Improved Biocontrol of Botrytis cinerea on Detached Strawberry Leaves by Adding Nutritional Supplements to a Mixture of Pichia guilermondii and Bacillus mycoides

Nutrient availability is one of the factors that govern the efficacy of biocontrol. The contribution of 14 different vitamins, amino acids and nutritional supplement combinations to suppression of Botrytis cinerea by Bacillus mycoides and Pichia guilermondii , alone or in a mixture, was tested on detached strawberry leaflets. Some of the nutritional supplements reduced B. cinerea development and improved biocontrol efficacy. Addition of nutritional supplements to a mixture of the biocontrol agents further improved control efficacy.     

Real-time RT-PCR expression analysis of chitinase and endoglucanase genes in the three-way interaction between the biocontrol strain Clonostachys rosea IK726, Botrytis cinerea and strawberry

Clonostachys rosea is a well-known biocontrol agent against Botrytis cinerea, the causal agent of gray mold in strawberry. The activity of cell wall-degrading enzymes might play a significant role for successful biocontrol by C. rosea. The expression pattern of four chitinases, and two endoglucanase genes from C. rosea strain IK726 was analyzed using real-time RT-PCR in vitro and in strawberry leaves during interaction with B. cinerea. Specific primers were designed for beta-tubulin genes from C. rosea and B. cinerea, respectively, and a gene encoding a DNA-binding protein (DBP) from strawberry, allowing in situ activity assessment of each fungus in vitro and during their interaction on strawberry leaves. Growth of B. cinerea was inhibited in all pathogen-antagonist interactions while the activity of IK726 was slightly increased. In all in vitro interactions, four of the six genes were upregulated while no change in expression of two endochitinases was measured. In strawberry leaves, the chitinase genes were upregulated 2-12-fold, except one of the endochitinases, whereas no change in expression of the two endoglucanases was measured. The results suggest that three out of four chitinase genes of IK726 are involved in biocontrol on leaves. This is the first example of monitoring of expression of chitinolytic genes in interactions between biocontrol agents and pathogens in plant material.     

Using combinations of biocontrol agents to control Botrytis cinerea on strawberry leaves under fluctuating temperatures

Experiments were conducted with Botrytis cinerea on strawberry leaves to investigate where combinations of commercially available biological control agents (BCAs) might control B. cinerea more effectively than individual BCAs. Specifically, we studied the persistence of biocontrol activities, spread of BCAs among leaves, and biocontrol efficacy in relation to application regimes: mixed versus single BCA, pre-versus post-inoculation application, and sequential versus simultaneous application. Three BCA products (Sentinel, Serenade and Trianum) were used for this study. Overall, Serenade did not significantly reduce sporulation of B. cinerea on strawberry leaf discs whereas Sentinel and Trianum gave a similar and significant biocontrol efficacy. Biocontrol efficacy remained almost unchanged 10 days after application at 20/20°C (day/night) or 24/16°C temperature regimes. In contrast, reduced biocontrol efficacy at 26/14°C suggests BCA survival was reduced under these conditions. Incidence of B. cinerea sporulation on leaf discs was ca. 60% higher on leaves that emerged after the BCA application than on leaves directly exposed to BCA, indicating insufficient amount of the BCA had managed to spread to new leaves. Combinations of BCAs, whether applied simultaneously or sequentially (48 h apart), did not improve disease control over the most effective BCA within the combination applied alone. This indicated possible antagonism or interference between the BCAs. Results suggested that there was significant antagonism for most combinations of the three BCAs tested and the degree of antagonism increased as the time from BCA application to pathogen introduction lengthened.     

Biological control of strawberry crown rot,root rot and grey mould by the beneficial fungus Aureobasidium pullulans

Utilization of biocontrol agents is a sustainable approach to reduce plant diseases caused by fungal pathogens. In the present study, we tested the effect of the candidate biocontrol fungus Aureobasidium pullulans (De Bary) G. Armaud on strawberry under in vitro and in vivo conditions to control crown rot, root rot and grey mould caused by Phytophthora cactorum (Lebert and Cohn) and Botrytis cinerea Pers, respectively. A dual plate confrontation assay showed that mycelial growth of P. cactorum and B. cinerea was reduced by 33–48% when challenged by A. pullulans as compared with control treatments. Likewise, detached leaf and fruit assays showed that A. pullulans significantly reduced necrotic lesion size on leaves and disease severity on fruits caused by P. cactorum and B. cinerea. In addition, greenhouse experiments with whole plants revealed enhanced biocontrol efficacy against root rot and grey mould when treated with A. pullulans either in combination with the pathogen or pre-treated with A. pullulans followed by inoculation of the pathogens. Our results demonstrate that A. pullulans is an effective biocontrol agent to control strawberry diseases caused by fungal pathogens and can be an effective alternative to chemical-based fungicides.

     

Phenazines and biosurfactants interact in the biological control of soil-borne diseases caused by Pythium spp

In this study, the putative role of phenazines and rhamnolipid-biosurfactants, antagonistic metabolites produced by Pseudomonas aeruginosa PNA1, was tested in the biological control of Pythium splendens on bean ( Phaseolus vulgaris L) and Pythium myriotylum on cocoyam ( Xanthosoma sagittifolium L Schott). A rhamnolipid-deficient and a phenazine-deficient mutant of PNA1 were used either separately or jointly in plant experiments. When the mutants were applied separately, no disease-suppressive effect was observed, although both mutants still produced one of the antagonistic compounds (phenazines or rhamnolipids). When the mutants were concurrently introduced in the soil, the biocontrol activity was restored to wild-type levels. Bean seeds developed significantly less pre-emergence damping-off caused by P. splendens when treated with a mixture of purified phenazine-1-carboxamide and rhamnolipids than with any of the chemicals alone. When phenazines and rhamnolipids were combined at concentrations that had no observable effects when the metabolites were applied separately, mycelial growth of P. myriotylum was significantly reduced. In addition, microscopic analysis revealed substantial vacuolization and disintegration of Pythium hyphae after incubation in liquid medium amended with both metabolites. Results of this study indicate that phenazines and biosurfactants are acting synergistically in the control of Pythium spp.     

Evaluation of potential biocontrol rhizobacteria from different host plants of Verticillium dahliae Kleb

AIMS: A screening approach was developed to assess the potential of rhizobacterial strains to control Verticillium wilt caused by Verticillium dahliae Kleb. METHODS AND RESULTS: Sixty randomly chosen antagonistic bacterial strains originally isolated from rhizosphere of three different host plants of V. dahliae--strawberry, potato and oilseed rape--were evaluated for biocontrol and plant growth promotion by analysing in vitro antagonism towards V. dahliae and other plant pathogenic fungi, production of fungal cell wall-degrading enzymes and plant growth-promoting effects on strawberry seedlings. To test the plant growth-promoting effect, a microplate assay with strawberry seedlings was developed. Although the rhizobacterial strains were isolated from different plants they showed effects on the growth of strawberry seedlings. According to the in vitro biocontrol and plant growth-promoting activity, the three best candidates Pseudomonas putida B E2 (strawberry rhizosphere), Ps. chlororaphis K15 (potato rhizosphere) and Serratia plymuthica R12 (oilseed rape rhizosphere) were selected for greenhouse experiments to verify the in vitro screening results. Under greenhouse conditions the isolates selected according to this strategy were as effective, or more effective than commercial biocontrol agents and may therefore possibly be valuable as antagonists of V. dahliae. CONCLUSIONS: In this study, the screening strategy resulted in a selection of three interesting biocontrol candidates against Verticillium: Ps. putida B E2 (strawberry rhizosphere), Ps. chlororaphis K15 (potato rhizosphere) and Ser. plymuthica R12 (oilseed rape rhizosphere). SIGNIFICANCE AND IMPACT OF THE STUDY: A new combination of in vitro screening methods including a microplate assay with strawberry seedlings to test the plant growth promoting effect which allow to more efficiently select potential biological control agents was developed successfully.     

Separate and combined disruptions of two exo-beta-1,3-glucanase genes decrease the efficiency of Pichia anomala (strain K) biocontrol against Botrytis cinerea on apple

The modes of action of the antagonistic yeast Pichia anomala (strain K) have been studied; however, thus far, there has been no clear demonstration of the involvement of exo-beta-1,3-glucanase in determining the level of protection against Botrytis cinerea afforded by this biocontrol agent on apple. In the present study, the exo-beta-1,3-glucanase-encoding genes PAEXG1 and PAEXG2, previously sequenced from the strain K genome, were separately and sequentially disrupted. Transfer of the URA3-Blaster technique to strain K, allowing multiple use of URA3 marker gene, first was validated by efficient inactivation of the PaTRP1 gene and recovery of a double auxotrophic strain (uracil and tryptophan). The PAEXG1 and PAEXG2 genes then were inactivated separately and sequentially with the unique URA3 marker gene. The resulting mutant strains showed a significantly reduced efficiency of biocontrol of B. cinerea when applied to wounded apple fruit, the calculated protection level dropping from 71% (parental strain) to 8% (mutated strain) under some experimental conditions. This suggests that exo-beta-1,3-glucanases play a role in the biological control of B. cinerea on apple. Furthermore, biological control experiments carried out in this study underline the complexity of the host-antagonist-pathogen interaction. Two experimental parameters (yeast inoculum concentration and physiological stage of the fruit) were found to influence dramatically the protection level. Results also suggest that, under some conditions, the contribution of exo-beta-1,3-glucanase to biological control may be masked by other modes of action, such as competition.     

Effect of three food types on the population growth of Brachionus calyciflorus and Brachionus patulus (Rotifera: Brachionidae).

We compared the population growth of B. calyciflorus and B. patulus using the green alga Chlorella vulgaris, baker's yeast Saccharomyces cerevisiae or their mixture in equal proportions as food. Food was offered once every 24 h in two concentrations (low: 1 x 10(6) and high: 3 x 10(6) cells ml-1) separately for each species. The experiments were terminated after 15 days. In general, at any food type or concentration, B. patulus reached a higher population density. A diet of Chlorella alone supported a higher population growth of both rotifer species than yeast alone. B. calyciflorus and B. patulus achieved highest population densities (103 +/- 8 ind. ml-1 and 296 +/- 20 ind. ml-1, respectively) on a diet of Chlorella at 3 x 10(6) cells ml-1. When cultured using the mixture of Chlorella and yeast, the maximal population densities of B. calyciflorus were lower than those grown on Chlorella. Under similar conditions, the maximal abundance values of B. patulus were comparable in both food types. Regardless of food type and density the rate of population increase per day (r) for B. calyciflorus varied from 0.13 +/- 0.03 to 0.63 +/- 0.04. These values for B. patulus ranged from 0.19 +/- 0.01 to 0.37 +/- 0.01. The results indicated that even though Chlorella was a superior food for the tested rotifers, yeast can be effectively used at low concentrations to supplement algal requirements in rotifer culture systems.     

Efficacy of Bacillus subtilis and Trichoderma harzianum combination on chickpea Fusarium wilt caused by F. oxysporum f. sp. ciceris

Fusarium wilt is caused by F. oxysporum Schlecht end. Fr. f. sp. ciceris (FOC) is a devastating disease of chickpea in Algeria. In this study, antagonistic effects of B. subtilis MF352017 (Bs1) and Trichoderma harzianum KX523899 (T5) isolated from the rhizosphere of chickpea were investigated separately and in combination for their efficacy in controlling the disease in vivo. The efficacy of the antagonistic biocontrol agents on Fusarium wilt was evaluated based on vegetative and root growth parameters of chickpea. Seed bacterisation with B. subtilis MF352017 (Bs1) and seed treatment with T. harzianum (T5) significantly protected chickpea seedlings from FOC as compared to untreated plants. Plant protection was more pronounced in T. harzianum-treated plants than in bacterised plants. The application of both antagonists effectively suppressed 93.67% of the disease and also enhanced plant growth leading to increased plant height, root length, fresh and dry weights of shoot and root. The mixture of antagonists increased the effectiveness of B. subtilis MF352017 (Bs1) isolate on Fusarium wilt and improved chickpea growth.     

Effects of a biocontrol agent of apple powdery mildew (Podosphaera leucotricha) on the host plant and on non-target organisms: an insect pest (Cydia pomonella) and a pathogen (Venturia inaequalis)

A number of studies have focused on the selection and use of new biocontrol agents, but the effects of the introduction of these microorganisms on non-target organisms, including the crop plants themselves, are not well known. Non-target effects of sprayed applications of a potential biocontrol agent of apple powdery mildew (Podosphaera leucotricha Ell. Et Ev.), on scab infections (Venturia inaequalis Cooke Winter), on codling moth [Cydia pomonella L. (Lepidoptera: Tortricidae)] oviposition and damage and apple (Malus x domestica) fruit quality are examined. This biocontrol agent, an epiphytic yeast isolate called Y16, affected neither conidia germination of V. inaequalis nor their penetration of the leaf tissue but suppressed the disease caused by this pathogen. The quantity of eggs laid by the codling moth during its second flight period on yeast treated trees was significantly different to the quantity of eggs laid on the untreated trees. In the first season of the experiments, more eggs were laid on the treated trees, especially on those tree parts closest to the fruit. These results, however, were not confirmed the following season: fewer eggs were laid on the treated trees than on the untreated trees. These conflicting observations are attributed to year-to-year variation in environmental conditions, which may affect yeast survival and activity. A 2-month-long assay was conducted in the orchard during the codling moth's second flight period from mid-July until mid-September. The yeast treatment did not affect the damage caused by the codling moth to the fruits. Finally, the yeast treatment did not affect any of the examined fruit quality parameters.     

生防真菌分生孢子抗氧化能力与多糖含量的关系

作为重要的丝孢类昆虫病原真菌,球孢白僵菌和玫烟色棒束孢因其易于生产和环境友好等优点而在害虫生防防治中受到广泛青睐。为初步探求孢子耐氧化力及其与孢子多糖含量的关系,球孢白僵菌和玫烟色棒束孢11株菌经胁迫后的残存指数随氧化剂H2O2浓度增加而减小。所有菌株的残存指数均能良好地与Logistic方程拟合,并计算出各菌株在氧化胁迫条件下的半致死浓度。结果显示玫烟色棒束孢孢子的耐氧化力强于球孢白僵菌。两种真菌的分生孢子耐氧力与各自多糖含量呈现良好线性正相关。培养基碳源成分和浓度变化可影响球孢白僵菌孢子耐氧化力,但耐氧化力与多糖含量依旧呈现线性正相关。由此可见,生防真菌分生孢子的耐氧化力的确与多糖积累有关,并在一定程度上受培养条件的调节。研究结果有望为提高生防真菌孢子环境稳定性提供新的策略。     

In vitro attachment of phylloplane yeasts to Botrytis cinerea, Rhizoctonia solani, and Sclerotinia homoeocarpa

The ability of yeasts to attach to hyphae or conidia of phytopathogenic fungi has been speculated to contribute to biocontrol activity on plant surfaces. Attachment of phylloplane yeasts to Botrytis cinerea, Rhizoctonia solani, and Sclerotinia homoeocarpa was determined using in vitro attachment assays. Yeasts were incubated for 2 d on potato dextrose agar (PDA) prior to experimentation. A total of 292 yeasts cultured on PDA were screened for their ability to attach to conidia of B. cinerea; 260 isolates (89.1%) attached to conidia forming large aggregates of cells, and 22 isolates (7.5%) weakly attached to conidia with 1 or 2 yeast cells attached to a few conidia. Ten yeasts (3.4%), including 8 isolates of Cryptococcus laurentii, 1 isolate of Cryptococcus flavescens, and an unidentified species of Cryptococcus, failed to attach to conidia. All non-attaching yeasts produced copious extracellular polysaccharide (EPS) on PDA. Seventeen yeast isolates did not attach to hyphal fragments of B. cinerea, R. solani, and S. homoeocarpa after a 1 h incubation, but attachment was observed after 24 h. Culture medium, but not culture age, significantly affected the attachment of yeast cells to conidia of B. cinerea. The 10 yeast isolates that did not attach to conidia when grown on agar did attach to conidia (20%-57% of conidia with attached yeast cells) when cultured in liquid medium. Attachment of the biocontrol yeast Rhodotorula glutinis PM4 to conidia of B. cinerea was significantly greater at 1 x 10(7) yeast cells x mL(-1) than at lower concentrations of yeast cells. The ability of yeast cells to attach to fungal conidia or hyphae appears to be a common phenotype among phylloplane yeasts.     

Isolation of bioactive antibiotic peptides from Bacillus brevis and Bacillus polymyxa against Botrytis grey mould in strawberry

Abstract

Grey mould caused by Botrytis cinerea is a devastating disease that results in extensive yield losses to strawberry. Bacillus brevis (Brevibacillus brevis) and Bacillus polymyxa (Paenibacillus polymyxa), which showed strong antifungal activity against B. cinerea, were isolated from the phyllosphere of strawberry plants. The advantage of using these bacteria is that the biochemistry and physiology of production of antibiotic peptides antimicrobial substances is well documented. A study was conducted to assess the activity of both Bacilli and their antibiotic peptides produced against B. cinerea in strawberry plants in vitro and in vivo. In vitro bioassay, both Bacilli have strongly inhibited pathogen germination, growth and extra-cellular enzyme production. Bacillus brevis was generally the most effective in reducing Botrytis growth. Gramicidin S and polymyxin B peptide antibiotics were extracted from culture filtrate of B. brevis and B. polymyxa, respectively, purified by silica thin chromatography and identified by high performance liquid chromatography. Germination, growth rate and production of extra-cellular enzymes were more sensitive to both antibiotics. Gramicidin S was the most active against B. cinerea with a minimal inhibitory concentration of 15 μmol/l. Polymyxin B also showed activity against B. cinerea at 25 μmol/l. Under controlled conditions (18 – 22°C, 90% relative humidity and 12 h photoperiod), strawberry plants were sprayed with pathogens (10⁵ spores/ml), antagonists (from 10⁵ to 10⁸ cells/ml) and antibiotic peptides (0 – 30 μmol/l) for reducing grey mould. Disease incidence was decreased in the presence of B. brevis. Both antibiotic peptides inhibited Botrytis growth that was observed by scanning electron microscopy. The plant leaves adsorbed significant amounts of antibiotics which reached from 46.1 to 67.5% of the original solution. Under natural field conditions, these biocontrol and antibiotic peptides at different concentrations were evaluated in 2003/2004 and 2004/2005 seasons against Botrytis grey mould. Treating plants with B. brevis exhibited a significant high activity against the development of Botrytis disease on strawberry. Gramicidin S showed a strong potential in reducing disease incidence, followed by polymyxin B, and acted as a fungicide to the pathogen growth. Inhibition of B. cinerea by both Bacilli was similar to equivalent levels of their antibiotics produced. In addition, these treatments significantly reduced the development of Botrytis and increased fruit yield. It can be suggested that B. brevis and B. polymyxa may be considered as potential biocontrol agents against Botrytis grey mould on strawberry based on the production of antifungal peptides. Therefore, gramicidin S and polymyxin B products are considered as biocontrol agents and may play a significant role in the future for practical applications in strawberry management systems.     

Bioassay methods for the detection of antifungal activity by Pseudomonas antimicrobica against the grey mould pathogen Botrytis cinerea

Antagonism against the grey mould pathogen Botrytis cinerea by Pseudomonas antimicrobica was demonstrated in vitro and in vivo. Cell-free filtrates showed activity against B. cinerea growing on Potato Dextrose Agar (PDA) in a media-dependent manner with the most distinct antagonism being produced in Czapek Dox Broth (CDB). Cell-free filtrates of CDB-grown cultures also significantly reduced conidial germination of B. cinerea. An assays based on the inhibition of conidial germination was compared with two assays measuring the antagonism of mycelial growth on PDA. The conidial germination bioassay was more sensitive in the detection of this antifungal activity than the Petri dish bioassay while a bioassay using Microdetection plates did not detect antagonism due to the small loading capacity of the latter. The conidial germination bioassay was modified for detection of antibiosis on the surface of strawberry leaves. Significant reductions in percentage conidial germination were recorded on the surface of leaves of both micropropagated and glasshouse grown strawberry plants when the antifungal compounds of Ps. antimicrobica were applied to the leaf tissue with the conidia. In addition, antifungal compounds were also detectable when conidia were applied to leaf tissue which had previously been sprayed with cells of Ps. antimicrobica. These tests indicate that Ps. antimicrobica would be a suitable biocontrol agent for the control of B. cinerea.     

Biocontrol of Rhizoctonia crown and root rot of sugar beet by binucleate Rhizoctonia spp. and Laetisaria arvalis

Two isolates of Laetisaria arvalis and 10 of binucleate Rhizoctonia spp. (BNR) from the Ohio sugar beet production area, were tested in the greenhouse and field for biocontrol of Rhizoctonia crown and root rot of sugar beet, caused by Rhizoctonia solani anastomosis group 2, type 2. L. arvalis was ineffective in standard greenhouse tests, and the single isolate used in the field was generally ineffective. Seven of 10 BNR isolates effectively controlled crown and root rot in greenhouse tests. Delayed application of biocontrol agents to plants 5 – 10 wk old was generally more effective than applications made at planting. A BNR isolate significantly reduced % plant loss and disease ratings and increased yield in a 1985 field test as compared with the control infested with R. solani alone. Two BNR isolates were effective in a 1986 field test and increased yields c. 22% in comparison to a L. arvalis treatment, which did not differ from the R. solani-infested control. The Ohio binucleate Rhizoctonia isolates appear to have considerable potential as applied biocontrol agents and may play a role in the natural ecology of R. solani in the sugar beet production area of Ohio.     

一株寒地高效解无机磷细菌的分离鉴定及拮抗作用

【目的】从北方寒地种植的不同农作物根际土壤中分离高效解磷的细菌,为微生物制剂和磷肥的开发提供适于本地区的优良菌种。【方法】通过初筛和复筛从26株解磷菌中筛选获得一株高效解磷细菌,对其进行生理生化和分子生物学鉴定,同时采用钼蓝比色法测定解磷能力。采用平板对峙法测定拮抗植物病原菌能力。【结果】通过筛选后获得的菌株B51-7经鉴定为伯克霍尔德菌属。菌株在发酵液中可溶性磷含量最高达到832.74 mg/L,同时具有很强的广谱抑菌作用,抑菌率最高为89.71%,可以显著促进水稻生长。【结论】菌株B51-7是一株具有生物防治作用的高效解磷细菌,可应用于生物菌肥和生防制剂中。     

Evaluation of antifungal activity of carbonate and bicarbonate salts alone or in combination with biocontrol agents in control of citrus green mold

The aim of this research was to determine if the attacks of green mold on orange could be reduced by edible salts alone or in combination with biocontrol agent. For this purpose toxicity to Pantoea digitatum and practical use of sodium carbonate (SC), sodium bicarbonate (SBC) and potassium carbonate, and potassium bicarbonate alone or in combination with antagonistic bacteria (Pseudomonas fluorescens isolate PN, Bacillus subtilis isolate VHN, Pantoea agglomerans isolate CA) to control green mold were determined. All were fungistatic. SC and SBC were equal and superior to the other salts for control of green mold on oranges inoculated 6h before treatment and were chosen for subsequent trails under cold storage conditions. The biocontrol agents were found completely tolerant to 3% sodium bicarbonate and sodium carbonate at room temperature; although their culturability was reduced by > 1000-fold after 60 min in 1% other salt solutions. Satisfactory results were also obtained with the combined treatment for control of green mold. A significant increase in biocontrol activity of all isolate was observed when combined with sodium carbonate and sodium bicarbonate. The treatments comprising CA combined with SB was as effective as fungicide treatment. Thus, use of sodium bicarbonate treatment at 3% followed by the antagonist P. agglomerans CA could be an alternative to chemical fungicides for control of green mold on oranges.     

Competition and antibiosis in the biological control of potato scab

Nonpathogenic, antibiotic-producing streptomycetes have been shown to reduce potato scab when added to disease-conducive soil. Spontaneous mutants of the pathogenic Streptomyces scabies RB4 that are resistant to at least one antibiotic activity produced by the nonpathogenic suppressive isolates Streptomyces diastatochromogenes strain PonSSII and S. scabies PonR have been isolated. To determine the importance of antibiosis in this biocontrol system, these mutants were investigated for their ability to cause disease in the presence of the two pathogen antagonists in a greenhouse assay. Disease caused by one of the mutant strains was reduced in the presence of both suppressive isolates, whereas disease caused by the other five mutants was not significantly reduced by either suppressive strain. In addition, a nonpathogenic mutant of S. scabies RB4 was isolated, which produced no detectable in vitro antibiotic activity and reduced disease caused by its pathogenic parent strain when the pathogen and mutant were coinoculated into soil. Population densities of the pathogen were consistently lower than those of the suppressive strains when individual strains were inoculated into soil. When a pathogen was coinoculated with a suppressive strain, the total streptomycete population density in the pot was always less than that observed when the suppressive isolate was inoculated alone. When the pathogens were inoculated individually into soil, a positive correlation was seen between population density and disease severity. In coinoculation experiments with pathogen and suppressive strains, higher total streptomycete population densities were correlated with lower amounts of disease.     

Effect of <Emphasis Type="Italic">Trichoderma</Emphasis> spp. isolates for biological control of tan spot of wheat caused by <Emphasis Type="Italic">Pyrenophora tritici-repentis</Emphasis> under field conditions in Argentina

Trichoderma spp. have been used as biocontrol agents to protect plants against foliar diseases in several crops, but information from field assays is scarce. In the present work, experiments were carried out to determine the effect of six isolates of Trichoderma harzianum and one isolate of T. koningii on the incidence and severity of tan spot, caused by Pyrenophora tritici-repentis (anamorph: Drechslera tritici-repentis) under field conditions. Significant differences between years, wheat cultivars and treatments were found. In 2003, two of the isolates assayed (T5, T7) showed the best performance against the disease applied as seed treatments or sprayed onto wheat leaves at different stages. The application of six of the treatments on wheat plants significantly reduced disease severity by 16 to 35% in comparison with the control. Disease control provided by isolate T7 was similar to that provided by the fungicide treatment (56% reduction). This is the first report on the efficacy of Trichoderma spp. against tan spot under field conditions in Argentina.