Effects of Nitrogen, Plant Density, Moisture Stress and Artificial Inoculation with Macrophomina phaseolina on Charcoal Rot Incidence in Grain Sorghum

The occurrence of pythiaceous fungi in pot plant cultures grown in ebb and flow bench systems was investigated monthly from May to December, Phytophthora, Pythium and Saprolegnia (in all 351) were isolated from water samples and identified. Nearly all the isolates of Pythium produced zoospores in water. A pathogenicity test involving 15 isolates of Pythium“group P”, and 7 of Pythium“group F” showed that 73 % were pathogenic on cucumber, 66 % on Gerbera, 59 % on lettuce, 50 % on tomato, and 32 % on cress. Control of Pythium and Phytophthora is important in order to improve the health of plants grown in ebb and flow systems.     

Survival and recovery of Phytophthora capsici and oomycetes in tailwater and soil from vegetable fields in Florida

A current trend in Florida agriculture to conserve water is to irrigate with surface runoff water (tailwater) recovered in retention ponds and canals. Water filtration and lemon leaf baiting recovered Phytophthora capsici and other plant pathogenic Oomycetes in runoff water from ponds and canals. A total of 196 isolates of Phytophthora spp. and 471 isolates of Pythium spp. were recovered. Phytophthora spp. included P. capsici, P. cinnamomi, P. lateralis, P. nicotianae, P. citricola, P. cryptogea and P. erythroseptica. Species of Pythium were P. aphanidermatum, P. catenulatum, P. helicoides, P. irregulare, P. myriotylum, and Pythium‘group F’. Isolates of P. aphanidermatum, P. irregulare, P. myriotylum, and Pythium‘group F’ were pathogenic on pepper and tomato. Recovery of P. capsici propagules was related to soil moisture‐holding capacity and time interval but not temperature. Recovery of P. capsici propagules at 100% soil moisture‐holding capacity and 30° C was 57 days. In tailwater, recovery of propagules of P. capsici was 63 days at 24°C to 25°C. The potential exists to reintroduce and disseminate species of Phytophthora and Pythium when using tailwater for irrigation or other practices.     

Occurrence of Pythium spp. and Phytophthora spp. in Norwegian Greenhouses and their Pathogenicity on Cucumber Seedlings

Samples of tomato, lettuce and cucumber submitted for diagnosis to the Plant Protection Centre at the Norwegian Crop Research Institute and samples of soil, water and cucumber collected from greenhouses employing hydroponic cultures were examined for the occurrence of Pythium spp. and Phytophthora spp. Two species of Phytophthora and 16 species of Pythium were identified. Phytophthora cryptogea was found on tomato and lettuce. Phytophthora nicotianae was found on tomato fruit. Phytophthora was not found on cucumbers. Pythium irregulare and Pythium group F were the two most commonly found Pythium species in hydroponically cultivated cucumbers. A pathogenicity test with 56 isolates was performed on cucumber seedlings. The most aggressive species were Pythium aphanidermatum, P. irregulare, Pythium paroecandrum and Pythium ultimum.     

The influence ofPythium on the growth of barley seedlings as affected by soil water and inoculum density

Summary Pythium spp. were more abundant in the southerly and more temperature regions of the barley growing region of South Australia than in the drier and hotter north. Populations were more abundant in the top 10 cm than in the 10 to 20 cm soil zone. Eleven species ofPythium were identified from barley crops.P. irregulare appeared to be the most abundant and was one of the most pathogenic species on barley.P. volutum was also highly pathogenic; it had not been recorded in South Australia before. A factorial experiment using ninePythium spp. and four levels of soil water indicated that unlike other species,P. irregulare, P. volutum andP. graminicolum were most pathogenic in soils with a water content close to field capacity. A factorial experiment usingP. irregulare at four levels of soil water and six inoculum levels showed that inhibition of growth in barley seedlings byP. irregulare increased as the level of water in the soil increased. The experiments support the hypothesis that inhibition of growth of barley seedlings byPythium spp. is most severe in the southerly parts of the barley growing area of South Australia particularly where there is a combination of high soil water and high population densities.     

Effects of Pythium species on the growth of apple and their possible causal role in apple replant disease

Several lines of circumstantial evidence collectively indicated that poor early growth of apple (‘replant disease’) might be associated with the effects of soil-borne pythiaceous fungi. This hypothesis was supported by pathogenicity tests. All isolates tested of P. sylvaticum and certain isolates of seven other Pythium spp. significantly reduced the growth of apple seedlings. The growth reductions caused by certain Pythium isolates were of comparable magnitude to the growth increases occurring after chloropicrin-fumigation of apple orchard soils. The Pythium isolates most virulent to apple were of low virulence to a clonal cherry rootstock. Reappraisal of the nature of the disease as a non-specific soil malaise is consistent with established features of the pathology of Pythium spp. The disease, however, is an ill-defined ‘poor growth phenomenon’ with no diagnostic symptoms and conclusive evidence that Pythium spp. are widely causal is likely to be elusive.     

Fungi pathogenic to ryegrass seedlings

Summary Fungi borne on or in ryegrass (Lolium spp.) seeds or invading ryegrass seedlings grown on field soils were isolated and identified. Selected isolates were tested to determine their pathogenicity to ryegrass seedlings. Seed-borne fungi were generally weakly virulent or non-pathogenic to ryegrass seedlings. Pathogenic seed-borne fungi includedChaetomium globosum Kunze: Fr.,Curvularia trifolii (Kauffm.) Boedijn, and species ofPenicillium Link andAspergillus Mich. ex Link. Species of fungi isolated from seedlings grown on field soils de pended on soil and temperature. Soil-borne fungi pathogenic to seedlings includedFusarium avenaceum (Fr.) Sacc.,F. culmorum (W. G. Smith) Sacc.,F. equiseti (Corda) Sacc.,F. oxysporum Schlecht.: Fr.,F. solani (Mart.) Sacc.,Pythium afertile Kanouse and Humphrey,P. debaryanum auct. non Hesse,P. irregulare Buisman,P. ultimum Trow, a sphaerosporangiatePythium sp.,Chaetomium globosum, Thanatephorus cucumeris (Frank) Donk,Trichoderma koningii Oudem., and aPhomopsis sp. Individual isolates of fungi differed in virulence to ryegrass seedlings, and ryegrass cultivars differed in susceptibility to seedling pathogens.     

Some effects of differently-acting nematicides on the cereal cyst-nematode (Heterodera avenae) and on the appearance of ''scorch'' in spring wheat on light loamy sand

In fungitoxicity tests against Phytophthora cinnamomi on Chamaecyparis lawsoniana cv. Ellwoodii, a drench of furalaxyl (1000 mg a.i./l) applied to the compost in which 1-yr-old plants were growing, 1 wk before they were inoculated with 650 000 zoospores, controlled disease for at least 12 months. With an inoculum dose of 650 zoospores/plant, furalaxyl at 500 mg a.i./l controlled disease even when inoculation was 12 wk after fungicide treatment. Aluminium tris (ethyl phosphonate) (2000 mg a.i./l) applied as a drench 1 wk before inoculation with 650 000 zoospores/plant did not prevent root infection but delayed foliar symptoms for 9 months: the same treatment, using etridiazole (500 mg a.i./l) only slightly reduced disease incidence. When applied as a single drench 2 days before inoculation, prothiocarb (2000 mg a.i./l) and cuprammonium compounds (200 mg a.i./l) were much less effective than furalaxyl (1200 mg a.i./l), sodium ethyl phosphonate (1500 mg a.i./l), aluminium tris (ethyl phosphonate) (1500 mg a.i./l) or etridiazole (500 mg a.i./l). However, a drench of furalaxyl at 1000 mg a.i./l, aluminium tris (ethyl phosphonate) at 2000 mg a.i./l or etridiazole at 500 mg'a.i./l did not eradicate P. cinnamomi from compost containing infected root debris. Pre-planting drenching of the compost was ineffective. All fungicide treatments were non-phototoxic to 1-yr-old C. lawsoniana cv. Ellwoodii. These results are of special relevance to the control of P. cinnamomi on container-grown woody ornamentals.     

An annotated checklist of Pythium species from Iran

The genus Pythium, with slightly over 280 described species has been classified traditionally with other filamentous, coenocytic, sporangia-producing fungi as “Phycomycetes”. However, with recent advances in chemical, ultrastructural and molecular studies Pythium spp. are now considered as “fungus-like organisms” or “pseudo-fungi” and are placed in the kingdom Chromista or kingdom Straminopila, distinct from the true fungi or the kingdom Fungi. Little is known about the biodiversity of Pythium in Iran. This paper attempts to assess the position of the genus Pythium and provides details of the historical development of the study of Pythium in Iran. The survey list contains 33 species, 4 species groups and 1 unknown species of Pythium.     

Characterization of Pythium spp. associated with root rot of tobacco seedlings produced using the float tray system in Zimbabwe

The study was undertaken to identify and characterize Pythium isolates associated with root rot disease of tobacco seedlings as a first step towards developing management strategies for the pathogen. A total of 85 Pythium isolates were collected from diseased tobacco seedlings during 2015–2016 tobacco growing season. The isolates were identified to species level using sequencing of the internal transcribed spacer region. Thereafter, a subset of the isolates was tested for sensitivity to the commonly used fungicides, metalaxyl, azoxystrobin and a combination of fenamidone/propamocarbby growing isolates on Potato Dextrose Agar plates amended with the fungicides. The sequence analysis of the ITS‐rDNA identified Pythium myriotylum as the dominant Pythium species associated with the root rot of tobacco seedlings in Zimbabwe. Pythium aphanidermatum and P. insidiosum were also identified albeit at lower frequencies. Phylogenetic analyses of the ITS region of the P. myriotylum isolates showed little sequence diversity giving rise to one distinct clade. The fungicide sensitivity tests showed that metalaxyl provided the best control of P. myriotylum in vitro, as compared to other fungicides. To the best of our knowledge, this is the first comprehensive study to determine and characterize Pythium species associated with root rot of tobacco in the float seedling production system in Zimbabwe.     

Host/parasite relationships between tomato and pathogenic isolates of Verticillium

A close relationship was established between the virulence in the field of six isolates of Verticillium and their ability to penetrate and colonize sterile tomato seedlings grown in culture. The highly pathogenic species V. albo-atrum and V. tricorpus rapidly colonized host tissue in culture, host reactions being absent or only slight. V. nigrescens and V. nubilum, mild pathogens, penetrated sterile roots more slowly and caused host reactions. The variation in pathogenicity in the field between two isolates of V. dahliae suggests that they are different physiological strains, but they induced no difference during the first stages of invasion-reaction of sterile seedlings. Hyaline variants of all these isolates were less pathogenic than the original parent types. Variations in temperature from 25° C. (near optimum for the growth of both tomato plants and the fungi) caused changes in host reactions. Ability to penetrate was not affected within the pH range 4.0–8.0, but at extreme values (3.2, 9.4 and 10.0) all isolates entered without any host reaction. Variations in nitrogen supply to the pathogens induced modifications in their ability to penetrate, whereas changes in supply of nitrogen to the seedlings had no apparent effect upon host/parasite relations. The effects of simultaneous contact of non-pathogenic and pathogenic isolates with seedling roots suggested that resistance of host tissue was controlled by the growing tip.     

<Emphasis Type="Italic">Pythium apinafurcum</Emphasis>: its morphology,molecular phylogeny,and infectivity for plants

During a survey of Pythium species in soils of Japan, Pythium isolates growing at high temperatures were obtained from an uncultivated field soil in Wakayama Prefecture. All six isolates showed similar morphology to each other and had complexly branched secondary hyphae, globose nonproliferating sporangia, and smooth-surfaced oogonia that have one or two oospores per oogonium. The combination of these characteristics differentiated these isolates from other Pythium species reported. Phylogenetic analyses based on sequences of the ribosomal DNA ITS and D1/D2 region of the large subunit showed that all Pythium isolates were clustered in a single clade that was distantly related to other known clades of the genus. We described these isolates as a new Pythium species, Pythium apinafurcum, based on morphology and molecular phylogeny. The P. apinafurcum isolates nonsymptomatically infected the roots of seedlings of bermudagrass, cabbage, and cucumber in a pot inoculation test.     

A web-based identification programme for Pythium species

The genus Pythium, with slightly over 140 described species, has been classified traditionally with other filamentous, coenocytic, sporangia-producing fungi as “Phycomycetes”. However, with recent advances in chemical, ultrastructural and molecular studies, Pythium spp. are now considered as “fungus-like organisms” or “pseudo-fungi” and are placed in the kingdom Chromista or kingdom Stramenopila, distinct from the true fungi or the kingdom Fungi. There is no comprehensive web-based identification key available for the identification of all described Pythium species. This article describes a web-based identification tool (which has been described by the authors under the name of Fungid) that uses morphological features. For the isolate in question, the software compares the unknown isolate with near to 90 known species submitted in the database. One or more than one species is suggested to the user by the software.     

Biocontrol of root-knot nematode Meloidogyne incognita by the isolates of Pseudomonas on tomato

Biocontrol of root-knot nematode Meloidogyne incognita was studied on tomato using 15 isolates of fluorescent Pseudomonads isolated from pathogen suppressive soils. Pseudomonas aeruginosa (isolates Pa8, Pa9 and Pa3) caused greater inhibitory effect on hatching of M. incognita than other isolates. In addition, isolates Pa8, Pa9 and Pa3 caused greater colonisation of tomato roots and also caused a greater increase in the growth of tomato seedlings. These isolates also caused a greater increase in growth of tomato and higher reduction in galling and nematode multiplication in a green house test than is caused by other isolates. Isolates Pf1, Pf5, Pf6 and Pa13 were unable to increase growth of tomato and caused less reduction in galling and nematode multiplication compared to other isolates. Only 10 isolates produced siderophores on chromo-azurol sulfonate (CAS) agar medium and isolate Pa12 showed greater production of siderophore followed by Pa11, Pa9, Pf10, Pa3 and Pf5. Similarly, isolates Pa14, Pa12, Pf10, Pa9, Pa8, Pa7 and Pa6 produced greater amount of HCN than the other isolates tested. Isolates Pa8 and Pa9 showed greater production of IAA than the other 13 isolates tested. This study suggests that P. aeruginosa isolates Pa8 and Pa9 may be used for the biocontrol of M. incognita on tomato.     

Endophytic Actinomycetes from <Emphasis Type="Italic">Azadirachta indica</Emphasis> A. Juss.: Isolation,Diversity, and Anti-microbial Activity

Endophytic actinomycetes from Azadirachta indica A. Juss. were screened and evaluated for their anti-microbial activity against an array of pathogenic fungi and bacteria. A total of 55 separate isolates were obtained from 20 plants, and 60% of these showed inhibitory activity against one or more pathogenic fungi and bacteria. Actinomycetes were most commonly recovered from roots (54.5% of all isolates), followed by stems (23.6%), and leaves (21.8%). The dominant genus was Streptomyces (49.09% of all isolates), while Streptosporangium (14.5%), Microbispora (10.9%), Streptoverticillium (5.5%), Sacchromonospora sp. (5.5%), and Nocardia (3.6%) were also recovered. Streptomyces isolates AzR 006, 011, and 031 (all from roots) had acute activity against Pseudomonas fluorescens, while AzR027, 032, and 051 (also all from roots) showed activity against Escherichia coli. Meanwhile, an isolate of Nocardia sp. from leaves (AzL025) showed antagonism against Bacillus subtilis. Overall, 32 of the 55 were found to have broad spectrum significant antimicrobial activity, while about 4% of them showed strong and acute inhibition to pathogenic fungi and bacteria. Isolates of Streptomyces AzR031, 008, and 047, Nocardia sp. AzL025, and Streptosporangium sp. AzR 021 and 048 are of particular interest because they showed significant antagonistic activity against root pathogens, including Pythium and Phytophthora sp. Thus, many of the isolates recovered from A. indica in this study may be used in developing potential bio-control agents against a range of pathogenic fungi and bacteria and in the production of novel natural antimicrobial compounds. These results not only further our understanding of plant–microbe interactions but also indicate that there is an untapped resource of endophytic microorganisms that could be exploited in the biotechnological, medicinal, and agricultural industries.     

Epicoccum Nigrum as biocontrol agent of Pythium damping-off and root-rot of cotton seedlings

Efficacy of Epicoccum nigrum and its exudate was tested in control the pre- and post-emergence damping-off and root-rot of Egyptian cotton (cv. Giza 83) in vitro and in vivo. Different isolates of Epicoccum nigrum reduced the radial growth of both Pythium debaryanum and P. ultimum significantly, by production considerable inhibition zones. In liquid cultures E. nigrum exudate showed a high fungicidal effect resulting in a significant reduction of the mycelial dry weight of the two investigated Pythium spp. Also E. nigrum exudate inhibited cellulase and pectinase activity by P. debaryanum and P. ultimum. Soaking of cotton seeds in E. nigrum exudate for different intervals resulted in significant reduction of root-rot severity of seedlings as well as the contamination of seeds and seedlings by fungi during and after germination. These treatments also stimulated germination of cotton seeds and enhanced the seedlings vigour significantly. In pot experiments, the use of E. nigrum as a soil mixture or seed dressing significantly alleviated the hazard effect of P. debaryanum. Pythium ultimum seemed to be weak or non pathogen to the used cotton cultivar (Giza 83). Application of E. nigrum or its exudate not only involved in protection of cotton seedlings against Pythium damping-off and root-rot but also enhanced their vigour and growth characteristics. The main conclusion of this study is that E. nigrum could be used successfully as environmentally safe and economic biological control agent to protect cotton (cv. Giza 83) from damping-off and root-rot diseases caused by P. debaryanum.     

Attraction and oviposition responses of the fungus gnat Bradysia impatiens to microbes and microbe‐inoculated seedlings in laboratory bioassays

Laboratory tests were conducted to examine preferences of Bradysia impatiens Johannsen (Diptera: Sciaridae) larvae and adults for various microbes associated with greenhouse crops. Fungus gnat larvae and adults exhibited a preference for cultures of Pythium spp. over the medium used to grow the pathogens. Larvae also exhibited a preference for geranium seedlings infected with pathogenic Pythium spp. [P. aphanidermatum (Edson) Fitz., P. ultimum Trow, and P. irregulare Buis. (Oomycota: Peronosporales)] over non‐inoculated plants. Adult fungus gnats exhibited a strong ovipositional preference for the aforementioned Pythium spp. as well as a variety of other microorganisms, including the pathogenic fungus Thielaviopsis basicola (Berk. & Br.) (Ascomycota: Microascales), the geranium‐infecting bacterium Xanthomonas campestris pv. pelargonii (Brown) Dye (Proteobacteria: Xanthomonadales), the non‐pathogenic species Pythium torulosum Coker & P. Patt. and Pythium graminicola Subramaniam, the pathogen‐suppressive fungus Trichoderma harzianum Rifai (Ascomycota: Hypocreales), and the insect pathogenic fungus Beauveria bassiana (Balsamo) Vuillemin (Ascomycota: Hypocreales). Our study is the first to demonstrate that fungus gnats are attracted to and/or stimulated to oviposit by a wide array of living microorganisms both in pure culture and in association with plant seedlings. These findings have important implications with respect to the potential role of fungus gnats in plant pathogen transmission.     

Selection of Chickpea-Rhizosphere-Competent Pseudomonas fluorescens NBRI1303 Antagonistic to Fusarium oxysporum f. sp. ciceri,Rhizoctonia bataticola and Pythium sp.

A procedure that consumes less screening time was developed for screening chickpea rhizosphere-competent bacteria for suppression of the chickpea pathogenic fungi Fusarium oxysporum f. sp. ciceri, Rhizoctonia bataticola and Pythium sp. Of the 478 bacteria obtained by random selection of the predominant, morphologically distinct colonies, 386 strains that effectively colonize chickpea roots could be divided broadly into three different groups. The first group consisted of 44 good chickpea rhizosphere colonizers with 10⁷ to 10⁸ colony-forming units (CFU)/g root; the second group consisted of 253 medium chickpea rhizosphere colonizers with 10⁴ to 10⁶ CFU/g root; and the third group consisted of 89 poor chickpea rhizosphere colonizers with 10⁰ (nondetectable) to 10³ CFU/g root. Forty-four Rif^r strains from the first group of good chickpea rhizosphere colonizers were further screened for their in vitro biocontrol activity against F. oxysporum f. sp. ciceri, R. bataticola, and Pythium sp. One bacterial strain was selected for further work because of its unique ability to inhibit all three fungi and its good chickpea rhizosphere colonization ability. This is the first report of a single biocontrol bacterium active against three most devastating pathogenic fungi of chickpea. In a greenhouse test, chickpea seed bacterization with P. fluorescens NBRI1303 increased the germination of seedlings by 25%, reduced the number of diseased plants by 45%, compared with nonbacterized controls. Increases in seedling dry weight, shoot length, and root length ranged from 16% to 18%. Significant growth increases in shoot length, dry weight, and grain yield, averaging 11.59%, 17.58%, and 22.61% respectively above untreated controls, were attained in field trials in Agra and Jhansi. A rifampicin-resistant mutant P. fluorescens NBRI1303R of the P. fluorescens NBRI1303, used to monitor chickpea root colonization, confirmed the rapid and aggressive colonization by the bacterium, making it a potential biocontrol agent against chickpea phytopathogenic fungi. The results, demonstrating an increase in the efficiency of screening and detection of plant beneficial strains, should greatly benefit future studies. Received: 23 December 1996 / Accepted: 28 January 1997     

Morphological and phylogenetic analyses of Pythium species in South Africa

The genus Pythium is important in agriculture, since it contains many plant pathogenic species, as well as species that can promote plant growth and some that have biocontrol potential. In South Africa, very little is known about the diversity of Pythium species within agricultural soil, irrigation and hydroponic systems. Therefore, the aim of the study was to characterise a selection of 85 Pythium isolates collected in South Africa from 1991 through to 2007. The isolates were characterised morphologically as well as through sequence and phylogenetic analyses of the internal transcribed spacer regions (ITS) and the 5.8S gene of the nuclear ribosomal DNA. Phylogenetic analyses showed that the isolates represented ten of the 11 published Pythium clades [Lévesque & De Cock, 2004. Molecular phylogeny and taxonomy of the genus Pythium. Mycological Research 108: 1363–1383]. Characterisation of isolates in clade D and J suggested that the phylogenetic concept of Pythium acanthicum and Pythium perplexum respectively, needs further investigation in order to enable reliable species identification within these clades. Our phylogenetic analyses of Pythium species in clade B also showed that species with globose sporangia group basal within this clade, and are not dispersed within the clade as previously reported. The 85 South African isolates represented 34 known species, of which 20 species have not been reported previously in South Africa. Additionally, three isolates (PPRI 8428, 8300 and 8418) were identified that may each represent putative new species, Pythium sp. WJB-1 to WJB-3.     

Biochemical characterisation of Pythium spp. involved in cavity spot of carrots in France

The pathogenicity and growth rate in vivo were assessed on 27 isolates of Pythium spp. recovered from cavity spot lesions on carrots grown in various parts of northwest France. Polyacrylamide gel electrophoresis of isoesterases was used to identify the Pythium spp. involved. Slow-growing isolates were more aggressive than fast-growing ones when inoculated on carrot tap roots. Isoesterase patterns identified the slow-growing isolates as P. violae and P. sulcatum; P. ultimum and P. intermedium were identified among the less aggressive fast-growing isolate group, in which some isolates were also classed as P. sylvaticum or P. irregulare, which have similar electrophoretic profiles. The incidence of Pythium spp. associated with the disease in France is discussed in regard to cavity spot in other countries.     

Resident bacteria, nitric oxide emission and particle size modulate the effect of Brassica napus seed meal on disease incited by Rhizoctonia solani and Pythium spp

Amendment of orchard soil with low-glucosinolate Brassica napus (rape) seed meal (RSM) suppresses infection of apple roots by Rhizoctonia solani but increases incidence of Pythium spp. infection. Following incorporation of Brassica sp. seed meals, soils were monitored for changes in populations of selected saprophytic and plant pathogenic microorganisms. When conducted in pasteurized soil, which possessed high numbers of Bacillus spp. and lower than detectable numbers of Streptomyces spp., RSM amendment did not provide control of R. solani. Populations of streptomycetes in RSM-amended soil increased to stable levels >20-fold higher than in non-amended soil. Disease suppressiveness was restored to pasteurized RSM-amended soil by adding any of several Streptomyces strains. Maximal rates of nitrification in orchard soil, determined by nitric oxide emission, were observed within two weeks following RSM amendment and inhibition of nitrification via application of nitrapyrin abolished the capacity of RSM to suppress R. solani infection of apple roots when seedlings were planted one day after soil amendment. Apple seedling mortality and Pythium spp. root infection were highest for seedlings planted immediately following incorporation of B. napus cv. Athena RSM, particularly when meal was added in a flake rather than powder form. Lower infection frequencies were observed for seedlings planted four weeks after RSM incorporation, even for soil in which densities of culturable Pythium spp. had not declined. Our results demonstrate that suppression of Rhizoctonia root rot in response to RSM amendment requires the activity of the resident soil microbiota and that initial disease control is associated with the generation of nitric oxide through the process of nitrification.