Infection of wheat seminal roots by varieties of Phialophora radicicola and Gaeumannomyces graminis

The growth of isolates of Phialophora radicicola var. radicicola, P. radicicola var. graminicola, Gaeumannomyces graminis var. graminis, G. graminis var. tritici and Leptosphaeria narmari was compared on the coleoptiles and roots of wheat seedlings. Fungal growth was measured as the extent and density of dark runner hyphae. All except P. radicicola var. graminicola grew on coleoptiles and all grew on roots although only G. graminis var. tritici extensively colonized the root stele. Growth rate on roots was positively correlated with that on agar, P. radicicola var. graminicola and L. narmari growing at about half the rate of the other fungi; hyphal density was high for P. radicicola var. graminicola but relatively low for the other fungi. For P. radicicola var. radicicola, P. radicicola var. graminicola and G. graminis var. tritici growing from buried inocula, the extent and density of hyphae up roots towards the seed was similar to that down, but G. graminis var. tritici caused chocolate-brown stelar discoloration up roots only.Root invasion by P. radicicola var. radicicola, P. radicicola var. graminicola and G. graminis var. tritici was described from sections. Each gave a different pattern of hyphae and host response within an inoculum layer, and progressive changes occurred away from the inoculum. Studies of the rate of penetration by each fungus and the rate and pattern of death of cortical cells explained the differences between fungi. G. graminis var. tritici penetrated living cells in advance of other soil micro-organisms, and hence by hyaline hyphae inducing much lignituber formation as a host resistance reaction. P. radicicola var. graminicola penetrated only senescent or dead cells in association with other soil microorganisms, and hence by dark hyphae, inducing little lignituber formation. P. radicicola var. radicicola was intermediate in all these respects. The high hyphal density of P. radicicola var. graminicola was due to the colonization of cortical cells and spaces by dark, clearly visible, rather than hyaline hyphae, which are invisible in unstained roots. Cell death in the outer cortex explained the observed progressive restriction of growth by all fungi to the inner cortex with increasing distance from the inoculum. Spread by G. graminis var. tritici up roots was ectotrophic relative to the stele but down roots hyphae spread rapidly within the stele. Stelar reactions suggested as resistance mechanisms occurred up roots only. Their absence down roots is attributed to infection disrupting stelar transport.     

Susceptibility of Pythium spp and other fungi to antagonism by the mycorparasite Phytium Oligandrum

Phytium spp and isolates within species differed in susceptibility to the mycoparasite Pythium oligandrum Drechs., as evidenced by their degree of inhibition by it on cellulose and ability to support its growth across their colonies on agar. Yet no Phythium sp. was highly susceptible to it, and P. graminicola Subramanian was highly resistant.No evidence was found that P. oligandrum produces toxins active against other fungi. In liquid culture, P. oligandrum grew on undisturbed colonies of Phialophora sp. (highly susceptible) but not P. ultimum Trow or Fusarium culmorum (W. G. Sm.) Sacc. (moderately resistant) and not Rhizoctonia solani Kuhn (highly resistant). It grew in culture filtrates of Phialophora sp., P. ultimum and F. culmorum, utilizing organic nitrogen and thiamine released by these fungi, but not in culture filtrates of R. solani. It grew on mycelial macerates of all these fungi, though poorly on those of R. solani. Resistance to parasitism by P. oligandrum seems to reside, at least partly, in low levels of nutrient release from host hyphae.     

Biological control of the take-all fungus, Gaeumannomyces graminis, by Phialophora radicicola and similar fungi

Phialophora radicicola is an avirulent fungal root-parasite of grasses and cereals, with runner hyphae like those of Gaeumannomyces graminis. Weakly and non-pathogenic varieties of these fungi control the pathogens, G. graminis vars. tritici and avenue. Biology of these fungi is considered and the evidence for biological control and possible mechanisms reviewed; control is probably widespread in natural plant communities, and host-mediated, perhaps by induction of plant resistance mechanisms.Prospects for application of biological control seem best for P. radicicola var. graminicola established on grass crops, as this is already exploited in British agriculture. New evidence is presented on the effects of grassland factors on this fungus, especially sward composition, age, mineral nutrition and management practices: its population might often be limited by the rate of new root production to replace those with cortices already colonized. Prospects for control by seed inoculation with P. radicicola var. radicicola and G. graminis var. graminis also seem good, but possible dangers of introducing them into cereal cropping are emphasized. The weak pathogens might be used also for indirect control by establishing hyper-parasites or inducing disease suppression (like take-all decline) in soils, but there is no evidence for ‘Phialophora decline’, at least in well-managed grasslands. Finally, different biocontrols of take-all might be combined, and biological with chemical ones for ophiobolus patch disease of turf.P. radicicola var. graminicola has a slight beneficial effect on grass yield, even when the pathogens, G. graminis vars. tritici and arenae arc absent; this probably contributes to its abundance in natural grasslands in Britain. The scale of biological control by this and similar fungi might explain why, in their absence, effective plant resistance to G. graminis is uncommon in the Gramineae.     

Non-pathogenic Fusarium strains protect the seedlings of Lepidium sativum from Pythium ultimum

Two root-colonizing Fusarium strains, Ls-F-in-4-1 and Rs-F-in-11, isolated from roots of Brassicaceae plants, induced the resistance in Lepidium sativum seedlings against Pythium ultimum. These strains caused an increase in the content of benzyl isothiocyanate, and of its precursor glucotropaeolin, in the roots of the host plants. The increased isothiocyanate content is one of the factors contributing to the resistance of L. sativum against P. ultimum. To be transformed into the fungitoxic compound benzyl isothiocyanate, glucotropaeolin has to be hydrolyzed by myrosinase, which can be produced either by plants or microorganisms. The Fusarium strain Ls-F-in-4-1 has a myrosinase activity but the strain Rs-F-in-11 has not. These results suggest that both strains are able to trigger the metabolic pathway leading to benzyl isothiocyanate production in the plant. In the case of the myrosinase-negative strain Rs-F-in-11, hydrolyzation into isothiocyanate is only due to the myrosinase activity of the plant, and in the other case, the myrosinase produced by the strain Ls-F-in-11 also would contribute to the production of isothiocyanate. This paper reports a new mode of action of non-pathogenic Fusarium strains in controlling P. ultimum.     

In vitro effects of Fusarium blight-controlling fungicides on pathogens of Poa pratensis

An experimental iprodione fungicide,3-(3,5dichlorophenyl)-N-(1-methylethyl)-2,4-dioxo-1-imidazolidinecarboxamide, controls Fusarium blight of Kentucky bluegrass (Poa pratensis L.), but it also amplifies the proportion of crowns colonized by Fusarium and the number of its propagules in soils. In contrast, the disease, the proportion of infected crowns, and the numbers of propagules in soil are generally suppressed by benomyl, methyl 1-(butylcarbamoyl)-2-benzimidazolecarbamate. Triadimefon, 1-(4-chlorophenoxy)-3,3-dimethyl-1-(1H-1,2,4-triazol-1-yl)-2-butanone, also controls the disease but is not stimulatory or inhibitory of fusaria. Iprodione and benomyl were studied for their effects on growth and sporulation of Fusarium acuminatum isolated from diseased crowns; iprodione had no or slightly stimulatory effects, and benomyl greatly suppressed these processes, except in a benomyl-tolerant strain.Toxicities of iprodione and benomyl to 1555 identified Fusarium isolates from Kentucky bluegrass turf were determined, as were the toxicities of iprodione to 23 turfgrass pathogens. Of the Fusarium spp, only F. solani was significantly inhibited by iprodione, whereas all were inhibited by benomyl. Iprodione-sensitive fungi included species of Bipolaris, Corticium, Curvularia, Drechslera, Rhizoctonia, Sclerotinia, and Typhula. Insensitive fungi included Colletotrichum, Fusarium, Gaeumannomyces, and Pythium.Investigations with selective fungicides indicate that the primary causal agent of Fusarium blight is not among the fusaria, and that re-interpretation of the disease and its etiology is necessary.     

The incidence of Gaeumannomyces graminis var. Tritici and Phialophora radicicola var. Graminicola on wheat grown after different cropping sequences

The effects of three treatment cropping sequences (fallow, lucerne or grass-clover ley) on the incidence of Gaeumannomyces graminis var. tritici and Phialophora radicicola var. graminicola were measured in a field experiment. Increases in G. g. tritici population in the soils of the first wheat crop and the incidence of take-all in the second wheat crop were greater after fallow or lucerne than after grass-clover. These differential increases were not associated with differences in survival of G. g. tritici during the treatment cropping but were correlated negatively with the population of P.r. graminicola in the soil. After the third wheat crop the P. r. graminicola population after grass-clover had decreased and take-all was as prevalent as after fallow or lucerne.     

Behaviour of cercosporella herpotrichoides and Ophiobolus graminis on buried wheat plant tissues

The survival of Ophiobolus graminis (Sacc.) Sacc. in buried wheat straw, but not of Cercosporella herpotrichoides Fron, was prolonged when nitrogen was added to soil. Five isolates of C. herpotrichoides survived similarly despite differences in their abilities to decompose cellulose and wheat straw. However, survival of O. graminis, at different temperatures, was entirely consistent with its cellulolytic activity. Unlike O. graminis, C. herpotrichoides survived up to 19 weeks in buried agar discs. These differences suggest contrasting means of survival of the pathogens.Both fungi showed short-term saprophytic activity in soil, growing into autoclaved wheat straws or coleoptiles only during the first few days after burial. An apparently increased competitive saprophytic colonization of straws at 10 than at 21°C, however, was largely an artefact of the “Cambridge Method” and not an indication of increased competitive saprophytism. Burial in unsterilized soil reduced the decomposition-rate of straws colonized by Chaetomium globosum Kunze and increased that of straws colonized by C. herpotrichoides. It also caused O. graminis to form lysis-resistant hyphae from previously unadapted ones.     

The ecology of a Pythium community in relation to the epidemiology of carrot cavity spot

Carrot cavity spot (CCS) is characterised by the appearance of small sunken elliptical lesions on the tap root. It is caused by a complex of Pythium species, but the species diversity and interactions within the complex have never been studied for modelling CCS epidemics. The diversity of a pathogenic Pythium community was assessed during 3 consecutive years in a field experiment after an initial artificial soil infestation with P. violae. 1241 lesions were examined, yielding 728 Pythium isolates. Conventional microbiological methods and restriction polymorphism of the internal transcribed spacer regions of the rDNA of 209 representative Pythium isolates allowed us to identify 655 isolates as belonging to six Pythium species, including P. violae and five indigenous species (P. sulcatum, P. intermedium, P. sylvaticum/irregulare, P. coloratum, and P. ultimum). Biological traits, such as pathogenicity, optimum temperature for mycelial growth and saprophytic survival of the inoculum, explained the fluctuations in the composition of the complex over 17 successive samplings during the 3-year period, most notably the prevalence of first P. violae and then P. sulcatum. P. violae and P. sulcatum were occasionally isolated in mixture from single lesions (10.4% and 9.6%, respectively). Other species were more frequently isolated in mixture: 30.8% for P. intermedium, 33.8% for P. sylvaticum/irregulare, 42.9% for P. ultimum, and 66.7% for P. coloratum. A contingency analysis allowed us to define ‘major’ and ‘minor’ species on both pathological and ecological criteria (frequency of occurrence in the complex, pathogenicity and ability to induce lesions by themselves), and demonstrated that infection by one ‘major’ pathogen species (P. violae or P. sulcatum) is not positively correlated with the presence of a second Pythium species. The ratio between ‘observed’ and ‘expected’ mixed infection frequency under the assumption of independent infection (mir) was less than 1 for P. violae, P. sulcatum, P. intermedium, and P. sylvaticum/irregulare (P < 0.05). For all Pythium species, there was a negative linear relationship between mir and pathogenicity (R² = 0.638): the less a Pythium species was pathogenic on carrot, the more often it was isolated from a CCS lesion in mixture with at least one other species. The non-significance of interactions between species during the infection phase suggests that CCS epidemics can be analysed as if they were caused by a single Pythium species.     

Influence of temperature on solvent-pesticide interaction effects towards fungi

The effect of temperature on interactions between combinations of the solvent acetone and the fungicide captan was determined using the fungi Pythium uhimum, Sclerotinia homeocarpa, and Pestalotia sp. Seven concentrations of acetone, ranging from 0.1 to 3.0% (v/v), were interacted with four concentrations of captan, ranging from 1.0 to 10.0 ppm (mg L^?1 ). This procedure was repeated at 15, 20, 25, and 30 °C using potato dextrose agar at pH 5.5. Acetone and captan interacted synergistically towards P. ultimum and S. homeocarpa, and antagonistically towards Pestalotia sp., regardless of the temperature. The exact temperature response was dependent upon both the captan level and culture used. As the temperature increased from 15 to 30 °C, the toxicity of captan decreased by up to 40% with P. ultimum, and from < 10 to 20 with S. homeocarpa and Pestalotia sp. With only a few exceptions, the magnitude of interactions observed generally decreased as the temperature was increased from 15 to 30 °C. This was most pronounced with P. ultimum. Generally, the lowest interaction magnitudes were recorded at 20 °C for P. ultimum, 20 to 25 °C for S. homeocarpa, and 30 °C for Pestalotia sp. The greatest interaction magnitudes were usually obtained at 15 or 30 °C with P. ultimum, 15 °C with S. homeocarpa, and 25 °C with Pestalotia sp.     

Quantification of Pythium populations in ginseng soils by culture dependent and real-time PCR methods

Ginseng (Panax quinquefolius L.) is widely cultivated in North America as a medicinal herb. However, yields are often reduced by various root pathogens, including Pythium species, which cause damping-off in young plants. In order to improve the prediction of disease risk, real-time PCR assays were developed and used in conjunction with dilution plating on selective media to quantify populations of Pythium irregulare Buisman and Pythium ultimum Trow directly from soil. The assays were tested on artificially infested soils and then used on a variety of naturally infested, ginseng-cultivated soils in south-western Ontario. Data on P. ultimum DNA concentrations were positively correlated with the number of P. ultimum viable propagules on selective media. However, in the case of P. irregulare, the presence of cryptic species resulted in incongruent relationships between the dilution plate and real-time PCR data. We therefore sequenced the ITS region of a large number of P. irregulare isolates in order to determine the proportion that would be detected by the real-time PCR assay. The ability to quantify pathogen populations directly from soils using real-time PCR (calibrated with data on inoculum potential) will improve disease risk assessment and lead to a reduction in pesticide application.     

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.     

Host status of 10 fungal isolates for two nematode species, Filenchus misellus and Aphelenchus avenae

The classification of nematodes in the family Tylenchidae into plant parasites, plant associates or fungal-feeders for community analyses, have been much discussed by nematode ecologists. For an appropriate classification, fungal-feeding habits in the family need to be studied. To evaluate the host status of 10 fungal isolates for Filenchus misellus (Tylenchidae) and Aphelenchus avenae (Aphelenchida, Aphelenchidae), population growth rates, body length and width and sex ratios of the nematodes were measured after 40-day culture on fungal colonies at 25 °C. For F. misellus, the fungi determined as good hosts were two Basidiomycota fungi (Agaricus bisporus, Coprinus cinereus), three Ascomycota fungi (Chaetomium cochlioides, Chaetomium funicola, Chaetomium globosum) and a plant-pathogenic fungus (Rhizoctonia solani) on the basis of nematode population growth rate and female body length. Interestingly Pleurotus ostreatus, known as a predaceous fungus for the other nematodes, was also a good host for F. misellus. While, for A. avenae, good hosts were four plant-pathogenic fungi (Fusarium oxysporum f. sp. conglutinans, F. oxysporum f. sp. cucumerinum, Pythium ultimum, R. solani) and A. bisporus. A. avenae was trapped and preyed upon by Pleurotus hyphae. In F. misellus, males were 7-21% of adults, but the ratio did not correlate significantly with the population growth rate. In A. avenae, no male occurred. Differences in habitat preference between Filenchus and Aphelenchus were explained on the basis of the host status and habitat preferences of the tested fungi.     

Suppression of damping-off of cucumber caused by Pythium ultimum with live cells and extracts of Serratia marcescens N4-5

Environmentally friendly control measures are needed for the soil-borne pathogen, Pythium ultimum. This pathogen can cause severe losses to field- and greenhouse-grown cucumber and other cucurbits. Live cells and ethanol extracts of cultures of the bacterium Serratia marcescens N4-5 provided significant suppression of damping-off of cucumber caused by P. ultimum when applied as a seed treatment. Live cells of this bacterium also suppressed damping-off caused by P. ultimum on cantaloupe, muskmelon, and pumpkin. Culture filtrates from strain N4-5 contained chitinase and protease activities while ethanol extracts contained the antibiotic prodigiosin, the surfactant serrawettin W1, and possibly other unidentified surfactants. Production of prodigiosin and serrawettin W1 was temperature-dependent, both compounds being detected in extracts from N4-5 grown at 28 °C but not in extracts from N4-5 grown at 37 °C. Ethanol extracts from strain N4-5 grown at 28 °C inhibited germination of sporangia and mycelial growth by P. ultimum in in vitro experiments. There was no in vitro inhibition of P. ultimum associated with ethanol extracts of strain N4-5 grown at 37 °C. Prodigiosin, purified from two consecutive thin-layer chromatography runs using different solvent systems, inhibited germination of sporangia and mycelial growth of P. ultimum. Another unidentified compound(s) also inhibited germination of sporangia but did not inhibit mycelial growth. There was no in vitro inhibition associated with serrawettin W1. These results demonstrate that live cells and cell-free extracts of S. marcescens N4-5 are effective for suppression of damping-off of cucumber caused by P. ultimum possibly due in part to the production of the antibiotic prodigiosin.     

Colonisation of seminal roots of wheat and barley by egg-parasitic nematophagous fungi and their effects on Gaeumannomyces graminis var. tritici and development of root-rot

This study provides evidence that egg-parasitic nematophagous fungi, Pochonia chlamydosporia, Pochonia rubescens and Lecanicillium lecanii, can also reduce root colonisation and root damage by a fungal pathogen. Interactions of nematophagous fungi with the take-all fungus, Gaeumannomyces graminis var. tritici (Ggt), and their influence on severity of the root disease it causes were studied in laboratory and pot experiments. In Petri dish experiments the three nematophagous fungi reduced colonisation of barley roots by Ggt and also reduced necrotic symptoms. On the contrary, root colonisation by nematophagous fungi was unaffected by Ggt. In growth tube experiments, the three nematophagous fungi again reduced Ggt root colonisation and increased effective root length of barley seedlings. This was true for both simultaneous and sequential inoculation of nematophagous fungi versus Ggt. In the pot experiments the inoculum of the tested fungi in soil was applied in the same pot, as a mixture or in layers, or in coupled pots used for wheat grown with a split-root system. The nematophagous fungi P. chlamydosporia (isolate 4624) and L. lecanii (isolate 4629), mixed with Ggt or in split root systems with the pathogen, promoted growth of wheat (i.e. increased shoot weight), although no disease reduction was found. In split root systems, lower levels of peroxidase activity were found in seedlings inoculated with Ggt in combination with the nematophagous isolates 4624 and 4629 than when the take-all fungus was applied alone.Our results show that nematophagous fungi reduce root colonisation by Ggt, root damage and stress induced senescence in Ggt-inoculated plants.     

Phytophthora species producing toxin active on tobacco

Extracts of oat cultures of 53 isolates of Pythium and Phytophthora and an isolate of Achlya were tested for their ability to cause watersoaking, laminar collapse, and dehydration of excised tobacco leaves. Extracts of oat cultures of 10 of 11 isolates of P. cryptogea Pethy. & Laff., 4 of 5 isolates of P. megasperma Drechs., all 5 isolates of P. drechsleri Tucker, and 3 of 4 isolates of P. erythroseptica Pethy, were toxic to tobacco laminae. Mycelial extracts of three P. cryptogea isolates obtained from three diverse sources had properties similar to a previously studied toxin from P. cryptogea. Extracts of oat cultures of three P. cryptogea isolates and an isolate of P. megasperma caused foliar toxicity and growth reduction of tobacco transplants. Mycelial extracts of isolates of four species of Phytophthora active on excised leaves were also active in inhibiting plant growth. Most isolates of these four species of Phytophthora appear to produce identical or similar toxins. Although not parasitic to tobacco, those species are potentially detrimental to tobacco.     

Influence of nitrogen on cellulolysis rate and saprophytic survival in soil of some cereal foot-rot fungi

Records of longevity of saprophytic survival in standard segments of colonized wheat straw buried in soil have been analysed for five species of cereal foot-rot fungi: Gaeumannomyces graminis var. tritici, Fusarium roseum f. sp. cerealis, Cercosporella herpotrichoides, Curvularia ramosa and Cochliobolus sativus. Also included in this survey has been Phialophora radicicola var. tgraminicola, which is a harmless, non-pathogenic parasite of grass and cereal roots. Longevity of survival has been expressed as the median period (S50), i.e. weeks the fungus survives in 50% of the straws sampled. S50 values for four of these fungi vary widely with the N status of the soil: ample soluble N promotes longevity of G. graminis and F. roseum but shortens survival of P. radicicola and C. sativus. The survival of C. herpotrichoides and C. ramosa is unaffected by N supply. These different responses can be explained through the operation of two mechanisms (1) cellulolysis of the wheat straw tissue (ca. 40% cellulose) by the primary fungal colonizer (2) liability of the fungus to be prematurely displaced from its straw substrate by the intense microbial competition developing in soils of high N status and fertility. C. herpotrichoides falls outside the scope of this analysis, because its cellulolytic ability is poor and it seems to survive in the straw tissue mainly as resting mycelium.Cellulolytic ability as a determinant of saprophytic survival has been expressed by the cellulolysis adequacy index (CAI), which is the ratio of cellulolysis rate to linear growth rate (as a parameter of general metabolic rate) for each fungal species. Studies on growth of three species in axenic culture on filter-paper cellulose with mineral nutrients have shown that as CAI value increases, so the need for N for prolonged survival of the fungal colony decreases; this is because a high CAI value reduces the minimum rate of hyphal growth and consequent extension of cellulolysis that is necessary for maintenance of the fungal colony. This relationship between CAI and S50 values appears to apply not only to survival in axenic cultures but also to survival in soils of low N status, in which microbial competition is limited by a critical shortage of this nutrient. In soils of high N status, on the other hand, longevity of survival seems to be determined chiefly by the degree of success in saprophytic competition for survival with other micro-organisms.     

Fungal-feeding habits of six nematode isolates in the genus Filenchus

The family Tylenchidae is a large group of soil nematodes but their feeding habits are not fully known. We studied the fungal-feeding abilities of nematodes in the genus Filenchus. We measured population growth rates (PGRs) of six nematode isolates, representing three Filenchus species, when feeding on seven fungal species on two types of culture media. On Potato Dextrose Agar (PDA) Filenchus misellus, Filenchus discrepans and an unidentified Filenchus sp. generally showed moderate to large PGRs on saprophytic fungi (Rhizoctonia solani, Chaetomium globosum, Coprinus cinereus, Flammulina velutipes) and low PGRs on plant-pathogenic fungi (Fusarium oxysporum, Pythium ultimum). In soil medium amended with chopped soybean plant material or wheat bran, the status of most of the fungi as food for the nematodes was similar to that on PDA, although PGRs tended to be lower in the soil medium. However, C. globosum, a good food on PDA, only supported low PGR in soil for each of the three nematodes. The PGRs of F. misellus on C. globosum in soil were still low even when types and amounts of organic matter amendments were varied. A nematophagous fungus, Pleurotus ostreatus (oyster mushroom), was determined to be a food for Filenchus on PDA or in soil, based on PGR measurements corrected for extraction efficiency. To determine whether fungal species and culture media affected nematode extraction efficiencies and, consequently, the apparent PGRs, we compared efficiencies between R. solani, C. globosum and C. cinereus, and between PDA and soil. The relatively low extraction efficiencies across fungal species in soil seemed responsible for the lower nematode PGRs in soil than on PDA. On PDA generally, fungal species did not affect the assessment. In soil, effects of fungal species on extraction were significant, but not consistent, across nematode species. Nevertheless, the extraction efficiency differences in soil were considered not to affect assessment of the three fungi as food for the nematodes. The confirmation that three Filenchus species reproduce by feeding on fungi in soil suggests that fungal-feeding is not an unusual habit in the field, in this genus. We believe that in community studies, nematodes in the genus Filenchus should be considered fungal feeders or root and fungal feeders, rather than only plant feeders. Our confirmation of fungal-feeding habits in the genus Filenchus supports the hypotheses that plant-feeding nematodes evolved from those feeding on fungi.     

Earthworms promote the reduction of Fusarium biomass and deoxynivalenol content in wheat straw under field conditions

A field experiment was conducted to elucidate ecosystem services provided by earthworms on the repression of phytopathogenic and toxinogenic fungi. The study focussed on decomposing Fusarium culmorum-infected and deoxynivalenol (DON)-contaminated wheat straw remaining on the soil surface as part in conservation tillage. Mesocosms were established in the topsoil of a winter wheat field located in Northern Germany, where conservation tillage has been practised for 20 years. Besides a non-earthworm treatment, two earthworm species were inoculated in the mesocosms either separately or combined: Lumbricus terrestris (anecic, detritivorous) and Aporrectodea caliginosa (endogeic, geophagous). The earthworms were exposed either to artificially Fusarium-infected wheat straw highly contaminated with DON or to non-infected straw serving as a control. The experiment was conducted during an eight week period after harvest from mid August to mid October. For both species, the artificially Fusarium-infected and DON-contaminated wheat straw was a more attractive food source than the non-infected control. In contrast to A. caliginosa, L. terrestris incorporated infected straw faster into the soil compared to control straw. Furthermore, the reduction of Fusarium biomass and DON concentration in wheat straw was significantly higher in the presence of L. terrestris than in treatments with A. caliginosa and without earthworms. Here, no significant differences could be measured between the Fusarium biomass and DON concentration in wheat straw. A. caliginosa seems not to be relevant for the reduction of Fusarium biomass and DON concentration. We concluded that amongst earthworms, anecic detritivorous species are the drivers to compensate possible negative consequences (like crop infection) of conservation tillage. They take an important role in the control of phytopathogenic and toxinogenic fungi surviving on plant residues and in the degradation of their mycotoxins.     

Influence oF Pseudomonas putida on nodulation of Phaseolus vulgaris

The effect of Pseudomonas putida (isolate M17) on Rhizobium phaseoli nodulation of the common bean, Phaseolus vulgaris, was investigated under field and greenhouse conditions. The results indicated that P. putida markedly increased nodulation compared to R. phaseoli controls. Furthermore, 2-ketogluconic acid, a phosphate-solubiliring compound, was detected in P. putida M17. This could imply an increased P supply to roots of P. vutgaris, which may function to increase nodules. Bean yields and shoot fresh weight were not significantly altered by the addition of P. putida M17.     

Microbial community shifts in Pythium ultimum-inoculated suppressive substrates

In this study, the role of compost amendments for the biocontrol of Pythium ultimum was evaluated in bioassays with cucumber (Cucumis sativa L. variety “Chinesische Schlangen”). The addition of compost to the peat-based growing substrates resulted in a significant reduction of disease symptoms of cucumber plants in the presence of P. ultimum compared to pure substrate. Microbial community composition of compost-amended substrates and with different levels of P. ultimum inoculum (0, 5‰) was analyzed by polymerase-chain-reaction-based techniques. To detect and compare dominant bacterial and fungal representatives of suppressive substrate mixes with different pathogen inoculum, 16S and 18S rRNA clone libraries were established. Phylogenetic analysis of the 16S rRNA clones revealed Actinobacteria and α-Proteobacteria to be the prominent classes in the presence of P. ultimum, which are not part of the dominant microflora in the mixes without the pathogen. 18S rRNA sequences for the Pythium-inoculated compost supplemented samples were dominated by Chytridiomycota and Sordariomycetes, whereas in uncontaminated soil–compost mixes, a large part of the sequences were related to Homobasidiomycetes. Thus, it is assumed that the presence of P. ultimum induces distinct shifts in microbial communities favoring to groups known to comprise potential biocontrol agents.