Biocontrol bacteria selected by a direct plant protection strategy against avocado white root rot show antagonism as a prevalent trait.

AIM: This study was undertaken to study bacterial strains obtained directly for their efficient direct control of the avocado white root rot, thus avoiding prescreening by any other possible mechanism of biocontrol which could bias the selection. METHODS AND RESULTS: A collection of 330 bacterial isolates was obtained from the roots and soil of healthy avocado trees. One hundred and forty-three representative bacterial isolates were tested in an avocado/Rosellinia test system, resulting in 22 presumptive protective strains, all of them identified mainly as Pseudomonas and Bacillus species. These 22 candidate strains were screened in a more accurate biocontrol trial, confirming protection of some strains (4 out of the 22). Analyses of the potential bacterial traits involved in the biocontrol activity suggest that different traits could act jointly in the final biocontrol response, but any of these traits were neither sufficient nor generalized for all the active bacteria. All the protective strains selected were antagonistic against some fungal root pathogens. CONCLUSIONS: Diverse bacteria with biocontrol activity could be obtained by a direct plant protection strategy of selection. All the biocontrol strains finally selected in this work were antagonistic, showing that antagonism is a prevalent trait in the biocontrol bacteria selected by a direct plant protection strategy. SIGNIFICANCE AND IMPACT OF THE STUDY: This is the first report on the isolation of biocontrol bacterial strains using direct plant protection strategy in the system avocado/Rosellinia. Characterization of selected biocontrol bacterial strains obtained by a direct plant protection strategy showed that antagonism is a prevalent trait in the selected strains in this experimental system. This suggests that antagonism could be used as useful strategy to select biocontrol strains.

Avocado Dieback Caused by Neofusicoccum parvum in the Andalucia Region, Spain.

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.

Progression and clinical recurrence of symptomatic middle cerebral artery stenosis: a long-term follow-up transcranial Doppler ultrasound study.

BACKGROUND AND PURPOSE: Patients with symptomatic intracranial atherosclerotic stenosis have a high rate of recurrence. We conducted a prospective study to determine which factors are associated with the progression of symptomatic middle cerebral artery (MCA) stenosis and to evaluate the relationship between progression and clinical recurrence. METHODS: Between January 1996 and February 2000, of a total of 2564 consecutive first-ever transient ischemic attack (TIA) or stroke patients admitted to our cerebrovascular unit, 145 showed an MCA stenosis signal on transcranial Doppler (TCD) on admission, and 40 fulfilled all criteria to enter this study, including angiographic confirmation. Patients were prescribed antiplatelet or anticoagulant agents following the criteria of the neurologist in charge. TCD recordings and clinical interviews were performed regularly during follow-up. Progression of MCA stenosis was defined as an increase >30 cm/s in TCD-recorded maximum mean flow velocity. Logistic regression analyses were used to identify predictors of progression and clinical recurrence. RESULTS: With a median follow-up of 26.55 months, 13 (32.5%) MCA stenoses progressed, 3 (7.5%) regressed, and 24 (60%) remained stable. Absence of significant extracranial internal carotid artery (ICA) stenosis (P=0.049) and the use of oral anticoagulants (P=0.045) were significantly associated with a lower progression rate in univariate analysis, and anticoagulation remained an independent predictor when a logistic regression model was applied (OR 7.25, CI 1.1 to 48.1, P=0.019). A new ischemic event during follow-up in the territory supplied by the stenosed MCA occurred in 8 cases (20%), and 13 patients had a major vascular event. Progression of the MCA stenosis detected by TCD was independently associated with a new ipsilateral ischemic event (OR 2.89, CI 1.09 to 7.71, P=0.031) and with the occurrence of any major vascular event (OR 7.03, CI 1.6 to 30.9, P=0.0071). CONCLUSIONS: Progression of symptomatic MCA stenosis detected by means of TCD predicts clinical recurrence. Anticoagulation is independently associated with a lower progression rate of symptomatic MCA stenosis.