Control of Pythium damping-off of sugar beet by seed treatment with crop straw powders and a biocontrol agent

Seed treatment with non-sterilized powdered straws from 39 crops was tested for the control of Pythium damping-off of sugar beet. Four straws, including flax, coriander, pea, and lentil were effective in controlling the disease in soil artificially infested with Pythium sp. “group G.” Sterilizing flax and pea straws eliminated the efficacy of these straws. Wheat straw powder coated on sugar beet seeds increased the incidence of Pythium damping-off but this effect was reversed by the co-inoculation of wheat straws with the biocontrol agent Pseudomonas fluorescens 708. Coating sugar beet seeds with P. fluorescens 708 and flax or pea straws also increased the efficiency of the bacterial strain for the control of Pythium damping-off. Pea straws and to a lesser extent lentil straws produced volatile substances that affected mycelial growth of Pythium sp. “group G” on potato dextrose agar in Petri plates when the straws were mixed with water and left to ferment for two days. Fermentation of pea straws led to the accumulation of volatile ammonia, which was produced by the reduction of the large amount of nitrate stored in the straw. Reduction of nitrate and therefore the release of volatile ammonia did not occur in sterilized pea straws. However, fermenting sterile pea straws with bacteria from different genera restored nitrate reduction and the release of volatile ammonia, suggesting that microorganisms associated with pea straws are responsible for the conversion of nitrate into volatile ammonia which in turn control Pythium damping-off disease in sugar beet.     

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.     

Analogous wheat root rhizosphere microbial successions in field and greenhouse trials in the presence of biocontrol agents Paenibacillus peoriae SP9 and Streptomyces fulvissimus FU14

Two Pythium-infested soils were used to compare the wheat root and rhizosphere soil microbial communities from plants grown in the field or in greenhouse trials and their stability in the presence of biocontrol agents. Bacteria showed the highest diversity at early stages of wheat growth in both field and greenhouse trials, while fungal diversity increased later on, at 12 weeks of the crop cycle. The microbial communities were stable in roots and rhizosphere samples across both soil types used in this study. Such stability was also observed irrespective of the cultivation system (field or greenhouse) or addition of biocontrol coatings to wheat seeds to control Pythium disease (in this study soil infected with Pythium sp. clade F was tested). In greenhouse plant roots, Archaeorhizomyces, Debaryomyces, Delftia, and unclassified Pseudeurotiaceae were significantly reduced when compared to plant roots obtained from the field trials. Some operational taxonomic units (OTUs) represented genetic determinants clearly transmitted vertically by seed endophytes (specific OTUs were found in plant roots) and the plant microbiota was enriched over time by OTUs from the rhizosphere soil. This study provided key information regarding the microbial communities associated with wheat roots and rhizosphere soils at different stages of plant growth and the role that Paenibacillus and Streptomyces strains play as biocontrol agents in supporting plant growth in infested soils.     

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.     

Root rot disease incidence and severity on some legume species grown in Samsun and the fungi isolated from roots and soils

Abstract

Root rot disease is very common in the bean, soybean, faba bean and pea plants growing areas in Samsun province. Disease incidence and severity were detected the highest at 93.8% and 55.4% in the bean growing area, and the lowest at 64.0% and 24.3% in the faba bean growing area respectively. In this study, a total of 2714 fungal isolates were obtained from some legume plants and soil samples. The most common fungi isolated from root and soil samples were Fusarium spp., multinucleate Rhizoctonia (MNR), binucleate Rhizoctonia (BNR) and Pythium spp. respectively. Fusarium spp. were isolated at high rates from all the examined areas. MN Rhizoctonia and BN Rhizoctonia were isolated both from inner and coastal areas of the province, whereas Pythium spp. were isolated in costal areas, except for the Vezirköprü district which is situated in the inner area. When looking at the interactions among pathogens causing root rot, it was found the great majority of the samples (30.4%) isolated both Fusarium spp. and MNR-BNR group fungi, whereas Fusarium spp. and Pythium spp. were isolated together from 10.9% of the samples and MNR-BNR and Pythium spp. from only 1.5% of the samples.     

Field evaluation of plant growth-promoting Rhizobacteria amended transplant mixes and soil solarization for tomato and pepper production in Florida

Field trials were performed in Florida to evaluate tomato and pepper transplants amended with formulations of several plant growth-promoting rhizobacteria (PGPR) in a production system that included soil solarization. Transplants grown in five different formulations of PGPR were planted into plots treated by soil solarization, MeBr fumigation, or untreated soil. Treatments were assessed for incidence of several naturally occurring tomato and pepper pathogens including root-knot nematode (Meloidogyne incognita) and species of Pythium, Phytophthora, and Fusarium. Highly significant increases in tomato and pepper transplant growth occurred in response to most formulations of PGPR tested. Transplant vigor and survival in the field were improved by PGPR treatments in both tomato and pepper. Diseases of tomato caused by root-knot nematodes, Fusarium, Phytophthora, and Pythium were not affected by PGPR treatments. PGPR formulation LS261 reduced numbers of root-knot nematode galls on pepper while pepper root condition was improved with formulations LS213, LS256 and LS261. Individual PGPR strains affected the number of Pythium colonies isolated from pepper roots, but did not affect isolation of Pythium from tomato roots. Greater numbers of colonies of Pythium were isolated from pepper roots in the MeBr treatment and fewest in the solarization treatment. Numbers of colony forming units of Fusarium were significantly higher in the untreated soil than in MeBr fumigated or solarized soil with no effect of PGPR on isolation of Fusarium from either crop. Incidence of wilt symptoms on tomato was significantly lower in MeBr treated plots and highest in the untreated plots. Yield of extra large tomato fruit and total yield increased with PGPR formulation LS256. Yield of pepper was increased with formulations LS255 and LS256. Solarization combined with LS256 on pepper produced yields comparable to MeBr.     

DORMANCY FACTORS IN IRIS (IRIDACEAE) SEEDS

Utilizing a newly developed radish seed bioassay, it has been demonstrated that Iris seed dormancy is caused by water-soluble inhibitor(s). Leached Iris seeds germinate freely when planted in soil without previous chilling. The inhibitor(s) can prevent germination of representative species of both monocotyledonous and dicotyledonous seeds.     

Efficiency of various selective media in determining Pythium population in soil

Of 15 selective media recommended for isolation and enumeration ofPythium spp. directly from soil, corn meal agar (CMA) supplemented with agar, sucrose, minor elements, thiamine, rose bengal, pimaricin, pentachloronitrobenzene and vancomycin (MPVM) was the most efficient. Streptomycin (30–50 ppm) and rose bengal (33–60 ppm) as used in certain tested media effectively suppressed development of bacteria and actinomycetes. However, these chemicals adversely affected germination of spores and mycelial growth and thereby the recovery ofPythium spp. from soil. Media containing pimaricin (5 to 100 ppm) were more effective than those with nystatin (40 ppm) in suppressing development of nonphycomycetous fungi on isolation plate. MPVM with pimaricin at 5 ppm was more efficient than that with 10 ppm of the antibiotic in recoveryingPythium from soil. However, there was no difference in recovery ofPythium by this medium containing rose bengal at 5 ppm or at 10 ppm, butPythium colonies were more dense and better delineated when the medium contained 10 ppm of rose bengal. CMA containing pimaricin (5–100 ppm) and vancomycin (200 ppm) permitted occasionally development of a large number ofMortierella and bacterial colonies from certain soils, that interfered with accurate determination of colonies of certainPythium spp. on the plates. Vancomycin at 300 ppm, as used in MPVM, substantially reduced development of bacterial colonies compared to 200 ppm of the antibiotic. Surface-soil dilution-plate was more effective than the soil-dilution-plate method in reducing bacteria andMortierella colonies on isolation plates without affecting recovery ofPythium. The importance of basal medium, complement of antimicrobial agents, and isolation methods for efficiency of selective medium in recovery ofPythium spp. directly from soil is discussed.     

The genus Pythium in Taiwan, China (1) — a synoptic review

The genus Pythium, with slightly over 280 described species, has been classified traditionally with other filamentous, coenocytic, sporangia-producing fungi as “Phycomyetes”. 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 of the Kingdom Fungi or Kingdom Mycota. They are widely distributed throughout the world as soil saprophytes or plant pathogens. Because of the warm moist maritime climate, Taiwan, China, is especially rich in Pythium species. To date, 48 species of Pythium have been reported from Taiwan, China, with the dominant species being Py. vexans, Py. spinosum, Py. splendens, Py. aphanidermatum, Py. dissotocum and Py. acanthicum. There is no definite geographical distribution of Pythium spp. in Taiwan, China. Twenty nine species of Pythium have proven to be plant pathogens attacking a wide variety of woody and herbaceous plants primarily causing pre- and post-emergence seedling damping-off, root rot, stem rot and rotting of fruits, tubers and ginger rhizomes, resulting in serious economic losses. The most important plant pathogenic species include Py. aphanidermatum and Py. Myriotylum, which are most active during the hot and wet summer months; whereas Py. splendens, Py. spinosum, Py. ultimum and Py. irregulare cause the greatest damage in the cool winter. Most Pythium spp. are non-specific pathogens, infecting mainly juvenile or succulent tissues. This review attempts to assess the taxonomic position of the genus Pythium and provide details of the historical development of the study of Pythium as pathogens in Taiwan, China, causing diseases of sugarcane, trees, vegetables, fruits, specialty crops and flowering plants, as well as measures to control these diseases. Of special note is the introduction of the S-H mixture which, when used as soil amendment, effectively controls many soil-borne Pythium diseases during the early stages of plant growth. The diversity of Pythium species in Taiwan, China, is discussed in comparison with the situation in the mainland of China and suggestions are made to fully utilize Pythium spp. as agents for biological control, in industry and medicine.     

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.     

Antagonistic Activities of Trichoderma harzianum Against Pythium aphanidermatum and Pythium myriotylum on Tobacco

Trichoderma harzianum rendered Pythium aphanidermatum and P. myriotylum non-viable in Petri dish dual culture. The Pythium mycelia from such cultures showed natural autofluorescence in the regions of interactions, indicating their death. Non-volatile and volatile fungicidal activities were detected in T. harzianum culture. Lytic activity of β-(l,3)-glucanase was detected on the cell walls of the Pythium spp. There was a significant decrease in the disease incidence when T. harzianum was incorporated into sterile soil, whereas the effect was insignificant in natural soil.     

Species Composition and Habitat Preference of Watermolds in the Nainital District of Kumaun Himalaya,India

A total of thirty two species of watermolds have been identified from the entire study area. Of these, 22 % species were restricted to water, 44 % to soil and 34 % occurred in water and soil both. The maximum number of species were found during rainy season in both the ecosystems. Pythium s pp. were dominant in terrestrial ecosystem whereas Pythium s pp. and Saprolegnia spp. were dominant in water system. There were two clusters indicative of presence of two distinct communities in both the systems.     

Enhanced activity of introduced biocontrol agents in solarized soils and its implications on the integrated control of tomato damping-off caused by Pythium spp

Soil solarization in combination with introduction of biocontrol agents (BCA) was evaluated as a potential disease management strategy for tomato damping-off caused by Pythium spp. A rifampicin resistant Pseudomonas fluorescens strain (PfT-8) and a carbendazim resistant Trichoderma harzianum strain (ThM-1) were introduced into soil following solarization. Tomato seeds were planted into treated field plots. The influence of soil solarization and application of biocontrol agents on damping-off incidence, plant biomass, rhizosphere population of introduced antagonists, and native Pythium spp. was assessed by two consecutive field trials. Damping-off incidence was significantly reduced in solarized plots compared to control. Soil inoculation of biocontrol agents into solarized plots resulted in the highest suppression of damping-off incidence (PfT-8 up to 92%; ThM-1 up to 83%), and increase in plant biomass (PfT-8 up to 66%; ThM-1 up to 48%) when compared to un-solarized control plots. Rhizosphere population of introduced biocontrol agents gradually increased (PfT-8 up to 102% and ThM-1 up to 84%) in solarized soils when compared to unsolarized control. The population of Pythium spp in rhizosphere soil was reduced up to 55% in solarized plots; whereas, application of BCA to solarized soils reduced the rhizosphere population of Pythium spp. by 86 and 82% in P. fluorescens and T. harzianum applied plots respectively.     

Biological control of a soil-borne pythium infection by seed inoculation

Summary 1. A three-fold effect was produced on white mustard seedlings grown in soil infected withPythium sp. Seed germination, the number of healthy plants which survived and the fresh weight of the shoots were reduced.2. Disease symptoms were controlled to some extent by dusting the seeds with spores of some common soil saprophytes includingTrichoderma viride, Penicillium nigricans, P. jrequentans andP. godlewskii.3. Of three strains ofTrichoderma viride which were tested for antagonism toPythium sp., a gliotoxin-producing strain was more effective in controlling the disease than a viridin-producing strain and an antibiotically inactive strain gave least protection to the seedlings.4. The disease symptoms were less severe in soil treated with acid or calcium hydroxide. Inoculation of the seeds withT. viride gave further control of the disease in soil treated with calcium hydroxide but not in acidified soils. These results are discussed in relation to the production of gliotoxin byT. viride.     

Untersuchungen zum Vorkommen von Pythium spp. im Boden

Investigations on the occurrence of Pythium spp. in soil: I. The isolation of Pythium spp., their distinction to macroscopically characteristics and their determination The aim of the present paper consisted in detection of Pythium spp. directly in the soil. This was possible by using a selective medium and by crumbling smallest particles of agar-covered soil on its surface. On the basis of simple morphologically criteria (growth patterns) this method allows to decide concerning the presence of high and less pathogen or apathogen Pythium spp. in a soil sample within 48 hours. About 700 isolates have been cultivated from hyphal tips, determinated and about 230 tested for pathogenicity to sugar beet seedlings in vitro. Most of the Pythia pathogen to sugar beet belong to P. ultimum Trow followed by P. paroecandrum Drechsler and P. debaryanum sensu Drechsler non Hesse. The taxonomically characteristics are demonstrated by figures of the three species.     

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.     

Effect of green manure on Pythium spp. population and microbial communities in intensive cropping systems

Saprophytic soil-borne pathogens can be either actively suppressed by organic amendments or enhanced, depending on soil health conditions. This can be deleterious in the event of selection of a soil-borne population by previous soil management and short crop rotation. Trials were performed in the open field and in pots, using naturally infected soil from intensive crop systems, i.e., soil from fields with 8 years of strawberry cultivation. The aim was to study short-term response of Pythium and soil microbial populations to green manure. The use of green manure in these naturally infested soils, 3–10 weeks after fresh tissue incorporation, caused Pythium populations to increase concurrent with an increase in soil microbial populations, and did not result in the suppression of the pathogen. A more elaborate trial was performed under controlled conditions, amending soil with fresh wheat plant material, air-dried wheat plant material and an organic fertilizer with a high level of humic substances. Although compared to the original soil, all amendments caused a similar increase in organic matter content and small differences in soil respiration, incorporation of fresh, not decomposed, plant material strongly increased Pythium, while the organic fertilizer did not affect the original level of the pathogen population. The increase in total number of fungi and bacteria did not have any suppressive effect on the Pythium population in naturally infested soil used for this study.     

Influence of film mulches and soil pesticides on root diseases and populations of soil-borne fungi in vegetables

Summary Field plots of Tifton loamy sand were treated with various soil pesticides and left exposed or covered with biodegradable paper or black polyethylene film mulch. Cucumber, squash, muskmelon, sweetcorn, and polebean were planted in one or more experiments. Trickle irrigation under the film was used in several tests. Isolations were made from roots of all crops except sweetcorn, and the fungi most commonly isolated wereFusarium oxysporum, Pythium spp.,F. solani, F. roseum, andRhizoctonia solani from cucurbits andF. solani andF. oxysporum from polebean. Significantly fewer fungi were isolated from plants grown in soil treated with DD-MENCS (20% methyl isothiocyanate +80% chlorinated C₃ hydrocarbons) or methyl bromide-chloropicrin (2:1) (MBC) than from controls, and populations ofPythium spp.,F. solani, andF. oxysporum, were reduced in soil. Sodium azide, sodium azide +ethoprop or carbofuran, and sodium methyl dithiocarbamate were less effective than DD-MENCS and MBC.Nematologist, ARS, USADA; Soil Scientist, ARS, USDA; and Associate Professor, Department of Entomology, University of Georgia, respectively, Coastal Plain Station, Tifton, Ga., 31794, United States of America     

Field assessment of soil biological and chemical quality in response to crop management practices

Soil microbiological and chemical aspects were evaluated to determine the effects of conservation tillage and crop rotation on soil fertility over a 16-year period. A field trial was established to compare two cropping systems (continuous soybean and maize/soybean, soybean/maize rotation). In addition, maize (Zea mays L.) and soybean (Glycine max L., Merr) were grown in two different tillage systems: no tillage and reduced tillage. Soil populations of Trichoderma spp., Gliocladium spp. and total fungi were more abundant when maize or soybean were under conservation tillage and in the maize/soybean and soybean/maize rotation, than in continuous soybean. Furthermore, higher levels of microbial respiration and fluorescein diacetate hydrolysis (FDA), were recorded under no tillage systems. However, soil counts of Actinomycetes and Pythium spp., and Pythium diversity together with soil microbial biomass were not affected by the field treatments. To establish a correlation with soil biological factors, soil chemical parameters, such as pH, organic matter content, total N, electrical conductivity, N–NO₃ ⁻ and P were also quantified, most of the correlations being significantly positive. Under no tillage there was a clear increase of the amount of crop residues and the C and N soil content due to the presence of residues. Also the distribution of crop residues in surface soil due to zero tillage and the quality of these residues, depending on the crop rotation employed, improved on soil biological and chemical characteristics. Crop yield was also enhanced by zero tillage through the management of residues. Although yield values were not directly associated with the development of microorganisms, both yield and microorganisms were influenced by crop management. These results suggest that measuring soil properties over a long period helps to define effective management strategies in order to preserve soil conditions.