Recirculating nutrient systems offer a good method to control nutrient leaching from greenhouses into the environment. However, the potential for the rapid spread of root diseases is the main hindrance to adoption of recirculating nutrient systems by the greenhouse industry. This review discusses and compares five broadly different methods of disease control in these systems, namely heat, filtration, chemical, radiation and biological control. Each has strengths and weaknesses, but all have been found to be effective in terms of pathogen control. Sterilization (heat, oxidizing chemicals, UV radiation) and membrane filtration methods are generally very effective, but may adversely affect beneficial microorganisms in the recirculated solution. Slow filtration and microbial inoculation methods are less disruptive of the microflora, but effectiveness may vary with the pathogen. Microbial inoculation holds the promise of very targeted disease suppression, but few products are commercially available. recirculation / disinfestation / hydroponics / disinfection / root diseaseRésumé -Désinfestation des solutions nutritives recyclées en horticulture sous serre. L'utilisation de systèmes de recirculation des nutriments est une bonne façon de contrôler le lessivage des nutriments des serres dans l'environnement. Toutefois, le risque de propagation rapide de maladies des racines est le principal obstacle à l'adoption de tels systèmes par l'industrie serricole. La présente étude examine et compare cinq façons distinctes de contrer les maladies dans ces systèmes, à savoir le traitement thermique, la filtration, le traitement chimique, le rayonnement et la lutte biologique. Chacune de ces méthodes a ses points forts et ses points faibles, mais toutes se sont révélées efficaces pour combattre les pathogènes. La stérilisation (par la chaleur, l'utilisation d'agents oxydants ou le rayonnement ultraviolet) et la filtration sur membrane sont habituellement très efficaces, mais peuvent nuire aux microorganismes utiles dans la solution recirculée. La filtration lente et l'inoculation microbienne sont moins nuisibles à la microflore, mais leur efficacité peut varier selon le pathogène. L'inoculation microbienne permet une élimination très sélective des maladies, mais peu d'inoculants microbiens sont disponibles dans le commerce. recirculation / désinfestation / culture hydroponique / désinfection / maladie des racinesAgronomie 21 (2001) 323-339 323

The increasing use of biological control agents (BCAs) against Botrytis cinerea in strawberry raises the question of whether there are any undesirable impacts of foliar applications of BCAs on nontarget microorganisms in the phyllosphere. Therefore, our objective was to investigate this issue within a field study. Strawberry plants were repeatedly sprayed with three BCAs-namely, RhizoVital 42 fl. (Bacillus amyloliquefaciens FZB42), Trianum-P (Trichoderma harzianum T22), and Naturalis (Beauveria bassiana ATCC 74040)-to suppress Botrytis cinerea infections. Microbial communities of differentially treated leaves were analyzed using plate counts and pyrosequencing and compared with the microbial community of nontreated leaves. Plate count results indicate that the applied Bacillus and Trichoderma spp. survived in the strawberry phyllosphere throughout the strawberry season. However, no significant impacts on the leaf microbiota could be detected by this culture-dependent technique. Pyrosequencing of internal transcribed spacer ribosomal RNA and 16S RNA sequences revealed a change in fungal composition and diversity at class level after the introduction of T. harzianum T22 to the phyllosphere, whereas the bacterial composition and diversity was not affected by either this Trichoderma preparation or the other two BCAs. Our results suggest that pyrosequencing represents a useful method for studying microbial interactions in the phyllosphere.

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Despite the overruling impact of light in the phyllosphere, little is known regarding the influence of light spectra on non-phototrophic bacteria colonizing the leaf surface. We developed an in vitro method to study phenotypic profile responses of bacterial pure cultures to different bands of the visible light spectrum using monochromatic (blue: 460 nm; red: 660 nm) and polychromatic (white: 350–990 nm) LEDs, by modification and optimization of a protocol for the Phenotype MicroArray™ technique (Biolog Inc., CA, USA). The new protocol revealed high reproducibility of substrate utilization under all conditions tested. Challenging the non-phototrophic bacterium Pseudomonas sp. DR 5–09 with white, blue, and red light demonstrated that all light treatments affected the respiratory profile differently, with blue LED having the most decisive impact on substrate utilization by impairing respiration of 140 substrates. The respiratory activity was decreased on 23 and 42 substrates under red and white LEDs, respectively, while utilization of one, 16, and 20 substrates increased in the presence of red, blue, and white LEDs, respectively. Interestingly, on four substrates contrasting utilization patterns were found when the bacterium was exposed to different light spectra. Although non-phototrophic bacteria do not rely directly on light as an energy source, Pseudomonas sp. DR 5–09 changed its respiratory activity on various substrates differently when exposed to different lights. Thus, ability to sense and distinguish between different wavelengths even within the visible light spectrum must exist, and leads to differential regulation of substrate usage. With these results, we hypothesize that different light spectra might be a hitherto neglected key stimulus for changes in microbial lifestyle and habits of substrate usage by non-phototrophic phyllospheric microbiota, and thus might essentially stratify leaf microbiota composition and diversity.

Currently, there is little knowledge of the establishment of repeatedly applied biological control agents (BCAs) in the phyllosphere of plants and, in particular, their interactions with the resident microbiome. Under field conditions, the BCA Aureobasidium pullulans was applied as a model organism to organically grown strawberries during two subsequent years (2011, 2012), either as single strain treatment or with the co-application of the entomopathogenic fungus Beauveria bassiana. Fungal and bacterial communities of strawberry leaves were investigated by means of plate counts and 454 pyrosequencing. The establishment of the introduced A. pullulans strains considerably differed between the two years, presumably due to distinct environmental conditions. Short-term and long-term effects of BCA applications on the composition and diversity of fungal communities could be observed as a result of successful establishment of A. pullulans, in 2011, showing, for instance, reduced diversity of fungal communities by competitive displacement shortly after BCA introduction. Due to considerable dynamics in untreated resident microbial communities in the phyllosphere in general, however, we suggest that even the effects caused by the applied BCA preparations in 2011 are negligible under practical conditions

No abstract

There is little knowledge about the effects of simultaneously applied biological control agents (BCAs) on the incidence of grey mould (Botrytis cinerea) in strawberries under field conditions. Our objective was, therefore, to investigate the use of three BCAs (Bacillus amyloliquefaciens FZB42, Aureobasidium pullulans DSM 14940 and DSM 14941 and Beauveria bassiana ATCC 74040) with respect to their efficacy against B. cinerea in field grown strawberries following single compared to combined application. In two subsequent years (2011 and 2012), improved control of B. cinerea resulted from combined BCA treatments, whereas the single BCAs were not efficient. Efficient combinations of BCA treatments were not the same in 2011 and 2012, however. The development of B. cinerea was reduced by single treatments of B. amyloliquefaciens and A. pullulans as well as by most combined BCA treatments during storage for one of two fruit samples in 2011. In 2012, however, no effects on B. cinerea during storage resulted from BCA treatments. The results of the present study suggest that the use of combined BCA treatments only cannot overcome inconsistent efficacies of BCAs against strawberry grey mould under field conditions. Further investigations on the interactions between the BCAs in combined BCA treatments as well as between the BCAs and the pathogen in dependence on multiple factors prevailing in the phyllosphere, however, need to be included to tailor combined BCA treatments with constant efficacies in different years.

Leaf lesions of Mandevilla sanderi were shown to be caused by Pseudomonas savastanoi. While BOX fingerprints were similar for P. savastanoi isolates from different host plants, plasmid restriction patterns and sequencing of plasmid-located pathogenicity determinants revealed that Mandevilla isolates contained similar plasmids distinct from those of other isolates. A repA-based detection method was established.T he ornamental plant Mandevilla sanderi (Dipladenia sanderi [family Apocynaceae]) originating from Middle and South America has become increasingly popular over the last decade, mainly because of its copiously formed red flowers. In 2008, breeders of Mandevilla sanderi observed for the first time large necrotic lesions with chlorotic rings on leaves and tumor formation on stems (see Fig. 1). The potential causal agents isolated from the lesions of leaves of diseased plant material were identified initially by metabolic profiling (Biolog) as Pseudomonas savastanoi pv. glycinea or pv. nerii (data not shown), pathogens of soybean (Glycine max) or oleander (Nerium oleander), respectively. Stem and leaf inoculation of healthy Mandevilla plants with these isolates indeed caused identical symptoms ( Fig. 1) fulfilling Koch's postulates.P. savastanoi strains are Gammaproteobacteria which belong to genomospecies 2 of the Pseudomonas syringae complex (6). Although the species P. savastanoi was established only in 1992 (5), sequencing of the genomes and plasmids of several P. syringae and P. savastanoi isolates reinforced the discussion about the taxonomic affiliation of P. savastanoi, as the core genome is clearly shared by P. syringae and P. savastanoi strains. Several pathovars of P. savastanoi infect woody plants, e.g., P. savastanoi pv. savastanoi is known as an important pathogen of olive trees (Olea europaea) in the Mediterranean area (11). The typical symptoms of olive knot disease, the formation of tumors in the stem and branches, were already described by Theobrast Eresos in the year 300 BC. The disease causes massive yield losses, and breeding for tolerant varieties is the main strategy to overcome the commercial losses due to infections with P. savastanoi pv. savastanoi (16).Taking into account that other known hosts of P. savastanoi are oleander (Nerium oleander) and privet (Liguster vulgare), leaf and stem inoculations of Mandevilla isolates were done also on oleander, olive, and privet plants. Nine weeks after inoculation, the response patterns of oleander were similar to those observed on Mandevilla plants; however, while no symptoms were observed for olive plants, privet plants displayed only leaf lesions (unpublished results). Thus, it was hypothesized that the most likely causal agent of the novel bacterial disease of Mandevilla sanderi might have originated from infested oleander plantations in the vicinity of the Mandevilla sanderi-producing companies in the South of France. To shed light on this hypothesis, P. savastanoi isolates from Mandevilla sanderi were characterized and compared to isolate...