Aleksa Obradović

Management of Tomato Bacterial Spot in the Field by Foliar Applications of Bacteriophages and SAR Inducers

Various combinations of the harpin protein, acibenzolar-S-methyl, and bacteriophages were compared for controlling tomato bacterial spot in field experiments. Harpin protein and aciben-zolar-S-methyl were applied every 14 days beginning twice before transplanting and then an additional four applications throughout the season. Formulated bacteriophages were applied prior to inoculation followed by twice a week at dusk. A standard bactericide treatment, consisting of copper hydroxide plus mancozeb, was applied once prior to inoculation and then every 7 days, while untreated plants served as an untreated control. Experiments were conducted in north and central Florida fields during fall 2001, spring 2002, and fall 2002. In three consecutive seasons, acibenzolar-S-methyl applied in combination with bacteriophage or bacteriophage and harpin significantly reduced bacterial spot compared with the other treatments. However, it did not significantly affect the total yield compared with the standard or untreated control. Application of host-specific bacteriophages was effective against the bacterial spot pathogen in all three experiments, providing better disease control than copper-mancozeb or untreated control. When results of the disease severity assessments or harvested yield from the bacteriophage-treated plots were grouped and compared with the results of the corresponding nonbacteriophage group, the former provided significantly better disease control and yield of total marketable fruit.

Improved efficacy of newly formulated bacteriophages for management of bacterial spot on tomato

Bacteriophages are currently used as an alternative method for controlling bacterial spot disease on tomato incited by Xanthomonas campestris pv. vesicatoria. However, the efficacy of phage is greatly reduced due to its short residual activity on plant foliage. Three formulations that significantly increased phage longevity on the plant surface were tested in field and greenhouse trials: (i) PCF, 0.5% pregelatinized corn flour (PCF) + 0.5% sucrose; (ii) Casecrete, 0.5% Casecrete NH-400 + 0.5% sucrose + 0.25% PCF; and (iii) skim milk, 0.75% powdered skim milk + 0.5% sucrose. In greenhouse experiments, the nonformulated, PCF-, Casecrete-, and skim milk-formulated phage mixtures reduced disease severity on plants compared with the control by 1, 30, 51, and 62%, respectively. In three consecutive field trials, nonformulated phage caused 15, 20, and 9% reduction in disease on treated plants compared with untreated control plants, whereas plants treated with PCF- and Casecrete-formulated phage had 27, 32, and 12% and 30, 43, and 24% disease reduction, respectively. Plants receiving copper-mancozeb treatments were included in two field trials and had a 20% decrease in disease in the first trial and a 13% increase in the second one. Skim milk-formulated phage was tested only once and caused an 18% disease reduction. PCF-formulated phage was more effective when applied in the evening than in the morning, reducing disease on plants by 27 and 13%, respectively. The Casecrete-formulated phage populations were over 1,000-fold higher than the nonformulated phage populations 36 h after phage application.

Integration of Biological Control Agents and Systemic Acquired Resistance Inducers Against Bacterial Spot on Tomato

Two strains of plant growth-promoting rhizobacteria, two systemic acquired resistance inducers (harpin and acibenzolar-S-methyl), host-specific unformulated bacteriophages, and two antagonistic bacteria were evaluated for control of tomato bacterial spot incited by Xanthomonas campestris pv. vesicatoria in greenhouse experiments. Untreated plants and plants treated with copper hydroxide were used as controls. The plant growth-promoting rhizobacteria or a tap water control were applied as a drench to the potting mix containing the seedlings, while the other treatments were applied to the foliage using a handheld sprayer. The plant growth-promoting rhizobacteria strains, when applied alone or in combination with other treatments, had no significant effect on bacterial spot intensity. Messenger and the antagonistic bacterial strains, when applied alone, had negligible effects on disease intensity. Unformulated phage or copper bactericide applications were inconsistent in performance under greenhouse conditions against bacterial spot. Although acibenzolar-S-methyl completely prevented occurrence of typical symptoms of the disease, necrotic spots typical of a hypersensitive reaction (HR) were observed on plants treated with acibenzolar-S-methyl alone. Electrolyte leakage and population dynamics experiments confirmed that acibenzolar-S-methyl-treated plants responded to inoculation by eliciting an HR. Application of bacteriophages in combination with acibenzolar-S-methyl suppressed a visible HR and provided excellent disease control. Although we were unable to quantify populations of the bacterium on the leaf surface, indirectly we determined that bacteriophages specific to the target bacterium reduced populations of a tomato race 3 strain of the pathogen on the leaf surface of acibenzolar-S-methyl-treated plants to levels that did not induce a visible HR. Integrated use of acibenzolar-S-methyl and phages may complement each other as an alternative management strategy against bacterial spot on tomato.

Characterization and PCR-based Typing of Xanthomonas campestris pv. vesicatoria from Peppers and Tomatoes in Serbia

During the last two decades bacterial strains associated with necrotic leaf spots of pepper and tomato fruit spots were collected in Serbia. Twenty-eight strains isolated from pepper and six from tomato were characterized. A study of their physiological and pathological characteristics, and fatty acid composition analysis revealed that all of the strains belong to Xanthomonas campestris pv. vesicatoria. Being non-amylolytic and non-pectolytic, pathogenic on pepper but not on tomato, containing lower amounts of fatty acid 15 : 0 ante–iso, the pepper strains were designated as members of the A group of X. campestris pv. vesicatoria. However, the tomato strains hydrolyzed starch and pectate, caused compatible reactions on tomato but not on pepper, had higher percent of 15 : 0 ante–iso fatty acid, and were classified into B phenotypic group and identified as X. vesicatoria. PCR primers were developed which amplified conserved DNA regions related to the hrp genes of different strains of X. campestris pv. vesicatoria associated with pepper and tomato. Restriction analysis of the PCR product resulted in different patterns and enabled grouping of the strains into four groups. When xanthomonads isolated from pepper and tomato in Serbia were analyzed, they clustered into two groups corresponding to the grouping based on their physiological and pathological characteristics. According to the reaction of pepper and tomato differential varieties, the strains from pepper belong to races P7 and P8 and tomato strains belong to the race T2. All strains were sensitive to copper and streptomycin. Advantages and disadvantages of various bacterial spot management practices are discussed.

Considerations for using bacteriophages for plant disease control

The use of bacteriophages as an effective phage therapy strategy faces significant challenges for controlling plant diseases in the phyllosphere. A number of factors must be taken into account when considering phage therapy for bacterial plant pathogens. Given that effective mitigation requires high populations of phage be present in close proximity to the pathogen at critical times in the disease cycle, the single biggest impediment that affects the efficacy of bacteriophages is their inability to persist on plant surfaces over time due to environmental factors. Inactivation by UV light is the biggest factor reducing bacteriophage persistence on plant surfaces. Therefore, designing strategies that minimize this effect are critical. For instance, application timing can be altered: instead of morning or afternoon application, phages can be applied late in the day to minimize the adverse effects of UV and extend the time high populations of phage persist on leaf surfaces. Protective formulations have been identified which prolong phage viability on the leaf surface; however, UV inactivation continues to be the major limiting factor in developing more effective bacteriophage treatments for bacterial plant pathogens. Other strategies, which have been developed to potentially increase persistence of phages on leaf surfaces, rely on establishing non-pathogenic or attenuated bacterial strains in the phyllosphere that are sensitive to the phage(s) specific to the target bacterium. We have also learned that selecting the correct phages for disease control is critical. This requires careful monitoring of bacterial strains in the field to minimize development of bacterial strains with resistance to the deployed bacteriophages. We also have data that indicate that selecting the phages based on in vivo assays may also be important when developing use for field application. Although bacteriophages have potential in biological control for plant disease control, there are major obstacles, which must be considered.

Soil-based systemic delivery and phyllosphere in vivo propagation of bacteriophages: Two possible strategies for improving bacteriophage persistence for plant disease control

Soil-based root applications and attenuated bacterial strains were evaluated as means to enhance bacteriophage persistence on plants for bacterial disease control. In addition, the systemic nature of phage applied to tomato roots was also evaluated. Several experiments were conducted applying either single phages or phage mixtures specific for Ralstonia solanacearum, Xanthomonas perforans or X. euvesicatoria to soil surrounding tomato plants and measuring the persistence and translocation of the phages over time. In general, all phages persisted in the roots of treated plants and were detected in stems and leaves; although phage level varied and persistence in stems and leaves was at a much lower level compared with persistence in roots. Bacterial wilt control was typically best if the phage or phage mixtures were applied to the soil surrounding tomatoes at the time of inoculation, less effective if applied 3 days before inoculation, and ineffective if applied 3 days after inoculation. The use of an attenuated X. perforans strain was also evaluated to improve the persistence of phage populations on tomato leaf surfaces. In greenhouse and field experiments, foliar applications of an attenuated mutant X. perforans 91-118:∆OPGH strain prior to phage applications significantly improved phage persistence on tomato foliage compared with untreated tomato foliage. Both the soil-based bacteriophage delivery and the use of attenuated bacterial strains improved bacteriophage persistence on respective root and foliar tissues, with evidence of translocation with soil-based bacteriophage applications. Both strategies could lead to improved control of bacterial pathogens on plants.

First report of brown rot caused by Monilinia fructicola on stored apple in Serbia.

Monilinia fructicola (G. Winter) Honey is a causal agent of brown rot of stone fruits, occasionally affecting pome fruits as well. The pathogen is commonly present in North and South America, Oceania, and Asia, but listed as a quarantine organism in Europe (4). After its first discovery in France in 2001, its occurrence has been reported in Germany, Hungary, Italy, Poland, Romania, Slovenia, Spain, Switzerland, Austria, and the Slovak Republic (1). In February 2011, during a survey for fungal postharvest pathogens in cold storage conditions, apple fruits (Malus domestica Borkh.) grown and stored in the Grocka Region, Serbia, were collected. All pathogens from symptomatic fruits were isolated on potato dextrose agar (PDA). One isolate from apple fruit cv. Golden Delicious with brown rot symptoms was identified as M. fructicola based on morphological and molecular characters. Colonies cultivated on PDA at 22°C in darkness were colorless, but later became grayish, developing mass of spores in concentric rings. Colony margins were even. Conidia were one-celled, limoniform, hyaline, measured 12.19 to 17.37 (mean 13.8) × 8.62 to 11.43 μm (mean 9.9), and were produced in branched monilioid chains (3). Morphological identification was confirmed by PCR (2) using genomic DNA extracted from the mycelium of pure culture, and an amplified product of 535 bp, specific for the species M. fructicola, was obtained. Sequence of the ribosomal (internal transcribed spacer) ITS1-5.8S-ITS2 region was obtained using primers ITS1 and ITS4 and deposited in GenBank (Accession No. JN176564). Control fruits were inoculated with sterile PDA plugs. After 3 days of incubation in plastic containers with high humidity at room temperature, typical symptoms of brown rot developed on inoculated fruits, while control fruits remained symptomless. The isolate recovered from symptomatic fruits showed the same morphological and molecular features of the original isolate. To our knowledge, this is the first report of M. fructicola in Serbia. Further studies are necessary for estimation of economic importance and geographic distribution of this quarantine organism in Serbia. References: (1) R. Baker et al. European Food Safety Authority. Online publication. www.efsa.europa.eu/efsajournal . EFSA J. 9(4):2119, 2011. (2) M.-J. Côté et al. Plant Dis. 88:1219, 2004. (3) J. E. M. Mordue. CMI Descriptions of Pathogenic Fungi and Bacteria. No. 616, 1979. (4) OEPP/EPPO. EPPO A2 List of Pests Recommended for Regulation as Quarantine Pests. Online publication. Version 2010-09. Retrieved from http://www.eppo.org/QUARANTINE/listA2.htm , June 27, 2011.

Characterization of Xanthomonas euvesicatoria strains pathogens of pepper in Serbia

During spring and summer of 2008, 101 bacterial strains was isolated from the diseased pepper leaves collected from different pepper growing areas in the Republic of Serbia. The aim of this research was to characterize the isolated strains and determine their taxonomic position according to the most recent nomenclature. Pathogenic, biochemical and physiological characteristics of isolated bacteria were tested using standard bacteriological tests. The pathogen races were determined according to the reaction of differential varieties of Early Calwonder (ECW), their isogenic lines (ECW-10R, ECW-20R, ECW-30R) and Capsicum pubescens. The sensitivity of strains to bactericides was studied in vitro by culturing bacteria on sucrose pepton agar (SPA) plates, amended with filter-sterilized aqueous solution of streptomycin and kasugamycin (50, 100, 200 ppm) or copper-sulphate (100, 200 ppm). Based on pathogenic, biochemical and physiological characteristics, the investigated strains belonged to Xanthomonas euvesicatoria. The reaction of pepper differential varieties indicated that these strains belonged to pepper races P1, P3, P7, P8. Streptomycin resistant strains were not detected, but 6 strains were resistant to kasugamycin (50 ppm) and 13 strains to copper-sulphate (200 ppm), indicating bacterial resistance development.

Antifungal Activity of Three Essential Oils against Colletotrichum acutatum, the Causal Agent of Strawberry Anthracnose

Abstract The antifungal effects of thyme, cinnamon bark and clove bud essential oils (EOs) were investigated in vitro on Colletotrichum acutatum mycelial growth, conidial germination, appressoria formation, and in vivo on strawberry fruit disease incidence. All tested EOs, incorporated in potato-dextrose agar, inhibited C. acutatum mycelial growth, and had a fungistatic effect at concentration 667 µl/l of medium. Volatiles of cinnamon bark, thyme and clove bud EOs completely prevented conidial germination at the lowest concentrations of 1.53, 15.3 and 76.5 µl/l of air, respectively, and disabled appressoria formation at concentration of 1.53 µl/l of air. On inoculated strawberry fruit, thyme and cinnamon bark EO volatiles reduced anthracnose incidence at concentrations above 15.3 and 76.5 µl/l of air, respectively. GC-FID and GC-MS analysis showed that major components of thyme EO were p-cymene, thymol, α-terpineol, carvacrol; cinnamon bark EO: trans- cinnameldehyde, trans-cinnamyl acetate; clove bud EO: eugenol and β-caryophyllene. Our results suggest that volatiles of thyme and cinnamon bark EOs are effective against C. acutatum both in vitro and in vivo.

Characterization and phylogenetic diversity of Agrobacterium vitis from Serbia based on sequence analysis of 16S-23S rRNA internal transcribed spacer (ITS) region

Serious outbreaks of grapevine crown gall disease were observed in major Serbian viticultural regions during the last five years. Tumorigenic Agrobacterium vitis was identified as a causal agent by using conventional bacteriological and molecular tests. The 36 studied strains of A. vitis showed homogeneous biochemical and physiological characteristics, but were heterogeneous in their pathogenic properties, especially on tomato and sunflower. Furthermore, genetic differences related to chromosomal and plasmid DNA were observed. The Ti plasmid of 35 strains was classified as the octopine/cucumopine (O/C) type, whereas one was classified as the vitopine (V) type. The O/C strains were further divided into O/C-1 and O/C-2 groups based on PCR analysis. Moreover, the sequence analysis of the 16S-23S rRNA ITS region provided robust and precise delineation of studied strains. Although a high level of genetic diversity in A. vitis strains from Serbia was revealed by using this approach, their genotypic relatedness with the strains from other countries suggested their common origin. Also, association between the chromosomal and plasmid DNA was determined for some phylogenetic groups and clusters.