(BIOLOGICAL CONTROL OF AGROBACTERIUM TUMEFACIENS (SCHMIT ET TOWNSEND) CONN ON CHRYSANTHEMUM WITH K84 AGROBACTERIUM RADIOBACTER (BEIJERINCK ET VAR DELDER) CONN STRAIN).

For several years, we could observe an increase of the disease known as crown-gall, caused by Agrobacterium tumefaciens ; important damages has sometimes been noted on some crops such as rosaceous fruit crops, chrysanthemum… Several facts seem to be at the origin of this increase which was aggravated due to the lack of really effective chemical or biological means of control (against that disease). Kerr, in 1972, proved that the K84 strain of the non-pathogen Agrobacterium radiobacter could protect young plants, offering then new perspective for the control of the disease. Numerous works on nursery crops conforted these observations. On chrysanthemum, Miller and Miller in the U.S.A. obtained good results, while Kerr in Australia did not succeed in the same way. Before recommanding the use of K84 for the control of Agrobacterium crown-gall, we thought it was necessary to precise some points, particularly the result of the treatment on some native strains and the behaviour of some chrysanthemum cultivars highly susceptible under natural conditions.

The trials done with four cultivars gave strong evidence for the efficiency of the treatment of cuttings when immerged in a suspension of strain K84 just before putting them in a substrate previously infected with a pathogen strain. The five pathogen strains, which of two were isolated from chrysanthemum have shown some susceptibility to Kerr's strain. Among 880 cuttings, 97.5 per cent of the untreated plants showed crown-gall symptoms versus 63.6 per cent for the treated ones. Nevertheless, this method can only be used as a preventive one.