Microbiological control of plant pathogens

The effect of a consortium of Penicillium sp. and Bacillus spp. in suppressing banana fungal diseases caused by Fusarium sp. and Alternaria sp

AIMS

This study sought to utilize indigenous soil micro-organisms to suppress wilt-causing fungal pathogens of the banana.

METHODS AND RESULTS

Fungal pathogens were isolated from wilt-affected rhizospheric soil, and potential antagonistic bacterial strains were isolated from healthy rhizospheric soil in the same area from which fungal pathogens were isolated. The antifungal activity of isolated micro-organisms against fungal pathogens was studied both in vitro and in vivo against fungal pathogens. It was found that Fusarium oxysporum and Alternaria sp. were pathogenic, while Penicillium sp., Bacillus velezensis and Bacillus subtilis were antagonistic. Moreover, it was seen that B. velezensis, B. subtilis and Penicillium sp. inhibited the growth of the two fungal pathogens in both in vitro and in vivo experiments. Further investigation indicated that B. velezensis, B. subtilis and Penicillium sp. were able to produce enzymatic antifungal compounds (chitinase and β-1,3-glucanase). The spray application around rhizome revealed that a combination of Bacillus spp. and Penicillium sp. in greenhouse conditions gave the highest reduction in disease severity by up to 60% to both fungal pathogens among the treatments.

CONCLUSIONS

Banana disease is seen to be induced not only by F. oxysporum but also by Alternaria sp. The isolated indigenous micro-organisms can effectively control both the pathogens. The combination of isolated antagonistic micro-organisms has thus demonstrated substantial potential for suppressing banana disease.

SIGNIFICANCE AND IMPACT OF THE STUDY

An antagonistic consortium isolated in this study has demonstrated remarkable potential for controlling fungal diseases caused by Fusarium sp. and Alternaria sp. Therefore, the use of indigenous microflora to improve disease suppression of banana plants against soil-borne pathogens is a preferable approach.

Pseudomonas stutzeri YPL-1 Genetic Transformation and Antifungal Mechanism against Fusarium solani, an Agent of Plant Root Rot

An actively antagonistic bacterium that could be used as a biocontrol agent against Fusarium solani, which causes root rots with considerable losses in many important crops, was isolated from a ginseng rhizosphere and identified as a strain of Pseudomonas stutzeri. In several biochemical tests with culture filtrates of P. stutzeri YPL-1 and in mutational analyses of antifungal activities of reinforced or defective mutants, we found that the anti-F. solani mechanism of the bacterium may involve a lytic enzyme rather than a toxic substance or antibiotic. P. stutzeri YPL-1 produced extracellular chitinase and laminarinase when grown on different polymers such as chitin, laminarin, or F. solani mycelium. These lytic extracellular enzymes markedly inhibited mycelial growth rather than spore germination and also caused lysis of F. solani mycelia and germ tubes. Scanning electron microscopy revealed degradation of the F. solani mycelium. Abnormal hyphal swelling and retreating were caused by the lysing agents from P. stutzeri YPL-1, and a penetration hole was formed on the hyphae in the region of interaction with the bacterium; the walls of this region were rapidly lysed, causing leakage of protoplasm. Genetically bred P. stutzeri YPL-1 was obtained by transformation of the bacterium with a broad-host-range vector, pKT230. Also, the best conditions for the transformation were investigated.

Purification and Characterization of an Extracellular Chitinase from the Antifungal Biocontrol Agent Streptomyces halstedii

An extracellular chitinase from Streptomyces halstedii AJ-7, a broad spectrum antifungal biocontrol agent, was characterized and purified. The apparent molecular weight of the purified protein was 55 kDa, Km value and Vmax of the protein for colloidal chitin were 3.2 mg ml(-1) and 118 micromol h(-1), respectively. The growth and chitinase activity of S. halstedii AJ-7 were enhanced by adding of 0.1% killed mycelium of Fusarium oxysporium in a medium containing 0.2% colloidal chitin.

Evaluation of Composts for Suppression of Dollar Spot (Sclerotinia homoeocarpa) of Turfgrass

The use of composts in turfgrass disease management allows for a reduction in pesticide use in chemical control practices. Disease suppressive properties of composts rely on a number of factors including microbial activity, microbial population dynamics, nutrient concentrations, and other associated chemical and physical factors. Five composts were evaluated for suppression of dollar spot caused by Sclerotinia homoeocarpa. The dollar spot disease suppressive properties of selected compost formulations prepared in different years was evaluated. A third objective was to examine the effects of storage of compost (1 year) on the suppression of dollar spot. Field experiments were conducted in 1998 with compost prepared in 1997 to 1998. Applications of compost every 3 weeks throughout the season suppressed dollar spot of turf to levels not significantly different than applications of chlorothalonil fungicide applied at the manufacturer's lowest recommended preventative rate of 38.4 ml a.i./100 m² every 2 weeks (P = 0.05). Single applications of composts at the start of the 1998 season were not effective in reducing disease. Field experiments in 1999 evaluated batches of two selected compost formulations, one batch produced in 1998 to 1999, another stored since production in 1997 to 1998. Composts were effective in suppressing disease to levels not significantly different than the fungicide controls, which showed up to 33% disease in 1998 and up to 31% disease in 1999 (P = 0.05). Storage of composts for up to 1 year did not affect their ability to reduce dollar spot severity (P = 0.05). The use of composts as plant disease suppressants is not likely to replace the use of commercial fungicides in dollar spot management. However, multiple applications of compost may reduce incidence and severity of dollar spot to levels at which chemical control may be reduced or eliminated for a significant portion of the season.

Effects of Cellulytic Enzymes on Phytophthora cinnamomi

ABSTRACT Two enzyme systems, cellulase (beta-1,4-glucanase) and laminarinase (beta-1,3-glucanase), were added to soil extracts to simulate (in vitro) lytic components found in mulches suppressive to Phytophthora cinnamomi. Concentration ranges of each enzyme were incubated with Phytophthora cinnamomi mycelium, zoospores, zoospores cysts, and zoospore-infected excised roots to evaluate the roles of each enzyme in potential control of avocado root rot disease. Cellulase significantly retarded the development of zoosporangia and chlamydospores when mycelia were incubated in soil extract containing the enzyme at concentrations greater than 10 units/ml. Zoospore production was also reduced by cellulase but not by laminarinase. Laminarinase had little effect on zoosporangia or chlamydospore formation. At high concentrations, laminarinase was consistently more effective at preventing encystment than cellulase. Chlamydospores preformed in root tips were immune to the lytic effects of all treatments except cellulase at 100 units/ml. Zoospores placed in enzyme solutions and plated on a selective medium survived high cellulase concentrations and formed colonies, but there were fewer surviving zoospores when laminarinase was present at greater than 10 units/ml. Low concentrations of cellulase stimulated infection of excised roots, however, low concentrations of laminarinase prevented infection. Cellulase and laminarinase have different effects on the structures of the Phytophthora cinnamomi life history, however, each enzyme may have a role in reduction of inoculum.

A method for selecting Trichoderma strains antagonistic against Sclerotinia minor

Thirty strains of T. hamatum, T. harzianum, T. koningii, T. longibrachiatum and T. viride were selected for in vitro mycoparasitic activity against Sclerotinia minor. High destruction of sclerotia (up to 70% in two weeks) was caused by antagonists only at 25 degrees C, in autoclaved soil, because the optimum temperature for radial growth of Trichoderma strains (25-30 degrees C) was significantly higher than those of phytophatogenic fungus (20 degrees C). The most active strains, which were selected also for competitive saprophytic ability could enhance the percentage of decayed sclerotia 5-9 fold over the control even after 1 week under non sterile conditions. A silicon rubber fixation technique was developed for recovery of irregularly shaped S. minor sclerotia.

Chitinase production by Streptomyces viridificans: its potential in fungal cell wall lysis

Streptomyces viridificans was found to be a good chitinase producer among nine species of Streptomyces screened. Minimum levels of constitutive enzyme were observed with both simple and complex carbon substrate. Arabinose doubled the enzyme production amongst the various pentoses and hexoses used with chitin. However, with glucose end-product inhibition and catabolite repression were observed. The enzyme tolerated a wide range of temperature (30-55 degrees C) and pH (3-7.5). Among various divalent cations Mn2+ and Hg2+ completely inhibited the purified enzyme while beta-mercaptoethanol stimulated its activity. Crude and purified enzyme had potential for cell wall lysis of many fungal pathogens tested.

Characterization of ech-42, a Trichoderma harzianum endochitinase gene expressed during mycoparasitism

A gene (ech-42; previously named ThEn-42) coding for one of the endochitinases produced by the biocontrol agent Trichoderma harzianum IMI206040 was cloned and characterized. Expression of the cDNA clone in Escherichia coli resulted in bacteria with chitinase activity. This chitinase has been shown to have lytic activity on Botrytis cinerea cell walls in vitro. The ech-42 gene was assigned to a double chromosomal band (chromosome V or VI) upon electrophoretic separation and Southern analysis of the chromosomes. Primer extension analysis indicated that transcription of the gene begins preferentially 109 bp upstream of the translation initiation codon. Expression of ech-42 was strongly enhanced during direct interaction of the mycoparasite with a phytopathogenic fungus when confronted in vitro and by growing it in minimal medium containing chitin as sole carbon source. Similarly, light-induced sporulation resulted in high levels of transcript, suggesting developmental regulation of the gene. The implications of these findings are discussed.

Detection of mycoparasitism by infrared photomicrography

The fungusTrichoderma harzianum which parasitizes its hostRhizoctonia solani (AG 1-6) was observed under a light microscope and the interaction sites photomicrographed with infrared film. Bright regions indicating infrared irradiation were observed at the interaction sites, apparently due to the high parasitic activity occurring there. The possible use of infrared photomicrography in cell-cell interactions is discussed.

Influence of Environmental Factors on Antagonism of Fungi by Bacteria in Soil: Clay Minerals and pH

The soil replica plating technique was used to evaluate the influence of clay minerals and pH on antagonistic interactions between fungi and bacteria in soil. In general, the antagonistic activity of bacteria towards filamentous fungi was greater in soil than on agar. The spread of Aspergillus niger through soil was inhibited by Serratia marcescens when the organisms were inoculated into separate sites in soil, and this antagonistic effect was maintained when the soil was amended with 3, 6, 9, or 12% (vol/vol) montmorillonite, whereas the addition of kaolinite at a concentration of 3% reduced the antagonism and at 6, 9, or 12% totally eliminated it. Similar results were obtained with the inhibition of A. niger by Agrobacterium radiobacter and of Penicillium vermiculatum by either S. marcescens or Nocardia paraffinae . When A. niger and S. marcescens were inoculated into the same soil site, A. niger was inhibited in all soils, regardless of clay content, although the extent of inhibition was greater as the concentration of montmorillonite, but not of kaolinite, increased. A. niger was inhibited more when inoculated as spores than as mycelial fragments and when inoculated 96 h after S. marcescens, but a 1% glucose solution reduced the amount of inhibition when the fungus was inoculated 96 h after the bacterium. When the pH of the soil-clay mixtures was altered, the amount of antagonism usually increased as the pH increased. Antagonism appeared to be related to the cation-exchange capacity and the pH of the soil-clay mixtures. Bacillus cereus and another species of Bacillus showed no activity in soil towards A. niger under any of the environmental conditions tested, even though the Bacillus sp. significantly inhibited A. niger and seven other fungi on agar.