The effects of various crops on the survival and carry-over of the wheat take-all fungus Gaeumannomyces graminis var. tritici

The effects of various crops on the saprophytic survival and carry-over of Gaeumannomyces graminis var. tritici were studied in the glasshouse and field.
In the glasshouse, survival of G. g. tritici was greater in unplanted pots than in pots planted with either cereals or non-cereals. In the field, fallow and various non-cereal crops reduced carryover of take-all but the disease was severe after wheat, barley and triticale. Grain yields were higher after fallow and non-cereals than after cereal crops.     

Volatile analogues of penconazole and their activity against the take-all fungus,Gaeumannomyces graminis var. tritici

Eight new sterol biosynthesis-inhibiting fungicides, structurally related to penconazole and having vapour pressures up to 118 mPa, were synthesised. Their toxicities to the cereal take-all fungus, Gaeumannomyces graminis var. tritici, on agar were measured; intrinsic activities were measured after incorporating the compounds into the agar and vapour activities were measured after their evaporation from glass and from moist soil. Vapour activity following evaporation from soil was shown to be a function of both the intrinsic activity of the compound and its partition coefficient between the air and moist soil (K_as). The latter is itself a function of vapour pressure, 1-octanol/water partition coefficient (K_ow) and the soil type. The compound most active as vapour from soil in the in-vitro test, 1-(pyridin-3-yl)-2-(4-flurorophenyl)pentane, was ineffective against take-all in wheat in a pot test in which the inoculated soil was treated unevenly, providing further evidence that the redistribution of fungicides in moist soil occurs predominantly via the water phase rather than the vapour phase.     

Explaining variation in the effects of take-all (Gaeumannomyces graminis var. tritici) on nitrogen and water uptake by winter wheat

Two field experiments, in 1999 and 2000, were used to test whether reductions in root growth and function explained the effects of take-all on crop water and nitrogen uptake. The fungicide silthiofam was used to manipulate take-all independently of other factors. Soil water content was manipulated from heading to determine effects on disease progress and resource capture. Epidemic progress was significantly delayed in the presence of silthiofam, leading to reductions in disease in both experiments. Effects of silthiofam were reduced by increasing soil water late in the season, although only in 2000 did increased soil water content have a direct effect, leading to a higher rate of disease increase. Higher levels of disease in the absence of silthiofam did not affect root growth as measured by total root length density (TRLD), but did lead to significantly reduced healthy root length density (HRLD, a measure of functional roots) in both experiments. Only in 2000 were there any significant effects of increased take-all on water and nitrogen uptake. This was attributed to the higher TRLD in the (1999) crop, which allowed HRLDs to be maintained above a critical threshold (where water and nitrogen uptake start to be severely affected) despite loss of functional root to disease. The effects of take-all on nitrogen uptake were more likely to affect crop canopy size and duration than the relatively small effects observed on water uptake. Increasing soil water content allowed the crop to take up more water in absolute terms despite, in 2000, increasing levels of disease and reducing HRLD.     

Methods for studying population structure, including sensitivity to the fungicide silthiofam, of the cereal take-all fungus, Gaeumannomyces graminis var. tritici

Field isolates ( n  = 144) of the wheat take-all fungus Gaeumannomyces graminis var. tritici ( Ggt ) were tested for sensitivity to silthiofam, a take-all-specific fungicide used as a seed treatment, and identified as A- or B-type by PCR–RFLP analysis of nuclear rDNA. A possible association was identified between polymorphisms in ITS2 of the nuclear rDNA and sensitivity to silthiofam. A Ggt -specific PCR assay was developed which simultaneously identified isolates of Ggt as A- or B-type, based on the polymorphisms in the nuclear rDNA. A highly significant correlation between Ggt type using the PCR assay and sensitivity to silthiofam was demonstrated in a collection of 358 isolates from three field experiments designed to test the effects of seed-treatment fungicides on take-all and Ggt populations in winter wheat. In one experiment the percentages of silthiofam-sensitive and B-type isolates were significantly less in populations from plots sown with silthiofam-treated seed in two consecutive years than in populations from plots sown with nontreated seed. However, silthiofam still provided a significant amount of control of take-all. The natural occurrence of fungicide-insensitive isolates, up to about 30% in soils in which the fungicide had never been used, is unusual. The new PCR assay provides a useful tool for studying the population structure of Ggt , and may provide a novel method for assessing the incidence of insensitivity to silthiofam (the target site for which has not yet been identified) in field populations of Ggt .     

The effectiveness of eleven sterol biosynthesis-inhibiting fungicides against the take-all fungus,Gaeumannomyces graminis var. tritici,in relation to their physical properties

Eleven sterol biosynthesis-inhibiting fungicides were compared in experiments to determine the physico-chemical properties required for most effective control of take-all by soil treatment. All were active in agar culture against an isolate of the pathogen which causes take-all, Gaeumannomyces grammis var. tritici, with prochloraz being the most toxic (EC₅₀ 0.02 μg ml^?1) and PP 969 the least (EC₅₀ 0.44 μg ml^?1). Penconazole and PP 969 had vapour activity against the fungus in further bioassays on agar. In soil in pots, the most strongly lipophilic compound, buthiobate, was ineffective against take-all in wheat; triadimenol was most effective and, like flutriafol, nuarimol and PP 969, retained some effectiveness after 12 weeks in soil. PP 969, unlike penconazole or nuarimol, was effective in soil treated unevenly by mixing the fungicides in layers. PP 969 is relatively polar, and it is suggested that this property, allowing redistribution in soil water rather than as vapour, outweighed its poor intrinsic toxicity. The ideal soil-treatment fungicide should therefore be polar and also have good intrinsic activity and moderate persistence. None of the compounds tested had all these properties.     

Enhanced field control of wheat take-all using cold tolerant isolates of Gaeumannomyces graminis var. graminis and Phialophora sp. (lobed hyphopodia)

Cold tolerant isolates of Gaeumannomyces graminis var. graminis ( Ggg ) and Phialophora sp. (lobed hyphopodia), which produced at least comparable growth rates at 5°C to those of pathogenic G. graminis var. tritici ( Ggt ), were shown to control take-all disease in wheat effectively in 2 years of field experiments in New South Wales, Australia. The addition of oat inoculum of these fungi at the rate of 60 kg/ha to the seeding furrow significantly ( P  ≤ 0.05) reduced disease and increased grain yields by 33–45% compared to the Ggt alone treatment. The use of 30 kg/ha of oat inoculum also significantly ( P  ≤ 0.05) reduced disease and increased grain yields by 21–44%. These high levels of take-all control were obtained consistently from four field experiments on three different soil types with different pHs. A treatment inoculated with Ggg alone showed no disease symptoms and produced grain yields similar to that of untreated wheat. This fungus is, therefore, non-pathogenic to wheat. At high rates of inoculation of Ggg and Phialophora sp. (lobed hyphopodia), 65–80% of tillering wheat plants (GS 32) had root systems colonized by these fungi. In contrast, two Pseudomonas spp. and an isolate each of Ggg and Phialophora sp. (lobed hyphopodia), which did not grow at 5°C, were ineffective in controlling take-all. Take-all assessments during heading (GS 61-83) were highly correlated ( R ²=0.6047, P ≤0.0005) with the relative yield increase or decrease of inoculated treatments compared to the Ggt alone treatment. The use of a Ggg isolate (90/3B) and a Phialophora sp. (lobed hyphopodia) isolate (KY) for take-all control has been patented. These fungi are being developed for commercial use.     

河北省小麦全蚀病菌变种类型鉴定

2007年从河北省的保定、石家庄、邢台小麦主产区采集小麦全蚀病株,共分离得到62个菌株,对其所属的变种类型进行了形态学和分子生物学鉴定。根据形态、培养性状、生理特性以及在小麦、高粱、水稻、玉米、燕麦等禾本科作物上的致病性,初步认定测定的所有菌株均为禾顶囊壳小麦变种(Gaeumannomyces graminisvar.tritici)。进一步采用4个变种的特异性引物进行PCR扩增,在所有菌株中扩增出870bp的条带,该片段为禾顶囊壳小麦变种特异性片段,证实所测菌株均为禾顶囊壳小麦变种。     

Sensitivity to pH and ability to modify ambient pH of the take‐all fungus Gaeumannomyces graminis var. tritici

pH is one of the major ambient factors affecting life history traits of soilborne phytopathogenic fungi. The diversity of phenotypic responses to pH changes has not been extensively explored within fungal populations. To investigate this question, the ability of 82 strains of a worldwide collection of the take‐all agent Gaeumannomyces graminis var. tritici (Ggt) to grow in controlled pH conditions, reflecting their pH sensitivity, was measured. Of these 82 strains, 37 belonged to the G1 type and 45 to the G2 type, the two main genetic groups identified in Ggt populations. The experiments were conducted in Petri dishes on Fahraeus solid media buffered at pH 4·6, 6·0 or 7·0 with citrate–disodium phosphate solutions. The 82 strains exhibited a wide range of hyphal growth rates at the three pH levels. Ten statistically different pH profiles were described. The G2 strains grew significantly better than the G1 on the slightly acidic (pH 6·0) and the neutral (pH 7·0) buffered media. The ability of three strains to change ambient pH was also measured on unbuffered Fahraeus solid media initially adjusted to pH 5·6 or 8·0. All three strains were able to alkalinize the acidic medium. However, important variations between strains in the intensity, range and persistence of this alkalinization were measured. These results provide the first evidence of intraspecific variability in pH sensitivity within soilborne fungal species.     

Mechanisms of inhibition of Gaeumannomyces graminis var. tritici by fluorescent pseudomonads

The take-all fungus Gaeumannomyces graminis var. tritici reduced the weight of wheat plants grown in tubes containing sterilized sand and plant nutrient solution. Fluorescent pseudomonads, when added to the tubes, inhibited the take-all fungus and increased plant weight. Iron (as FeNaEDTA) had no effect on the inhibition of the fungus by the pseudomonads. Some of the pseudomonads produced a compound in the wheat rhizosphere with a UV absorption peak at 365 nm, but the inhibition of G. graminis by pseudomonads was not proportional to the UV absorbance of rhizosphere extracts. A yellow crystalline compound, absorbing at 365 nm, was extracted from broth cultures and shown to be toxic to G. graminis under acid conditions. This compound is considered to be phenazine-I-carboxylic acid.     

禾顶囊壳小麦变种对小麦种子根的侵染过程

 光镜和电镜观察表明,禾顶囊壳小麦变种(Gaeumannomyces graminis var.tritici,小麦全蚀病菌)对小麦种子根的侵染过程可分为侵入前、侵入表皮层、进入皮层和进入中柱等4个连续阶段。麦根接菌后在15℃下培养,48 h后侵入表皮层细胞,60 h后进入皮层,120 h后进入中柱。病原菌主要以侵染菌丝直接侵入表皮层,表皮细胞间隙和根毛基细胞是主要侵入部位,少数由附着枝侵入。菌丝穿透细胞壁有明显的酶解作用特征,菌丝先端前方胞壁上还产生电子密物质。皮层细胞是病原菌定殖和发展的主要场所,病原菌还能离解胞间层,形成胞外空间,特别有利于菌丝和菌丝束的扩展。在侵入位点的寄主细胞壁和质膜之间,形成多种形状的木质管,其数量与侵入菌丝的数目相对应,但木质管不能阻止菌丝进入细胞。菌丝进入中柱后,可阻塞导管和筛管。小麦细胞发生退行性病变,尤以细胞壁膨大崩坏和早期质壁分离最明显,细胞间隙还产生性质不明的黄色物质。     

Sensitivity to silthiofam, tebuconazole and difenoconazole of Gaeumannomyces graminis var. tritici isolates from China

BACKGROUND: Wheat take‐all caused by Gaeumannomyces graminis var. tritici (Ggt) has become an emerging threat to wheat production in the last few years. Silthiofam is very effective against Ggt, and recently it has been widely used for the control of take‐all in China. However, farmers have noted a decline in control efficacy with this compound in some wheat fields, suggesting that the pathogen may have developed resistance to silthiofam. RESULTS: Of the 66 Ggt isolates collected from different locations in China, 27 were resistant to silthiofam. There was no cross‐resistance between silthiofam and tecuconazole or difenoconazole. The effectiveness of silthiofam in controlling take‐all was compromised on wheat inoculated with silthiofam‐resistant isolates. Based on the DNA fingerprinting generated by microsatellite PCR, two predominant genetic clusters were found among these isolates and were clearly associated with the sensitivity to silthiofam. CONCLUSION: Silthiofam has a high risk in the development of resistance in Ggt. Tebuconazole and difenoconazole show great potential for control of take‐all on wheat. Results from this study provide useful information for take‐all control and the management of fungicide resistance. Copyright © 2012 Society of Chemical Industry     

Evidence of a Multicopper Oxidase in Mn Oxidation by Gaeumannomyces graminis var. tritici

ABSTRACT Manganese (Mn) oxidation by the plant-pathogenic fungus Gaeumannomyces graminis var. tritici has been correlated with virulence in take-all disease. The mechanism of Mn oxidation has not, however, been investigated adequately. Research on bacteria and other fungi indicates that Mn oxidation is most often the result of the activity of multicopper oxidases. To determine if G. graminis var. tritici oxidizes Mn by similar means, the Mn oxidizing factor (MOF) produced by G. graminis var. tritici was characterized by cultural, spectrophotometric, and cellulose acetate electrophoresis methods. Based on our results, the MOF is an extracellular enzyme with an estimated molecular weight of 50 to 100 kDa. Electrophoresis and spectrophotometry indicate that the MOF is a multicopper oxidase with laccase activity.     

荧光假单胞杆菌P2-5菌株对小麦全蚀病的抑制作用

室内测定了荧光假单胞杆菌P_2-5菌株对小麦全蚀病菌9826菌株的拮抗作用。结果表明,P_2-5菌株对小麦全蚀病菌具有较强抑制作用,抑制效果和防治效果分别为67％和59％,高于化学杀菌剂烯唑醇(特普唑)处理的效果。研究表明P_2-5菌株可以致使病菌菌丝畸变。应用双抗标记法测定了P_2-5菌株在小麦根系中的定殖情况,结果表明,该菌可以在小麦根系定殖。     

Evidence that wheat cultivars differ in their ability to build up inoculum of the take‐all fungus,Gaeumannomyces graminis var. tritici,under a first wheat crop

The effect of wheat cultivar on the build‐up of take‐all inoculum during a first wheat crop was measured after harvest using a soil core bioassay in field experiments over five growing seasons (2003–2008). Cultivar differences in individual years were explored by analysis of variance and a cross‐season Residual Maximum Likelihood (REML) variance components analysis was used to compare differences in those cultivars present in all years. Differences between cultivars in the build‐up of inoculum were close to or at significance in two of the five trial years (2004 P < 0·05; 2006 P < 0·07), and current commercially listed cultivars were represented at both extremes of the range. In 2007 and 2008, when environmental conditions were most favourable for inoculum build‐up, differences were not significant (P < 0·3). In 2005 the presence of Phialophora spp. at the trial site restricted the build‐up of take‐all inoculum under all cultivars. The cross season REML variance components analysis detected significant differences (range: 3·4–47·8% roots infected in the soil core bioassay; P < 0·01) between the nine cultivars present in all years (excluding 2005). This is the first evidence of relatively consistent differences between hexaploid wheat cultivars in their interactions with the take‐all fungus, and this could give an indication of those cultivars that could be grown as a first wheat crop, in order to reduce the risk of damaging take‐all in a second wheat crop. This phenomenon has been named the take‐all inoculum build‐up (TAB) trait.     

The relative resistance of wheat, rye and triticale to take-all caused by Gaeumannomyces graminis

In seven field experiments conducted over 6 years with a wide range of disease severities, triticale was intermediate in resistance to Gaeumannomyces graminis between wheat (susceptible) and rye (resistant). Use of triticale is suggested as an immediately available means of introducing take-all resistance into cereal cultivation.
Octoploid triticale was slightly more susceptible than hexaploid triticale. There was little evidence of consistent variation in resistance among wheat or rye cultivars but a few hexaploid triticale cultivars varied in resistance. The resistance of triticale was not reliably expressed in the glasshouse tests used, so selection for resistance to take-all in a breeding programme would need to be conducted in the field. Individual pairs of rye chromosomes added to wheat did not significantly reduce its susceptibility. The feasibility of transferring the resistance of rye to wheat is considered.     

小麦全蚀病菌重组酶聚合酶检测方法的建立及应用

由禾顶囊壳小麦变种Gaeumannomyces graminis var.tritici引起的小麦全蚀病是危害极大的小麦土传真菌病害。本研究以小麦全蚀病菌β-tubulin为靶标基因设计重组酶聚合酶扩增技术(recombinase polymerase amplification,RPA)引物Gg-RPA-F/Gg-RPA-R和RPA探针Gg-LF-Probe,结合侧流层析试纸条,建立了小麦全蚀病菌RPA快速检测方法,该方法在38℃恒温条件下30 min内完成可视化检测,摆脱PCR仪等仪器设备的限制。小麦全蚀病菌RPA快速检测方法具有较高的检测特异性,其检测极限为10 pg/μL,与普通PCR一致。此外,小麦全蚀病菌RPA快速检测方法可从土壤中快速检测到小麦全蚀病菌,具有较强的实用性。因此,本研究建立的小麦全蚀病菌RPA快速检测方法具备简便高效、实用性强的特点,为小麦全蚀病菌的快速检测和病害早期诊断提供新技术。     

小麦全蚀病生防细菌的筛选和鉴定

为获得对小麦全蚀病菌有良好拮抗效果的生防菌株,分别从河南省商丘市及驻马店市小麦全蚀病发生田块中采集小麦根际土样,采用稀释平板涂布法共分离到1051株细菌,通过与全蚀病菌G1037菌株进行平板对峙筛选,最终获得9株具有明显拮抗效果且生长状况良好的菌株。16S rDNA序列比对及生理生化性状分析结果表明：菌株P155、P154、P16及P147为荧光假单胞菌Pseudomonas fluorescens,菌株P188和P97为恶臭假单胞菌Pseudomonas putida,菌株LY3为产酶溶杆菌Lysobacter enzymogenes,菌株S38为嗜根寡养单胞菌Stenotrophomonas rhizophila,菌株B20为洋葱伯克霍尔德Burkholderia cepacia。产抗生素相关基因的检测结果发现,菌株P147含吩嗪和硝吡咯菌素合成基因,菌株B20含硝吡咯菌素合成基因。除B20、LY3、S38和P147外,其余菌株均可产生嗜铁素。9株细菌都产蛋白酶。除P97、B20和S38外,其余菌株均可产脂肽类物质。盆栽试验结果表明,9株生防菌对小麦全蚀病都具有较好的防治效果,菌株P155和P154的防治效果最好,相对防效分别为67.11%和63.82%,略高于3%的苯醚甲环唑种衣剂的防治效果。研究结果表明这些细菌具有作为小麦全蚀病生防菌的潜力。     

Virulence of Gaeumannomyces graminis var. tritici from fields under short-term and long-term wheat cultivation in the Pacific Northwest, U.S.A.

Gaeumannomyces graminis var. tritici was recovered from 63% of 731 winter wheat plants collected randomly from six sites where wheat had been grown in monoculture for the previous 7–22 years. Typical take-all was not evident at the time the plants were collected. The fungus was isolated by a baiting method without regard to the presence of take-all on the plants. Isolates from fields under short-term wheat cultivation (3 years or less after a break crop) were obtained by plating directly from infected roots of plants with typical take-all. Virulent isolates comprised 89 and 99% of those collected from long- and short-term wheat cultivation respectively. There was also only a slight difference in the proportions of virulent isolates among monoascosporic subcultures from the two groups of isolates. There was thus little evidence that, during prolonged wheat cultivation, declining virulence in the population of G. graminis var. tritici could account for the absence of take-all.