6.34
SPECIFIC INTERACTIONS BETWEEN FLUORESCENT PSEUDOMONAS SPP. STRAINS INFLUENCE EFFECTIVITY OF FUSARIUM WILT CONTROL BY COMBINATIONS OF THESE STRAINS
M DE BOER, L VAN DER SLUIS, LC VAN LOON AND PAHM BAKKER

Section of Plant Pathology, Department of Plant Ecology and Evolutionary Biology, Utrecht University, P.O.box 80084, 3508 TB Utrecht, The Netherlands

Background and objectives
Fusarium wilt diseases, caused by the fungus Fusarium oxysporum, can lead to significant yield losses of agricultural and horticultural crops. One strategy to control fusarium wilt is the use of antagonistic, root-colonizing Pseudomonas spp.. It has been demonstrated that these bacteria can be responsible for disease suppression occuring in fusarium wilt suppressive soils. Mechanisms by which disease can be suppressed by these pseudomonads are competition for substrate, siderophore mediated competition for iron, and induction of systemic resistance. Application of a mixture of these biocontrol strains, and thus of several suppressive mechanisms, may represent a viable strategy to control fusarium wilt. A prerequisite for effective disease control by combinations of biocontrol agents can be the compatibility of the co-inoculated micro-organisms[l]. Objective of this study was to determine whether in vitro interactions between fusarium wilt suppressing pseudomonads have predictive value for disease suppression by combinations of these strains.


Results and discussion
Compatibility between several Pseudomonas spp. strains, that suppress fusarium wilt of radish by different disease suppressive mechanisms, was tested in vitro on KB agar plates. Growth of P. fluorescens strain RS111 was strongly inhibited by Pseudomonas spp. strains RE8, RS13, RS56 and RS158, whereas a mutant of strain RS1 1 1 (RS111-a) is insensitive to inhibition by these strains. Strains RS111 and RS111 -a only slightly inhibited growth of the other strains. Suppression of fusarium wilt of radish in a potting soil bioassay by the incompatible combination of RE8 and RS111 was comparable to that obtained using the single strains. However, disease suppression by the compatible combination of RE8 and RS111 -a was significantly better as compared to the single strains. In contrast, the compatible combinations of RS13, RS158 with RS111-a did not result in a better disease suppression as compared to the single strains and the incompatible combinations. Surprisingly, the incompatible combination of RS56 and RS111 resulted in enhanced disease suppression whereas the compatible combination of RS56 and RS1 1 1 -a did not. So, in the combination of RE8 with RS111 in vitro compatibility seems to be a prerequisite for enhanced disease suppression whereas for the combinations of RS1 3 or RS1 58 with RS111 it is not. It is possible that RS13, RS158 and RS111 suppress disease by the same mechanism and therefore enhanced disease suppression by the compatible combination can not be expected. The incompatibility between strain RS56 and RS111 could lead to earlier expression of traits involved in competition and disease suppression and therefore result in enhanced disease suppression while this does not occur in the compatible combination RS56 + RS111 -a.
These results indicate that specific interactions between biocontrol strains can influence disease suppression by combinations of these strains. Currently the population dynamics and disease suppressive mechanisms of the strains RE8, RS56, RS111, RS111 -a and the combinations are under investigation.


References
1. Raaijmakers JM, Leeman M, Van Oorschot MMP, Van der Sluis 1, Schippers B, Bakker PAHM, 1995. Phytopathology 85,1075-1081.