Reaction of somaclonal variants of wheat to the take-all fungus (Gaeumannomyces graminis var. tritici)

More than 20000 wheat plants, derived from callus culture of immature wheat embryos were assessed for reaction to the take-all fungus ( Gaeumannomyces graminis var. tritici ) using a test-tube assay. Lines with less root and shoot blackening and more remaining green tissue than parental lines were identified in the SC₃ generation. However these lines had more root and shoot blackening and less remaining green tissue in subsequent generations. The changes in reaction to the fungus may therefore have been epigenetic. The potential of this tissue culture derived material for breeding take-all resistant wheats is discussed.     

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.     

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.     

Pathogenicity of Gaeumannomyces graminis isolates to wheat and rye seedlings

A total of 187 isolates of Gaeumannomyces graminis , obtained from diseased cereal roots between 1979 and 1983, mostly from the UK, were compared for their ability to produce disease on wheat and rye seedlings in the glasshouse. Isolates varied continuously in pathogenic adaptation to rye. from those infecting wheat substantially and rye slightly (designated normal or N-isolates) to those infecting both wheat and rye substantially (designated R-isolates). The degree of adaptation to rye was a consistent characteristic of individual isolates. All were morphologically similar to G. graminis var. tritici R-isolates were obtained from all the areas sampled, they did not appear to predominate on a particular host species and occurred together with N-isolates and intermediates in a single field.     

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.     

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.     

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     

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.     

新型吡唑-3-甲酸酯类化合物HNNK-1对小麦全蚀病菌的毒力及防治效果

为了研究新型吡唑-3-甲酸酯类化合物HNNK-1对小麦全蚀病的防治效果,采用菌丝生长速率法测定了其对小麦全蚀病菌的毒力;并通过拌种处理在温室和田间测定了其对小麦全蚀病的防治效果;同时观察了HNNK-1拌种对小麦生长的影响。菌丝生长速率法测定结果表明,HNNK-1对小麦全蚀病菌的EC₅₀为0.28 mg/L,低于硅噻菌胺的EC₅₀(0.58 mg/L);盆栽试验结果表明,用50 g/L HNNK-1 SC拌种(药种质量比1∶3 000)后对小麦全蚀病的防效为60.10%;对小麦株高、根长等生长指标无显著影响。田间药效试验表明,用50 g/L HNNK-1 SC拌种(药种质量比1∶3 000)对小麦全蚀病的防效为73.13%(开封)和71.15%(周口)。HNNK-1可以有效控制田间小麦全蚀病发生和危害。     

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。     

The classification of isolates of Gaeumannomyces graminis from wheat, rye and oats using restriction fragment length polymorphisms in families of repeated DNA sequences

Isolates of the take-all fungus, Gaeumannomyces graminis var. avenae , which affects oats, wheat and other grasses, and of G.g. var. tritici , which preferentially affects wheat, rye and barley, contain a high proportion of repeated sequences. Total DNA from 57 fungal isolates collected from many locations and different cereal hosts, and scored for virulence on wheat, rye and oats, revealed many restriction fragment length polymorphisms. These RFLP s were observed either by staining the DNA directly, by hybridization to radioactively labelled total fungal DNA , or by hybridization with labelled wheat ribosomal DNA . With only a few exceptions, the isolates with the same preferred cereal hosts showed more similar patterns of restriction fragments than isolates that had different pathogenicity properties on cereal hosts, irrespective of the geographical origins of the isolates. This was even the case for R isolates of G.g. var. tritici that were virulent on wheat and rye compared with N isolates that were virulent only on wheat. Isolates were identified by hybridizing DNA from infected root samples with ³²P-labelled total fungal DNA . The restriction fragment polymorphisms involving families of repeated sequence can therefore be used as a predictive assay for host preference of an isolate, and have probably arisen by host selection of fungal lineages. The variation between isolates in different pathogenicity groups suggests that there is little gene flow between isolates that can infect different hosts, even though they can coexist in the same field.     

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。     

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.     

2013年我国部分麦区小麦白粉菌群体对温度敏感性及其与毒性的关系

 研究小麦白粉病菌群体对温度的敏感性及其与毒性的关系,可为气候变化背景下小麦白粉病的长期发生趋势预测和防治提供依据。本研究对2013年采自云南、四川、河南、北京和陕西五省（市）的小麦白粉菌标样进行分离纯化,获得90个单孢子堆分离物,采用小麦离体叶段法进行温度敏感性测定,并利用32个已知抗病基因品种（系）进行毒性测定,且对两者之间的相关性进行分析。结果表明,供试菌株ET50分布于19.78℃~25.15℃,平均ET50为23.22℃,其中73.34%的供试菌株ET50值介于22℃到24℃之间,小于22℃的菌株占10%,大于24℃的菌株占16.67%。利用Popgen1.32软件对5个省（市）小麦白粉病菌群体的毒性进行多样性分析,结果表明五省市小麦白粉菌群体毒性水平上的多样性指数 H 值为0.237 0,其中四川群体毒性多样性水平最高,Nei’s基因多样性指数 H 值为0.221 2,河南群体最低, H 值为0.197 5。小麦白粉菌群体对温度敏感性与毒性多样性相关性分析结果表明,ET50与毒性多样性（ H 和 I 值）间存在显著的负相关。     

Glasshouse and field evaluation of benomyl and triadimefon applied at seeding to control take-all in wheat

The effects of two fungicides, benomyl and triadimefon, applied at seeding, on the incidence and severity of take-all ( Gaeumannomyces graminis var. tritici ) of wheat were investigated in the glasshouse and in the field. In glasshouse studies with an artificially inoculated and naturally infested sandy loam, both fungicides caused significantly less disease, whereas in a field study with a clay soil and artificial inoculum, triadimefon caused significantly less disease. In the field, both fungicides were more effective when placed near the seed as granules or as pellets than when applied as seed treatments.     

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.     

Characterization of Puccinia graminis f. sp. tritici isolates derived from an unusual wheat stem rust outbreak in Germany in 2013

An unusual stem rust infestation occurred in German wheat fields in summer 2013. This study analysed 48 isolates derived from 17 Puccinia graminis f. sp. tritici (Pgt) samples and six races were identified: TKTTF, TKKTF, TKPTF, TKKTP, PKPTF and MMMTF. Infection type and genotypic data confirmed that none of these races belonged to the TTKS (Ug99) race group. German isolates of race TKTTF are phenotypically different to the ones responsible for the stem rust epidemic in Ethiopia in 2013–2014. Forty isolates were genotyped using a custom SNP array. Phylogenetic analysis showed that these 40 isolates represented two distinct lineages (clade IV and clade V). Thirty‐eight isolates clustered into clade IV, which previously was defined by Ethiopian isolates of race TKTTF. Race TKKTP is of special concern due to its combined virulence to stem rust resistance genes Sr24, SrTmp and Sr1RS^Amigo. The vulnerability to race TKKTP in US and international winter wheat was confirmed as 55% of North American and international cultivars and breeding lines resistant to race TTKSK (Ug99) became susceptible to TKKTP. Races identified in Germany in 2013 confirmed the presence of virulence to important resistance genes that are effective against race TTKSK. This information should be useful for breeders to select diverse and effective resistance genes in order to provide more durable stem rust resistance and reduce the use of fungicides.