2.7.13
EVIDENCE FOR THE ROLE OF GAEUMANNOMYCES GRAMINIS VAR. TRITICI IN THE INDUCTION OF SOIL SUPPRESSIVENESS TO TAKE-ALL ON WHEAT AND IN FLUORESCENT PSEUDOMONADS SELECTION

A SARNIGUET1, P LUCAS 1, A-Y GUILLERM 1, D DE VOS2 and P CORNELIS2

1 INRA, Centre de Recherches de Rennes, Station de pathologie végétale, BP 29, 35650 Le Rheu, France; 2 Flanders Inter-university Institute for Biotechnology, Vrije Universiteit Brussel, Paardenstraat 65, B-1640 Sint-Genesius-Rode, Belgium

Background and objectives
Take-all decline is the most famous example of soil suppressiveness induction in the same soil. The role of the soil microflora, especially antagonistic fluorescent pseudomonads, has been demonstrated but little is known about the mechanism of the soil suppressiveness build up. The high disease severity always observed before it decreases during wheat monoculture and the large build-up of fluorescent pseudomonads around root necroses suggest an important role of the pathogenic fungus in bacterial colonization of roots. In pot experiments, we compared the influence of introduced Gaeumannomyces graminis var. tritici (Ggt) and of several consecutive wheat plantings on soil suppressiveness induction; we also analysed the structure of fluorescent pseudomonad populations with molecular descriptors.

Results and conclusions
A highly conducive soil became more resistant to the disease after inoculation with Ggt and three consecutive wheat plantings (1 ;month each) in pots. The soil remained highly conducive when there was just wheat plantings without introduction of Ggt. The amount of total aerobic bacteria isolated from the rhizoplane did not vary significantly but fluorescent pseudomonads were more numerous after root attacks by Ggt compared with the absence of Ggt at the first and at the third planting. The succession of planting cycles increased to a lesser extent the amount of fluorescent pseudomonads.

PCR amplification of the opri gene that codes for the major lipoprotein in Pseudomonas aeruginosa at first confirmed that all the isolated strains belonged to the RNA group 1 of hybridization in Pseudomonas spp. [1]. Analysis of the polymorphism of the opri gene and of the polymorphism of REP-PCR fragments revealed the presence of pseudomonad groups that were specific for the presence of introduced Ggt and also for the induction of soil suppressiveness.

The presence of Ggt and several planting cycles are necessary to induce soil suppressiveness. We conclude that root necroses are probably involved in the modification of the structure of pseudomonad populations. Some fluorescent Pseudomonas spp. groups colonize root necroses more specifically during soil suppressiveness induction [2].

References
1. De Vos D, Lim A Jr, De Vos P, Sarniguet A, Kersters K, Cornelis P, 1993. Journal of General Microbiology 139, 2215-2223.
2. Sarniguet A, Lucas P, Lucas M, Samson R, 1992. Plant and Soil 145, 29-36.