Microbial populations involved in the suppression of Rhizoctonia solani AG1-1B by lignin incorporation in soil

Rhizoctonia solani causes worldwide losses in numerous crops. Sclerotia of R. solani remain viable for several years in soil and are an important source of primary infection. In this study the effect of soil incorporation of Kraft pine lignin, a side product of the paper industry, on viability of R. solani AG1-1B sclerotia was investigated. The efficacy of lignin was assessed in a sandy loam (Oppuurs) and a silt loam soil (Leest) collected from commercial fields in Belgium. Evaluating sclerotial viability after 4 weeks incubation in the two soils amended with 1% (w/w) Kraft pine lignin demonstrated a soil-dependent effect. In Leest soil the addition of lignin resulted in a significantly reduced sclerotial viability, together with an increased mycoparasitism by Trichoderma spp.; in Oppuurs soil, on the other hand, only a slight and insignificant reduction in sclerotial viability was observed. Based on phospholipid fatty acid analysis, different changes in microbial community structure upon lignin amendment were detected in the two soils. Both amended soils showed a significant increase in Gram negative bacteria. In Leest soil this increase was accompanied with a significantly higher increase in fungi and actinomycetes compared with Oppuurs soil. In addition, Kraft pine lignin resulted in both soils in a small but significant increase in manganese peroxidase activity and this increase tended to be higher in Leest soil. Manganese peroxidase produced by lignin-degrading basidiomycetes has previously been shown to degrade melanin, which protects the sclerotia against biotic and abiotic stress. We hypothesize that lignin-degrading fungi increased the susceptibility of the sclerotia to sclerotial antagonists such as Trichoderma, Gram negative bacteria and actinomycetes. Clearly, the effect observed here did not rely on the stimulation of one microbial group, but is the result of an interaction of different groups.     

Population dynamics of Trichoderma in fumigated and compost-amended soil and on strawberry roots

Effectiveness of Trichoderma strains for biocontrol of soilborne pathogens requires an improved understanding of soil and root ecology of this fungus. We compared the population dynamics of Trichoderma hamatum strain T382 (T382) and indigenous Trichoderma spp. in soil and on roots in different strawberry production systems. Strawberry transplants, either amended or not-amended with Trichoderma biocontrol strains, were planted in field soil left untreated or treated with soil fumigant, compost, and compost-amended with T382. Soil and root samples were taken between October and June of two production seasons (2002-03 and 2003-04), and Trichoderma populations were assessed by plating soil dilutions and root pieces onto selective medium. Identity of T382 was confirmed using strain-specific primers. T382 became established and maintained a stable population of 10³ cfu/g soil throughout the growing season when added to field soil in amended compost, but T382 was rarely isolated from strawberry roots. Populations of indigenous Trichoderma spp. were up to 60-fold greater in fumigated soil than in any other soil treatment. Indigenous Trichoderma spp. were isolated from a greater proportion (20–50%) of roots in fumigated soil than from roots in the other treatments (0–20%). Transplant treatments did not significantly affect Trichoderma populations on roots or in soil during field production. This study showed that compost may be used as a substrate to establish and promote survival of Trichoderma in field soil, and illustrates how soil manipulation can affect population dynamics of indigenous Trichoderma spp. on roots and in soil.     

Survival of sclerotia of Macrophomina phaseolina and Sclerotium cepivorum after drying and wetting treatments

Exposure of sclerotia of Macrophomina phaseolina to 0 and 33% relative humidity (r.h.) for 12 weeks and of Sclerotium cepivorum to 0, 33 and 55% r.h. for 20 weeks did not reduce their germinability on agar. Exposure to 78% r.h. caused high loss of germinability in M. phaseolina and complete loss in S. cepivorum. After 7-day exposures respective moisture contents of sclerotia of M. phaseolina and S. cepivorum were 1 and 2% at 0% r.h.; and 10 and 14% at 78% r.h. M. phaseolina sclerotia held at 0% and 33% r.h. in desiccators for several times up to 12 days did not decrease in subsequent survivability in moist soil, unlike sclerotia held at 78% r.h. for 4 days.More sclerotia of M. phaseolina were colonized by fungi and Streptomyces spp. on alkaline soil than on acid soil. On alkaline soil twice as many sclerotia were colonized after exposure to 0% r.h. as after exposure to 33, 55 and 78% r.h. Colonization of S. cepivorum sclerotia was as high on acid as on alkaline soil and 3 times as high on sclerotia treated at 0% r.h. as on those treated at higher r.h. Attempts to ascertain the effects of colonization on sclerotial viability were unsuccessful. Incubation of sclerotia of M. phaseolina in moist Rumsford sandy loam (50% m.h.c.) for 20 weeks reduced survivability by 43%. At room temperature, alternate drying and wetting of soil containing sclerotia did not appreciably affect survivability of either pathogen. Survivability of S. cepivorum sclerotia was highest when the sclerotia were incubated in air-dried soil (2–3% m.h.c.) for 20 weeks.Incidence of white rot on onion seedlings transplanted to S. cepivorum-infested soil was higher in soil that had been air-dried for 20 weeks than in soil that had been alternately wetted and dried. Sclerotia that were exposed to 0% r.h. for 7 days before soil incubation produced little white rot.     

Effect of sclerotial distribution pattern of Sclerotinia sclerotiorum on biocontrol efficacy of Trichoderma harzianum

Field studies were conducted over two seasons to investigate effects of random versus highly aggregated spatial arrangements of sclerotia of S. sclerotiorum, and effects of biocontrol agent density and formulation additives, on colonization of sclerotia by Trichoderma spp. Application of T. harzianum encapsulated in alginate pellets with either bran or polyethylene glycol additives increased the percentages of sclerotia colonized in both years, but there was no difference between additives in either year. Higher pellet densities (200 pellets/m² versus 40 pellets/m²) resulted in higher proportions of sclerotia colonized by Trichoderma spp. in one season but not in the other. However, when sclerotia were in highly aggregated spatial patterns, significantly higher percentages were colonized in both years, compared to sclerotia in random distributions.     

Multivariate effects of plant canopy, soil physico-chemistry and microbiology on Sclerotinia stem rot of soybean in relation to crop rotation and urban compost amendment

The effects of canopy, soil physico-chemical and microbiological variables on Sclerotinia stem rot (SSR) on soybean were assessed in two soils (clay loam and sandy loam) using multiple regression and canonical redundancy analysis (RDA) and their partial form to control for the rotation (2 or 3-y-corn/soybean monoculture) and fertilization (mineral/urban compost) or spatial variables effects. The models revealed the minimal sets of variables that best explain the variance of the survival of Sclerotinia sclerotiorum’s sclerotia, carpogenic germination, disease severity and their associations. In clay loam, the 3-y-corn rotation reduced disease severity mainly through the reduction of weed biomass that favoured carpogenic germination. Urban compost has a conducive effect explained by a better soil surface drainage. Additionally, total N was found suppressive to sclerotial survival. In sandy loam, the carpogenic germination was negatively correlated with high C mineralization quotient and aggregate stability but correlated positively with Ca. Sclerotial survival was negatively correlated with pH and Ca, and positively correlated with biological fertility index. Aggregate stability, Ca and pH were associated with the urban compost. The regression and RDA analyses allowed to identify key variables that drived SSR development and explain their relationship with the cultural practices, soil health, as well as the spatial variation of disease variables.     

Potential of Trichoderma spp. as consistent plant growth stimulators

In a series of repeated trials, six Trichoderma spp. strains, applied as a dried powder from a liquid fermentation in molasses/yeast medium, proved to be consistent at promoting the growth of lettuce (Latuca sativa L.) seedlings grown in a peat-sand potting compost in the glasshouse. Strains WT, 92, 20, and 75 at 0.75% or 1% w:w concentrations increased shoot dry weight by up to 26%, although WT did inhibit germination. For example, after 4 days only 13% of seeds sown in WT 1% w:w treated compost had germinated, whereas in other treatments germination was consistently greater than 32%. WT increased shoot fresh and dry weights by 14.3 g and 0.6 g per pot, respectively, without affecting the root dry weights, to give concomitant increases in shoot: root ratios of fresh and dry weight. The potential use of these Trichoderma spp. strains for plant growth promotion is discussed.     

Mycostasis in relation to the microbial nutrient sinks of five soils

[^l4C]exudation from fungal propagules on 5 soils over 4–24 h was studied in relation to mycostasis. [^l4C]exudation from sclerotia of Macrophomina phaseolina, chlamydospores of Thielaviopsis basicola, and conidia of Cochliobolus victoriae after 24 h on two sandy loam soils and a loam was generally greater than exudation on the two clay loam soils. Results were similar for conidia of Stemphylium sarcinaeforme but differences were not statistically significant. When natural soils were pulsed with [¹⁴C]glucose, ¹⁴CO₂ evolved by the soil microflora over 2–12 h showed a similar trend. [¹⁴C]exudation from M. phaseolina sclerotia and C. victoriae conidia incubated on soils was greater than that from propagules incubated aseptically on a bed of sand through which water percolated at a flow rate sufficient to inhibit germination. Propagules of C. victoriae, M. phaseolinia and T. basicola germinated greater on one or more of the coarse-textured soils than on fine-textured soils. Using γ-irradiated soils, more [^l4C]exudate was adsorbed by the clay loams than by the loam and sandy loam soils, suggesting that the adsorptive capacity of soils may be an important factor in controlling fungal utilization of soluble nutrients. Fungal germination in soil appears to be jointly influenced by two opposing tendencies: the ease with which germination occurs in response to exogenous nutrients and the amount of endogenous substrate lost or retained.     

Quantitative real-time PCR effectively detects and quantifies colonization of sclerotia of Sclerotinia sclerotiorum by Trichoderma spp.

Some members of the fungal genus Trichoderma are able to colonize and destroy sclerotia, the thick-walled resting structures of the soilborne plant pathogenic fungus Sclerotinia sclerotiorum, thus providing a potential means of biological disease control. However, current methods to detect and quantify colonization of sclerotia are labor-intensive, and generally qualitative rather than quantitative in nature. Our objective was to develop quantitative real-time PCR (polymerase chain reaction) methods to detect and measure colonization of sclerotia by Trichoderma spp. Specific PCR primer/probe sets were developed for Trichoderma spp. and for S. sclerotiorum. A total of 180 ITS1 (internal transcribed spacer) and ITS2 sequences from different species in the genus Trichoderma were aligned, and consensus sequences were determined. Six candidate primer sets were based on conserved regions of the consensus sequence, and the specificity of each nucleotide sequence was examined using BLAST (Basic Local Alignment Search Tool; NCBI) software. Each candidate primer set was tested on genomic DNA of T. harzianum strain ThzID1-M3, as well as six different Trichoderma isolates from soil, and on genomic DNA of S. sclerotiorum. The optimum primer/probe set selected, TGP4, successfully amplified genomic DNA of all Trichoderma isolates tested, and showed high precision and reproducibility over a linear range of eight orders of magnitude, from 85 ng to 8.5 fg of T. harzianum genomic DNA. TGP4 did not amplify S. sclerotiorum DNA. A specific PCR primer/probe set (TMSCL2) was developed for S. sclerotiorum, based on the calmodulin gene sequence. TMSCL2 did not amplify Trichoderma DNA. Quantitative real-time PCR with the primers then was evaluated in experiments to test differential effects of two soil moisture levels (−50 kPa, −1000 kPa matric potential) on biocontrol of S. sclerotiorum by indigenous Trichoderma spp. Periodically over 40 days, Trichoderma and S. sclerotiorum DNA in sclerotia were quantified by PCR with appropriate primers. Over 90% of the sclerotia were colonized by indigenous Trichoderma spp. at −1000 kPa, over the 40-day period, compared to only 60% at −50 kPa. In addition to determining incidence of colonization, the PCR method allowed measurement of the extent of sclerotial colonization, which also was significantly greater in the drier soil. Quantitative real-time PCR with the TGP4 primer/probe set provides a sensitive detection and measurement tool to evaluate colonization of sclerotia by Trichoderma spp.     

Comparaison des aptitudes parasitaires de clones de Trichoderma vis-a-vis de quelques champignons a sclerotes

Nineteen monoconidial isolates (referred to as clones) of Trichoderma from different species aggregates, one isolate of Gliocladium virens, and one isolate of an Acrostalagmus sp. (that was naturally associated with sclerotia of Sclerotinia spp and Macrophomina phaseolina) were tested. They were incubated in controlled conditions, in sterile soil, with sclerotia of Corticium rolfsii, Sclerotinia minor, or S. sclerotiorum. At the end of appropriate periods of incubation (respectively 26, 20 and 8 days), the sclerotia were retrieved from soil and checked for invasion by the antagonist. Important differences between the parasitic ability of Trichoderma clones were noted. Clones from at least three different species (T. aureoviride, T. hamatum, T. harzianum) exhibited a high antagonistic activity. Activity of the G. virens isolate was at the same level as the best clones of Trichoderma, whereas no parasitic tendencies were found in the isolate of Acrostalagmus sp., thus confirming previous results.A rather good correlation was found between the capacity of the clones for attacking C. rolfsii sclerotia and their ability to parasitize both Sclerotinia.In conclusion, it is proposed that a screening with only one of the sclerotial species would give clones efficient against all three, and possibly against related sclerotial types.     

Sewage Sludge Compost's Cumulative Effects on Crop Growth and Soil Properties

? Onion (Allium cepa cv. Spanish Sweet Utah), lettuce (Lactuca sativa cv. Black Seeded Simpson), snapdragon (Antirrhinum majus cv. Sonnet Yellow), and turfgrass (Festuca arundinacea cv. Marathon) were grown twice annually (spring and fall) on a San Emigdio sandy loam (coarse-loamy, mixed calcareous thermic, Typic Xerorthents) soil for two years that was treated with a cumulative total of 0, 37 and 74 MT/ha of sewage sludge compost from San Diego. The soil received two compost treatments each year and crops were planted within a week of compost incorporation. Crop growth was monitored and the results of the fourth or final planting are described here. Seedlings of onion, snapdragon and lettuce transplanted to compost treated plots displayed more vigorous establishment than those in the control plots. Compost treatments produced higher yields of onion, turf and lettuce. Snapdragon yield was not affected by compost treatment. Soil analysis of compost treated plots revealed lowered pH and increased levels of organic matter, primary nutrients, soluble salts and heavy metals.     

Interactions between Sclerotium rolfsii and Trichoderma spp: Relationship between antagonism and disease control

Ten isolates of Trichoderma spp were examined for their ability to antagonize growth and to parasitize mycelium of Sclerotium rolfsii (Sr-1) on agar media, to inhibit germination of sclerotia of S. rolfsii on natural soil plates and to sporulate on the sclerotia, and to protect bean seedlings against the pathogen in the greenhouse. A high negative correlation (r = ?0.844) was observed between plant stand in the greenhouse and sclerotial germination on soil plates but not with antagonism on agar plates. Three isolates of T. harzianum (Th-7, Th-20, WT-6) and one of T. hamatum (TRI-4) were especially effective in reducing sclerotial germination and controlling disease in the greenhouse. Three isolates of Trichoderma spp (WT-6, TMP, and TRI-4), effective in reducing sclerotial germination of isolate Sr-1, also prevented sclerotial germination in four out of five additional S. rolfsii isolates studied.     

Cumulative effects of sludge compost on crop yields and soil properties

Abstract

Onion (Allium cepa cv. Spanish Sweet Utah), lettuce (Lactuca sativa cv. Black Seeded Simpson), snapdragon (Antirrhinum majus cv. Sonnet Yellow), and turfgrass (Festuca arundinacea cv. Marathon) were grown twice annually (spring and fall) on a San Emigdio sandy loam (coarse‐loamy, mixed calcareous thermic, Typic Xerorthents) soil for two years that was treated with a cumulative total of 0, 37, and 74 MT/ha of sewage sludge compost from San Diego. The soil received two compost treatments each year and crops were planted within a week of compost incorporation. Crop growth was monitored and the results of the fourth or final planting are described here. Seedlings of onion, snapdragon and lettuce transplanted to compost treated plots displayed more vigorous establishment than those in the control plots. Compost treatments produced higher yields of onion, turf and lettuce. Snapdragon yield was not affected by compost treatment. Soil analysis of compost treated plots revealed lowered pH and increased levels of organic matter, primary nutrients, soluble salts and heavy metals. A concurrent greenhouse study demonstrated that the presence of chipped Eucalyptus tree trimmings (60% by volume) in the sewage sludge compost did not inhibit the growth of the test crops.     

Loss of nutrient-independence for germination by fungal propagules incubated on soils or on a model system imposing diffusive stress

Nutrient independent conidia of Cochliobolus victoriae and sclerotia of Sclerotium cepivorum, Macrophomina phaseolina, and Verticillium dahliae were incubated aseptically on sand through which a dilute salts solution percolated at a flow rate sufficient to inhibit germination. Propagules were then transferred to a static salts solution to assess germination. Conidia of C. victoriae and sclerotia of S cepivorum became nutrient-dependent (6% germination in salts solution) after 9 and 15 days on sand, respectively. Thirty-five days of diffusive stress were required to attenuate the nutrient-independence of M. phaseolina sclerotia. Sclerotia of V. dahliae lost little of their nutrient-independence even after 45 days of diffusive stress. Viability of C. victoriae and S. cepivorum was reduced after 45 days of diffusive stress, but viability of V. dahliae and M. phaseolina was not. Conidia of C. victoriae gradually became nutrient-dependent when incubated for several weeks on each of five soils. A loam and two sandy loam soils were more effective in decreasing the nutrient-independence of conidia than were two clay loams. Sclerotia of M. phaseolina also lost nutrient-independence when incubated on four of the five soils.Interruption of artificially imposed diffusive stress resulted in increased [¹⁴C]exudation from conidia of C. victoriae and sclerotia of M. phaseolina. Germinability on salts solution of C. victoriae conidia previously made nutrient-dependent was significantly increased, when the conidia were kept at 4°C for 3.5 days before germination assay. Conidia of C. victoriae made nutrient-dependent and then incubated on soils labeled with [¹⁴C]glucose, absorbed twice as much ¹⁴C from a loam and two sandy loam soils as from two clay loam soils. Following incubation on four of the five ¹⁴C-labeled soils, the germinability of the conidia in the absence of nutrients was significantly increased over that of conidia not incubated on these soils.The results suggest that a continued minimal stress may be needed to maintain the nutrient-dependence of some fungal propagules in soil. Interruption of nutrient stress appears to allow nutrient-dependent propagules an opportunity to recoup nutrients from the soil solution or to reorganize endogenous energy reserves whereby the potential for germination is increased.     

Desorption behaviour of lead in two calcareous soils as affected by Pb level without and with compost supply

A laboratory study was performed to investigate the influence of soil texture (sandy loam vs. clay loam), Pb supply (as Pb(NO₃)₂ without or with compost) and Pb levels on the extraction of available Pb by diethylene triamine pentaacetic acid (DTPA) and its desorption patterns at ten shaking periods. The soils were polluted with five Pb levels without or with compost and incubated for 1 month. Kinetic models commonly used to study the release of the nutrients were used in this study. Results showed that Power function model described the pattern of Pb desorption better than other models. The amount of extracted Pb increased as the Pb levels increased and was found to be higher in sandy loam soil treated with Pb without compost than that of clay loam soil treated with Pb with compost. The a value (Pb desorption constant) was the highest in sandy loam soil amended with Pb without compost. The lowest value of a, however, was observed in clay loam soil amended with Pb with compost. The ab coefficient (initial desorption rate of Pb) was higher in sandy loam than clay loam soil, demonstrating higher initial release rates of Pb in the coarser-textured soil. Addition of Pb without compost resulted in a higher increase in ab value in comparison with Pb with compost, in both the soils.     

Microbial Activity of Cu Contaminated Soils and Effect of Lime and Compost on Soil Resiliency

In vineyards, the long-term use of copper fungicides has increased soil Cu concentrations that can adversely affect the number and activities of soil microorganisms. To better understand this phenomenon and to ameliorate such harmful effects, an incubation experiment was carried out with a sandy loam and a sandy soil to which increasing rates of CuSO₄ were added. By this treatment, the basal soil respiration (7-55%) and decomposition of added vine branches (46-86%) was inhibited. At the application rate of 500 mg Cu kg^?1, soil microbial biomass-C was inhibited (7-66%) in the sandy soil and stimulated (2-10%) in the sandy loam soil. The specific respiration rate was a reliable indicator for Cu stress, and it increased with time and higher Cu concentrations before lime and compost applications. Total number of bacteria and streptomycetes were also strongly inhibited. Fungal population was significantly more tolerant to copper toxicity than the bacteria. A stimulation of fungal population at a dose of 500 mg Cu kg^?1 in both soils was observed. A criterion such as “stimulation” lasting for more than 60 days can also be used as indication of Cu contamination of soils. The order of inhibition (on day 125) at a dose of 500 mg Cu kg^?1 soil was as follows: A. sandy loam soil (pH> 7.0) — fungi < biomass-C < basal soil respiration < bacteria < streptomycetes; B. sandy soil (pH< 6.0) — fungi < basal soil respiration < biomass-C < bacteria < streptomycetes. The application of lime increased soil recovering ability at a moderate rate (for CO₂ production – 22-70% and for biomass-C- 39-156%), but the combination of lime and compost significantly increased soil resiliency (for CO₂ production- 16-518% and for biomass-C- 103-693%). The soil resiliency assessed by number of bacteria in compost treatments was 30-120% in sandy loam soil and 92-700% in the sandy soil. Compost and lime application increased the number of streptomycetes from 52 to 500% in sandy loam soil and from 100 to 700% in sandy loam soil. Fungal population was less increased in sandy soil as compared to sandy loam soil. The ecological dose higher than 5% inhibition of microbial processes and microorganisms appears to be suitable to assess Cu contamination of soils. CO₂ production, biomass-C and specific respiration rate were less sensitive indicators as compared to streptomycetes and bacteria. It appears that compost application effectively promoted the recovery of soil microbial activity and soil fertility of Cu contaminated soils.     

Effect of High‐Molybdenum Compost on Soils and the Growth of Lettuce and Barley

A pot experiment evaluated the growth of lettuce (Lactuca sativa L.) and barley (Hordeum vulgar) and accumulation of molybdenum (Mo) in plants and soils following amendments of Mo compost (1.0 g kg^?1) to a Truro sandy loam. The treatments consisted of 0 (control), 12.5, 25, and 50% Mo compost by volume. The Mo compost did not affect dry‐matter yield (DMY) up to 25% compost, but DMY decreased at the 50% compost treatment. The 50% compost treatments increased the soil pH an average of 0.5 units and increased the nitric acid (HNO₃)–extractable Mo to 150 mg kg^?1 and diethylenetriaminepentaacetic acid (DTPA)–extractable Mo to 100 mg kg^?1 in the growth medium; the same treatment increased tissue Mo concentration to 569 and 478 mg kg^?1 in the lettuce and barley, respectively. Plants grown in the 25% compost produced about 55 mg kg^?1 of total Mo in the growth medium; this resulted in tissue Mo concentration of 348 mg kg^?1 in lettuce and 274 mg kg^?1 in barley without any phytotoxicity. Our results suggested that 55 mg Mo kg^?1 soil would be an appropriate limit for Mo loading of soil developed from compost additions, a value which is presently greater than the Canadian Council for Ministers of the Environment (CCME) Guidelines for the use of type B compost in Canada.     

Effect of type of compost and application rate on growth and quality of lettuce plant

Abstract

This study intends to assess the effect of compost types and rates on lettuce growth. A pot experiment was conducted at the greenhouse of the College of Agriculture and Veterinary Medicine experimental station, Saudi Arabia. Three types (chicken residues, animal residues, and mixed organic residues) and four rates (1%, 2%, 4%, and 6%) were added to sandy soil. Lettuce seeds were sown in a nursery. After two weeks, each seedling was transplanted to a pot that contained 15?kg of soil. The lettuce plants were irrigated regularly for 60?days, then harvested following dry weight (DW) and chemical analyses. The results showed great variation in the DW, nutrient content, and heavy metals concentrations between the different compost treatments. It noted that DW of lettuce decreased with increasing rates of compost in certain treatments. Moreover, no plant growth occurred in some treatments derived from the mixed organic residue with higher application rates. The nutrient status of lettuce tissues varies greatly among different types of compost. The concentration of heavy metals and microelements present within the lettuce tissues was significantly affected by the types of compost. Copper (Cu), Zinc (Zn), Lead (Pb), Cadmium (Cd), and Nickel (Ni) concentrations in lettuce tissues ranged between 7.4 and 9.6, 81.5 and 104, 23.3 and 28.2, 3.7 and 5.8, and 27 and 30?mg kg^?1, respectively. It revealed that heavy metals concentration increased in lettuce tissues with an increasing application rate of composts. The content of Zn, Pb, Cd, and Ni in lettuce plants were higher and did not meet the requirement set by national standards.     

Nitrogen transformations in copper-contaminated soils and effects of lime and compost application on soil resiliency

 A neutral and an acidic soil were treated with different doses (0–3,000 mg Cu kg^–1 soil as CuSO₄) of copper. The percentages of inhibition of nitrification in both soils varied from 5 to 97%, but for the N mineralization these percentages varied from 8 to 65%. The toxic effect of Cu for basal nitrification and N mineralization was assessed as critical. Nitrification was more sensitive than ammonification to copper toxicity. It appears that an ecological dose of inhibition for nitrification and N mineralization higher than 10% is suitable as an indicator for Cu contamination. Soil resiliency assessed by N mineralization in the lime treatments varied from 11 to 154% in the sandy soil and from 70 to 168% in the sandy loam soil. A combined application of lime and compost significantly increased soil resiliency. The percentage increase varied from 904 to 1,390% in the sandy soil and from 767 to 2,230% in the sandy loam soil. It appears that compost was a powerful agent for recovering the soil fertility of Cu-contaminated soils as assessed by N transformation. The acidic sandy soil showed a lower capacity for recovery after Cu toxicity stress. Received: 27 February 1999     

Survival of Pseudomonas solanacearum biovars 2 and 3 in soil: Effect of moisture and soil type

The survival of Pseudomonas solanacearum biovars 2 and 3 in three soils, a Nambour clay loam, a Beerwah sandy loam and a Redland Bay clay, was compared at pressure potentials of ?0.003, ?0.05 and ?0.15 kPa. The soils were inoculated with mutants of P. solanacearum biovars 2 and 3, resistant to 2000 μg streptomycin sulphate ml^?1 and their survival measured every 6 weeks for 86 weeks in the clay loam and clay and for 52 weeks in the sandy loam. Soil populations declined with the initial drying necessary to bring the soil moisture to the specific pressure potentials; the initial counts for biovar 2 varied between 0.20 and 2.00 × 10⁹ cfu g^?1 soil and for biovar 3 between 0.17 and 1.29 × 10⁹ cfu g^?1 soil.The population decline in soil maintained at a constant pressure potential was expressed as the rate of population decline. Biovar 2 declined more rapidly than biovar 3. The rate of population decline of each biovar at ?0.003 and ?0.05 kPa was greater in clay loam than in sandy loam and at all pressure potentials it was greater in clay loam and sandy loam than in clay. There was also a tendency for the rate of population decline of both biovars to decrease in the drier soil treatments.     

Partitioning the spatial and environmental variation of Sclerotinia stem rot on soybean

The variance in survival of Sclerotinia sclerotiorum's sclerotia, carpogenic germination (apothecia) as well as Sclerotinia stem rot (SSR) severity (Disease Severity Index (DSI)) on soybean was partitioned among canopy, soil physico-chemistry and microbiology, cultural practices (2 or 3-y-corn rotation/soybean monoculture and mineral fertilization/urban compost), and spatial matrices in two soils. Partial multiple regression was used to partition the individual SSR variables variance while partial canonical redundancy analysis partitioned the DSI-apothecia and apothecia-survival variance. In clay loam, the sclerotial survival and apothecia variance were mainly explained by the spatial structure of soil physico-chemistry while the DSI did not share this spatial structure and was largely explained by the effects of 3-y-corn rotation on canopy and soil, i.e. lower weed biomass, enhanced soybean yield and fewer apothecia were correlated with disease suppressiveness. In sandy loam, the DSI variance was mostly explained by the spatial structure of canopy and physico-chemistry. Disease suppressiveness, by the interaction of 3-y-corn rotation with urban compost, was largely explained by the enhancement of soil properties, i.e. higher aggregate stability, microbial activity and soil solution concentration in exchangeable ions correlated negatively with carpogenic germination. Partitioning the SSR variance among four matrices of spatial and environmental factors allowed for the first time to interpret and quantify the variance of disease development explained by cultural practices in interaction with the main characteristics of this agroecosystem.