Reducing Pathogen Survival Research Articles

Effect of Sequential Crop Termination and Bed Fumigation on Verticillium dahliae Soil and Plant Density in Strawberry

ABSTRACT Verticillium dahliae, the causal agent of Verticillium wilt of strawberry, is a major soilborne pathogen in California strawberry production. Recent shifts in disease management have necessitated alternative methods for disease control. Two field experiments were conducted to examine the efficacy of sequential fumigant applications for crop termination and bed fumigation to reduce Verticillium dahliae inoculum density in a naturally infested field. Fumigant efficacy was evaluated for crop termination and for reducing pathogen survival in host tissue. Crop termination was conducted using metam potassium and metam sodium. Bed fumigation was conducted using both metam products and chloropicrin. For plots that were subject to both crop termination and bed fumigation, inoculum levels were somewhat reduced (3–6 CFU/g of soil) but not to a level below the disease threshold for strawberry (2 CFU/g of soil). Plots subjected to crop termination with metam potassium and bed fumigation with chloropicrin resulted in the lowest V. dahliae inoculum density. Injury due to crop termination ranged from 53% to 77% at 7 days after fumigation (DAF) and 73–89% at 14 DAF. The odds ratio of V. dahliae survival in the terminated crop were lower than in crops not terminated and V. dahliae survival was less likely at 14, compared to 7 DAF. In the crop terminated with metam potassium, the odds ratio of pathogen survival in the petiole versus the crown was 0.9–1.4. This research shows that crop termination could constitute management strategies for Verticillium wilt of strawberry.

May Sulfonamide Inhibitors of Carbonic Anhydrases from Mammaliicoccus sciuri Prevent Antimicrobial Resistance Due to Gene Transfer to Other Harmful Staphylococci?

Mammaliicoccus sciuri, previously known as Staphylococcus sciuri, is a Gram-positive bacterium involved in gene transfer phenomena that confer resistance to multiple antibiotics. These plasmid-encoded genes can be easily transferred to other pathogenic staphylococci. Because antibiotic resistance is rising, inhibiting M. sciuri proliferation may be a credible strategy for restricting antimicrobial resistance gene transfer to other pathogenic bacteria. Recently, it has been shown that blocking bacterial carbonic anhydrases (CAs, EC 4.2.1.1), metalloenzymes sustaining bacterial metabolic activities, can reduce pathogen survival and fitness. Here, the recombinant M. sciuri γ-CA (MscCAγ) has been cloned and purified, utilizing the DNA recombinant technology. Its kinetic properties for the CO2 hydration reaction, as well as the sulfonamide inhibition profile, were investigated and compared with those reported earlier for MscCAβ (previously described as SauBCA) and the two off-target human CA isoforms (hCA I and hCA II). The recombinant MscCAγ showed significant hydratase activity. Moreover, the MscCAγ sulfonamide inhibitory profile was different from that of MscCAβ, implying that a varied amino acid set typifies the catalytic pocket of the two enzymes. These differences provide additional evidence for the possibility of developing novel CA class-specific inhibitors.

Impact of mixed biofilm formation with environmental microorganisms on E. coli O157:H7 survival against sanitization

Biofilm formation by foodborne pathogens is a serious threat to food safety and public health. Meat processing plants may harbor various microorganisms and occasional foodborne pathogens; thus, the environmental microbial community might impact pathogen survival via mixed biofilm formation. We collected floor drain samples from two beef plants with different E. coli O157:H7 prevalence history and investigated the effects of the environmental microorganisms on pathogen sanitizer tolerance. The results showed that biofilm forming ability and bacterial species composition varied considerably based on the plants and drain locations. E. coli O157:H7 cells obtained significantly higher sanitizer tolerance in mixed biofilms by samples from the plant with recurrent E. coli O157:H7 prevalence than those mixed with samples from the other plant. The mixed biofilm that best protected E. coli O157:H7 also had the highest species diversity. The percentages of the species were altered significantly after sanitization, suggesting that the community composition affects the role and tolerance level of each individual species. Therefore, the unique environmental microbial community, their ability to form biofilms on contact surfaces and the interspecies interactions all play roles in E. coli O157:H7 persistence by either enhancing or reducing pathogen survival within the biofilm community.

Survival of Escherichia coli O157 in autoclaved and natural sandy soil mesocosms.

While the soil microbiome may influence pathogen survival, determining the major contributors that reduce pathogen survival is inconclusive. This research was performed to determine the survival of E. coli O157 in autoclaved and natural (unautoclaved) sandy soils. Soils were inoculated with three different E. coli O157 strains (stx1+/stx2+, stx1-/stx2-, and stx1-/stx2+), and enumerated until extinction at 30°C. There was a significant difference in the survival of E. coli O157 based on soil treatment (autoclaved versus natural) at 30°C on days 1 (P = 0.00022), 3, (P = 2.53e-14), 7 (P = 5.59e-16), 14 (P = 1.072e-12), 30 (P = 7.18e-9), and 56 (P = 0.00029), with greater survival in autoclaved soils. The time to extinction (two consecutive negative enrichments) for all three strains was 169 and 84 days for autoclaved and natural soils, respectively. A separate E. coli O157 trial supplemented with 16S rRNA gene sequencing of the soil microbiome was performed at 15°C and 30°C on days 0, 7, 14, and 28 for each soil treatment. Greater species richness (Chao1, P = 2.2e-16) and diversity (Shannon, P = 2.2e-16) was observed in natural soils in comparison with autoclaved soils. Weighted UniFrac (beta-diversity) showed a clear distinction between soil treatments (P = 0.001). The greatest reduction of E. coli O157 was observed in natural soils at 30°C, and several bacterial taxa positively correlated (relative abundance) with time (day 0 to 28) in these soils (P < 0.05), suggesting that the presence of those bacteria might cause the reduction of E. coli O157. Taken together, a clear distinction in E. coli O157 survival, was observed between autoclaved and natural soils along with corresponding differences in microbial diversity in soil treatments. This research provides further insights into the bacterial taxa that may influence E. coli O157 in soils.

Role of PAFR-MHC-II-CD38 interactions in infections

Abstract Infectious diseases are the most prevalent cause of deaths around the world. Chronic contagious ailments will account for seven out of ten deaths in the world by 2020. The most significant problem in the treatment of the infectious disease is the ever-increasing rate of resistance to drugs mounted by pathogens. Our preliminary results suggest that during antigen presentation the major histocompatibility complex II (MHC-II) engages two associate protein molecules CD38 and Platelet Activating Factor Receptor (PAFR). CD38 and PAFR elicit two counteractive signaling pathways in APCs resulting in the release of either proinflammatory cytokine such as IL-12 or anti-inflammatory cytokines such as IL-10 respectively. Pathogens show remarkable adaptability to the drugs by either metabolizing the drug molecules or by changing the surface profile to neutralize binding of drugs to the pathogens. Therefore, there is a requirement to find new treatment modalities that do not target the pathogens but aid the immune system to eliminate the pathogens. Our preliminary data further suggest that the use of associate molecule during antigen presentation decides the fate of pathogens such as Salmonella Typhimurium or Staphylococcus aureus in macrophages. For example, if MHC-II engages CD38 to signal inside the macrophages leading to reduced pathogen survival in macrophages and mice, while MHC-II-signaling via PAFR aid the pathogen survival in macrophages and mice. Our studies also suggest that combinatorial use of PAFR inhibitor (WEB2086) and antibiotics ampicillin reduced the growth of S. Typhimurium or S. aureus in macrophages and mice.

In vivo investigation of the volatile metabolome of antiphytopathogenic yeast strains active against Penicillium digitatum using comprehensive two-dimensional gas chromatography and multivariate data analysis

Penicillium digitatum is the responsible for green mold decay on citrus fruits, which contributes to economic losses in agriculture industry during storage, transportation and in the market. Biocontrol agents may be a viable and low-cost alternative in the control of different plant pathogens: metabolites from some yeast strains can prevent infection, decreasing host tissue colonization and reducing pathogen survival and sporulation with varying degrees of efficiency. Eight S. cerevisae strains and one Candida stellimalicola strain all were evaluated in vitro as biological control agents against P. digitatum. After in vitro tests the headspace of yeast strains were extracted in vivo using an optimized HS-SPME + GC × GC–MS method. The correlation between antiphytopathogenic activity with the volatile metabolite profiles was assessed using principal component analysis. Nine compounds were associated with the volatile metabolome of strains with higher antifungal action: ethyl propionate, 3-methyl-1-butanol, 2-methyl-1-butanol, ethyl butyrate, 3-methylbutyl acetate, 3-methylbutyl propionate, n-pentyl butyrate, phenylethyl alcohol and 2-phenethyl acetate. This study allowed for the first time a comprehensive overview of the effect of individual volatile metabolites from yeast strains on P. digitatum infection.

In vivo investigation of the volatile metabolome of antiphytopathogenic yeast strains active against Penicillium digitatum using comprehensive two-dimensional gas chromatography and multivariate data analysis

Penicillium digitatum is the responsible for green mold decay on citrus fruits, which contributes to economic losses in agriculture industry during storage, transportation and in the market. Biocontrol agents may be a viable and low-cost alternative in the control of different plant pathogens: metabolites from some yeast strains can prevent infection, decreasing host tissue colonization and reducing pathogen survival and sporulation with varying degrees of efficiency. Eight S. cerevisae strains and one Candida stellimalicola strain all were evaluated in vitro as biological control agents against P. digitatum. After in vitro tests the headspace of yeast strains were extracted in vivo using an optimized HS-SPME + GC × GC–MS method. The correlation between antiphytopathogenic activity with the volatile metabolite profiles was assessed using principal component analysis. Nine compounds were associated with the volatile metabolome of strains with higher antifungal action: ethyl propionate, 3-methyl-1-butanol, 2-methyl-1-butanol, ethyl butyrate, 3-methylbutyl acetate, 3-methylbutyl propionate, n-pentyl butyrate, phenylethyl alcohol and 2-phenethyl acetate. This study allowed for the first time a comprehensive overview of the effect of individual volatile metabolites from yeast strains on P. digitatum infection.

Soil Conditions That Can Alter Natural Suppression of Escherichia coli O157:H7 in Ohio Specialty Crop Soils.

Food-borne pathogen persistence in soil fundamentally affects the production of safe vegetables and small fruits. Interventions that reduce pathogen survival in soil would have positive impacts on food safety by minimizing preharvest contamination entering the food chain. Laboratory-controlled studies determined the effects of soil pH, moisture content, and soil organic matter (SOM) on the survivability of this pathogen through the creation of single-parameter gradients. Longitudinal field-based studies were conducted in Ohio to quantify the extent to which field soils suppressed Escherichia coli O157:H7 survival. In all experiments, heat-sensitive microorganisms were responsible for the suppression of E. coli O157 in soil regardless of the chemical composition of the soil. In laboratory-based studies, soil pH and moisture content were primary drivers of E. coli O157 survival, with increases in pH after 48 h (P = 0.02) and decreases in moisture content after 48 h (P = 0.007) significantly increasing the log reduction of E. coli O157 numbers. In field-based experiments, E. coli O157 counts from both heated and unheated samples were sensitive to both season (P = 0.004 for heated samples and P = 0.001 for unheated samples) and region (P = 0.002 for heated samples and P = 0.001 for unheated samples). SOM was observed to be a more significant driver of pathogen suppression than the other two factors after 48 h at both planting and harvest (P = 0.002 at planting and P = 0.058 at harvest). This research reinforces the need for both laboratory-controlled experiments and longitudinal field-based experiments to unravel the complex relationships controlling the survival of introduced organisms in soil.

Identification and therapeutic potential of a vitronectin binding region of meningococcal msf.

The human pathogen Neisseria meningitides (Nm) attains serum resistance via a number of mechanisms, one of which involves binding to the host complement regulator protein vitronectin. We have shown previously that the Meningococcal surface fibril (Msf), a trimeric autotransporter, binds to the activated form of vitronectin (aVn) to increase Nm survival in human serum. In this study, we aimed to identify the aVn-binding region of Msf to assess its potential as an antigen which can elicit antibodies that block aVn binding and/or possess bactericidal properties. Using several recombinant Msf fragments spanning its surface-exposed region, the smallest aVn-binding recombinants were found to span residues 1-86 and 39-124. The use of further deletion constructs and overlapping recombinant Msf fragments suggested that a region of Msf comprising residues 39-82 may be primarily important for aVn binding and that other regions may also be involved but to a lesser extent. Molecular modelling implicated K66 and K68, conserved in all available Msf sequences, to be involved in the interaction. Recombinant fragments which bound to aVn were able to reduce the survival advantage conveyed by aVn-interaction in serum bactericidal assays. Antibodies raised against one such fragment inhibited aVn binding to Msf. In addition, the antibodies enhanced specific killing of Msf-expressing Nm in a dose-dependent manner. Overall, this study identifies an aVn-binding region of Msf, an adhesin known to impart serum resistance properties to the pathogen; and shows that this region of Msf can elicit antibodies with dual properties which reduce pathogen survival within the host and thus has potential as a vaccine antigen.

Evaluation of Current Industry Practices for Maintaining Tomato Dump Tank Water Quality during Packinghouse Operations

Maintenance of adequate sanitizer levels in tomato dump tanks is critical to preventing pathogen survival and transfer, and internalization in the fruit. However, the rapid accumulation of organic matter in dump tanks can cause a significant decline in sanitizer concentration, thus leaving wash solutions vulnerable to becoming a reservoir for pathogens. This study investigated the dynamic changes in wash water quality and sanitizer concentration during routine operations of three medium to large tomato packinghouses in Florida in 2010 and 2011. Overall, water quality declined continuously during packinghouse operations, as exhibited by significant increases in chemical oxygen demand and turbidity. Free chlorine concentration, oxidation reduction potential and pH fluctuated largely as a consequence of periodic addition of sodium hypochlorite and citric acid, and the rapid loss of free chlorine caused by organic matter. Although the packinghouses differed significantly in system configuration, operation and chlorine dosing rates, at least 25 mg/L of free chlorine concentrations were maintained in all of the dump tanks surveyed. Practical Applications Wash water quality and sanitizer strength are important factors influencing food safety of tomatoes washed in packinghouses. Soil, debris and juices from damaged fruits entering dump tanks along with tomatoes can cause water quality and sanitizer strength to decline dramatically, leaving wash solutions vulnerable to the growth of harmful bacteria. Maintenance of adequate sanitizer levels in tomato dump tanks is critical to reducing pathogen survival, transmission and incidence of internalization in the fruit. The researchers surveyed the water quality and sanitizer concentrations during routine operations of three major tomato packinghouses in Florida in 2010 and 2011. The results are useful for the regulatory agency and tomato industry in developing packinghouse handling guidelines to maintain the quality and food safety of tomatoes.

Phenotypic and Genotypic Characterization of Biofilm Forming Capabilities in Non-O157 Shiga Toxin-Producing Escherichia coli Strains

The biofilm life style helps bacteria resist oxidative stress, desiccation, antibiotic treatment, and starvation. Biofilm formation involves a complex regulatory gene network controlled by various environmental signals. It was previously shown that prophage insertions in mlrA and heterogeneous mutations in rpoS constituted major obstacles limiting biofilm formation and the expression of extracellular curli fibers in strains of Escherichia coli serotype O157:H7. The purpose of this study was to test strains from other important serotypes of Shiga toxin-producing E. coli (STEC) (O26, O45, O103, O111, O113, O121, and O145) for similar regulatory restrictions. In a small but diverse collection of biofilm-forming and non-forming strains, mlrA prophage insertions were identified in only 4 of the 19 strains (serotypes O103, O113, and O145). Only the STEC O103 and O113 strains could be complemented by a trans-copy of mlrA to restore curli production and Congo red (CR) dye affinity. RpoS mutations were found in 5 strains (4 serotypes), each with low CR affinity, and the defects were moderately restored by a wild-type copy of rpoS in 2 of the 3 strains attempted. Fourteen strains in this study showed no or weak biofilm formation, of which 9 could be explained by prophage insertions or rpoS mutations. However, each of the remaining five biofilm-deficient strains, as well as the two O145 strains that could not be complemented by mlrA, showed complete or nearly complete lack of motility. This study indicates that mlrA prophage insertions and rpoS mutations do limit biofilm and curli expression in the non-serotype O157:H7 STEC but prophage insertions may not be as common as in serotype O157:H7 strains. The results also suggest that lack of motility provides a third major factor limiting biofilm formation in the non-O157:H7 STEC. Understanding biofilm regulatory mechanisms will prove beneficial in reducing pathogen survival and enhancing food safety.

Effects of processing and storage variables on penetration and survival of Escherichia coli O157:H7 in fresh-cut packaged carrots

The effects of severity of slicing and peeling, and of storage atmosphere and temperature on the survival of Escherichia coli O157:H7 at and below the cut surfaces of fresh-cut carrot discs were determined. Slicing with a blunt machine blade enhanced penetration of E. coli O157:H7 and its subsequent survival during storage at 8 °C. Significantly (P < 0.05) higher numbers of cells (0.5 log CFU g−1) penetrated deeper (475–500, 725–750 and 975–1000 μm) into carrot tissue sliced with a blunt machine blade compared to those sliced with a razor blade. Counts on Day 3 and Day 5 of storage remained higher in carrot tissue sliced with a blunt machine blade at all depths sampled. There were no significant effects of peeling method on penetration and survival. Storage of carrot slices in sub-optimal atmospheres (20%CO2/1%O2 resulted in increased survival of cells at the surface and within the tissue compared to storage in more optimal atmospheres (5% CO2/3% O2). Increasing storage temperature from 4 to 10 °C resulted in growth of E. coli O157:H7 at all sample depths. For all experiments, E. coli O157:H7 cells colonising the surface generally survived better than cells that penetrated into the tissue. The data are relevant to improving microbial safety in the fresh-cut sector by demonstrating that cutting with sharp blades (e.g. during harvesting and processing) reduces the depth of potential contamination by E. coli O157:H7. This may also have implications for more efficient anti-microbial dipping and reduced pathogen survival during storage.

Effect of storage temperature on survival and growth of foodborne pathogens on whole, damaged, and internally inoculated jalapeños (Capsicum annuum var. annuum).

There is a lack of general knowledge regarding the behavior of foodborne pathogenic bacteria associated with jalapeño peppers. The survival and growth behaviors of Listeria monocytogenes, Escherichia coli O157:H7, and Salmonella enterica on the interior and exterior of jalapeño peppers were determined under different storage conditions. Jalapeños were inoculated with a five-strain cocktail of L. monocytogenes, E. coli O157:H7, or S. enterica on the intact external surface, injured external surface, or intact internal cavity of jalapeño peppers and held at 7 or 12°C for a period of 14 days. Populations of each pathogen were determined at 0, 1, 2, 5, 7 10, and 14 days throughout storage. The uninjured, intact external surface of jalapeño peppers did not support growth of the pathogens tested under both storage conditions, with the exception of L. monocytogenes at 12°C. Populations of E. coli and S. enterica declined on the external injured surface of peppers at 7°C, but populations of L. monocytogenes remained consistent throughout the length of storage. At 12°C, L. monocytogenes and S. enterica populations increased throughout storage, and E. coli populations remained unchanged on injured surfaces. The uninjured internal cavity of the jalapeño supported growth of all pathogens at 12°C. Overall, L. monocytogenes was the microorganism most capable of growth and survival in association with jalapeño peppers for the scenarios tested. Results emphasize the importance of jalapeño pepper quality and proper storage conditions in preventing or reducing pathogen survival and growth.

Strategies for managing Fusarium head blight and deoxynivalenol accumulation in wheat

Many mycotoxigenic fungi infect plant hosts and cause disease in the field. Therefore, control of field infection by these fungi is a critical step in managing mycotoxin accumulation in the harvested product. Fusarium graminearum, also known as Gibberella zeae, is the causal agent of Fusarium head blight (FHB), or scab, in cereals and is also the primary agent responsible for contamination of grain with deoxynivalenol (DON). Research efforts worldwide are devoted to the development of strategies to control field infection of wheat and barley by this pathogen. Strategies include the use of fungicides and biological control agents to protect flowering heads from infection. There is extensive effort in breeding for host resistance to infection and spread of the pathogen within the heads. Scientists are also seeking exogenous traits to introduce into cereals to enhance resistance. Cultural practices are also being examined, primarily as measures to reduce pathogen survival and inoculum production in crop residues. The successes and limitations of these strategies in the management of Fusarium head blight and deoxynivalenol are discussed.

SUPPRESSION OF SOILBORNE PATHOGENS BY COMPOST: SUPPRESSIVE EFFECTS OF GRAPE MARC COMPOST ON PHYTOPATHOGENIC OOMYCETES

SUPPRESSION OF SOILBORNE PATHOGENS BY COMPOST: SUPPRESSIVE EFFECTS OF GRAPE MARC COMPOST ON PHYTOPATHOGENIC OOMYCETES

Occurrence of inhibitory compounds in spent growth media that interfere with acid-tolerance mechanisms of enteric pathogens

Understanding the acid-tolerance ability of enteric human pathogens is critical in determining microbial food safety and the associated risk. We have discovered naturally occurring compounds in the spent growth media, which inhibit the acid-tolerance ability of several enteric human pathogens when challenged at pH 3.0 for 2 h at 37 °C. The compounds were detected in the spent growth media obtained from Escherichia coli O157:H7, Salmonella spp., and Vibrio cholera, but not from Shigella flexneri. The compounds were effective in reducing pathogen survival by 5-logs during acid-challenge assay. The low molecular weight (<300 Da) and heat resistant nature (121 °C, 15 min, at 15 psi) of the compounds make them excellent candidates to explore their suitability as food additives that would increase microbial food safety.

Determination of the effect of sodium lactate on the survival and heat resistance of Escherichia coli O157:H7 in two commercial beef patty formulations

The effect of 4% sodium lactate (NaL) in beefburger patty formulations on the survival and heat resistance of Escherichia coli O157:H7 was investigated. Fresh beef trimmings were inoculated with E. coli O157:H7 to a concentration of 6·0–7·0 log10 cfu g−1 and subjected to the processing stages of beefburger patty production. Two commercial beefburger patty formulations were produced: a ‘quality’ patty (100% beef) and an ‘economy’ patty (70% beef, 30% other ingredients, including onion, water, salt, seasoning, rusk and soya concentrate). Sodium lactate (4% w/v) was added to the beefburger patties during mincing and the formed patties were frozen and stored for 1 month. Beefburger patties without added NaL were used as controls. After frozen storage for 1 month, patties were examined for E. coli O157:H7 counts. There was a synergistic effect between freezing and NaL, which resulted in a small but significant reduction (P<0·05) (approximately 0·5 log10 cfu g−1) in E. coli O157:H7 numbers. The frozen beefburger patties were also heat-treated at 50, 55 and 60°C and the data analysed to derive D -values for E. coli O157:H7 cells. At each temperature treatment, theD -values of the quality and economy beefburger patties with 4% NaL were significantly lower (P<0·001) than the D -values of the patty formulations without NaL. The study demonstrates that the presence of 4% NaL in beefburger patty formulations can reduce the overall risks posed to consumers by the presence ofE. coli O157:H7 by, first; reducing pathogen survival during freezing and frozen storage of the uncooked product; and, second, by increasing the susceptibility of the pathogen to heat during normal cooking processes.

Management of soil-borne plant pathogens with organic soil amendments: a disease control strategy salvaged from the past

The utilization of organic amendments for control of soil-borne plant pathogens has often been considered at best variable, but more often as a snake-oil remedy. Understanding the mode of action of these products, however, provides insight as to where and how to exploit these underutilized energy sources for the benefit of plant and soil health. Organic amendments containing high nitrogen, such as poultry manure, meat and bone meal, and soymeal, significantly reduced populations of a wide spectrum of soil-borne plant pathogens. Pathogen control was shown to arise from the ammonia and (or) nitrous acid generated, the concentrations of which are controlled by pH, organic matter content, soil buffering capacity, and nitrification rate. Swine manure can reduce pathogen populations by both these mechanisms as well as by an additional process involving volatile fatty acids. Volatile fatty acids are active only at pH conditions below 6.0 and were present in most, but not all liquid manures tested. Disease control with organic amendment occurs when soil and biological factors are conducive to activating the processes that reduce pathogen survival. Understanding the mechanisms allows prediction of efficacy based on analysis of the soil and the organic amendment. For some products, such as ammonium lignosulfonate, a by-product of the pulp and paper industry, disease control was demonstrated but the mode of action is as yet unclear. Although organic amendments reduce populations of plant pathogens, overall they lead to an increase in soil microorganisms populations by up to 1000-fold following application. Thus, the displacement of pathogens is selective and can persist in fields for several years after single application. As we unravel the potential benefits of organic treatments, there is indication that they may return as a valuable tool in disease management strategies. L'emploi d'amendements organiques pour la lutte aux agents phytopathogènes du sol a souvent été considéré d'une efficacité incertaine dans les meilleurs des cas et, encore plus souvent, comme un remède de charlatan. Cependant, la compréhension du mode d'action de ces produits nous permet de savoir où et comment utiliser ces sources d'énergie sous-exploitées au bénéfice de la santé des plantes et des sols. Des amendements organiques à teneur élevée en azote tels que le fumier de volaille, la farine de viande et d'os et le tourteau de soja réduisent significativement les populations d'une multitude d'agents phytopathogènes du sol. Il a été démontré que l'activité contre les agents pathogènes est due à l'ammoniac ou l'acide nitreux générés, dont les concentrations sont contrôlées par le pH, la teneur en matière organique, le pouvoir tampon du sol et le taux de nitrification. Le fumier de porc peut réduire les populations d'agents pathogènes par ces deux mécanismes aussi bien que par des processus supplémentaires impliquant des acides gras volatils. Les acides gras volatiles sont actifs seulement dans des conditions de pH inférieur à 6,0 et étaient présents dans presque tous les purins testés. La lutte aux maladies par les amendements organiques se produit lorsque les facteurs édaphiques et biologiques contribuent à l'activation des processus qui diminuent la survie des agents pathogènes. La compréhension de ces mécanismes permet de prédire l'efficacité après l'analyse du sol et des amendements organiques. Pour certains produits, comme le lignosulfate d'ammonium, un sous-produit de l'industrie des pâtes et papier, l'efficacité de la lutte aux maladies a été démontrée bien que le mode d'action demeure inconnu. Alors que les amendements organiques réduisent les populations d'agents phytopathogènes, en général, ils induisent une augmentation des populations des microorganismes du sol par un facteur de 1000, suite à l'application. Donc, il y a déplacement sélectif des agents pathogènes, lequel peut persister plusieurs années en champ après une seule application. Alors que nous découvrons les bénéfices potentiels des traitements organiques, il y a des indications que ceux-ci pourraient être considérés à nouveau comme outils importants dans les stratégies de gestion des maladies.

Suppression of Phytophthora cinnamomi in Potting Mixes Amended with Uncomposted and Composted Animal Manures

ABSTRACT We examined the effects of fresh and composted animal manures on the development of root rot, dieback, and plant death caused by Phytophthora cinnamomi. Fresh chicken manure, or chicken manure composted for 5 weeks before incorporation into the potting mix (25%, vol/vol), significantly reduced pathogen survival and the development of symptoms on Lupinus albus seedlings. Chicken manure composted for 2 weeks was less suppressive. Cow, sheep, and horse manure, whether fresh or composted, did not consistently suppress populations of P. cinnamomi or disease symptoms at the rates used (25%, vol/vol). All composts increased organic matter content, total biological activity, and populations of actinomycetes, fluorescent pseudomonads, and fungi. Only chicken manure stimulated endospore-forming bacteria, a factor that was strongly associated with seedling survival. Fallowing the potting mix for an additional 8 weeks after the first harvest increased the survival of lupin seedlings in a second bioassay, with survival rates in chicken manure compost-amended potting mix exceeding 90%. These data suggest that the ability of composted manure to stimulate sustained biological activity, in particular the activity of endospore-forming bacteria, is the key factor in reducing disease symptoms caused by P. cinnamomi. Supporting these results, the survival of rooted cuttings of Thryptomene calycina was significantly higher in sand-peat potting mix following amendment with commercially available chicken manure (15% vol/vol). However, this protection was reduced if the potting mix was steam pasteurized before amendment, indicating that suppression was due to endogenous as well as introduced microbes. Chicken manure compost incorporated at 5% (vol/vol) or more was strongly phytotoxic to young Banksia spinulosa plants and is not suitable as an amendment for phosphorus-sensitive plants.

A new formulation system for the application of biocontrol fungi to soil

A new biocontrol formulation system was devised that does not require sterile conditions during preparation. It involves mixing vermiculite and powdered wheat bran with wet or dry fermentor biomass of Trichoderma spp. or Gliocladium virens, moistening with 0.05 N HCl, and drying the mixture. Before application to soil, the preparation (VBA‐FB) is activated by re‐moistening with 0.05 N HCl and incubated at room temperature for 2–3 days to stimulate development of young hyphae of the biocontrol fungus. Populations of biocontrol fungi proliferated to greater than 107 colony‐forming units (cfu) per g of soil when activated VBA‐FB was added to soil. In soil artificially infested with Rhizoctonia solani, seven isolates of the 14 studied added as VBA‐FB reduced survival and 12 reduced saprophytic growth of the pathogen. Of these, two isolates of T. hamatum (TRI‐4, Tm‐23) and one of T. harzianum (Th‐87) were the most effective. Preparations formulated with either wet or dry biomass effectively reduced pathogen survival, but activated VBA‐FB was more effective than non‐activated VBA‐FB. Storage of VBA‐FB at 25°C for 24 weeks before activation reduced viability of isolates considerably more than storage at 5°C for 24 weeks. In addition, VBA‐FB stored at 5°C before activation more effectively reduced survival of R. solani than VBA‐FB stored at 25° C. Survival of R. solani was reduced by activated VBA‐FB applied to several soil types (sandy loam, sandy clay loam, clay). Some nitrogen fertilizers increased the efficacy of VBA‐FB preparations of several isolates.