Abstract
Pseudomonas fluorescens FPH9601 (referred to as FPH) is known to suppress an important soil-borne disease, tomato crown and root rot caused by Fusarium oxysporum f. sp. radicis-lycopersici. However, the effect of FPH on disease suppression is unstable. To evaluate the effect of FPH on tomato crown and root rot and to clarify the cause of the instability, appropriate conditions for disease suppression and the effect of the FPH treatment on the invasion of Fusarium into tomato roots were examined. The FPH treatment was effective against the pathogen at up to 10^7 cells per milliliter based on several disease parameters, i.e., prevalence, severity based on prevalence, extent of browning of vessels, and extent of root rot. The invaded sites in tomato roots were as few as 0.4/cm root after 30 days in the FPH-treated seedlings, while one or more invasion sites per centimeter were observed in untreated seedlings during the 10day period after inoculation. The number of infectious Fusarium microorgamisms in tomato roots tended to be lower in the FPH-treated seedlings than in the untreated seedlings at all time points studied. In the field test, the first occurrence of the disease in the untreated plots was recognized at about three weeks after transplantation. The number of infected plants subsequently increased. On the other hand, the initial occurrence in the plots treated with FPH9601 or biopesticide containing FPH9601 were at about seven weeks after transplantation. Furthermore, subsequent disease development was also slow in the plots with FPH9601, indicatetly that FPH has an effect to retard the disease occurrence in the field.
