sample for growing tomato seedlings, Vermiculite and Red soils (a term of Japanese soil classification) were blended in a ratio of 2:1, respectively, and heated at 18C for 1 ;h. In addition, the bacterial strains, FPT and FPH, were prepared by the method of commensal culture in which sterilized roots were cultured with the bacteria. These propagated cells were mixed with Vermiculite-soil mixture by adding 20% (v/v) of the total mixture. Biocontrol trials in fields were carried out on 10 occasions at nine different locations in 1995 and 1996. The seedlings grown in the blended plug mixture containing FPT and FPH were transplanted into the fields. The mobility of FPT and FPH at roots of tomato seedlings and the occurrence of tomato bacterial wilt were investigated.

Results and conclusions
The antagonistic bacteria were easily incorporated into roots of young seedlings of tomato and colonized within the root tissues when the plants were grown in a plug mixture containing those bacteria. When the roots were endophytically colonized by FPT, the antagonistic strain formed colonies and crystals of 2,4-diacetylphloroglucinol were formed in and around the roots. When FPH colonized the tomato roots, antagonistic substances were exuded from the roots and suppressed growth of pathogen around the roots.

Protection values in field trials ranged from 31.3 to 86.8. The treatments with the antagonistic strains suppressed occurrence of tomato bacterial wilt in all field trials in both 1995 and 1996. The seedlings grown with the antagonistic bacteria showed evidence of tolerance to bacterial wilt, not only in laboratory tests, but also in fields of tomato in which bacterial wilt occurred. The antagonistic pseudomonads colonized endophytically the root tissues, possibly relating to the fact that actual activity of biological control by these strains was maintained for a long period.