Motility is one of the most important traits for efficient rhizosphere colonization by Pseudomonas fluorescens F113rif (F113). In this bacterium, motility is a polygenic trait that is repressed by at least three independent pathways, including the Gac posttranscriptional system, the Wsp chemotaxis-like pathway, and the SadB pathway. Here we show that the kinB gene, which encodes a signal transduction protein that together with AlgB has been implicated in alginate production, participates in swimming motility repression through the Gac pathway, acting downstream of the GacAS two-component system. Gac mutants are impaired in secondary metabolite production and are unsuitable as biocontrol agents. However, the kinB mutant and a triple mutant affected in kinB, sadB, and wspR (KSW) possess a wild-type phenotype for secondary metabolism. The KSW strain is hypermotile and more competitive for rhizosphere colonization than the wild-type strain. We have compared the biocontrol activity of KSW with those of the wild-type strain and a phenotypic variant (F113v35 [V35]) which is hypermotile and hypercompetitive but is affected in secondary metabolism since it harbors a gacS mutation. Biocontrol experiments in the Fusarium oxysporum f. sp. radicis-lycopersici/Lycopersicum esculentum (tomato) and Phytophthora cactorum/Fragaria vesca (strawberry) pathosystems have shown that the three strains possess biocontrol activity. Biocontrol activity was consistently lower for V35, indicating that the production of secondary metabolites was the most important trait for biocontrol. Strain KSW showed improved biocontrol compared with the wild-type strain, indicating that an increase in competitive colonization ability resulted in improved biocontrol and that the rational design of biocontrol agents by mutation is feasible.Pseudomonas fluorescens F113rif (F113) is a biocontrol agent isolated from the sugar beet rhizosphere (36) that is able to suppress take-all disease produced by the oomycete Pythium ultimum (18). This strain, which is able to efficiently colonize the rhizosphere of a variety of plants (12,14,27,33), is being used as a model for rhizosphere colonization (3,26,33) and has also been genetically modified for polychlorinated biphenyl (PCB) rhizoremediation (5, 13, 41). The biocontrol ability of this strain has been related to the production of a series of secondary metabolites, including siderophores, diacetyl-phloroglucinol (DAPG), hydrogen cyanide, and an extracellular protease (1, 36), whose production is regulated by the GacAS posttranscriptional system (1).The GacAS system, as well as SadB and the Wsp system, independently regulates swimming motility in strain F113 (29). In this strain mutants affected in sadB and wspR showed increased swimming motility. In F113, the sadB and wspR genes showed 84% and 83% identities with their Pseudomonas aeruginosa orthologues and are highly conserved in sequence and genomic context with orthologues in all the sequenced P. fluorescens strains. SadB and WspR possibly repress swimming mot...

Rosellinia necatrix, an ascomycete soil-inhabiting fungus, causes white root rot disease in a large number of plant species, especially fruit trees. The fungus, which occurs worldwide, is very aggressive and can kill infected trees. The biology and pathogenicity of the fungus are reviewed here, together with the current principal methods of disease control used in different pathosystems.www.blackwell-synergy.com

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Huelva in southern Spain is a major production area for strawberry (Fragaria × ananassa). At the end of the 2006 season (May-June) collapsed and dying strawberry plants were observed on several cultivars in four fields. Cut crowns of affected plants revealed dark brown necrotic areas on the margins and along the woody vascular ring. Roots of these plants were also shown to be necrotic. Macrophomina-like isolates developed from surface-disinfested affected tissues plated on potato dextrose agar amended with 250 mg L-1 of chloramphenicol. Dark, oblong-shaped sclerotia were observed in affected crown tissue and in culture after 5 to 7 days incubation at 25 ° C. They had an average length of 107 (217 to 62) μ m and width of 71 (110 to 35) μ m. Sequenced rDNA fragments of a single sclerotium isolate CH 724 (Spanish Type Culture Collection, CECT 20715; GenBank Accession No. AM410964) presented a 99% identity with Macrophomina phaseolina. Morphological and molecular results confirmed this species as M. phaseolina (Holliday & Punithalingam, 1970). Six single sclerotium isolates of M. phaseolina from strawberry were used for pathogenicity tests. Each isolate was used to inoculate six strawberry runner plants (cv. Camarosa) growing in pots of coconut fibre substrate for 5 weeks. Plants were inoculated by inserting a fungal colonised toothpick into each crown (Mertely et al ., 2005). An equal number of uninoculated plants treated similarly were left as controls. After 58 days, the incidence of plant death ranged from 67 to 100% depending on isolate. Macrophomina phaseolina was reisolated from all plants showing symptoms. Uninoculated plants remained symptomless.