Some Aspects of the Biology of Fusarium oxysporum Schl. in Soil

From either a mycelial or a conidial inoculum the fungus survivedin soil as inactive chlamydospores. The level of its soil populationat equilibrium was too low to be studied by dilution plating.Plant materials placed on or beneath the surface of inoculatedsoil were colonized deeply by the fungus, which produced conidiaon them. Dispersal of conidia can occur with water movementin soil, and at right angles to, as well as in the directionof, that movement. No evidence was found of dispersal of thefungus in soil by continuous growth, even over continuous stretchesof organic matter. This finding was related to the inabilityof the fungus to colonize those organic materials that werepreviously colonized by other organisms from the soil, unlessits inoculum potential were greatly augmented. The fungus isthus seen to be a pioneer fungus. The strain used here grewoutwards a short distance from colonized organic food basesin the soil, leaving in the soil resting spores which couldcolonize fresh pieces of organic material subsequently addedthere. The organism could thus spread by discontinuous growthon successively available, fresh, organic materials.     

Methods of Inoculation of Faba Bean Plants with Sclerotinia sclerotiorum

Different organic materials colonized by the fungus were applied, as an energy-rich inoculum, on faba bean plants cv. Polycarpe in growth chamber experiments. The organic materials used were colonized pieces of celery stems, faba bean petioles, and carrot root, blocks of cultures on potato dextrose agar and pieces of sclerotia. The inoculum was left attached to the plant stems for 48 h. After removal of the inoculum the plants were scored for disease incidence and severity. It was found that colonized pieces of carrot root produced the most uniform disease incidence and severity.     

Behaviour of the fungus Rhizoctonia solani Kiihn in the soil

Rhizoctonia solani was found able to grow as a saprophyte through natural unsterilized soil. Its rate of growth under different soil conditions in glass tumblers was studied by the Rossi-Cholodny soil-plate method. Growth was most rapid at the lowest soil-moisture content tested, viz. 30 % saturation, and was accelerated by forced aeration of the soil. The maximum distance to which mycelial growth could be supported on the food reserves of the agar inoculum alone was some 5 cm., as shown by extent of growth through tubes of moist sand, but in 23 days the fungus grew 21–24 cm through tubes of soil. Removal of the agar disk 2 days after inoculation of the tubes reduced growth through sand by more than half, but through soil by only a small proportion. In soil, Rhizoctonia was able to cause 100% damping-off of radish seedlings planted at a radial distance of 4 cm. from the agar inoculum, and some 40 % damping-off at a distance of 9 cm. The depressing effect of additions of 1 % ground-wheat straw or dried grass to the soil upon growth of the fungus was attributed to (1) the negligible cellulose-decomposing ability of Rhizoctonia, (2) nitrogen starvation of the mycelium, through rapid utilization of the available soil nitrogen by the cellulose-decomposing micro-organisms multiplying upon the fresh organic material, (3) fungistatic action on Rhizoctonia of the respiratory carbon dioxide produced by the cellulose-decomposers.     

Chilli rhizosphere fungus Aspergillus spp. PPA1 promotes vegetative growth of cucumber (Cucumis sativus) plants upon root colonisation

A rhizosphere fungus was isolated from roots of chilli plants and identified as Aspergillus spp. PPA1. The fungus was tested for its ability to promote the growth of cucumber plants in a pot experiment. Cucumber seeds were sown in sterilised field soil amended with wheat grain inoculum (WGI) of PPA 1 at the rate of 0.5, 1 and 1.5% w/w, and plants were grown for 21 days in a net house. The treatment with PPA1 significantly increased shoot length, shoot fresh weight, shoot dry weight, root length, root fresh weight, root dry weight, plant length, leaf area and leaf chlorophyll content of cucumber plants compared to non-treated control. The growth promotion rate increased with the increasing concentration of inoculum of PPA1 applied to the soil. The fungus was re-isolated from the roots of cucumber plants at higher frequencies. These results suggest that Aspergillus spp. PPA1 is a root colonising plant-growth promoting fungus for cucumber.     

Effect of organic soil amendments on damping-off of lettuce caused by Corticium praticola

Ten organic amendments were added to unsterile soil which was contaminated 14 days later with Corticium praticola and sown with lettuce seeds. Substantial increases in final stands of seedlings were obtained with grass meal, bran and wood cellulose. Corn and barley meal, linseed cake and fish meal decreased final stands; molassine meal, potato starch and peptone had relatively little effect. Seedlings grown with wood cellulose were very chlorotic and stunted. Up to 30% of lettuce seeds sown in soil which, 180 days earlier, had been amended with corn meal and contaminated with C. praticola became colonized by the fungus. None was colonized in unamended soil or in soil amended with grass meal. Ninety days after amendment and contamination fewer seeds were colonized in soil amended with grass meal than in unamended soil. The amendment of soil with grass meal was as effective as thiram seed treatment in protecting lettuce seedlings against C. praticola and grass meal was particularly effective in reducing both the numbers of seedlings attacked and the survival of the fungus in the soil.     

Inoculum size effect in dimorphic fungi: extracellular control of yeast-mycelium dimorphism in Ceratocystis ulmi

We studied the inoculum size effect in Ceratocystis ulmi, the dimorphic fungus that causes Dutch elm disease. In a defined glucose-proline-salts medium, cells develop as budding yeasts when inoculated at > or = 10(6) spores per ml and as mycelia when inoculated at <10(6) spores per ml. The inoculum size effect was not influenced by inoculum spore type, age of the spores, temperature, pH, oxygen availability, trace metals, sulfur source, phosphorous source, or the concentration of glucose or proline. Similarly, it was not influenced by added adenosine, reducing agents, methyl donors, amino sugars, fatty acids, or carbon dioxide. Instead, growing cells excreted an unknown quorum-sensing factor that caused a morphological shift from mycelia to budding yeasts. This yeast-promoting effect is abolished if it is extracted with an organic solvent such as ethyl acetate. The quorum-sensing activity acquired by the organic solvent could be added back to fresh medium in a dose-dependent fashion. The quorum-sensing activity in C. ulmi spent medium was specific for C. ulmi and had no effect on the dimorphic fungus Candida albicans or the photomorphogenic fungus Penicillium isariaeforme. In addition, farnesol, the quorum-sensing molecule produced by C. albicans, did not inhibit mycelial development of C. ulmi when present at concentrations of up to 100 microM. We conclude that the inoculum size effect is a manifestation of a quorum-sensing system that is mediated by an excreted extracellular molecule, and we suggest that quorum sensing is a general phenomenon in dimorphic fungi.     

On farm production of arbuscular mycorrhizal fungi inoculum using lignocellulosic agrowastes

The present study evaluated the efficiency of lignocellulosic agrowastes produced in Brazil as substrates for production of on farm AMF inoculum and tested different diluents and inoculation techniques. In a first experiment, Sorghum bicolor seedlings were colonized with Rhizophagus clarus or Claroideoglomus etunicatus and transplanted to 20 L bags containing sugarcane bagasse (SC), king palm leaf sheets (KP), or barley hulls (BH) mixed (1:1:1 or 2:1:1, v/v/v) with sand and rice shell. SC and KP were conducive for production of spores and infectious propagules. A number of infectious propagules obtained were greater than with BH and ranged from 233–350 propagules cm^?3 for both isolates in SC and KP at the1:1:1 mix dilution. Number of spores of both fungi was affected mainly by the SC agrowaste, and spore densities were significantly higher compared to KP and BH. In a second experiment, SC was mixed with soil or sand and inoculation consisted of transplanting colonized seedlings or adding soil inoculum. Number of propagules tended to differ for each fungus according to the inoculation technique or diluent. It is concluded from the data that SC and KP are suitable agrowastes to be incorporated in substrates for producing AMF inoculum using the on farm method.     

Potential of Allyl Isothiocyanate to Control Rhizoctonia solani Seedling Damping Off and Seedling Blight in Transplant Production

The synthetic mustered flavouring essential oil, allyl isothiocyanate (AITC), was evaluated for its effect on suppression of Rhizoctonia solani growth in vitro, and in field soils for reducing inoculum density, saprophytic substrate colonization and seedling damping off and blight using snap bean and cabbage as indicator plants. In vitro growth was completely inhibited at the concentration of 50 μl/l. Inoculum density and saprophytic substrate colonization by the fungus in soil were not affected by AITC concentrations of 50 or 75 μl/kg soil. The inoculum density estimation by the use of soil‐drop technique created an artefact leading to an erroneous conclusion that the fungus was eradicated from soil within 1–3 days after AITC treatment at 150 or 200 μl/kg soil. The saprophytic substrate colonization showed that although the activity of R. solani was greatly reduced, the fungus still colonized 45% of the substrate units at these concentrations, and up to 100% at lower concentrations within 1 day after treatment. At higher concentrations the recovery rate from the substrates gradually declined over time to <6%. Drenching R. solani infested sandy‐loam or silty‐clay‐loam soil with water containing the emulsified AITC to provide 150 or 200 μl/l soil, a few days prior to planting, gave over 90% disease control in snap bean and cabbage, with no apparent phytotoxic effect. The effect of AITC was not influenced by the physical soil texture. AITC appears to have a good potential to replace methyl bromide fumigation of the substrate used for transplant production.     

Immobilization of <Emphasis Type="Italic">Psilocybe castanella</Emphasis> on ceramic (slate) supports and its potential for soil bioremediation

The objective of the present study was to develop and characterize a support for the immobilization of Psilocybe castanella in order to optimize the process of incorporation of fungal inoculum into the soil. The ceramic supports were fabricated from slate powder in the shape of hollow spheres by the slip casting technique (suspension: 40% v/v). The sintering temperature was evaluated in the range of 850–1,070°C and porosity was analyzed by mercury intrusion. The temperature of 1,050°C was the most adequate for sintering of the ceramic supports, with the porosity obtained being less than 1%. The fungus was immobilized on the ceramic supports containing lignocellulosic substrate using disks of fungal mycelium grown on 2% malt agar as the inoculum. Fungal biomass was estimated by the quantification of ergosterol. Peroxidase and laccase activities were determined by the oxidation of ABTS in the presence and absence of H₂O₂, respectively. The efficiency of the immobilized inoculum was tested in a grinder containing coarse sand for 45 min at 75 rpm. The supports were colonized with P. castanella and enzymatic activities were detected after the fifth day of fungal growth. Immobilization of the fungus on the ceramic support provided 80% protection of the inoculum against loss of efficiency during mixture with soil. The results demonstrate the potential of the ceramic supports produced with slate powder for immobilization of basidiomycetous fungi and for application to soil bioremediation processes.     

Growth promotion effect of Fusarium spp. PPF1 from bermudagrass (Cynodon dactylon) rhizosphere on Indian spinach (Basella alba) seedlings are linked to root colonisation

A rhizosphere fungus was isolated from roots of bermudagrass (Cynodon dactylon) and identified as Fusarium spp. PPF1. A pot experiment was conducted to test its ability to promote the vegetative growth of Indian spinach seedlings (Basella alba). Indian spinach seeds were sown in sterilised field soil amended with wheat grain inoculum of PPF1 at the rate of 0.5 and 1.0% w/w, and plants were grown for 21?days in a net house. Significantly, higher germination percentage and vigour index were observed due to application of PPF1 in the potting soil. Treatment with PPF1 also significantly increased shoot length, shoot fresh weight, shoot dry weight, root length, root fresh weight, root dry weight, leaf area and leaf chlorophyll content of cucumber plants compared to non-treated control. The growth promotion rate increased with the increasing concentration of inoculum of PPF1 applied to the soil. The fungus was re-isolated from the roots of cucumber plants at higher frequencies, while a positive co-relation was found between the root colonisation ability and the plant growth enhancement by the isolate. These results suggest that growth promotion effect of Fusarium spp. PPF1 on Indian spinach (B. alba) are linked to root colonisation ability of the fungus.     

An arbuscular mycorrhizal inoculum enhances root proliferation in, but not nitrogen capture from, nutrient-rich patches in soil

Most work on root proliferation to a localized nutrient supply has ignored the possible role of mycorrhizal fungi, despite their key role in nutrient acquisition. Interactions between roots of Plantago lanceolata , an added arbuscular mycorrhiza (AM) inoculum and nitrogen capture from an organic patch ( Lolium perenne shoot material) dual-labelled with ¹⁵N and ¹³C were investigated, to determine whether root proliferation and nitrogen (N) capture was affected by the presence of AM fungi. Decomposition of the organic patch in the presence and absence of roots peaked in all treatments at day 3, as shown by the amounts of ¹³CO₂ detected in the soil atmosphere. Plant N concentrations were higher in the treatments with added inoculum 10 d after patch addition, but thereafter did not differ among treatments. Plant phosphorus concentrations at the end of the experiment were depressed by the addition of the organic residue in the absence of mycorrhizal inoculum. Although uninoculated plants were also colonized by mycorrhizal fungi, colonization was enhanced at all times by the added inoculum. Addition of the AM inoculum increased root production, observed in situ by the use of minirhizotron tubes, most pronouncedly within the organic patch zone. Patch N capture by the end of the experiment was c . 7.5% and was not significantly different as a result of adding an AM inoculum. Furthermore, no ¹³C enrichments were detected in the plant material in any of the treatments showing that intact organic compounds were not taken up. Thus, although the added AM fungal inoculum benefited P. lanceolata seedlings in terms of P concentrations of tissues it did not increase total N capture or affect the form in which N was captured by P. lanceolata roots.     

Inoculum Size Effect in Dimorphic Fungi: Extracellular Control of Yeast-Mycelium Dimorphism in Ceratocystis ulmi

We studied the inoculum size effect in Ceratocystis ulmi, the dimorphic fungus that causes Dutch elm disease. In a defined glucose-proline-salts medium, cells develop as budding yeasts when inoculated at ≥10⁶ spores per ml and as mycelia when inoculated at <10⁶ spores per ml. The inoculum size effect was not influenced by inoculum spore type, age of the spores, temperature, pH, oxygen availability, trace metals, sulfur source, phosphorous source, or the concentration of glucose or proline. Similarly, it was not influenced by added adenosine, reducing agents, methyl donors, amino sugars, fatty acids, or carbon dioxide. Instead, growing cells excreted an unknown quorum-sensing factor that caused a morphological shift from mycelia to budding yeasts. This yeast-promoting effect is abolished if it is extracted with an organic solvent such as ethyl acetate. The quorum-sensing activity acquired by the organic solvent could be added back to fresh medium in a dose-dependent fashion. The quorum-sensing activity in C. ulmi spent medium was specific for C. ulmi and had no effect on the dimorphic fungus Candida albicans or the photomorphogenic fungus Penicillium isariaeforme. In addition, farnesol, the quorum-sensing molecule produced by C. albicans, did not inhibit mycelial development of C. ulmi when present at concentrations of up to 100 μM. We conclude that the inoculum size effect is a manifestation of a quorum-sensing system that is mediated by an excreted extracellular molecule, and we suggest that quorum sensing is a general phenomenon in dimorphic fungi.     

Identification of host plants and description of sclerotia of the truffle <Emphasis Type="Italic">Mattirolomyces terfezioides</Emphasis>

The truffle, Mattirolomyces terfezioides, is a hypogeous ascomycete with uncertain host relationships. The fungus has been regularly collected on sandy soils in the Carpathian Basin. During the study of the natural host plants of the fungus, strange, amorphous, belowground hyphal aggregates incorporating soil and sand particles have been found attached to the surface of the roots. The fruitbodies of M. terfezioides develop from these hyphal aggregates. This structure, similar to that formed by morels, could be interpreted as a sclerotium. Sclerotia were found both on roots of woody and herbaceous plants. To detect the roots colonized by M. terfezioides, a species-specific polymerase chain reaction was developed. Seven natural hosts of the fungus were identified by this method. No specificity regarding taxa or life form of the plants was found. The colonization of the roots by the septate hyphae of M. terfezioides was weak, particularly compared to the colonization by arbuscular–mycorrhizal fungi. This suggests that this fungus is not the dominant fungal partner of these plants. Therefore, using M. terfezioides as the only inoculum may be inappropriate in truffle cultivation experiments. Nevertheless, further in vitro experiments are needed to develop reliable knowledge on the still ambiguous symbiotic strategy of this fungus.     

The survival on chrysanthemum roots of epiphytic mycelium of Mycosphaerella ligulicola

Mycosphaerella ligulicola has been shown to survive as epiphytic mycelium on the root surface of chrysanthemum cuttings: such survival could continue throughout the life of the glasshouse crop. Symptomless surface colonization of roots of cuttings could be induced in non-sterile soil from an inoculum of (a) mycelium and sclerotia or (b) conidia (Ascochyta state); the colonization could spread upwards over the root surface. After 12 weeks survival as an epiphyte on chrysanthemum roots the fungus was still pathogenic to unrooted cuttings. Although the root surfaces of twelve other plants could be colonized by M. ligulicola the fungus survived on these roots for not more than 8 weeks.     

Soil-Behaviour of Phytophthora clandestina

Investigations were undertaken to study the nature and behaviour of P. clandestina in soil. The pathogen was recovered only from soil sievings 250–499 μm and 500 μm–0.99 mm, containing small root fragments. In soil the introduced inoculum of the fungus was incapable of saprophytically and competitively colonizing the dead cotyledons of subterranean clover used as bait material. Exposure of the inoculum to increasing numbers of microbes by adding greater proportions of nonsterile fields, oil to the growth medium of the plant had no significant effect on survival rate and fresh shoot weight of subterranean clover. However, microbes present in the field soil reduced the severity of root rot of subterranean clover. P. clandestina was, able to spread between 15–30 mm through pasteurized soil within a period of 20 days.     

Effect of Host Plants and Inoculum Forms on the Inoculum Potential of Arbuscular Mycor rhizal Fungi

The effects of inoculum forms (single-spore, multi-spores, or colonized root pieces) and host plants (Nicotiana tabacum L., Sorghum sudanense(Piper) Stapf, and Trifolium repens L. ) on the development and inoculum potential (IP) of the arbuscular mycorrhizal fungi (AMF): Glomus macrocarpum Tul & Tul, Glomus mosseae (Nicol & Gerd. ) Gerdemann & Trappe, Glomus versiforme (Karsten) Berch, and Sclerocystis sinuosa Gerdemann & Bakhi cultured in pots were investigated. The lag phase of treatment with 50 spores or 0.5 g (fresh weight) of colonized root pieces was 4 weeks, much shorter than that of the treatment with 1 spore (8 weeks); the value of IP(VIP) and percentage of root colonization(PRC) of the former were greater than those of the latter. Only on the early stages of colonization was there difference between the 50 spores and the 0.5 g (fresh weight) of colonized root piece inoculation treatments. The IP per plant inoculated with 0. 5 g (fresh weight) of colonized mot pieces of AMF was greater than that of the other two treatments except G. vers/forme on Nicotiana tabacum, while the PRC of the plants inoculated with 50 spores and 0. 5 g (fresh weight) of colonized root pieces of AMF was higher than that of the 1 spore inoculation after 10 weeks. The VIP of AMF on Trifolium repens was significantly higher than that on the other two hosts. The VIP of G. mosseae, G. versiforme, and S. sinuosa was respectively greater than that of G. macrocarpum. This suggested that different species of AMF produced different VIP of the inoculum. Nicotiaha tabacum was much better than the other host plants which used to be inoculated with single spore, and to produce inocula of AMF.     

Mycorrhizas in the Kakadu region of tropical Australia

This research represents the first part of a study which aimed to characterize the role of mycorrhizal associations in undisturbed and disturbed habitats in the Alligator Rivers Region of the Northern Territory of Australia. This is a seasonally dry tropical region with a climate consisting of a long dry season and a monsoonal wet season. Intact soil cores were sampled from 22 sites in this region, representing eucalypt savanna woodland, wetland, rocky hill and rainforest habitats. Clover, sorghum and eucalypt seedlings were grown in these cores in bioassays to measure the inoculum potential of vesicular-arbuscular mycorrhizal (VAM) and ectomycorrhizal (ECM) fungi. Propagules of VAM fungi were concentrated in the surface horizon, and were not adversely affected by 6 months dry storage of soil. Bioassays detected VAM fungus propagules at all sites, but these were less numerous in three sites with sparse herbaceous vegetation (a shrub-dominated woodland site, a sandstone area and a disturbed gravel pit without topsoil), than in other woodland sites. Propagules of VAM fungi were particularly numerous in soil from a rainforest habitat, which had much denser plant cover than any of the savanna sites. Propagules of ECM fungi colonized eucalypt seedling roots in some cores from all sites, except two wetland areas and a disturbed area without eucalypt trees. Physical and chemical properties of soils varied between sites and some properties (texture, organic carbon, etc.) were correlated with the inoculum potential of VAM fungi.     

Influence of soil organic matter decomposition on arbuscular mycorrhizal fungi in terms of asymbiotic hyphal growth and root colonization

Soil organic matter is known to influence arbuscular mycorrhizal (AM) fungi, but limited information is available on the chemical components in the organic matter causing these effects. We studied the influence of decomposing organic matter (pure cellulose and alfalfa shoot and root material) on AM fungi after 30, 100, and 300 days of decomposition in nonsterile soil with and without addition of mineral N and P. Decomposing organic matter affected maize root length colonized by the AM fungus Glomus claroideum in a similar manner as other plant growth parameters. Colonized root length was slightly increased by both nitrogen and phosphorus application and plant materials, but not by application of cellulose. In vitro hyphal growth of Glomus intraradices was increased by soil extracts from the treatments with all types of organic materials independently of mineral N and P application. Pyrolysis of soil samples from the different decomposition treatments revealed in total 266 recognizable organic compounds and in vitro hyphal growth of G. intraradices in soil extract positively correlated with 33 of these compounds. The strongest correlation was found with 3,4,5-trimethoxybenzoic acid methyl ester. This compound is a typical product of pyrolysis of phenolic compounds produced by angiosperm woody plants, but in our experiment, it was produced mainly from cellulose by some components of the soil microflora. In conclusion, our results indicate that mycelia of AM fungi are influenced by organic matter decomposition both via compounds released during the decomposition process and also by secondary metabolites produced by microorganisms involved in organic matter decomposition.     

On-farm production of AM fungus inoculum in mixtures of compost and vermiculite

On-farm production of arbuscular mycorrhizal (AM) fungus inoculum can reduce the cost of the inoculum and increase utilization of this symbiosis in plant production. Bahiagrass (Paspalum notatum Flugge) seedlings, colonized by AM fungi, were transplanted into raised bed enclosures. Media within the enclosures was vermiculite mixed with either field soil or yard clippings compost in Experiment I and vermiculite mixed with yard clippings compost or dairy manure/leaf compost in Experiment II. Compost and vermiculite mixtures yielded more propagules of AM fungi than soil-based mixtures in Experiment I. Growth of plants in a 1:4 (v/v) mixture of yard clippings compost and vermiculite produced more inoculum (503 propagules cm(-3)) than growth in 1:9 and 1:99 (v/v) mixtures (240 and 42 propagules cm(-3), respectively). Water, inorganic nutrient solution minus P, and fish protein digest were added to inoculum production enclosures in Experiment II. Results indicated that supplemental nutrient addition was unnecessary. This method produces a concentrated inoculum of AM fungi in a form readily used as an amendment to horticultural potting media for the production of vegetable seedlings.     

Effect of seasonal abiotic conditions and field margin habitat on the activity of Pandora neoaphidis inoculum on soil

The ability of the aphid pathogenic fungus Pandora neoaphidis to remain active in the absence of a resting stage through a combination of continuous infection and as conidia deposited on soil was assessed alongside the potential for planted field margins to act as a refuge for the fungus. P. neoaphidis was able to infect the pea aphid, Acyrthosiphon pisum, when maintained under controlled conditions that simulated those that occur seasonally in the UK. Although there was a significant inverse relationship between temperature and time-to-kill, with death occurring after 4.2, 6.9 and 13.6 days when maintained under fluctuating summer, autumn and winter temperatures, respectively, there were no additional statistically significant effects of photoperiod. The activity of inoculum on soil was indirectly assessed by baiting with A. pisum. Under controlled conditions P. neoaphidis remained active on soil and was able to infect aphids for up to 80 days. However, the percentage of aphids that became infected decreased from 76% on day 1 to 11% on day 80. Whereas there was little difference in the activity of conidia that had been maintained at 4 degrees C and 10 degrees C, activity at 18 degrees C was considerably reduced. Under field conditions the activity of inoculum was strongly influenced by season. On day 49 there was little or no activity during spring, summer or winter. However, during autumn a mean proportion of 0.08 aphids still became infected with P. neoaphidis. Margin type did not affect the activity of conidia nor was there a difference in activity between blocks that had regenerated naturally and those that had been planted. These results suggest that P. neoaphidis can infect aphids and remain active on soil under the abiotic conditions that occur seasonally in the UK and that this fungus may be able to persist annually without a resting stage.