Mechanisms of biocontrol and plant growth-promoting activity in soil bacterial species of Bacillus and Pseudomonas: a review

Plant pathogens are responsible for many crop plant diseases, resulting in economic losses. The use of bacterial agents is an excellent option to fight against plant pathogens and an excellent alternative to the use of chemicals, which are offensive to the environment and to human health. Two of the most common biocontrol agents are members of the Bacillus and Pseudomonas genera. Both bacterial genera have important traits such as plant growth-promoting (PGP) properties. This review analyzes pioneering and recent works and the mechanisms used by Bacillus and Pseudomonas in their behaviour as biocontrol and PGP agents, discussing their mode of action by comparing the two genera. Undoubtedly, future integrated research strategies for biocontrol and PGP will require the help of known and novel species of both genera.     

Effects of biocontrol agents on lipid and protein composition of Indian mustard seeds from plants infected with Alternaria species

Field experiments were carried out with Indian mustard (Brassica juncea L. Cv RLM 1359) to investigate the influence of biocontrol agents on seeds from plants infected with Alternaria blight. The biocontrol agents viz, Trichoderma harzianum, Pseudomonas fluorescens and Bacillus subtilis were applied as seed treatment/seed treatment coupled with spray on 30 and 60 days after sowing of seeds in experimental fields. The plants treated with different biocontrol agents were more developed than non-treated plants throughout the experiment. Biochemical analysis revealed that application of biocontrol agents resulted in increase in lipid and protein content in seeds from treated plants. The proportion of various lipidic fractions i.e. phospholipids, glycolipids and sterol content in seeds increased with a corresponding decrease in total glycerides. The proportion of 18:3, 20:1 and 22:1 fatty acids increased while that of 18:1 and 18:2 fatty acids decreased in seeds with application of biocontrol agents. There were both qualitative and quantitative differences in the banding patterns of albumin and globulin proteins after application of biocontrol agents. The data suggested that biochemical alterations in the host induced by treatment with biocontrol agents could be associated with defence mechanisms and enhanced growth of the plant.     

What makes a successful biocontrol agent? A meta-analysis of biological control agent performance

We qualitatively reviewed the biocontrol literature in two major journals, Biological Control and Environmental Entomology, over the past 10 years by scoring 878 studies into 11 biocontrol-oriented questions. Quantitative meta-analyses were then used on data from 145 studies to examine the effects of different types of biocontrol agents (parasitoids, predators, and pathogens) on several attributes of weed and pest populations. Results for our qualitative review showed that most biocontrol studies were focused on lepidopteran pests, and that parasitoids were the most common biocontrol agents used. Our quantitative review showed that, for weeds, biocontrol agents significantly reduced weed biomass (−82.0%), flower (−98.9%), and seed production (−89.4%). For pests, our quantitative review showed that biocontrol agents significantly reduced pest abundance by 130% compared to control groups, increased parasitism (+139.0%) and increased overall pest mortality (+159.0%) compared to targets not exposed to biocontrol agents. Effects on pest mortality tended to be stronger for parasitoids than predators, although reductions caused in pest abundance were much stronger when predators were used as biocontrol agents. Addition of two or more biocontrol agents increased mortality by 12.97% and decreased pest abundance by 27.17% compared to single releases. Separate sets of meta-analyses demonstrated that the negative impacts of biocontrol on non-target species were much smaller than those for target species, although adverse effects of biocontrol on non-target organisms are based on small sample sizes and should be interpreted with caution. Our results also showed that biocontrol efficacy tended to be higher when agents were generalists than when they were specialists. Large fail–safe numbers found for most of the estimated effects indicate the robustness of the results found for the efficacy of biological control programs.     

Growth promotion activity and biological control for the management of leaf blight incited by Alternaria helianthi

The Pseudomonas fluorescens (Pf1), Bacillus subtilis (Bs1) are the major potential biocontrol agents against foliar pathogens. MPf1 and MBs1 were found to be the most effective in inhibiting the mycelial growth of Alternaria helianthi. These biocontrol agents have the maximum capacity in controlling the spore germination, and data showing the growth-promoting effect of biocontrol agents and inhibition of seed-borne fungi are available. Seed-borne infections of A. helianthi are controlled by seed treatment with P. fluorescens, which showed least seed infection. The root length and shoot length has also been increased.     

Bacillus-based bionematicides: development,modes of action and commercialisation

Agricultural crops are severely damaged by root-knot nematodes causing extensive financial losses globally. Historically, agrochemicals have been the preferred method to combat these pests; however, threats to humans and the environment posed by these agrochemicals led to the need for developing new biocontrol agents. Importantly, the latter should adhere to biosafety regulations while being highly effective. Root-knot nematodes live in soil and thus the use of rhizobacteria such as Bacillus for biocontrol development have shown potential. Although various Bacillus species have been tested in this capacity, little is known about their secondary metabolites and the mechanisms of action responsible for their nematicidal activity. If these secondary metabolites can be qualitatively and quantitatively characterised, metabolic features could be synthetically engineered and used to combat root-knot nematodes. Although there is great potential for bionematicides, the commercialisation and development of such products can be difficult. This review summarises the importance of Bacillus species as natural antagonists of root-knot nematodes through the production of secondary metabolites. It provides an overview of the significance of root-knot nematodes in agriculture and the advances of chemical nematicides in recent years. The potential of Bacillus species as biocontrol agents, the known mechanisms of action responsible for the nematicidal activity demonstrated by Bacillus species, non-target effects of biocontrol agents and the commercialisation of Bacillus-based bionematicides are discussed.     

Osmotic stress adaptation, compatible solutes accumulation and biocontrol efficacy of two potential biocontrol agents on Fusarium head blight in wheat

Fusarium head blight (FHB) caused by Gibberella zeae (anamorph = Fusarium graminearum) is a devastating disease that causes extensive yield and quality losses to wheat in humid and semi-humid regions of the world. Biological control has been demonstrated to be effective under laboratory conditions but a few biocontrol products have been effective under field conditions. The improvement in the physiological quality of biocontrol agents may improve survival under field conditions, and therefore, enhance biocontrol activity. Bacillus subtilis RC 218 and Brevibacillus sp. RC 263 were isolated from wheat anthers and showed significant effect on control of FHB under greenhouse assays. This study showed the effect of water availability measured as water activity (a_W) using a growth medium modified with NaCl, glycerol and glucose on: (i) osmotic stress tolerance, (ii) viability in modified liquid medium, (iii) quantitative intracellular accumulation of betaine and ectoine and (iv) the biocontrol efficacy of the physiologically improved agents. Viability of B. subtilis RC 218 in NaCl modified media was similar to the control. Brevibacillus sp. RC 263 showed a limited adaptation to growth in osmotic stress. Betaine was detected in high levels in modified cells but ectoine accumulation was similar to the control cells. Biocontrol activity was studied in greenhouse assays on wheat inoculated at anthesis period with F. graminearum RC 276. Treatments with modified bacteria reduced disease severity from 60% for the control to below 20%. The physiological improvement of biocontrol agents could be an effective strategy to enhance stress tolerance and biocontrol activity under fluctuating environmental conditions.     

Beneficial rhizospheric microorganisms mediated plant growth promotion and suppression of aflatoxigenic fungal and aflatoxin contamination in groundnut seeds

The potential of root‐colonising antagonistic microbial biocontrol agents was evaluated for their ability to improve plant growth and suppress aflatoxigenic fungal and aflatoxin contamination in groundnut. By considering root colonisation of groundnut seedlings, plant growth promotion and antagonism against aflatoxigenic Aspergillus flavus as preliminary criteria, eight rhizobacteria and nine Trichoderma spp. were selected and characterised for their beneficial traits. These strains gave varying results for IAA production, phosphate solubilisation, ACC deaminase, chitinase and siderophore production. Under laboratory and greenhouse conditions, these strains significantly (P < 0.05) suppressed seed‐borne and rhizospheric population of A. flavus and improved seed quality variables. However, cdELISA results revealed that none of the biocontrol strains were effective in reducing aflatoxin level in seed. Based on the overall performance, Pseudomonas fluorescens 2bpf, Bacillus sp. Bsp‐3/aM and Trichoderma atroviride UMDBT‐Dha.Tat8 were used for field trials in the form of talcum powder formulations. Under field conditions, biocontrol agents improved seedling emergence, plant biomass and pod yield. Seeds harvested from plots treated with biocontrol agents showed significant (P < 0.05) reduction in A. flavus infection and aflatoxin production after 6 months' storage. Use of microbial strains with multiple beneficial traits is advantageous in bioformulation development. Hence, in future, these formulations will play a major role as biofertilisers and biopesticides, which can reduce the usage of agrochemicals up to greater extents in groundnut production.     

Relationship of in vitro and in planta screening: improving the selection process for biological control agents against Fusarium root rot in row crops

Fusarium root rot in row crops is typically managed by cultural practices and fungicide seed treatments. Biological control using microbial agents is another option but needs further development for improved disease management. Screening to identify biocontrol agents are crucial. However, relationships among the steps and how to improve the screening process are unresolved questions. Strains of Burkholderia (4), Bacillus (5) and Trichoderma (26) were studied in vitro against six Fusarium pathogens. All the bacteria and five selected Trichoderma strains were tested in planta in the greenhouse against diseases of Fusarium graminearum and Fusarium oxysporum. Burkholderia ambifaria C628, Bacillus simplex R180, and all Trichoderma isolates showed high reduction in disease levels in corn, soybean and wheat, ranging from 16 to 63%. Responses of the biocontrol agents during in vitro and in planta screening did not always correlate. In vitro and in planta tests should be considered independently in selecting biocontrol candidates.     

Analogous wheat root rhizosphere microbial successions in field and greenhouse trials in the presence of biocontrol agents Paenibacillus peoriae SP9 and Streptomyces fulvissimus FU14

Two Pythium-infested soils were used to compare the wheat root and rhizosphere soil microbial communities from plants grown in the field or in greenhouse trials and their stability in the presence of biocontrol agents. Bacteria showed the highest diversity at early stages of wheat growth in both field and greenhouse trials, while fungal diversity increased later on, at 12 weeks of the crop cycle. The microbial communities were stable in roots and rhizosphere samples across both soil types used in this study. Such stability was also observed irrespective of the cultivation system (field or greenhouse) or addition of biocontrol coatings to wheat seeds to control Pythium disease (in this study soil infected with Pythium sp. clade F was tested). In greenhouse plant roots, Archaeorhizomyces, Debaryomyces, Delftia, and unclassified Pseudeurotiaceae were significantly reduced when compared to plant roots obtained from the field trials. Some operational taxonomic units (OTUs) represented genetic determinants clearly transmitted vertically by seed endophytes (specific OTUs were found in plant roots) and the plant microbiota was enriched over time by OTUs from the rhizosphere soil. This study provided key information regarding the microbial communities associated with wheat roots and rhizosphere soils at different stages of plant growth and the role that Paenibacillus and Streptomyces strains play as biocontrol agents in supporting plant growth in infested soils.     

Post-fire management targeting invasive annual grasses may have inadvertently released the exotic perennial forb Chondrilla juncea and suppressed its biocontrol agent

Top-down and bottom-up factors affecting invasive populations are rarely considered simultaneously, yet their interactive responses to disturbances and management interventions can be essential to understanding invasion patterns. We evaluated post-fire responses of the exotic perennial forb Chondrilla juncea (rush skeletonweed) and its biocontrol agents to landscape factors and a post-fire combined herbicide (imazapic) and bacteria (Pseudomonas fluorescens strain MB906) treatment that targeted invasive annual grasses in a sagebrush steppe ecosystem. Biocontrol agents released against C. juncea in previous decades included Cystiphora schmidti (gall midge), Aceria chondrillae (gall mite), and Puccinia chondrillina (rust fungus). C. juncea abundance was greater in sprayed than unsprayed plots, and where soils were coarser, slopes faced southwest, solar heat loads and topographic water accumulation were greater, and cover of deep-rooted native perennials was lower. Mite infestation was greater in unsprayed plots, midge infestation was greater at higher elevations on steeper slopes, and midges were more abundant while rust was less abundant on gravelly soils. Biocontrol infestation levels varied considerably between years and could not be predicted in 2019 from 2018 infestation levels. Multiple biocontrol species were often present at the same plots but were rarely present on the same C. juncea individuals. These results suggest that spatial patterns of invasion by C. juncea are related to deep-soil water availability, warmer conditions, and alleviation of competition. Treatments designed to reduce invasive annual grasses may inadvertently release C. juncea by both reducing plant competition for soil resources and affecting biocontrol agent (mite) abundance.

     

Development of a mode of application of bioorganic fertilizer for improving the biocontrol efficacy to <Emphasis Type="Italic">Fusarium</Emphasis> wilt

More effective ways of applying biocontrol products should be developed based both on the characteristics of the biocontrol agents and the normal practices of the agricultural producer. A new system was developed to improve the biocontrol efficacy of Fusarium wilt for watermelon production, and this system was tested in pot and field experiments. Biocontrol was achieved by applying a novel bioorganic fertilizer product (BIO) to Fusarium-infested soil. The best biocontrol was obtained by application of a bioorganic fertilizer, BIO, into soil during the nursery phase of watermelon seedling followed by a second application to Fusarium-infested soil when watermelon seedlings were transplanted. In comparison with the controls, the incidence of the disease was reduced by 60–100% in the pot experiment and by 59–73% in the field experiment when the BIO was applied during the nursery stage. After application of BIO during the nursery stage, the number of colony-forming units of Fusarium oxysporum in rhizospheric soil was significantly (P < 0.05) inhibited compared to the controls. An in vitro experiment showed that the antagonist Paenibacillus polymyxa in the BIO could effectively colonize the rhizosphere of watermelon and proliferate along the extending plant roots. This inhibited growth of Fusarium oxysporum in the rhizosphere of watermelon and protected the watermelon roots from attack by the pathogens. The method used for biocontrol Fusarium wilt disease in watermelon should be a useful strategy to improve field efficacy of other biocontrol agents.     

拮抗植物病原真菌链霉菌菌株ZH-356的鉴定及其生防评价

[目的]对实验室分离到的菌株ZH-356进行鉴定并评价其对植物病原真菌的生物防治效果,为研发针对植物真菌病害的生防菌剂提供理论指导。[方法]通过平板对峙法确定菌株ZH-356抗菌谱,并通过16S rRNA基因序列分析确定其种属,利用离体枝条的苹果树腐烂病菌感染预防试验和患腐烂病苹果树的防治试验评价其生防效果。[结果]菌株ZH-356鉴定为链霉菌属,与直丝紫链霉菌(Streptomyces rectiviolaceus)相似性最高,为99.71%。抗菌谱试验表明,菌株ZH-356对苹果树腐烂病菌、小麦赤霉病菌、小麦根腐病菌和番茄早疫病菌等多种植物病原真菌均具有较强的抑制作用,这种抑制作用可导致苹果树腐烂病菌菌丝变粗、交叉扭曲、分支变少且容易断裂。此外,ZH-356产生的抑菌活性物质对温度和酸碱度具有高度稳定性,并且该活性物质只存在于其胞内,只有当ZH-356遇到植物病原真菌时才会被分泌出来以抑制它们的生长。在离体枝条的苹果树腐烂病菌感染预防试验中,ZH-356对苹果树腐烂病防效可达94%以上,而在患腐烂病苹果树的防治试验中,ZH-356菌制剂对苹果树腐烂病的防效高达100%。[结论]链霉菌ZH-356抑菌谱广,对多种植物病原真菌均具有良好的拮抗活性,可作为防治植物真菌病害的生防菌株,为基于ZH-356菌株的生防菌剂的开发和防治苹果树腐烂病等植物真菌病害奠定了基础。     

Genetic diversity and recombination in natural populations of the nematode‐trapping fungus Arthrobotrys oligospora from China

Nematophagous fungi can trap and capture nematodes and other small invertebrates. This unique ability has made them ideal organisms from which to develop biological control agents against plant‐ and animal‐parasitic nematodes. However, effective application of biocontrol agents in the field requires a comprehensive understanding about the ecology and population genetics of the nematophagous fungi in natural environments. Here, we genotyped 228 strains of the nematode‐trapping fungus Arthrobotrys oligospora using 12 single nucleotide polymorphic markers located on eight random DNA fragments. The strains were from different ecological niches and geographical regions from China. Our analyses identified that ecological niche separations contributed significantly, whereas geographic separation contributed relatively little to the overall genetic variation in our samples of A. oligospora. Interestingly, populations from stressful environments seemed to be more variable and showed more evidence for recombination than those from benign environments at the same geographic areas. We discussed the implications of our results to the conservation and biocontrol application of A. oligospora in agriculture and forestry.     

Contribution of a parasitoid species to multiplication and transmission of a multiple nucleopolyhedrovirus in caterpillars

Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) and Microplitis pallidipes are both used as biocontrol agents of the beet armyworm (Spodoptera exigua). However, it has not been determined how beet armyworms respond when these agents interact. Here, we studied the effects of M. pallidipes on virus multiplication and transmission using quantitative detection of SeMNPV. Our results indicated that parasitoids promoted virus multiplication in caterpillars (10⁵ copies per caterpillar) and that it was more advantageous when the M. pallidipes oviposited one day prior to infection with NPV. Interestingly, SeMNPV was transmitted by M. pallidipes in four ways. Transmission efficiency was higher for parasitoids whose body surfaces were contaminated with NPV, and for parasitoids ovipositing on NPV-infected caterpillars, than for those emerging from NPV-infected caterpillars, or feeding on mixtures of honey, water and NPV. Our study reveals that parasitoids do affect the proliferation and transmission of NPV in caterpillars and suggests that M. pallidipes could be used to strengthen the effectiveness of SeMNPV as a biocontrol agent.     

Root Hairs Play a Key Role in the Endophytic Colonization of Olive Roots by <Emphasis Type="Italic">Pseudomonas</Emphasis> spp. with Biocontrol Activity

The use of indigenous bacterial root endophytes with biocontrol activity against soil-borne phytopathogens is an environmentally-friendly and ecologically-efficient action within an integrated disease management framework. The earliest steps of olive root colonization by Pseudomonas fluorescens PICF7 and Pseudomonas putida PICP2, effective biocontrol agents (BCAs) against Verticillium wilt of olive (Olea europaea L.) caused by the fungus Verticillium dahliae Kleb., are here described. A gnotobiotic study system using in vitro propagated olive plants, differential fluorescent-protein tagging of bacteria, and confocal laser scanning microscopy analysis have been successfully used to examine olive roots–Pseudomonas spp. interactions at the single-cell level. In vivo simultaneous visualization of PICF7 and PICP2 cells on/in root tissues enabled to discard competition between the two bacterial strains during root colonization. Results demonstrated that both BCAs are able to endophytically colonized olive root tissues. Moreover, results suggest a pivotal role of root hairs in root colonization by both biocontrol Pseudomonas spp. However, colonization of root hairs appeared to be a highly specific event, and only a very low number of root hairs were effectively colonized by introduced bacteria. Strains PICF7 and PICP2 can simultaneously colonize the same root hair, demonstrating that early colonization of a given root hair by one strain did not hinder subsequent attachment and penetration by the other. Since many environmental factors can affect the number, anatomy, development, and physiology of root hairs, colonization competence and biocontrol effectiveness of BCAs may be greatly influenced by root hair’s fitness. Finally, the in vitro study system here reported has shown to be a suitable tool to investigate colonization processes of woody plant roots by microorganisms with biocontrol potential.     

Characterization of <Emphasis Type="Italic">Penicillium</Emphasis> Species by Ribosomal DNA Sequencing and BOX,ERIC and REP-PCR Analysis

The genus Penicillium is one of the largest and widely distributed fungal genera described to date. As a result, its taxonomic classification and species discrimination within this genus has become complicated. In this study, 52 isolates that belonged to the Penicillum genus and other related genera were characterized using two DNA-based methods: (i) analysis of the nucleotide sequences of internal transcribed spacers in ribosomal DNA and (ii) analysis of DNA fingerprints that were generated by polymerase chain reactions with specific primers for enterobacterial repetitive intergenic consensus (ERIC) and repetitive extragenic palindromic (REP) sequences, and BOX elements. Using both methods, Penicillium species were discriminated from other fungal genera. Furthermore, Penicillium species that include strains which are used as biocontrol agents, such as P. glabrum, P. purpurogenum, and P. oxalicum, could be distinguished from other Penicillium species using these techniques. Based on our findings, we propose that a polyphasic approach that includes analysis of the nucleotide sequences of ribosomal DNA and detecting the presence of highly conserved, repeated nucleotide sequences can be used to determine the genetic relationships between different Penicillium species. Furthermore, we propose that our results can be used as a start point to develop a strategy to monitor the environmental presence of particular strains of Penicillium species when they are used as biocontrol agents.     

Screening of freeze-dried protective agents for the formulation of biocontrol strains, Bacillus cereus AR156, Burkholderia vietnamiensis B418 and Pantoea agglomerans 2Re40

Aims: The effects of different freeze‐drying protective agents on the viabilities of biocontrol strains Bacillus cereus AR156, Burkholderia vietnamiensis B418 and Pantoea agglomerans 2Re40 were investigated. Method and Results: Several concentrations of protective and rehydration media were tested to improve the survival of biocontrol agents after freeze‐drying. The subsequent survival rates during storage and rehydration media of freeze‐dried biocontrol strains were also examined. Conclusions: The results indicated that cellobiose (5%) and d ‐galactose (5%) gave maximum viability of strains Bu. vietnamiensis B418 and P. agglomerans 2Re40 (98 and 54·3% respectively) while the perfect one (100%) of strain B. cereus AR156 was obtained with sucrose (5%) during freeze‐drying, and the highest survival of the three strains was reached when they were rehydrated with 10% nonfat skim milk. In the following storage, the survival rates showed that B. cereus AR156 could still reach 50% after 12 months. Significance and Impact of the study: This study showed that freeze‐drying could be used to stabilize cells of these three biocontrol strains. Further studies should focus on the scale‐up possibilities and formulation development.     

Tritrophic interactions between a fungal pathogen,a spider predator,and the blacklegged tick

The blacklegged tick Ixodes scapularis is the primary vector for the bacterium causing Lyme disease in eastern North America and for other medically important pathogens. This species is vulnerable to attack by fungal pathogens and arthropod predators, but the impacts of interactions between biocontrol agents have not been examined. The biocontrol agent Met52^®, containing the entomopathogenic fungus Metarhizium brunneum (=M. anisopliae), controls blacklegged ticks with efficacy comparable to chemical acaricides. The brush‐legged wolf spider Schizocosa ocreata is a predator of I. scapularis that reduces their survival under field conditions. We conducted a field microcosm experiment to assess the compatibility of Met52 and S. ocreata as tick biocontrol agents. We compared the fits of alternative models in predicting survival of unfed (flat) and blood‐fed (engorged) nymphs. We found the strongest support for a model that included negative effects of Met52 and S. ocreata on flat nymph survival. We found evidence for interference between biocontrol agents, with Met52 reducing spider survival, but we did not find a significant interaction effect between the two agents on nymph survival. For engorged nymphs, low recovery rates resulted in low statistical power to detect possible effects of biocontrol agents. We found that nymph questing activity was lower when the spider was active above the leaf litter than when the spider was unobserved. This provides the first evidence that predation cues might affect behavior important for tick fitness and pathogen transmission. This study presents field microcosm evidence that the biopesticide Met52 and spider Schizocosa ocreata each reduced survival of blacklegged ticks Ixodes scapularis. Met52 reduced spider survival. Potential interference between Met52 and the spider should be examined at larger scales, where overlap patterns may differ. Ticks were more likely to quest when the spider was inactive, suggesting the ticks changed their behavior to reduce danger.     

Biocontrol of aerial plant diseases in agriculture and horticulture: current approaches and future prospects

Until recently, the majority of research on the biological control of aerial plant diseases was focused on control of bacterial pathogens. Such research led to the commercialization of the biocontrol agent Pseudomonas fluorescens A506, as BlightBan A506™, for control of fire blight of pear. In contrast, chemical fungicides typically have provided adequate control of most foliar fungal pathogens. However, fungicide resistance problems, concerns regarding pesticide residues and revocation of registration of certain widely used fungicides have led to increased activity in the development of biocontrol agents of foliar fungal pathogens. Much of this activity has centered around the use of Trichoderma spp and Gliocladium spp to control Botrytis cinerea on grape and strawberry. The biocontrol agent Trichoderma harzianum T39 is commercially available in Israel, as Trichodex ™, for control of grey mold in grapes and may soon be registered for use in the US. Also targeted primarily against a foliar disease of grapes, in this case powdery mildew caused by Uncinula necator, is the biocontrol agent Ampelomyces quisqualis AQ10, marketed as AQ^{10  TM} biofungicide. Another promising development in the area of foliar disease control, though one which is not yet commercialized, is the use of rhizobacteria as seed treatments to induce systemic resistance in the host plant, a strategy which can protect the plant against a range of bacterial and fungal pathogens. Received 06 February 1997/ Accepted in revised form 05 June 1997     

Could Bacillus biofilms enhance the effectivity of biocontrol strategies in the phyllosphere?

Maize (Zea mays L.), a major crop in Argentina and a staple food around the world, is affected by the emergence and re-emergence of foliar diseases. Agrochemicals are the main control strategy nowadays; however, they can cause resistance in insects and microbial pathogens and have negative effects on the environment and human health. An emerging alternative is the use of living organisms, i.e. microbial biocontrol agents, to suppress plant pathogen populations. This is a risk-free approach when the organisms acting as biocontrol agents come from the same ecosystem as the foliar pathogens they are meant to antagonize. Some epiphytic microorganisms may form biofilm by becoming aggregated and attached to a surface, as is the case of spore-forming bacteria from the genus Bacillus. Their ability to sporulate and their tolerance to long storage periods make them a frequently used biocontrol agent. Moreover, the biofilm that they create protects them against different abiotic and biotic factors and helps them to acquire nutrients, which ensures their survival on the plants they protect. This review analyzes the interactions that the phyllosphere-inhabiting Bacillus genus establishes with its environment through biofilm, and how this lifestyle could serve to design effective biological control strategies.