Pythium Spp Research Articles

Effect of Seed Treatment with Rhizobium leguminosarum on Pythium Damping‐off, Seedling Height, Root Nodulation, Root Biomass, Shoot Biomass, and Seed Yield of Pea and Lentil

AbstractField experiments were conducted in 2004 and 2005 to determine the effects of seed treatment with Rhizobium leguminosarum bv. viceae on damping‐off, seedling height, root nodule mass, root biomass, shoot biomass and seed yield of pea and lentil in a field naturally infested with Pythium spp. Compared with the untreated controls, treatment of pea seeds with R. leguminosarum bv. viceae strains R12, R20 or R21 significantly (P < 0.05) reduced incidence of damping‐off, promoted seedling growth and increased root nodule mass, root biomass and shoot biomass. Seed treatments with R12 or R21 also resulted in a significant (P < 0.05) increase in seed yield of pea. The strain R21 was most effective among the four strains of R. leguminosarum bv. viceae tested in peas. Although, the level of disease control by strain R21 was similar to seed treatment with the fungicide ThiramTM, R21 was more effective in enhancing root nodule production and promoting plant growth. For lentil, treatment of seeds with R. leguminosarum bv. viceae strains R12 or R21 significantly (P < 0.05) reduced incidence of damping‐off compared with the untreated control. All of the four strains of R. leguminosarum bv. viceae tested increased lentil seedling height, root nodule mass and shoot biomass, and all except R20 increased root biomass. Seed yield was higher for the treatments of R12 and R21. The strain R12 was most effective among the four strains of R. leguminosarum bv. viceae tested in lentil. Although, strain R12 was as effective as ThiramTM for control of damping‐off of lentil, it was more effective than ThiramTM for the production of root nodules and promotion of plant growth. The study concludes that seed treatment with R. leguminosarum bv. viceae is effective in control of Pythium damping‐off of pea and lentil and that the efficacy of control is strain specific, strain R21 for control of the disease on pea and strain R12 for control of the disease on lentil.

Root Diseases of Wheat and Barley During the Transition from Conventional Tillage to Direct Seeding.

The use of direct seeding (no-till) in place of tillage can reduce soil erosion and improve water infiltration. However, despite these improvements in soil quality, growers in the Pacific Northwest are reluctant to adopt direct seeding, partially because of fears of increased root diseases caused by Gaeumannomyces graminis var. tritici, Rhizoctonia spp., and Pythium spp. To examine the effect of the transition from conventional tillage to direct seeding, field plots were established at two locations. One site had been managed with direct seeding for 12 years, and the second had been conventionally tilled. Over 4 years, a portion of each plot was tilled or direct seeded, and planted to wheat or barley. Plants in the tilled plots had consistently more crown roots than plants in direct-seeded plots. Rhizoctonia root rot and yield did not differ between tillage types during the first 2 years of the study. However, in the third and fourth years of the transition to direct seeding, a higher incidence of Rhizoctonia root rot, increased hyphal activity of R. solani, and reduced yields were observed in direct-seeded plots. Populations of R. oryzae and Pythium spp., and incidence of take-all were the same for both management practices.

Control of <i>Phytophthora cinnamomi</i> in <i>Erica</i> spp in New Zealand

Soilborne diseases (caused by Phytophthora and Pythium spp.) continue to cause severe problems for potato growers in the Manawatu region of New Zealand. A long-term trial (5 years) is being conducted at a commercial property on land that has a history of severe infestation by soilborne diseases. Five rotational crops (potato, pasture, brassica, cereal and squash) are being tested for their ability to reduce the inoculum of the soilborne pathogens. Metalaxyl fungicide is used as a standard treatment. Soil samples were taken before and after each crop rotation treatment and plated on agar media to study the pathogen populations. In the first year, results showed that there was a general reduction in Phytophthora spp. inoculum in the soil (to between 36% and 58% of pre-treatment levels) after the crop rotation and metalaxyl treatments but there was no significant difference between treatments. The crop rotation treatments did not reduce the levels of Pythium spp, but numbers of this pathogen were reduced by metalaxyl treatment.

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.

Identification and Quantification of Pathogenic Pythium spp. from Soils in Eastern Washington Using Real-Time Polymerase Chain Reaction

ABSTRACT Traditional methods of quantifying Pythium spp. rely on the use of selective media and dilution plating. However, high variability is inherent in this type of enumeration and counts may not be representative of the pathogenic population of Pythium spp. Variable regions of the internal transcribed spacer of the rDNA were used to design species-specific primers for detection and quantification of nine Pythium spp. from soils in eastern Washington. Primer pairs were designed for Pythium abappressorium, P. attrantheridium, P. heterothallicum, P. irregulare group I, P. irregulare group IV, P. paroecandrum, P. rostratifingens, P. sylvaticum, and P. ultimum and used with real-time polymerase chain reaction. Standard curves were generated for each of the species using SYBR Green I fluorescent dye for detection of amplification. Seventy-seven isolates of Pythium were screened to confirm specificity of each primer set. DNA was extracted from soil and standard curves were generated for P. irregulare group I, P. irregulare group IV, and P. ultimum to correlate populations of each species in the soil with quantities of DNA amplified from the same soil. Examination of raw field soils revealed results similar to those observed in previous studies. This new technique for the quantification of Pythium spp. is rapid and accurate, and will be a useful tool in the future study of these pathogenic Pythium spp.

Seed Treatment with Phosphonate (AG3) Suppresses Pythium Damping-off of Cucumber Seedlings.

A formulation of phosphonate (AG3) was tested as a seed treatment for the control of Pythium damping-off of cucumber (Cucumis sativus L.) plants under controlled environment and field conditions. Cucumber seed were treated by soaking for 10 min in phosphonate solution. They were then planted into peat-based mix or sandy-loam soil mixed with Pythium aphanidermatum or P. ultimum inoculum or into muck soil naturally infested with P. irregulare, P. ultimum, and other Pythium spp. Under growth-room conditions, phosphonate seed treatment provided more than 80% control of damping-off in all infested substrates tested. Effective disease control was obtained even when treated seed were stored for 5 weeks and up to 18 months prior to planting. In microplots containing naturally infested muck soil, phosphonate seed treatment decreased the percentage of diseased cucumber plants and increased total fresh weights compared with untreated seed and phosphonate post-planting drench. In field-plot tests 6 weeks after planting treated seed in Pythium-infested muck soil, cucumber stands were 63% compared with 18% in the control, which had no phosphonate exposure, and 53% in the post-planting drench. Tests for potential phytotoxicity in the greenhouse showed that radish and bok choy germination was reduced by phosphonate treatment but corn, cucumber, soybean, sugar beet, tomato, and wheat were not affected. Phosphonate seed treatment is a cost-effective way of protecting cucumber plants from Pythium damping-off.

British Columbia Regional Meeting, 2005

Composts containing fluorescent pseudomonads are suppressive to the development of fusarium root and stem rot on greenhouse cucumber. G. Bradley, S. Rose, and Z.K. Punja. Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, Canada. Interactions between Gliocladium catenulatum, Fusarium oxysporum, and cucumber plants leading to biological control of fusarium root and stem rot. S. Chatterton and Z.K. Punja. Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, Canada. Influences of organic amendments on nematodes in rootzone soil of apple and red raspberry. T.A. Forge, D. Neilsen, G. Neilsen, E. Hogue, and C. Kempler. Pacific Agri-Food Research Centre (PARC), Agriculture and Agri-Food Canada (AAFC), P.O. Box 1000, Agassiz, BC V0M 1A0, Canada; and (D.N., G.N., E.H.) PARC, AAFC, P.O. Box 5000, 4200 Highway 97, Summerland, BC V0H 1Z0, Canada. Detection of Fusarium spp. in rusty-root lesions on ginseng, using DNA-array hybridization. R.S. Goswami, T. Barasubiye, C.A. Lévesque, A. Wan, K.A. Seifert, and Z.K. Punja. Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, Canada; and (T.B., K.S., C.A.L.) Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada, 960 Carling Avenue, Ottawa, ON K1A 0C6, Canada. Evaluation of the role of the coat protein of Cucumber necrosis virus on particle structure and virus transmission. E. Hui and D. Rochon. Department of Plant Science, Faculty of Land and Food Systems, University of British Columbia, 2357 Main Mall, Vancouver, BC V6T 1Z4, Canada; and (D.R.) Pacific Agri-Food Research Centre, Agriculture and Agri-Food Canada, P.O. Box 5000, 4200 Highway 97, Summerland, BC V0H 1Z0, Canada. Identification of greenhouse plant pathogens on glass slides, using microarray technology. C.A. Koch, P.C.H. Li, and R.S. Utkhede. Pacific Agri-Food Research Centre, Agriculture and Agri-Food Canada, P.O. Box 1000, Agassiz, BC V0M 1A0, Canada; and (P.C.H.L.) Department of Chemistry, Simon Fraser University, Burnaby, BC V5A 1S6, Canada. Screening for resistance to Thielaviopsis basicola in Daphne spp. D. Noshad, Z.K. Punja, and A. Riseman. University of British Columbia Botanical Garden and Centre for Plant Research, University of British Columbia, 6804 SW Marine Drive, Vancouver, BC V6T 1Z4, Canada; and (Z.K.P.) Department of Biological Sciences, Simon Fraser University, 888 University Drive, Burnaby, BC V5A 1S6, Canada. Cellular and biochemical changes occurring in ginseng rusty-root tissue associated with infection by Fusarium spp. Z.K. Punja, M. Rahman, and N. Verma. Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, Canada. Influence of iron on severity of cylindrocarpon root rot on ginseng roots. M. Rahman and Z.K. Punja. Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, Canada. Rhizome blackening and root rot of wasabi (Wasabia japonica) caused by Pythium and Fusarium spp. G. Rodriguez and Z.K. Punja. Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, Canada. The screening of bacterial isolates for biological control of blue and gray molds of apples. S.C. Stokes, P.L. Sholberg, L.M. Nelson, and E. Sergeeva. Pacific Agri-Food Research Centre, Agriculture and Agri-Food Canada, P.O. Box 5000, 4200 Highway 97, Summerland, BC V0H 1Z0, Canada; and (L.M.N., E.S.) The University of British Columbia Okanagan, Irving K. Barber School of Arts and Sciences, Kelowna, BC V1V 1V7, Canada. Molecular technique and biological control for internal fruit rot of greenhouse sweet peppers. R.S. Utkhede and S. Mathur. Pacific Agri-Food Research Centre, Agriculture and Agri-Food Canada, P.O. Box 1000, Agassiz, BC V0M 1A0, Canada. Evaluation of promoter activity in carrot (Daucus carota) tissues for the expression of pathogenesis-related proteins in transgenic plants. O. Wally, J. Jayaraman, and Z.K. Punja. Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, Canada.

Pythium sterilumsp. nov. isolated from Poland, Spain and France: its morphology and molecular phylogenetic position

In a survey of Phytophthora species associated with forest decline in Spain, Poland and France, we found three Pythium isolates, which have been characterized with internal transcribed spacer rRNA gene sequences and with classical morphological descriptors for Pythium spp. These isolates showed unique internal transcribed spacer sequences, different enough from those of any described species to justify new species status. These three distinct isolates failed to produce any sex organs with an entirely asexual reproduction and were found to represent a new species for which the name Pythium sterilum is proposed. This paper describes and illustrates the morphology of P. sterilum and presents its taxonomic position and relationships with other, related Pythium species belonging to clade K.

Cultural Conditions Contributing to Vine Decline Syndrome in Watermelon

Since the mid-1980s, a syndrome known as mature watermelon vine decline (MWVD) has had a serious effect on watermelon (Citrullus lanatus Thunb.) crops in Southern Indiana. As efforts to identify a pathogen responsible for MWVD have been unsuccessful, we have examined cultural conditions that might contribute to the syndrome. Field conditions were simulated in greenhouse pot trials to assess the impact of one or more factors on watermelon growth. Alone, low organic matter, soil acidity, black plastic mulch, and liming did not significantly affect root fresh weight; however, when these conditions were combined, root fresh weight was significantly reduced. Alanap-treated watermelons in combination with simulated cultural conditions resulted in further reduction of root fresh weight and had symptoms similar to MWVD. Watermelon plants grown in Alanap-treated, aged soil (from the previous year's experiments) under combined deficient cultural conditions demonstrated increased symptoms of MWVD and susceptibility to the pathogens Rhizoctonia and Pythium spp. Alanap, N-1-naphthylphthalamic acid (NPA), is a preemergent herbicide that functions as an inhibitor of auxin efflux and is widely used by watermelon farmers to impede obnoxious weeds. Metabolism of Alanap in planta involves aryl amidases (aminopeptidases) that also function in defense responses. We hypothesize that negative cultural practices are likely to inhibit defense responses and watermelon resistance to residual Alanap, leading to MWVD. We suggest that MWVD incidence is increased by certain common cultural conditions and that the incidence of MWVD can be reduced by altering these cultural practices.

RpoN (σ54) Controls Production of Antifungal Compounds and Biocontrol Activity in Pseudomonas fluorescens CHA0

Pseudomonas fluorescens CHA0 is an effective biocontrol agent of root diseases caused by fungal pathogens. The strain produces the antibiotics 2,4-diacetylphloroglucinol (DAPG) and pyoluteorin (PLT) that make essential contributions to pathogen suppression. This study focused on the role of the sigma factor RpoN (sigma54) in regulation of antibiotic production and biocontrol activity in P. fluorescens. An rpoN in-frame-deletion mutant of CHAO had a delayed growth, was impaired in the utilization of several carbon and nitrogen sources, and was more sensitive to salt stress. The rpoN mutant was defective for flagella and displayed drastically reduced swimming and swarming motilities. Interestingly, the rpoN mutant showed a severalfold enhanced production of DAPG and expression of the biosynthetic gene phlA compared with the wild type and the mutant complemented with monocopy rpoN+. By contrast, loss of RpoN function resulted in markedly lowered PLT production and plt gene expression, suggesting that RpoN controls the balance of the two antibiotics in strain CHA0. In natural soil microcosms, the rpoN mutant was less effective in protecting cucumber from a root rot caused by Pythium ultimum. Remarkably, the mutant was not significantly impaired in its root colonization capacity, even at early stages of root infection by Pythium spp. Taken together, our results establish RpoN for the first time as a major regulator of biocontrol activity in Pseudomonas fluorescens.

Molecular characterisation, pathogenesis and fungicide sensitivity ofPythiumspp. from table beet (Beta vulgarisvar.vulgaris) grown in the Lockyer Valley, Queensland

Table beet production in the Lockyer Valley of south-eastern Queensland is known to be adversely affected by soilborne root disease from infection by Pythium spp. However, little is known regarding the species or genotypes that are the causal agents of both pre- and post-emergence damping off. Based on RFLP analysis with HhaI, Hinfl and MboI of the PCR amplified ITS region DNA from soil and diseased plant samples, the majority of 130 Pythium isolates could be grouped into three genotypes, designated LVP A, LVP B and LVP C. These groups comprised 43, 41 and 7% of all isolates, respectively. Deoxyribonucleic acid sequence analysis of the ITS region indicated that LVP A was a strain of Pythium aphanidermatum, with greater than 99% similarity to the corresponding P. aphanidermatum sequences from the publicly accessible databases. The DNA sequences from LVP B and LVP C were most closely related to P. ultimum and P. dissotocum, respectively. Lower frequencies of other distinct isolates with unique RFLP patterns were also obtained with high levels of similarity (>97%) to P. heterothallicum, P. periplocum and genotypes of P. ultimum other than LVP B. Inoculation trials of 1- and 4-week-old beet seedlings indicated that compared with isolates of the LVP B genotype, a higher frequency of LVP A isolates caused disease. Isolates with the LVP A, LVP B and LVP C genotypes were highly sensitive to the fungicide Ridomil MZ, which suppressed radial growth on V8 agar between approximately four and thirty fold at 5 μg/mL metalaxyl and 40 μg/mL mancozeb, a concentration far lower than the recommended field application rate.

Fungicidial Activity of Benthiavalicarb-isopropyl against <i>Phytophthora infestans</i> and Its Controlling Activity against Late Blight Diseases

Benthiavalicarb-isopropyl, a novel member of the amino acid amide carbamate group of fungicides, is effective against all Oomycete fungal plant pathogens except Pythium spp. Our results demonstrate that this fungicide effectively controls potato and tomato late blight caused by metalaxyl-sensitive and -resistant strains of Phytophthora infestans. Experiments in vitro demonstrated that benthiavalicarb-isopropyl was ineffective in stopping the discharge of zoospores from zoosporangia and suppressing their motility; but strongly inhibited mycelial growth, sporulation, and the germination of sporangia and cystospores. Experiments in a greenhouse showed that benthiavalicarb-isopropyl has not only a strong preventive, but also a curative effect; its translaminar properties are effective along with its rainfastness and residual activity. In field trials, it was effective in controlling tomato and potato late blight at 25–75 g a.i./ha. © Pesticide Science Society of Japan

Aphelenchus avenae and Antagonistic Fungi as Biological Control Agents of Pythium spp.

To examine the control effect of damping-off on radish caused by Pythium spp., researchers used the isolates of a fungivorous nematode, Aphelenchus avenae, and antagonistic fungi, Trichoderma spp. These were used as biocontrol agents, either alone, or in combination. Growth rates of the A. avenae isolates and fungal damages by the nematodes varied depending on Trichoderma spp., which contained lower T. koningii and T. virens cultures than other Trichoderma cultures. Pythium spp. were damaged by all five Aphelenchus isolates, but the multiplication rate of nematode isolate Aa-3 was very poor. Antibiotic activity of T. virens and T. harzianum to Pythium spp. was stronger than that of T. koningii. Control efficacy against damping-off of radish was most enhanced under the treatment using the nematode-T. harzianum combination. On the contrary, the combinations of the nematodes and T. virens or T. koningii mostly did not increase or decreased their control effect vis-a-vis that of the nematodes or antagonistic fungi being used alone. The results suggest that the fungivorous nematodes may play a leading role in the disease control, and that the activity of the fungivorous nematodes may be activated by T. harzianum, but inhibited by T. koningii and T. virens.

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.

Pythium attrantheridium sp. nov.: taxonomy and comparison with related species

Pythium attrantheridium sp. nov. is a new species isolated from cavity spot lesions of carrots as well as apple and cherry seedlings from various locations widely distributed in Canada and the USA. This fungus is closely related to the heterothallic P. intermedium, but is distinguished by: (1) unique molecular characteristics; (2) unique morphological characteristics; and (3) mating incompatibility with P. intermedium. The ITS region of the nuclear rDNA of all strains of P. attrantheridium studied is different from that of all other known Pythium spp. The oogonia attract a large number of antheridia when compatible mating types contact each other. The positive mating type produces zoospores unlike those of P. intermedium. Thus, biological, morphological and molecular data support the recognition of a new species.

Synthesis and fungicidal activity of ethaboxam against Oomycetes.

This study describes the chemical synthesis and intrinsic fungicidal activity of ethaboxam [(RS)-N-(alpha-cyano-2-thenyl)-4-ethyl-2-(ethylamino)-1,3-thiazole-5-carboxamide], a new Oomycetes fungicide. In in vitro tests, ethaboxam showed inhibitory activity against isolates of Phytophthora and some Pythium spp, with MIC values ranging from 0.1 to 0.5 mg litre(-1) for nine isolates of Phytophthora infestans (Montagne) de Bary and from 1.0 to 5.0 mg litre(-1) for eight isolates of Phytophthora capsici Leonian. In tests to determine time and concentration for complete inactivation of each pathogen (five isolates of P infestans and five isolates of P capsici), ethaboxam inactivated all isolates of P infestans within 48h at 10 mg litre(-1) and those of P capsici within 96 h at 10 mg litre(-1). Ethaboxam effectively suppressed development of tomato late blight caused by P infestans and pepper Phytophthora blight caused by P capsici in the studies conducted to determine its preventive, curative, persistent and systemic activity. These results show that ethaboxam has desirable fungicidal characteristics as an Oomycetes fungicide.

Biological control of Pythium damping-off of pea and sugar beet by Rhizobium leguminosarum bv. viceae

Rhizobium leguminosarum Jordan bv. viceae strains from pea and lentil root nodules were tested for control of damping-off of pea (Pisum sativum L., host) and sugar beet (Beta vulgaris L., nonhost) crops caused by Pythium sp. "group G". Of the 18 Rhizobium isolates tested, only strain R5 inhibited mycelial growth of Pythium sp. "group G". None of the strains showed any protease activity. Results of indoor experiments in soil artificially infested with Pythium sp. "group G" showed that 10 strains of R. leguminosarum bv. viceae were effective in increasing sugar beet emergence compared with the untreated control, when bacteria were coated onto seeds. Three of the most promising strains, R12, R20, and R21, were further tested for control of damping-off of field pea and sugar beet in a field naturally infested with Pythium spp. R12 and R20 significantly increased seedling emergence of field pea in the two field tests, compared with the untreated control. The efficacy of strains R12 and R20 was similar to that of Pseudomonas fluorescens Migula 708, a biological control agent of Pythium sp. "group G". Rhizobium leguminosarum bv. viceae strains R12 and R21 were the most effective biological control agents for control of sugar beet damping-off in the field experiments. They were as effective as seed treatment with the fungicide ThiramTM in one field experiment. The present study reveals that some R. leguminosarum bv. viceae strains, in addition to their use as biofertilizer, also have the potential to be used for biological control of Pythium damping-off of field pea and sugar beet.Key words: Rhizobium leguminosarum bv. viceae, Pythium sp. "group G", damping-off, biological control, sugar beet, pea.

Soil Fumigant Effects on Three Genera of Potential Soilborne Pathogenic Fungi and Their Effect on Potato Yield in the Columbia Basin of Oregon.

During three years of trials in commercial production fields, populations of three genera of potato pathogens, Fusarium spp, Pythium spp, and Verticillium dahliae, were followed before and after the single or combination use of 1,3-dichloropropene (1,3-D), 1,3-D + 17% chloropicrin (1,3-D + chloropicrin), or metam sodium (MS). Populations of these fungi did not always increase during the growing season but the relative population at a soil depth of 0 to 30 cm was nearly always higher than at 30 to 60 cm, regardless of year, sampling time, or fungal pathogen. The use of MS alone or in combination with 1,3-D generally suppressed recovery of all three genera and also increased yields. 1,3-D or 1,3-D + chloropicrin did not reduce fungal populations, but 1,3-D increased yield in 1 of 3 years. Reduced rates of MS and 1,3-D used in combination were as effective as higher rates of MS used alone. Multiple regression analysis comparing yield with fungal populations before planting indicated that population size was correlated negatively with yield. Propagules of V. dahliae had the greatest impact in reducing yield, but propagules of Pythium spp. and Fusarium spp. may have been important when populations of V. dahliae were low. Soil populations of Fusarium spp. and Pythium spp. have not been reported previously to be associated with yield reductions in potato grown in the Columbia Basin. Threshold estimates suggested that yield of number one tubers was reduced by 1 metric ton/ha for each 0.6 to 3.0 V. dahliae CFU/g dry soil present at planting.

Evaluation of endophytic actinobacteria as antagonists of Gaeumannomyces graminis var. tritici in wheat

Endophytic actinobacteria isolated from healthy cereal plants were assessed for their ability to control fungal root pathogens of cereal crops both in vitro and in planta. Thirty eight strains belonging to the genera Streptomyces, Microbispora, Micromonospora, and Nocardioidies were assayed for their ability to produce antifungal compounds in vitro against Gaeumannomyces graminis var. tritici (Ggt), the causal agent of take-all disease in wheat, Rhizoctonia solani and Pythium spp. Spores of these strains were applied as coatings to wheat seed, with five replicates (25 plants), and assayed for the control of take-all disease in planta in steamed soil. The biocontrol activity of the 17 most active actinobacterial strains was tested further in a field soil naturally infested with take-all and Rhizoctonia. Sixty-four percent of this group of microorganisms exhibited antifungal activity in vitro, which is not unexpected as actinobacteria are recognized as prolific producers of bioactive secondary metabolites. Seventeen of the actinobacteria displayed statistically significant activity in planta against Ggt in the steamed soil bioassay. The active endophytes included a number of Streptomyces, as well as Microbispora and Nocardioides spp. and were also able to control the development of disease symptoms in treated plants exposed to Ggt and Rhizoctonia in the field soil. The results of this study indicate that endophytic actinobacteria may provide an advantage as biological control agents for use in the field, where others have failed, due to their ability to colonize the internal tissues of the host plant.

Composition and Distribution of Pythium Communities in Wheat Fields in Eastern Washington State

ABSTRACT Pythium spp. were isolated from a mixture of soil and roots collected from 80 wheat fields in eastern Washington in the summer of 2000 from an area encompassing approximately 27,000 km(2). These sites covered a range of soil textures (coarse to fine, silty loess), average annual precipitation (200 to 600 mm), and average annual temperatures (7 to 11 degrees C). Soil type and annual precipitation run in an east-west gradient, while temperature has a north-south gradient. Species were identified using classical methods and by sequencing the internal transcribed spacer (ITS)-1 region of the rDNA and comparing these sequences to a database from a worldwide collection of Pythium spp. The species with the highest frequency of occurrence among all the sites were P. abappressorium sp. nov. (A) (50%), P. rostratum (R) (40%), P. debaryanum (D) (37.5%), P. heterothallicum (H) (33.7%), P.oligandrum (O) (31.2%), an unidentified P. sp. (aff. echinulatum) (E) (25%), and P. ultimum (U) (18%). P. intermedium, P. irregulare, P. paroecandrum, P. sylvaticum, P. dissimile, and P. dissoticum were isolated at a low frequency. From one to six species were isolated at each site, and there were 46 different species combinations detected. The species presence/absence data from all sites were analyzed with Jaccard's similarity coefficient hierarchical cluster analysis. Six communities were identified (species within each community designation in order of frequency among the sites within the community)-AD, AOU, AR, DEH, HE, and RU. In general, P. abappressorium was evenly distributed over all zones. AOU was more prevalent in zones with lower precipitation and coarser soil, while DEH and HE were associated with zones with higher precipitation and finer-textured soils on the basis of comparison of frequency distributions with the expected distribution over all the sites. The RU community was more prevalent in higher temperature zones. Canonical correspondence analysis was performed to examine the relationship between species and environmental variables. Soil type and precipitation were highly correlated with each other and with axis 1, which separated P. ultimum and P. abappressorium (lower variable values) from P. heterothallicum (higher variable values). Axis 2 and 3 were most correlated with temperature, and these axes separated P. oligandrum (higher value) from P. debaryanum (lower value) and P. ultimum-P. rostratum from the other species. The results suggest that Pythium species composition, distributions, and associations on a given crop may be influenced by environmental factors at a mesoscale level (100 to 1,000,000 ha).