EFFECTIVENESS OF FLUORESCENT PSEUDOMONADS ON CUCUMBER AND TOMATO PLANTS UNDER PRACTICAL CONDITIONS AND PRELIMINARY STUDIES ON THE MODE OF ACTION OF THE ANTAGONISTS

Three preselected fluorescent Pseudomonas isolates controlling dumping -off of cucumber seedlings caused by Pythium ultimum and Rhizoctonia s olani in laboratory experiments were tested as biological control agen ts tinder practical conditions. The studies were carried out in plasti c greenhouses of an organic horticultural farm (''Bioland''). In the t rials of 1995, cucumber plants (4 weeks old) of the cvs. 'Nadine' and 'Petita' were treated for the first rime immediately after transplanti ng in the plastic greenhouse by soil drenches with cell suspensions (5 0 or 100 ml, 2 x 10(9) cfu x ml(-1)) of three Pseudomonas strains WB1, WB15 and WB52. Soil drenches were repeated every 2 or 3 weeks. In pla nts treated with cell suspensions of strain WB1 and WB52, the number o f fruits per plant was increased. The comparative studies with the thr ee fluorescent Pseudomonas strains revealed that strain WB1 was superi or to the other two strains in its effectiveness. Because of its prope rties, strain WB1 may be grouped to the ''plant growth promoting rhizo bacteria'' (PGPR) or ''yield increasing bacteria'' (YIB). In the studi es of 1996, only strain WB1 was used in the practical trials with cucu mber and tomato plants. Severe disease incidence of damping-off caused by Pythium spp. occurred in cucumber plants cvs. 'Perita' (100 % dise ase incidence) and 'Nadine' (70 % disease incidence). The degree of pr otection provided by soil drenches of strain WB1 cell suspension was h igher in the less susceptible: cv. 'Nadine' than in the more sensitive cv. 'Petita'. Soil drenches applied immediately after transplanting r esulted in yield increases (number of fruits plant) of cvs. 'Petita' a nd 'Nadine' compared to plants created with WB1 cell suspension 8 days after transplanting or water-treated control plants. In trials with t he cucumber cvs. 'Bella', 'Aroma','Tyria' and 'Kalunga', soil drenches with WB1 proved to be effective in reducing the wilt disease caused b y Sclerotinia sclerotiorum. Soil drenches with strain WB1 cell suspens ions immediately after transplanting of tomato plants cv. 'Obst' (4 we rks old) in plastic greenhouse affected yield increase (number of frui ts per plant) as compared to control plants. Preliminary studies on th e mode of action revealed a marked stimulation of callose deposition i n cell wails of roots of cucumber seedlings in the presence of cells o f strain WB1.