Comparisons of single versus multiple bacterial species on biological control of anthurium blight

Biocontrol and Plant-Growth-Promoting Traits of Talaromyces apiculatus and Clonostachys rosea Consortium against Ganoderma Basal Stem Rot Disease of Oil Palm

Basal stem rot (BSR) disease caused by Ganoderma boninense basidiomycetous fungus is the most economically important disease in oil palms in South East Asia. Unfortunately, there is no single most effective control measure available. Tremendous efforts have been directed in incorporation of environmentally friendly biocontrol approaches in minimizing BSR disease. This study investigated the performance of two potential biocontrol agents (BCAs), AAT0115 and AAB0114 strains recovered from oil palm on suppression of BSR in planta, and also assessed their plant-growth-promoting (PGP) performance. ITS rRNA-sequence phylogeny discriminated the two ascomycetous Talaromyces apiculatus (Ta) AT0115 and Clonostachys rosea (Cr) AAB0114 biocontrol species with PGP characteristics. In vitro studies have demonstrated both Ta and Cr are capable of reducing linear mycelial growth of G. boninense. Inoculation of individual Cr and Ta—as well as Cr+Ta consortium—induced a significant increment in leaf area and bole girth of oil-palm seedlings five months post-inoculation (MPI) under nursery conditions. At five months post-inoculation, shoot and root biomass, and nutrient contents (nitrogen, phosphorus, potassium, calcium, magnesium and boron) were significantly higher in Ta-inoculated seedlings compared to control treated with non-Ta-inoculated maize. Chlorophyll and carotenoids contents in rapidly growing oil-palm seedlings challenged with Cr, Ta or a combination of both were not negatively affected. Cr, Ta and Cr+Ta consortium treated seedlings had 4.9–60% BSR disease reduction compared to the untreated control. Co-inoculation of Cr and Ta resulted in increased BSR control efficiencies by 18–26% (compared with Cr only) and 48–55% (compared with Ta only). Collectively, Cr and Ta, either individually or in consortium showed potential as BSR biocontrol agents while also possess PGP traits in oil palm.

The Great Five-an artificial bacterial consortium with antagonistic activity towards Pectobacterium spp. and Dickeya spp.: formulation, shelf life, and the ability to prevent soft rot of potato in storage

Abstract

“The Great Five” (GF) is an artificial bacterial consortium developed to protect potato tubers from soft rot caused by Pectobacterium spp. and Dickeya spp. To investigate the commercialization potential of the GF, we developed liquid and powder formulations of the consortium and of each of the comprising strains (Serratia plymuthica strain A294, Enterobacter amnigenus strain A167, Rahnella aquatilis strain H145, Serratia rubidaea strain H440, and S. rubidaea strain H469). To form powders, the cells were lyophilized using a newly developed lyoprotectant: Reagent PS. The shelf life of the formulations stored at 8 and 22 °C was monitored for a period of 12 months. The longest shelf life was obtained for formulations stored at 8 °C; however, the viability of all formulations was negatively affected at 22 °C. For the consortium, a 2.5 log₁₀ cfu (colony forming units) drop in cell number was recorded for the liquid formulation after 6 months, while in case of powders, the drop remained below 1 log₁₀ cfu following 12 months. The ability of the powder formulations to preserve biocontrol activity of the consortium was tested on potato tubers treated with the formulations and a mixture of the soft rot pathogens. The inoculated tubers were stored for 6 months at 8 °C to mimic commercial storage conditions. Soft rot severity and incidence on potato tubers treated with formulations were significantly reduced (62–75% and 48–61%, respectively) in comparison to positive control with pathogens alone. The potential use of the newly developed formulations of “The Great Five” for the biocontrol of soft rot is discussed.

Key Points

• An innovative reagent to protect bacterial cells during lyophilization was developed.

• Powder formulations of “The Great Five” prolonged its shelf life.

• The powder-formulated “The Great Five” was active against soft rot bacteria on potato tubers.

Electronic supplementary material

The online version of this article (10.1007/s00253-020-10550-x) contains supplementary material, which is available to authorized users.

Specificity of Pseudomonas isolates on healthy and Fusarium head blight-infected spikelets of wheat heads

The specificity of culturable bacteria on healthy and Fusarium head blight (FHB)-infected spikelets of wheat heads was investigated to find a candidate of biocontrol agents against FHB. The bacterial genus Pseudomonas was commonly isolated from the tissues, and phylogenetic analysis using 16S ribosomal RNA gene sequences of isolates of the genera revealed that particular phylogenetic groups in the genus specifically inhabited either healthy or infected spikelet tissues. The specificity of each group was suggested to be due to differences in the ability to form biofilms and colonize spikelet tissues; isolates originated from healthy spikelets formed biofilms on polyvinyl chloride microplate wells and highly colonized the spikelet tissues. Other bacterial groups obtained from FHB-infected spikelets less formed biofilms and attached with low densities on the spikelet tissues. Their colonization on the tissues, however, was promoted when co-inoculated with the causal pathogenic fungus, Fusarium graminearum, and several isolates were observed to smash the mycelia in vivo. Moreover, based on results of in vitro mycelial growth inhibition activity, the diseased tissue-originated isolates were verified to have a negative effect on the fungal growth. These results suggest that Pseudomonas isolates obtained from infected spikelet tissues were highly associated with the FHB pathogen and have potential as candidates for biological control against FHB.

Combined use of biocontrol agents to manage plant diseases in theory and practice

Effective use of biological control agents (BCAs) is a potentially important component of sustainable agriculture. Recently, there has been an increasing interest among researchers in using combinations of BCAs to exploit potential synergistic effects among them. The methodology for investigating such synergistic effects was reviewed first and published results were then assessed for available evidence for synergy. Correct formulation of hypotheses based on the theoretical definition of independence (Bliss independence or Loewe additivity) and the subsequent and statistical testing for the independence-synergistic-antagonistic interactions have rarely been carried out thus far in studies on biocontrol of plant diseases. Thus, caution must be taken when interpreting reported "synergistic" effects without assessing the original publications. Recent theoretical modeling work suggested that disease suppression from combined use of two BCAs was, in general, very similar to that achieved by the more efficacious one, indicating no synergistic but more likely antagonistic interactions. Only in 2% of the total 465 published treatments was there evidence for synergistic effects among BCAs. In the majority of the cases, antagonistic interactions among BCAs were indicated. Thus, both theoretical and experimental studies suggest that, in combined use of BCAs, antagonistic interactions among BCAs are more likely to occur than synergistic interactions. Several research strategies, including formulation of synergy hypotheses in relation to biocontrol mechanisms, are outlined to exploit microbial mixtures for uses in biocontrol of plant diseases.

Pathogen refuge: a key to understanding biological control

Pathogen refuge is the idea that some potentially infectious pathogen propagules are not susceptible to the influence of an antagonistic microbial agent. The existence of a refuge can be attributable to one or more factors, including temporal, spatial, structural, and probabilistic, or to the pathogen's evolved ability to acquire antagonist-free space prior to ingress into a plant host. Within a specific pathosystem, refuge size can be estimated in experiments by measuring the proportion of pathogen propagules that remain infective as a function of the amount of antagonist introduced to the system. Refuge size is influenced by qualities of specific antagonists and by environment but less so by the quantity of antagonist. Consequently, most efforts to improve and optimize biological control are in essence efforts to reduce refuge size. Antagonist mixtures, optimal timing of antagonist introductions, integrated biological and chemical control, environmental optimization, and the utilization of disarmed pathogens as antagonists are strategies with potential to minimize a pathogen refuge.

Culturable leaf-associated bacteria on tomato plants and their potential as biological control agents

Culturable leaf-associated bacteria inhabiting a plant have been considered as promising biological control agent (BCA) candidates because they can survive on the plant. We investigated the relationship between bacterial groups of culturable leaf-associated bacteria on greenhouse- and field-grown tomato leaves and their antifungal activities against tomato diseases in vitro and in vivo. In addition, the isolated bacteria were analyzed for N-acyl-homoserine lactone (AHL) and indole-3-acetic acid (IAA) production, which have been reported to associate with bacterial colonization, and resistance to a tomato alkaloid (alpha-tomatine). Leaf washings and subsequent leaf macerates were used to estimate the population size of epiphytic and more internal bacteria. Bacterial population sizes on leaves at the same position increased as the leaves aged under both greenhouse and field conditions. Field-grown tomatoes had significantly larger population sizes than greenhouse-grown tomatoes. Analysis of 16S rRNA gene (rDNA) sequencing using 887 culturable leaf-associated bacteria revealed a predominance of the Bacillus and Pseudomonas culturable leaf-associated bacterial groups on greenhouse- and field-grown tomatoes, respectively. Curtobacterium and Sphingomonas were frequently recovered from both locations. From the 2138 bacterial strains tested, we selected several strains having in vitro antifungal activity against three fungal pathogens of tomato: Botrytis cinerea, Fulvia fulva, and Alternaria solani. Among bacterial strains with strong in vitro antifungal activities, Bacillus and Pantoea tended to show strong antifungal activities, whereas Curtobacterium and Sphingomonas were not effective. The results indicated the differences in antifungal activity among predominant bacterial groups. Analysis of alpha-tomatine resistance revealed that most bacterial strains in the dominant groups exhibited moderate or high resistance to alpha-tomatine in growth medium. Furthermore, some Sphingomonas and Pantoea strains showed AHL and IAA production activities. Strain 125NP12 (Pantoea ananatis) showed particular alpha-tomatine resistance, and AHL and IAA production had the highest protective value (91.7) against gray mold. Thus, the differences of these physiological properties among dominant bacteria may be associated with the disease suppression ability of BCAs on tomato plants.

Production of quorum-sensing-related signal molecules by epiphytic bacteria inhabiting wheat heads

The production of quorum-sensing-related signal molecules (QSRMs) among culturable bacteria comprising the community on wheat heads was investigated. The taxonomic position of 186 bacterial isolates obtained from ten heads was inferred based on 16S rRNA gene sequences, and their QSRM production was determined using two bioreporter strains of N-acylhomoserine lactones. Approximately 33% of isolates produced QSRMs, though the proportion of QSRM-producing isolates on a wheat head was significantly negatively correlated with population size. Most of the producing isolates were Pantoea species, most commonly Pantoea ananatis. Furthermore, the proportion of Pantoea ananatis that produced QSRMs was significantly negatively correlated with the number of bacterial genera (community richness) on each head. Finally, community richness was positively correlated with population size. Qualitative analysis using thin-layer-chromatography revealed that the QSRMs of Pantoea isolates were composed of at least two compounds. This is the first report indicating that Pantoea ananatis isolates inhabiting wheat heads are capable of producing QSRMs. QSRM production by Pantoea spp. may contribute to the predominance of this genus on wheat heads, particularly at relatively low population densities and community diversity.

Biological control of postharvest diseases of fruits

Losses from postharvest fruit diseases range from 1 to 20 percent in the United States, depending on the commodity. The application of fungicides to fruits after harvest to reduce decay has been increasingly curtailed by the development of pathogen resistance to many key fungicides, the lack of replacement fungicides, negative public perception regarding the safety of pesticides and consequent restrictions on fungicide use. Biological control of postharvest diseases (BCPD) has emerged as an effective alternative. Because wound-invading necrotrophic pathogens are vulnerable to biocontrol, antagonists can be applied directly to the targeted area (fruit wounds), and a single application using existing delivery systems (drenches, line sprayers, on-line dips) can significantly reduce fruit decays. The pioneering biocontrol products BioSave and Aspire were registered by EPA in 1995 for control of postharvest rots of pome and citrus fruit, respectively, and are commercially available. The limitations of these biocontrol products can be addressed by enhancing biocontrol through manipulation of the environment, using mixtures of beneficial organisms, physiological and genetic enhancement of the biocontrol mechanisms, manipulation of formulations, and integration of biocontrol with other alternative methods that alone do not provide adequate protection but in combination with biocontrol provide additive or synergistic effects.

Combining biocontrol agents to reduce the variability of biological control

ABSTRACT Two biocontrol agents, a yeast (Pichia guilermondii) and a bacterium (Bacillus mycoides), were tested separately and together for suppression of Botrytis cinerea on strawberry leaves. The aims of the research were to determine whether the use of their combination would broaden the environmental conditions under which biological control is effective, and to test the hypothesis that it would reduce the variability of control efficacy under diverse conditions. Applied separately, the biocontrol agents significantly inhibited spore germination, lesion formation, and lesion development at most temperatures, relative humidities, and spray-timing combinations (temperatures: 10, 15, 20, 23, 25, and 30 degrees C; relative humidities: 78, 85, 96, and 100%; and spray-timings: 0, 4, and 7 days before inoculation). However, control efficacy was highly variable, and under certain combinations it was not adequate. Control efficacy achieved by the biocontrol agents applied separately ranged between 38 and 98% (mean 74%) and the coefficient of variation ranged from 9.7 to 75%. The mixture of Bacillus mycoides and Pichia guilermondii suppressed Botrytis cinerea effectively (80 to 99.8% control) under all conditions, and the coefficients of variation were as low as 0.4 to 9% in all cases. Thus, application of both biocontrol agents resulted in better suppression of Botrytis cinerea, and also reduced the variability of disease control. Application of more than one biocontrol agent is suggested as a reliable means of reducing the variability and increasing the reliability of biological control.