Ralph Noble

Sexuality and Genetic Identity in the Agaricus Section Arvenses

ABSTRACT Twelve wild collections and one commercial strain were used to characterize breeding systems and to develop molecular identities in the Arvenses section of the genus Agaricus, which includes the “horse mushroom” A. arvensis. Two morphotypes were identified based on macro- and micromorphological features. However, not all collections could be delimited by conventional taxonomic characters. Sequencing of the small subunit intergenic spacer (ITS) region (368 to 370 bp) of the rRNA genes clearly resolved the 13 collections into two clusters consistent with the identified morphotypes. Single-spore progenies and mating type testers were established and used to test intra- and interstock compatibility. The two compatibility groups identified were consistent with ITS clusters. Compatibility group I stocks readily interbred within the constraints of a unifactorial heterothallic system with a multiallelic mating type factor. Compatibility group II had a more restricted breeding pattern, and interactions were difficult to predict on the basis of mating type. Morphological data, ITS sequences, and the ability to interbreed suggest that these collections are part of a complex of interrelated species. Single-spore, homokaryotic isolates from both compatibility groups were able to fruit in compost culture, and two of the collections may represent natural homokaryotic fruiting. We conclude that species from the section Arvenses have versatile unifactorial heterothallic life cycles that permit both interbreeding and homokaryotic fruiting.

Volatile C8 compounds and pseudomonads influence primordium formation of Agaricus bisporus

Primordium formation of Agaricus bisporus depends on the presence of a casing layer containing stimulatory bacteria and on sufficient air exchange. The influence of specific pseudomonad populations and volatile organic compounds (VOC) on primordium formation of A. bisporus was studied in microcosm cultures. VOC produced by A. bisporus mycelium were predominantly C8 compounds, some of which could inhibit primordium formation, with 1-octen-3-ol being most inhibitory. A VOC produced by the rye grain substrate, 2-ethyl-1-hexanol, on which A. bisporus was grown also inhibited primordium formation. 2-Ethyl-1-hexanol and 1-octen-3-ol were metabolized by pseudomonad populations and adsorbed by activated charcoal, with both modes of removal enabling primordium formation in the casing. Removal of VOC by ventilation also enabled primordium formation to occur under axenic conditions. The presence of 2-ethyl-1-hexanol and 1-octen-3-ol in the microcosms resulted in higher total bacterial and pseudomonad populations in the casing. The stimulatory effects of the casing and its microbiota and air exchange on primordium formation of A. bisporus at least partly are due to the removal of inhibitory C8 compounds produced by the mycelium and its substrate. Monitoring and controlling the levels of these inhibitory VOC in mushroom culture should enable primordium formation of A. bisporus to be more efficiently and precisely controlled.

Allium white rot suppression with composts and Trichoderma viride in relation to sclerotia viability

ABSTRACT Allium white rot (AWR) is a serious disease of Allium spp. caused by the sclerotium-forming fungus Sclerotium cepivorum. This work has examined the effects of onion waste compost (OWC) and spent mushroom compost (SMC), with and without Trichoderma viride S17A, on sclerotia viability and AWR in glasshouse and field experiments. Incorporation of OWC into soil reduced the viability of sclerotia and the incidence of AWR on onion plants in glasshouse pot bioassays, whereas SMC or T. viride S17A only reduced incidence of AWR. In two field trials, OWC reduced sclerotia viability and was as effective in reducing AWR as a fungicide (Folicur, a.i. tebuconazole). Field application of SMC had no effect on sclerotia viability and did not control AWR. However, the addition of T. viride S17A to SMC facilitated proliferation of T. viride S17A in the soil and increased the healthy onion bulb yield. The results indicate two mechanisms for the suppression of AWR: (i) reduction in the soil population of viable sclerotia, which may be due to volatile sulfur compounds detected in OWC but absent in SMC, and (ii) prevention of infection of onion plants from sclerotia following amendment of soil with OWC, SMC, or T. viride S17A.

Management of plant health risks associated with processing of plant-based wastes: a review

The rise in international trade of plants and plant products has increased the risk of introduction and spread of plant pathogens and pests. In addition, new risks are arising from the implementation of more environmentally friendly methods of biodegradable waste disposal, such as composting and anaerobic digestion. As these disposal methods do not involve sterilisation, there is good evidence that certain plant pathogens and pests can survive these processes. The temperature/time profile of the disposal process is the most significant and easily defined factor in controlling plant pathogens and pests. In this review, the current evidence for temperature/time effects on plant pathogens and pests is summarised. The advantages and disadvantages of direct and indirect process validation for the verification of composting processes, to determine their efficacy in destroying plant pathogens and pests in biowaste, are discussed. The availability of detection technology and its appropriateness for assessing the survival of quarantine organisms is also reviewed.

Population and phylogenetic structure within the Agaricus subfloccosus complex

The name Agaricus subfloccosus (J. E. Lange) Pilat, based originally on Danish specimens, has been applied to mushrooms growing in coastal northwestern Europe as well as to mushrooms associated with Picea (and Abies ) at higher elevations in western Europe. Corresponding populations are also found in these two habitat zones in western North America. Material of both lowland and highland forms from Europe and North America was studied using morphological, cultural, and genotypic approaches. Homothallism was consistently observed in those isolates studied. Individual and population level genetic data are also consistent with homothallic reproduction. Dissimilarity analysis of multilocus nuclear and mitochondrial genotypes based on allozyme and DNA RFLP markers provided strong evidence that, within either ecologically distinct group, the amphiatlantic populations were genotypically very similar. Conversely, there were large genotypic differences between the two ecologically distinct groups, whether within or between continents. Supported by cultural and morphological evidence, these data indicate that two ancient, phylogenetically distinct entities exist within current concepts of A. subfloccosus . A formal taxonomic resolution is complicated by the lack of a holotype for Langes species.

Variation in morphology, growth and fructification of isolates in the Agaricus subfloccosus complex

A number of homothallic isolates in the Agaricus subfloccosus complex, designated W4, was obtained from basidiome collections at four separate locations in the U.K. The relative prominence of floccules on the pileus surface, the ring structure, stipe length and vivid discoloration of the cut or bruised flesh distinguished these isolates from Canadian and Swiss isolates of A. subfloccosus and a morphologically similar species, the cultivated mushroom A. bisporus. The mycelial extension growth rates of the A. subfloccosus W4 isolates on agar and in composted substrate were less than those of Swiss and Canadian isolates of A. subfloccosus, which in turn were less than those of wild and commercial strains of A. bisporus. Basidiomes of all A. subfloccosus isolates were obtained from culture in composted substrates. There were significant differences in basidiome yield and mycelial extension growth rate between different sources of the A. subfloccosus W4 isolates, but the two parameters were not correlated. Single spore isolates from the same parent culture and same basidium were found to have significant differences in basidiome yield. The culture of A. subfloccosus isolates on a range of composted substrates was examined and their potential as a commercial edible mushroom is discussed.

Properties of peat-based casing soils and their influence on the water relations and growth of the mushroom (Agaricus bisporus)

Different combinations of peat and chalk or lime sources with differing moisture contents were used to determine how specific physical and chemical properties of the casing soil relate to the growth and water relations of the mushroom. The peat types varied in terms of decomposition and extraction method; the lime addition varied in terms of rate and type (chalk or sugar beet lime). During the colonisation of the casing soil before fruiting, the extension growth rate of mushroom mycelium was most closely correlated (negatively) with the volumetric moisture content of the casing soil. Scanning electron microscopy showed that mycelium growing at a lower casing soil matric potential (Ψm) had a much finer and branched structure than mycelium growing at a higher Ψm. Across all the peat and lime source treatments, a relationship was found between the mean Ψm of the casing soil and mushroom yield, with an optimum Ψm of -7.9 to -9.4 kPa. Mushrooms are produced in ‘flushes’ at about 8-day intervals and during the development of each flush of mushrooms, there was a significant decrease in casing soil Ψm . This decrease (to below -40 kPa) was greatest in the second flush, which was the highest yielding. There were no relationships between mushroom yield and casing soil osmotic potential Ψπ within the range -93 to -154 kPa or any of the other chemical properties and water and air holding characteristics of the casing soils which were determined. Across different casing soil treatments, mushroom dry matter content was negatively correlated with mushroom yield and positively correlated with mushroom tissue osmotic potential.

Indicator organisms for assessing sanitization during composting of plant wastes

The potential for using plant pathogens and seeds as indicator organisms for assessing sanitization of plant wastes during composting was tested in bench-scale flask and large-scale systems. Plasmodiophora brassicae was unsuitable due to high temperature tolerance in dry to moist composts, and detection of viable inoculum post-composting using bioassay plants not corresponding with that using TaqMan® PCR, possibly due to preservation of nucleic acids at elevated temperatures. Several other plant pathogens (Sclerotinia sclerotiorum, Microdochium nivale, Phytophthora cinnamomi and Phytophthora nicotianae) were unsuitable due their low temperature tolerance. Fusarium oxysporum f.sp. cepae and f.sp. radicis-lycopersici chlamydospores and tomato seeds were suitable indicators due to their moderate temperature tolerance and ease of viability testing post-composting. Abutilon seeds were more tolerant than tomato seeds of compost temperatures ≥52°C but more prone to degradation at lower temperatures and therefore less suitable as indicators. Relationships between compost temperature during exposures of 2-10 days and subsequent viability of the above chlamydospores or seeds enabled the sanitizing effect of composting processes to be predicted within 2-6 days. Plant waste type (woody or vegetable) had a small but significant effect on the relationship for tomato seeds but not for F. oxysporum chlamydospores.

CONTROL OF SCLEROTIAL PATHOGENS WITH THE MYCOPARASITE CONIOTHYRIUM MINITANS

Pressure to reduce the use of chemicals in the environment has led to the search for alternative sustainable methods to control soil-borne pathogens, especially those plant pathogens that form long-lived resting bodies (sclerotia). Mycoparasites that attack sclerotia have been explored as biocon- trol agents of these pathogens and some mycoparasites such as Coniothyrium minitans and Trichoderma species have been the focus of particular study. This paper reviews recent developments in the use, ecology, impact and modes of action of C. minitans especially against Sclerotinia sclerotiorum that may be influential in improving reproducibility of disease control in the future. Some studies of the use of Trichoderma viride to control Allium white rot caused by Sclerotium cepivorum are also discussed.

Aerating recycled water on mushroom composting sites affects its chemical analysis and the characteristics of odor emissions

Recycled water (RW) containing compost leachate can be a potent source of foul odor on mushroom composting sites. Samples of RW were repeatedly collected from storage tanks and pits of 14 mushroom composting sites in England and Ireland. Relationships between the effects of submerged aeration of the RW, the chemical and microbial characteristics of the RW, and the odors emitted were investigated. Recycled water samples were analyzed for pH, electrical conductivity (EC), redox potential, and dissolved oxygen concentration after 7 to 14 d cold storage. Freeze-dried material from the RW samples was chemically profiled by pyrolysis gas chromatography-mass spectrometry (GC-MS), and the content of odor precursor compounds was determined by the ninhydrin colorimetric method. The headspace air of containerized RW samples was analyzed by thermal desorption GC-MS and with gas detection tubes and assessed for odor intensity (OI) and concentration by panelists and serial dilution olfactometry. The predominant odorants in the headspace and freeze-dried residues of RW samples were volatile sulfur-containing compounds and carboxylic acids. The headspace OI, EC, dry matter content, and redox potential of RW were interrelated. The headspace OI and combined concentration of hydrogen sulfide (H(2)S) + dimethyl sulfide of RW were correlated (R(2) = 0.635; P < 0.001). Prediction of the OI of RW by measuring RW EC and the concentration of headspace sulfides using gas detector tubes enables rapid and low-cost monitoring of RW from mushroom composting sites. Submerged aeration of RW in storage tanks or pits reduced the RW headspace air OI and the combined H(2)S + dimethyl sulfide concentration by 88%.