Survival of sclerotia of Sclerotium cepivorum berk. In muck soil as influenced by drying and the location of sclerotia in soil

The proportion of viable sclerotia of Sclerotium cepivorum placed in field plots in Burnaby, British Columbia, decreased with time (P = 0.05). Sclerotia that had been air-dried for 48–72 hr had a lower percentage survival than those that had not been dried. Sclerotia placed on the soil surface decayed more rapidly than those buried at 15 cm (P = 0.05). Loss of viability was due to decay of sclerotia rather than to a reduction in the ability of the sclerotia to germinate which did not decline with time (P = 0.05). After 16 months in the field 23.6; 2.1; 11.7 and 8.9% of the sclerotia remained viable in the not-dried buried, not-dried surface, dried buried and dried surface treatments respectively.     

Some factors affecting survival of sclerotia of Macrophomina phaseolina in soil

At least 75% of the sclerotia of Macrophomina phaseolina survived for 1 yr in most natural soils kept at 26°C and at 50–55% of the soil moisture holding capacity (m.h.c.). Although survivability was reduced in a very acid soil (pH 4.5) collected under a pine stand, 33% of the sclerotia survived for 1 yr. Soil pH had very little or no effect on sclerotial survivability. Of three organic amendments tested (alfalfa hay, chitin, pine needles) only ground alfalfa hay at 0.8% (w/w) reduced survivability of sclerotia in soil by about 75% in a year. Alfalfa hay at 0.4% reduced survivability by 36%. Various N sources added at 200 μg Ng^?1 soil had no effect on survival. Of 13 fungicides tested, only benomyl and captan at 20 μg a.i. g^?1 soil appreciably reduced populations of sclerotia in soil.Soil temperature and moisture content were the two most important factors affecting survivability of sclerotia. At ?5 or 5°C the biggest drop in sclerotial survivability occurred when the soil was incubated moist (at 50% m.h.c. or more). At 26°C the biggest drop occurred in air-dried soil (2–3% m.h.c.) and survivability was decreased to some extent at 15 and 30% m.h.c. Survivability also dropped rapidly in moist soil (50–55% m.h.c.) exposed to four cycles each having 3-week freezing (?5°C) and 1 week thawing (26°C). Sclerotia in air-dried soil (2–3% m.h.c.) continuously kept at ?5°C maintained nearly complete survivability after 16 weeks. Sclerotia survived almost 80–90% in moist soil (50–55% m.h.c.) kept for 16 weeks at 26°C or in moist soil exposed to four cycles each having 3-week thawing (26°C) and 1-week freezing (?5°C).     

Diallyl-disulphide to reduce the numbers of sclerotia of Sclerotium cepivorum in soil

A single injection of 0.2 ml diallyl disulphide (DADS) at 0.156% (v/v) into soil containing naturally-produced sclerotia of Sclerotium cepivorum and maintained in the laboratory at 15°C stimulated sclerotial germination and reduced sclerotial numbers by 67%; ungerminated sclerotia remained viable. Higher concentrations of DADS had no additional effect except that at 20% (v/v), germination was slightly inhibited. A similar reduction in sclerotial numbers was obtained when the mixture of soil and sclerotia was exposed to DADS vapour. Four, monthly applications of DADS at 0.2 ml 0.15% (v/v) per application did not give a further reduction.The effect of DADS was temperature dependant, with a reduction in sclerotial numbers of 65 and 9% at 15 and 5°C respectively.     

Reduction in viability of sclerotia of Macrophomina phaseolina with polyethylene mulching of soil

Mulching of Macrophomina phaseolina-inksted soil (moist or dry) with transparent polyethylene sheets during the hot days of May increased temperature of wet soil at 5 cm from 37°C (unmulched) to 52°C (mulched) and of dry soil from 52°C (unmulched) to 65°C (mulched). At 20 cm mulching increased temperature from 30°C to 41°C (wet) and from 38°C to 42°C (dry). In artificially-infested soil. the sclerotia of M. phaseolina were eradicated at 5 cm by a mulch treatment for 1 week and at 20 cm depth 50% sclerotia lost viability in wet soil but were not affected in dry soil. In a naturally infested soil (5–7 sclerotia g^?1), which gave 20% infection on Vigna, the sclerotia were reduced to such an extent that after 1 week mulching no disease was observed on Vigna.     

Parasitism of sclerotia of Sclerotium rolfsii by trichoderma harzianum

The ability of Trichoderma harzianum isolate 203 to attack the soil-borne plant pathogen Sclerotium rolfsii is apparently connected with the production by the isolates of chitinase and β-(1,3)-glucanase inside the attacked sclerotia during parasitism.SEM and TEM micrographs show that the mycoparasite degraded walls of sclerotial cells and the attacked cells lost their cytoplasmic contents. It is assumed that T. harzianum utilizes sclerotial cell contents thus enabling it to sporulate intensively on the sclerotial surface and inside the digested cells.     

Polyethylene mulching of soil to reduce viability of sclerotia of Sclerotium oryzae

Mulching of Sclerotium oryzae infested soil (moist or dry) with polyethylene sheets during hot summer days of May and June increased the soil temperature at 5 cm from 36°C (unmulched) to 48°C (wet) and from 44 to 52°C (dry) and at 20cm from 32 to 38°C (wet) and from 35 to 39°C (dry). In artificially-infested soil, the sclerotia were not eradicated but 95–100% loss in viability was observed at 5 cm by a mulch treatment for 1 week and at 20 cm by mulching for 8 weeks. Mulching effects were not influenced by moisture content of soil or by amendments with lucerne or wheat straw. Mulching of naturally-infested soil at a second site did not eradicate S. oryzae but reduced sclerotial viability by 93%.     

Biological control of the charcoal root rot fungus Macrophomina phaseolina on slash pine seedlings by a hyperparasite

Macrophomina phaseolina. the cause of damping-off and charcoal root rot disease on slash pine seedlings (Pinus elliottii Engelm var. elliottii), was antagonized by an unidentified basidiomycete species under laboratory and nursery conditions. Aseptically-grown slash pine seedlings died 3 days after inoculation with M. phaseolina, whereas seedlings inoculated with the basidiomycete sp., or basidiomycete sp. plus M. phaseolina, and uninoculated controls were alive after 6 months. In nursery studies, survival of slash pine seedlings in plots inoculated with M. phaseolina was significantly lower (57 per cent) than survival resulting from inoculations with the basidiomycete sp. (90 per cent), or basidiomycete sp. plus M. phaseolina (84 per cent), and the uninoculated controls (81 per cent). Electron microscopy showed that the basidiomycete sp. is a hyperparasite on mycelia of M. phaseolina. Preliminary evidence indicates that the hyperparasite is not host-specific but will infect other fungal pathogens. Two ectomycorrhizal fungi were not parasitized by the basidiomycete.     

Survival of sclerotia of sclerotinia sclerotiorum in soil

Factors affecting longevity of sclerotia in sandy clay loam (s.c.l.) and sandy loam (s.l.) were examined, using sclerotia from a laboratory culture of S. sclerotiorum and from natural infestations on beans and lettuce.Survival of sclerotia from culture and lettuce was compared in s.l. Recovery and viability were less, and incidence of Fusarium, Mucor and Trichoderma spp. greater, in sclerotia from lettuce than from culture. Rinds of sclerotia from lettuce were more perforated than those from culture.Burial of sclerotia at 4 cm for 35 weeks reduced recovery of sclerotia to zero in s.c.l. and by 50% in s.l. At the soil surface recovery was reduced by 55% in s.c.l. and by 10% in s.l. Less than 50% of sclerotia recovered were viable. Neither a chloropicrin-methyl bromide fumigant nor a tomato compost treatment affected recovery or viability. Fumigation increased incidence of Trichoderma spp. and decreased incidence of Fusurium and Mucor spp. isolated from sclerotia.Apothecia were produced over 6 weeks in s.c.l. and over 20 weeks in s.l. Production was increased by the low rate of fumigant in s.c.l. and by tomato compost in s.l.     

Survival of Coniothyrium minitans associated with sclerotia of Sclerotinia sclerotiorum in soil

The development and survival of the mycoparasite Coniothyrium minitans associated with sclerotia of the plant pathogen Sclerotinia sclerotiorum was studied in pasteurised and non-sterile (untreated) soil. Using scanning electron microscopy, developing pycnidia were first seen within the sclerotial medulla at 7 days post-inoculation with the mycoparasite in pasteurised soil. However, by 14 days post-inoculation, pycnidia had developed fully in both pasteurised and non-pasteurised treatments, and conidial droplets were exuded onto the outer surface of the infected sclerotia. Thirty days post-inoculation, irrespective of soil treatment, the majority of the sclerotial medulla had been converted to pycnidia, with the sclerotial rind remaining largely intact. The pycnidia and dried intact droplets were still observed 6 months post-inoculation with C. minitans, although the conidia on the outer surface of the dried droplets had largely collapsed by this stage. Germinability studies at 10 months post-inoculation showed that approximately 13% of the conidia in dried droplets were still viable. This work shows the potential for infected sclerotia of S. sclerotiorum to provide a unique reservoir for the survival of C. minitans.     

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.     

Prolonged compost curing reduces suppression of Sclerotium rolfsii

Composts have long been recognized to facilitate biological control of soil borne plant pathogens. Composts can introduce biocontrol agents into growth media and serve as a food base for their establishment and activity. Mature biosolids compost (a blend of sewage sludge and yard waste) was found to be suppressive to germination of the sclerotia of S. rolfsii on compost plates and also suppresses the disease development in bean plants (Phaseolus vulgaris L.). Microscopic observations revealed that sclerotia placed on suppressive compost were attacked by mycoparasites. However, prolonged curing of compost negated this phenomenon. This research was aimed to study the changes in chemical and biological properties occurring during prolonged curing and their relation to compost suppressiveness. Correlations were found between the decrease and subsequent loss of suppression of sclerotia germination and the decrease in basal respiration, dissolved organic carbon (DOC) and concentrations, and the increase in concentration and specific UV absorbance. A shift of both bacterial and Ascomycetes populations as a consequence of curing was observed. Interactions between micro-organisms and their chemical environment are discussed.     

Water-dispersible clay after wetting and drying cycles in four Brazilian oxisols

Widespread intensive land use in the seasonal tropics can damage the physical stability of aggregates. Similar damage can be expected from wetting and drying cycles causing aggregate fragmentation and, consequently, leading to an increase in their specific area and exposure of internal electric charges. Thus, we hypothetised that the influence of wetting and drying cycles is dependent on the mineralogical composition of oxisols (latosols) and it is higher in soils with low aggregate stability. A greenhouse experiment was carried out to test this hypothesis in highly weathered soils from Brazil, all with variable-charge clays and highly stable aggregates. Wetting and drying cycles were defined from the quantity of water available between field capacity and the permanent wilting point. Soil columns were submitted to 0, 2, 6, 9, 12, 15 and 18 wetting and drying cycles. After each number of wetting and drying defined physical and chemical properties were determined. Statistical analysis, such as simple and multiple linear regression and Pearson's correlation were performed, showing significantly correlated WDC contents with wetting and drying cycles. The obtained results led to the conclusion that there was a close interdependence among mineralogical composition, aggregate stability and WDC influenced by wetting and drying cycles. Soils of reduced aggregate stability like kaolinitcs made them more susceptible to the action of wetting and drying on the WDC. Changes in the WDC with wetting and drying cycles showed correlated with eletrochemical properties.     

Evaluation of different Coniothyrium minitans inoculum sources and application rates on apothecial production and infection of Sclerotinia sclerotiorum sclerotia

The effects of three Coniothyrium minitans isolates (Conio, IVT1 and Contans^®), applied to soil as conidial suspensions or as maizemeal-perlite (MP) inocula (Conio), on apothecial production and infection of Sclerotinia sclerotiorum sclerotia were assessed in two soil pot bioassays and two novel box bioassays in the glasshouse at different times of the year. C. minitans isolate Conio applied as either MP or ground MP at full rate (10⁶-10⁷ cfu cm⁻³ soil) consistently decreased the carpogenic germination, recovery and viability of sclerotia and increased C. minitans infection of the sclerotia of S. sclerotiorum by in comparison with either MP or conidial suspension treatments applied at lower rates (10³-10⁴ cfu cm⁻³ soil). Additionally, when applied at the same rate, MP inoculum of C. minitans was consistently more effective at reducing carpogenic germination than a conidial suspension. The effect of MP and ground MP at full rate on carpogenic germination was expressed relatively early as those sclerotia recovered before apothecia appeared on the soil surface already had reduced numbers of apothecial initials. In general, there were few differences between the isolates of C. minitans applied as conidial suspensions. Box bioassays carried out at different times of the year indicated that temperature and soil moisture influenced both apothecial production and mycoparasitism. Inoculum concentration of C. minitans and time of application appear to be important factors in reducting apothecial production by S. sclerotiorum.     

Time and burial depth influencing the viability and bacterial colonization of sclerotia of Sclerotinia sclerotiorum

Sclerotia are the primary over wintering inoculum of Sclerotinia sclerotiorum (Lib.) de Bary. The effects of tillage on the primary inoculum are not well understood. The purpose of this research was to study sclerotial viability over time and between burial depths in soil, to identify bacteria colonizing and degrading the sclerotia, and determine whether these bacteria may be utilized as biological control agents. Correlation analysis indicated that a significant negative relationship existed between sclerotial viability and elapsed temporal factors (R²=−0.68, P<0.0001), and depth of burial (R²=−0.58, P<0.0001). After twelve months, sclerotia on the soil surface had the highest viability (57.5%), followed by those at the 5 cm depth (12.5%), and only 2.5% of those placed at the 10 cm depth remained viable. A significant negative relationship between sclerotial viability and bacterial populations also existed (R²=−0.60, P<0.0001). Two hundred and sixty-eight bacteria were isolated from sclerotia, 29 of which showed strong in vitro antagonism to the mycelial growth of S. sclerotiorum. Biodiversity of the inhibitory bacterial isolates was minimal on sclerotia from the soil surface and within all depths sampled at three months (i.e. in January). All burial depths within the April and July sampling dates produced bacterial diversities that were distinct from each other.     

The wetting and drying of a grazed and ungrazed clay soil

A 4 year field experiment clearly demonstrates the role played by desiccation cracks in the redistribution of rainfall during the autumn re-wetting of a Windsor Series clay soil. Where topsoil structure is damaged by the trampling of grazing animals, cracks are well developed and a portion of rain water is diverted down them, by-passing the topsoil. Redistribution takes place from two centres, the bottom of the cracks and the surface. But where structure is unaffected by trampling, the soil accommodates seasonal shrinkage without cracking and redistribution is by diffusion from the surface only. Summer evaporation is shown to be linearly related to Penman's potential evapotranspiration over a wide range of soil moisture deficits, and simple constants are provided to correct E_T to obtain E_A. The M.A.F.F. method of estimating evapotranspiration is shown to be too conservative and fails to identify the October maximum soil moisture deficit common in this soil.     

Carbon and nitrogen mineralization as affected by drying and wetting cycles

Drying and rewetting of soil is an important process in soil aggregation, soil organic matter (SOM) decomposition, and nutrient cycling. We investigated the source of the C and N flush that occurs upon rewetting of dry soil, and whether it is from microbial death and/or aggregate destruction. A moderately well drained Kennebec silt loam (Fine-silty, mixed, superactive, mesic Cumulic Hapludoll) was sampled to a 10 cm depth. Soil under constant water content (CWC) was compared with soil subjected to a series of four dry-wet (DW) cycles during the experimental period (96 d) and incubated at 25 °C. Mineralized C and N were measured during the drying and rewetting periods. Aggregate size distributions were studied by separating the soil into four aggregate size classes (>2000, 250-2000, 53-250, and 20-53 μm) by wet sieving. Repeated DW cycles significantly reduced cumulative N mineralization compared with CWC. The reduction in cumulative mineralized C resulting from DW compared with CWC increased as the DW treatments were subjected to additional cycles. The flush of mineralized C significantly decreased with repeated DW cycles. There was no significant effect on aggregate size distributions resulting from to the DW cycles compared with CWC treatment. Therefore, the flush of mineralized C and N seemed to be mostly microbial in origin in as much as aggregate distribution was unaffected by DW cycles.     

Nitrous oxide emissions from grain legumes as affected by wetting/drying cycles and crop residues

Grain legume production with rhizobial inoculation has drawn attention not only because of the economic value of nitrogen (N) fixation by grain legumes, but also because of the concern that N₂ fixation by grain legumes may enhance emissions of nitrous oxide (N₂O), a powerful greenhouse gas. However, the relationship between N₂O emissions and biological N₂ fixation by grain legumes is not well understood. The objective of this study was to quantify N₂O emissions associated with N₂ fixation by grain legumes as affected by wetting/drying cycles and crop residues. Two grain legumes, lentil (Lens esculenta Moench) and pea (Pisum sativum L.), either inoculated with two Rhizobium leguminosarum biovar viciae strains, 99A1 and RGP2, respectively, or fertilized with ¹⁵N-labeled fertilizer were grown in a controlled environment under three wetting/drying cycles. Profile N₂O concentrations and surface N₂O emissions were measured from the soil–plant systems, which were compared with those from a cereal, spring wheat (Triticum aestivum L. ac. Barrie). After harvest, crop residues were incorporated into soils that were seeded to spring wheat to evaluate the effect of crop residues on N₂O emissions. Results indicated that: (1) inoculating grain legumes with non-denitrifying rhizobia did not enhance N₂O emissions and the presence of grain legumes did not increase N₂O emissions compared with the cereal crop, and (2) profile N₂O accumulation and surface emissions were not related to the type of crop residues added to the soil, but related to the residual N applied previously as N fertilizer. This suggests that N₂O emissions are not directly related to biological N₂ fixation by grain legumes, and on a short time scale, N rich residues of N₂-fixing crops have a limited impact on N₂O emissions compared with N fertilization.     

Glucose additions to aggregates subjected to drying/wetting cycles promote carbon sequestration and aggregate stability

Biogeochemical mechanisms at microscale regions within soil macroaggregates strengthen aggregates during repeated DW cycles. Knowledge of additional biogeochemical processes that promote the movement of dissolved organic carbon (DOC) into and throughout soil aggregates and soil aggregate stabilization are essential before we can more accurately predict maximum carbon (C) sequestration by soils subjected to best management practices. We investigated the spatial distribution of ¹³C-glucose supplied to individual soil macroaggregate surfaces and subjected to multiple drying and wetting (DW) cycles. Subsequent distribution of added glucose-C, CO₂ respiration, increased microbial community activity and concomitant changes in soil aggregate stabilization were monitored. Moist macroaggregates were treated with no DW cycles and zero glucose C (Control), 5 DW cycles and zero glucose (DW0G), and 5 DW cycles with additions of 250 μg glucose-¹³C/g soil during each cycle (DW+G). Repeated additions of glucose-C to aggregate surfaces reduced the mineralization of pre-existing soil C by an average of 45% and established concentric gradients of glucose-derived C. It is concluded these increasing gradients promoted the diffusion of soluble C into interior regions and became less available to microbial respiration. Spatial gradients of glucose-derived C within aggregates influenced a shift in the abundance of unique ribotypes spatially distributed within aggregates. Rapid decreases in the mineralization rates of glucose-C during repeated DW cycles suggested greater C sequestration by either physical restriction of microbes or chemical sorption of new C that diffused into aggregates. Aggregate stability decreased significantly following 2-3 DW cycles, when glucose-C was not added. Additions of glucose-C with each DW cycle maintained soil aggregate stability equal to the moist but not cycled control throughout the 5 DW cycles of this study. These data simulate the strengthening of soil aggregates in no tillage agroecosystems which provides continuous additions of DOC compounds generated by decomposing plant residues on the soil surface, and root exudates and decomposition, as well as the mineralization of POM materials within nondisturbed soil profiles.     

Survival of Rhizobium japonicum in soil-sludge environment

The usage of sewage sludge on agricultural lands is an effective and inexpensive practice that provides nutrients for crops. A successful legume crop also depends on the survival of Rhizobium in the soil environment. The number of R. japonicum (USDA 110) in treatment groups containing various soil-to-sludge ratios (control, 13:1, 9:1 and 5:1) during incubation for 1, 21 and 42 days was investigated. The control group contained soil without sludge. Mecklenburg clay and Enon sandy loam soils (both are fine, mixed, thermic, ultic Hapludalfs) were used. All treatments were adjusted to pH 6.7 and brought to 75% of field capacity with 1 ml inoculum (9 × 10⁸ cells ml^? 1) and distilled water. Samples were incubated at 25 C and monitored periodically for the number of surviving R. japonicum (USDA 110) organism by the plant infcction-MPN method. Strains were identified by gel-immunodiffusion. Recovery of rhizobia from both soils was < 1% in all treatment groups after 42 days. However, for control, 13:1, 9:1 and 5:1 groups, the percentage recovery was higher in Enon sandy loams (7.9, 2.3, 2.3 and 2.3%, respectively) at 21 days. Recovery of rhizobia in the 5:1 group from both soils was 7.9% after 1 day, whereas control values were 92%. A decline in rhizobial populations in higher sludge soils may be due to the heavy metals present and available during mineralization of sludge in soils. However, the number of R. japonicum that survived to 21 days was 1.7 × 10₅g^?1 and 1.7 x 10⁶g^?1 for Mecklenburg clay and Enon sandy loam soils with highest sludge, respectively.     

Effects of wetting and drying cycles on in situ soil particle mobilization

Understanding particle mobilization and transport in soils is a major concern for environmental protection and water resource management as they can act as vectors for sorbing pollutants. In natural soils, the existence of a finite size and renewable pool of dispersible particles has been hypothesized. Even though freeze‐thaw and wetting‐drying cycles have been identified as possible mechanisms of pool replenishment between rainfall events, to date the underlying phenomena ruling the renewal of particle pools are still largely unexplored. We carried out a series of infiltration‐drainage experiments to study systematically the effects of periods without rain (pauses) on in situ particle mobilization in undisturbed soil columns. We found that, for a given column, pause duration between two rainfall events has a major influence on subsequent particle mobilization: the mass of leached particles increases with pause duration until it reaches a maximum (mass for a 200‐hours pause is 15 time greater than for a 1‐hour pause), and then it decreases for even longer pauses. This behaviour was correlated with soil water content, and can be explained by soil matrix weakening due to differential capillary stresses during drying. The consequences of this finding are important because the 15‐fold increase in mass of leached particles, when pause duration is changed from 1 hour to 4 days, might overwhelm variations caused by changes in other parameters such as the ionic strength of the incoming solution or the rainfall intensity.