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...

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.

No abstract

From 2002 to 2006, adult avocado trees, Persea americana Miller cv. Hass, located in the subtropical-fruit-producing area of Andalucia (southern Spain) developed symptoms of dieback characterized by death of twigs and branches in the tree canopy. Sections of surface-disinfested, necrotic branch tissues were plated on Difco potato dextrose agar (PDA) (Sparks, NV) and a Neofusicoccum-like fungus was isolated. On PDA, the isolates had white, appressed mycelium that turned dull gray as the colony aged, although conidia were not formed. Abundant pycnidia and conidia developed when isolates were cultured on 2% water agar with sterilized pine needles as substratum at 25°C under near-UV light for 2 weeks. Conidia were hyaline, unicellular, ellipsoid with an obtuse apex and subtruncate base, averaged 16.2 μm long by 5.8 μm wide and ranged from 12.0 to 20.0 by 4.0 to 8.0 μm, and becoming brown with one or two septa with age. Sequenced rDNA fragments (ITS1, 5.8S rDNA, and ITS2, amplified with ITS1 and ITS4 primers) of two avocado isolates were 100% homologous with Neofusicoccum parvum (Pennycook & Samuels) Crous, Slippers, & A.J.L. Phillips (1) (GenBank Accession Nos. AM410965 and AM410966). Morphological and molecular results confirmed this species as N. parvum, reported as the anamorph of Botryosphaeria parva (1). A pathogenicity test was conducted using two isolates on sets of five 2-year-old avocado plants produced from seeds of cv. Topa-Topa growing in 5-liter pots with soil. Unwounded and wounded plants were used for inoculations. Plants were wounded 2 to 3 cm below the apical tip with a lance (4 mm long and 1 mm deep). For inoculation, 4-mm 2-week-old PDA culture plugs were placed in contact with wounded tissues and covered with Parafilm. Five noninoculated plants treated similarly served as controls. Plants were maintained in the greenhouse with a temperature range of 18 to 26°C, and 1 month later, brown stem lesions, as much as 5 cm, originating from the inoculation site followed by dieback of branches were observed. Reisolations from necrotic branches were successful, and both isolates with identical morphology to those used for inoculations were recovered. Pathogenicity tests of seedlings using the same methods also caused stem lesions on unwounded plants and the pathogen was reisolated. To our knowledge, this is the first report of N. parvum causing dieback of avocado trees in Spain. Previously, B. parva has been reported causing stem-end rot of avocado fruit in New Zealand (2). In Spain, since diseased orchards are increasing rapidly, this pathogen could be efficiently distributed by pruning activities (tools and vegetal debris) as observed with other diseases (3). The presence of N. parvum in this subtropical area presents a serious disease problem not only to avocado but also to mango (Mangifera indica L.), which is another susceptible host (4). References: (1) P. W. Crous et al. Stud. Mycol. 55:235, 2006. (2) W. F. T. Hartill et al. N. Z. J. Crop Hortic. Sci. 30:249. 2002. (3) A. J. L. Phillips. Phytopathol. Mediterr. 41:3, 2002. (4) B. Slippers et al. Mycologia 97:99, 2005.