Dosis-Wirkungs-Beziehung und Populationsdynamik bei der Anwendung des Antagonisten Bacillus subtilis zur biologischen Bekämpfung von Fusarium oxysporum f. sp. dianthi

Summary For the biological control of the wilt pathogen on carnation, Fusarium oxysporum f. sp. dianthi , different technological variants of antogenist application with Bacillus subtilis T 99 were tested in comparison to untreated control and chemical treatments under greenhouse conditions. Against the pathogen of dianthus wilt, F. oxysporum f. sp. dianthi , occurring in greenhouse soil, the highly-active B. subtilis strain T 99 was obtained by screening. It was investigated for settlement, persistence, and infiltration in model experiments complying with practical conditions. A complex trial of 14 variants with different volumes and concentrations of antagonist suspension, based on practical conditions, culture media, sporulation rates, and application frequency was laid out. Antagonist efficiency could be statistically analyzed, because a high infection rate had been ensured by artificial infection and good infection and dispersions conditions. Until the 4th month of the trial nearly all T99 variants showed a clear decline in wilthing in comparison with the control. The variants with total doses of 20 and 40 ml T 99 suspension/m 2 substrate with a titer of 2 · 10 7 cfu/ml were most effective whilst overdosed and underdosed application at 160 and 10 ml/m 2 , respectively, caused higher infection as in the untreated control and chemical variants. From the 5th months until the end of the trial in the 6th month reduction in infection slowly decreased in the variants with the antagonist. Investigations of population dynamics revealed a high competitive ability of the introduced antagonist Bacillus subtilis T 99 in soil substrate, which had relatively constant titer over the 6-minth trial period. Only in the underdosed variants population decreased below 10 3 cfu/g soil. This turned out insufficient for fusarium control. Additional antagonist application favoured biological control only in the first 2–3 months. A 3rd application in the 4th month neither changed T 99 titer nor did it effect dispersal of the wilt pathogen. This indicates that B. subtilis is effective only in the first stages of carnation plants. The establishment of T 99 was, besides on dosis and application frequency, particulary dependent on the culture medium of the antagonist suspension. This resulted in different sporulation rates of T 99 in the culture broth. A high proportion of sporulating T 99 cells supports high survivability of the introduced organism and, therefore, a smaller dose is required for the pathogen control than in variants with lower sporulation rate of the culture medium. The effective antagonist titer of B. subtilis T 99 ranges from 10 3 to 10 5 cfu/g soil substrate.