Factors impacting the activity of 2,4-diacetylphloroglucinol-producing Pseudomonas fluorescens against take-all of wheat

Take-all, caused by Gaeumannomyces graminis var. tritici, is an important soilborne disease of wheat worldwide. Pseudomonas fluorescens producing the antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG) are biocontrol agents of take-all and provide natural suppression of the disease during wheat monoculture known as take-all decline. To identify factors that could contribute to the effectiveness of 2,4-DAPG producers in take-all suppression, P. fluorescens strains Q8r1-96 (genotype D) and Q2-87V1 (genotype B; reduced antibiotic production) were tested against three pathogen isolates differing in sensitivity to 2,4-DAPG (LD5, ARS-A1 and R3-111a-1) and two wheat cultivars (Tara and Buchanan). The antibiotic sensitivity of the pathogen and cultivar significantly affected the level of take-all suppression by Q8r1-96 and Q2-87V1; suppression was greatest with LD5 and Tara. Q8r1-96 suppressed ARS-A1 and R3-111a-1 on Tara but not Buchanan, and Q2-87V1 failed to suppress either pathogen isolate on either cultivar. Q8r1-96 colonized the rhizosphere of Tara and Buchanan grown in soil similarly, but 2,4-DAPG accumulation was higher on the roots of Buchanan than Tara. 2,4-DAPG at 7.5 μg mL⁻¹ reduced the growth of roots of both cultivars, and 10 μg mL⁻¹ caused brown necrosis and tissue collapse of seedling roots and reduced root hair development. The half-life of 2,4-DAPG in the rhizosphere was estimated to be 0.25 days. These results suggest that several interconnected factors including sensitivity of G. graminis var. tritici to 2,4-DAPG, wheat cultivar, fluctuations in populations of 2,4-DAPG producers, and antibiotics accumulation in the rhizosphere will impact the robustness of take-all suppression by P. fluorescens in the field.     

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.     

A new growth medium for the study of siderophore-mediated interactions

Summary A microbial growth medium, RSM, was developed to study the role of siderophores (microbial Fe-transport compounds) in the inhibition of the take-all pathogen, Gaeumannomyces graminis var. tritici, by Pseudomonas putida strain B10. The inorganic constituents of the medium were designed to mimic the rhizosphere while the organic composition was designed to promote rapid growth and siderophore production. The antibiosis experiments were highly reproducible and the antagonism appeared to be due to production of pseudobactin, the siderophore of B10. On plates amended with chrome azurol S, G. graminis did not produce siderophores while other fungi did. The growth of G. graminis on plates prepared with Fe chelate buffers was inhibited at a free ferric ion concentration of 10^–24.6 M, although three other fungi were not inhibited, even at 10^–25.5 M, presumably due to their greater production of siderophores. In liquid medium amended with Fe chelate buffers, both the doubling time and the lag phase of P. putida increased as the free ferric ion concentration was reduced. A wide variety of fungi and bacteria were found to grow on this medium. Because the inorganic composition of RSM is based on that of the rhizosphere, the development of this medium may be a first step towards the study of the chemistry and biology of the rhizosphere under well defined conditions.     

Suppression of wheat take-all and ophiobolus patch by fluorescent pseudomonads from a fusarium-suppressive soil

Fluorescent pseudomonads isolated from a soil suppressing Fusarium wilt significantly reduced take-all (Gaeumannomyces graminis var. tritici) in wheat and Ophiobolus patch (G. graminis var. avenae) in Agrostis turfgrass. The bacteria were mixed into a conducive soil at a concentration of 10⁷ colony-forming units (cfu)g^?1 soil at sowing. There were significantly fewer (P ? 0.05) diseased wheat roots in the treatments with the bacteria and pathogen than in those with the pathogen alone. Dry weights of the tops of wheat and Agrostis turfgrass were significantly greater (P ? 0.01) in treatments inoculated with the bacteria in the presence of the pathogens compared to controls with the pathogens alone. Dry weights of the tops of plants from treatments inoculated with the bacteria alone were not significantly different to those of healthy wheat non-inoculated with the bacteria, showing that the fluorescent pseudomonads did not stimulate plant growth. At the end of the experiments, the bacterial isolates (genetically-marked with rifampicin resistance) were recovered from wheat roots and rhizosphere soil at concentrations of 10⁵–10⁷cfu g^?1 fresh weight of roots or oven-dried rhizosphere soil.Many of the fluorescent pseudomonads and some non-fluorescent pseudomonads showed in vitro antibiosis on quarter-strength potato dextrose agar (QPDA) against the pathogens. However, there was no correlation between in vitro antibiosis on agar plates and suppression of disease in pot experiments. Further, while some isolates of G. graminis var. tritici and var. avenae were inhibited by certain bacterial isolates, other isolates of the same fungus were not similarly inhibited by the same isolates of bacteria. Most of the fluorescent pseudomonads that produced inhibition zones (>5mm) against G. graminis var. tritici on QPDA did not do so on King's medium B, where fluorescent siderophores were formed. In vitro antibiosis is, therefore, a poor criterion for selecting effective bacterial antagonists of the wheat take-all fungus. All of the fluorescent pseudomonads tested produced siderophores in low-Fe media while a non-fluorescent pseudomonad and the fungal pathogens did not produce siderophores of comparable activity. The addition of 500 μg FeEDTA g^?1 with a lower stability constant did not. The evidence suggests that iron competition at the rhizoplane or in the rhizosphere is one mechanism of suppression.     

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.     

Mycorrhizal fungi increase biocontrol potential of Pseudomonas fluorescens

Wheat roots are susceptible to colonisation by soil-borne pathogens, such as Gaeumannomyces graminis var. tritici (Ggt), which causes the globally important disease take-all, and mutualistic arbuscular mycorrhizal fungi (AMF). Certain rhizosphere fluorescent Pseudomonas strains have received much attention as potential biocontrol agents given their ability to produce antibiotics, such as 2,4-diacetylphloroglucinol (DAPG), that confer a measure of plant protection. Here we show that Pseudomonas fluorescens only produced DAPG in the presence of soluble carbon from soil containing either Ggt or AMF, and production increased by two orders of magnitude in response to both AMF and Ggt. Encouragement of mycorrhizal colonisation may therefore offer a sustainable strategy for protection against take-all.     

Effect of certain herbicides on soil microbial populations and their influence on saprophytic growth in soil and pathogenicity of take-all fungus

Summary The application of diquat + paraquat, glyphosate and trifluralin to unsterilized field soil increased take-all caused by the fungus, Gaeumannomyces graminis var. tritici Walker by 13.0% 16.6% and 10.8% respectively, while no effect on disease was recorded in sterilized soil treated with the same herbicides. The herbicides tested had no effect on the saprophytic growth of the pathogen with the exception of glyphosate, which increased its growth in unsterilized soil. The application of diquat + paraquat and glyphosate to unsterile soil had no effect on the numbers of actinomycetes. The diquat + paraquat treatment, however, increased populations of fungi while the glyphosate decreased the numbers of bacteria. The proportion of soil fungi antagonistic to the pathogen was reduced in glyphosate-treated soil. The frequency of occurrence of Eupenicillium euglaucum (v. Beyma) Stolk & Samson (strain B), and Penicillium verruculosum Peyr. (strain B), which were strong and low level antagonists of Ggt on agar, were reduced in their occurrence in soil by 7.7% and 2.5% respectively, following glyphosate treatment. Moreover, the numbers of Aspergillus viridinutans Ducker & Thrower, which showed moderate antagonism to the pathogen, was decreased by 1.9% and 4.1% in diquat + paraquat and glyphosate treatments respectively. The proportion of antagonists rather than total numbers of fungi appears to be related to the treatment effect observed on the soil growth and pathogenicity of G. graminis var. tritici in our investigation. The increase in disease of wheat in certain herbicide-treated soils may be due to the shift in soil microbial populations away from those which are antagonistic to the pathogen.     

EFFECT OF CHLORIDE FERTILIZERS AND LIME ON WHEAT GRAIN YIELD AND TAKE-ALL DISEASE

One experiment was initiated in the fall of 1991 to evaluate the effect of chloride (Cl) fertilizers on the suppression of take-all disease (Gaeumannomyces graminis var. tritici Walker) in winter wheat (Triticum aestivum L.). Preplant and topdress rates of potassium chloride (KCl) and calcium chloride (CaCl₂) (0, 34, 67 and 101 kg Cl ha^?1) were applied each year. In 1995, plots were split in half whereby one half received 2.24 Mg of 76% ECCE lime ha^?1 to elevate the pH and potentially increase disease incidence. Wheat grain yield was not affected by lime applications in any year (1995–1999). Plots exhibited visual symptoms of take-all in almost all years, however, grain yields increased in only two of eight years by the application of CaCl₂ and KCl. Applied fertilizer Cl for take-all disease suppression was inconsistent, even following the application of lime where increased soil pH can increase disease severity.

     

低氮胁迫下小麦抑草作用的化感效应与资源竞争分析

化感效应和资源竞争在作物抑草中起重要作用。为区分低氮胁迫下小麦抑草作用的化感效应与资源竞争,以强化感小麦品种‘115/青海麦’、‘92L89’和弱化感小麦‘抗10103’为材料,设置氮水平为25 mg·L-1、50mg·L-1、100 mg·L-1和200 mg·L-1的水培试验,通过小麦与看麦娘共培、应用含小麦根系分泌物的小麦-看麦娘共培液处理看麦娘,分别测定了小麦对看麦娘的生物干扰作用、化感效应及资源竞争效应。结果表明,不同化感潜力的小麦对看麦娘的抑制能力存在显著差异,抑制率大小为‘115/青海麦’(55.1%~73.9%)‘92L89’(48.9%~65.9%)‘抗10103’(15.4%~45.5%),且抑制率随氮水平的降低而增大。在小麦的抑草作用中,化感效应与资源竞争并存。低氮胁迫条件下,强化感小麦抑草能力增强主要通过提高化感效应实现;弱化感小麦抑草能力增强主要通过提高资源竞争实现。不同化感潜力小麦的抑草作用在低氮胁迫下表现出不同的生态策略。小麦抑草作用的化感效应均随氮水平降低而增大,其抑制率随氮水平的变化可用一元二次方程加以拟合。     

Wheat-rhizoplane pseudomonads as antagonists of Gaeumannomyces graminis

The role of rhizoplane-inhabiting Pseudomonas spp as inhibitors of take-all on wheat was investigated. Apparent numbers of pseudomonads in wheat rhizoplanes and numbers that were antagonistic in vitro toward Gaeumannomyces graminis var, tritici did not differ when wheat was supplied with NH⁺₄-N or NO^?₃-N. More intense antagonism was expressed by colonies selected from soil treated with NH⁺₄-N than with NO^?₃-N, and from isolation media prepared at pH 5.5 rather than at 7.0. Antagonists were not recovered from methyl bromide-treated soil. Highly antagonistic pseudomonads were recovered from a wheat-monoculture soil which is considered suppressive toward the pathogen in the field, and were not recovered from a “nonsuppressive” soil. Pseudomonad antagonism ratings were inversely correlated with take-all severity in the suppressive soil, but not in the nonsuppressive soil. Pseudomonads were considered to be antagonists of G. graminis on rhizoplanes of wheat in a soil exhibiting the “take-all decline” phenomenon, but the significance of this interaction remains to be determined.     

MICROBIAL POPULATIONS AND NITROGEN IN SOIL GROWING CONSECUTIVE CEREAL CROPS INFECTED WITH TAKE-ALL

Annually, for the past 12 years, a consecutive cereal-cropping sequence was begun. During 1969–72 soil was collected from plots with similar fertilizer treatments, but with different sequences of crops. In 1971 and 1972 wheat seedling bioassays showed that take-all disease became miximal after two or three crops, decreased to the fifth or sixth crop and then remained fairly constant in the‘take-all decline’state. The sequence with maximum disease had most NH₄⁺ -N and least NO₃^- -N in the rhizosphere soil in spring 1972, but there were no similar relationships in bulk soil in either of 2 years. Soil and rhizoplane populations of ammonifying and denitrifying bacteria were notably smaller in autumn 1970 and 1971 in short cereal sequences (2-5 yr.) than in the longer sequences. After γ-irradiating or autoclaving irradiated soils, all sequences contained much NH₄⁺ -N and although disease developing from added inoculum differed among soils, it did not follow the take-all decline pattern. Diffusate from sterilized soil favoured growth of the pathogen (Gaeumannomyces graminis var. tritici) least when it originated from the sequence that supported maximum disease.     

Phylogenetic positions of Mn<Superscript>2+</Superscript>-oxidizing bacteria and fungi isolated from Mn nodules in rice field subsoils

We isolated manganous ion (Mn²⁺) oxidizing bacteria and fungi from Mn nodules collected from two Japanese rice fields. The phylogenetic position of the Mn-oxidizing bacteria and fungi was determined based on their 16S rDNA and 18S rDNA sequences, respectively. Among 39 bacterial and 25 fungal isolates, Burkholderia and Acremonium strains were the most common and dominant Mn²⁺-oxidizing bacteria and fungi, respectively. Majority of the Mn-oxidizing bacteria and fungi isolated from the Mn nodules belonged to the genera that had been isolated earlier from various environments. Manganese oxide depositions on Mn²⁺-containing agar media by these microorganisms proceeded after their colony developments, indicating that the energy produced from Mn²⁺ oxidation is poorly used for microbial growth.     

Interaction between a peat carrier and bacterial seed treatments evaluated for biological control of the take-all diseases of wheat (Triticum aestivum L.)

Summary Field plots were established in Indiana, Oregon, and Montana to evaluate the potential for biological control of various strains of bacteria as seed treatments to reduce the severity of take-all root, crown, and foot rot of wheat. The bacteria were grown in liquid broth Cas-amino acid broth media, mixed with finely ground peat, and applied to seed with methyl cellulose as a glue just before planting in field soils conducive for severe take-all. Autoclave-sterilized peat (minus bacteria) seed treatments increased take-all, immobilized Mn, and reduced plant vigor and grain yields. These effects were intensified when the pH of the natural peat was adjusted from 5.2 to 7.0 with CaCO₃. The ability of the bacterial strains to counteract this peat-induced predisposition to take-all varied, and was influenced by planting site, genetic tolerance of the cultivar, and N treatment. Although the strains differed in their ability to suppress the peat-induced take-all, none of the isolates fully nullified the deleterious effects of the peat carrier. It is clear from this study that the carrier used with potential biological-contol agents may have a greater influence on disease than the biological agent.     

Cross-protection against the wheat and oat take-all fungi by Gaeumannomyces graminis var. graminis

Glasshouse experiments have shown that the prior colonisation of wheat roots by Gaeumannomyces graminis var. graminis, a fungus closely related to the wheat and oat take-all fungi but non-pathogenic to temperate cereals, reduced take-all infection along the roots. Cross-protected wheat plants produced grain yields significantly greater than those of unprotected plants but not significantly different to those of healthy wheat plants. A Phialophora-like fungus from grass roots did not confer the same degree of protection. There is some evidence that the cross-protection mechanism may be a specific host response nduced by var. graminis. The possible use of var. graminis in the biological control of take-all is discussed.     

A note about Triticum in Oman

Little is known about the diversity of wheat (Triticum spp.) in Oman. Results of a survey conducted in two remote mountain oases of northern Oman indicate that there exists considerable morphological variation within and among the five traditional landraces of wheat cultivated. Within two of the landraces grown on irrigated terraces, sized between 2 and 100 m², two new botanical wheat varieties (Triticum aestivum var. baladseetense and var. maqtaense) were identified of which the agronomic properties, in particular tolerance to drought and heat, and the nutritional value require further investigation.     

Ammonium and nitrate in the rhizosphere of spring barley, hordeum vulgare L., and take-all disease

The incidence and severity of take-all disease, due to Gaeumannomyces graminis (Sacc.) Arx & Olivier var. tritici Walker, was observed on spring barley plants growing in soil in two glasshouse experiments. Soil amendments of NH⁺₄-N significantly increased the number of diseased plants and roots during the first month after germination in comparison with controls unamended with N (P < 0.05). No significant difference in the incidence of take-all disease was detected between more mature barley plants growing in soil amended with either NH⁺₄ or NO^?₃-N and unamended controls. The least take-all disease in 3 month-old barley plants was observed when N was supplied as foliar sprays of urea at 0.5 mg N kg^?1 soil (P < 0.01). There was no significant correlation between the degree of infection and the NH⁺₄-N to NO^?₃-N ratio in the rhizosphere soil     

Transfer of leaf rust and stripe rust resistance from <Emphasis Type="Italic">Aegilops umbellulata</Emphasis> Zhuk. to bread wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.)

Aegilops umbellulata acc. 3732, an excellent source of resistance to major wheat diseases, was used for transferring leaf rust and stripe rust resistance to cultivated wheat. An amphiploid between Ae. umbellulata acc. 3732 and Triticum durum cv. WH890 was crossed with cv. Chinese Spring Ph ^I to induce homoeologous pairing between Ae. umbellulata and wheat chromosomes. The F₁ was crossed to the susceptible Triticum aestivum cv. ‘WL711’ and leaf rust and stripe rust resistant plants were selected among the backcross progenies. Homozygous lines were selected and screened against six Puccinia triticina and four Puccinia striiformis f. sp. tritici pathotypes at the seedling stage and a mixture of prevalent pathotypes of both rust pathogens at the adult plant stage. Genomic in situ hybridization in some of the selected introgression lines detected two lines with complete Ae. umbellulata chromosomes. Depending on the rust reactions and allelism tests, the introgression lines could be classified into two groups, comprising of lines with seedling leaf rust resistance gene Lr9 and with new seedling leaf rust and stripe rust resistance genes. Inheritance studies detected an additional adult plant leaf rust resistance gene in one of the introgression lines. A minimum of three putatively new genes—two for leaf rust resistance (LrU1 and LrU2) and one for stripe rust resistance (YrU1) have been introgressed into wheat from Ae. umbellulata. Two lines with no apparent linkage drag have been identified. These lines could serve as sources of resistance to leaf rust and stripe rust in breeding programs.     

Age of wheat and barley roots and infection by gaeumannomyces graminis var tritici

Seminal roots of wheat and barley seedlings were inoculated with G. graminis var tritici on regions 0, 5- and 15-days old, and assessed for intensity and extent of infection after standard times. Wheat roots were most heavily infected on young regions, whereas barley roots were most heavily infected on old ones. The effect of root age in wheat was similar in both unsterile and aseptic conditions, so it could not be ascribed to saprophytic rhizosphere micro-organisms interacting with G. graminis.The contrasting results for wheat and barley are explained by a single hypothesis, based on decreasing host-resistance in the root cortex but increasing resistance at or near the endodermis as the roots age. It is suggested that, under some conditions, even small amounts of non-pathogenic root cortex death can enhance infection by G. graiminis. This interpretation may explain several aspects of take-all and its biological control by other dark mycelial parasites.     

Cultivation and Taxonomic Classification of Wheat Landraces in the Upper Panjsher Valley of Afghanistan After 23 Years of War

After 23 years of war, current information about the biodiversity of crops in the Hindukush mountains of Afghanistan is scarce. This study aimed at assessing the genetic composition of farmers wheat (Triticum spp.) populations through a survey of 21 randomly chosen cereal fields on both sides of the Panjsher river in the upper Panjsher valley of Northern Afghanistan. A stratified sampling of wheat heads according to morphological differences was followed by estimates of field size and grain yield and a formal interview with the landowner about the cropping sequence and the inputs used. About 75% of the cereal fields were cropped in rotation systems with faba bean (Vicia faba L.), barley (Hordeum vulgare L.), potato (Solanum tuberosum L.), maize (Zea mays L.) or fallow. Manure application at between 2.3 and 5.3 t ha⁻¹ was the major source of nutrient inputs at grain yield levels between 1.2 and 4.7 t ha⁻¹. The morphological characterization of the collection revealed 19 taxonomically different varieties of bread wheat (Triticum aestivum L.) but also barley and triticale (Triticosecale Wittm.) grown in mixtures. Populations within one field consisted of up to seven botanical wheat varieties. Farmers did not differentiate between morphological differences within such mixtures but identified their populations instead according to grain color, cooking properties and resistance to mildew and frost. Triticum aestivum var. subferrugineum was the most widespread wheat variety and no effects of altitude on biodiversity of wheat was noted across the transect. Particularly interesting was the occurrence of T. aestivum var. subferrugininflatum and var. subgraecinflatum which so far have only been reported from Mongolia. The finding of triticale indicated the active seed exchange with lowland or long-distance seed sources.     

The role of bacteria in the biological control of Gaeumannomyces graminis by suppressive soils

The suppression of Gaeumannomyces graminis var. tritici by certain soils or following certain soil treatments is considered to be an expression of either specific or general antagonism sensu Gerlagh (1968). Specific antagonism is effective in dilutions as high as 1 in 1,000, can be transferred from soil to soil, operates near or on wheat roots, is destroyed by 60°C moist heat for 30 min. or desiccation, is fostered by wheat monoculture but may be lost from a soil by fallow or rotation with certain crops, especially legume hay or pasture crops. Strains of Pseudomonas fluorescens may be involved. General antagonism is a soil property which cannot be transferred and is resistant to 80°C moist heat for 30 min, to methyl bromide and chloropicrin, but not to autoclaving. Take-all control by organic amendments, minimum tillage, or a soil temperature of 28°C may be expressions of increased general antagonism.In much of the southern Australian wheat belt, where take-all can cause heavy crop losses, some general but rarely specific antagonism is apparently operative. Both types of antagonism are probably operative in long-term wheat growing areas of the Pacific Northwest U.S.A. where take-all is virtually nonexistent.