The Hordeum vulgare subsp. spontaneum–Blumeria graminis f. sp. hordei pathosystem: its position in resistance research and breeding applications

Barley (Hordeum vulgare L.) is an important cereal crop and powdery mildew, caused by Blumeria graminis f. sp. hordei, is one of the most serious diseases that occurs on barley throughout the world. In the Middle East, which is the centre of diversity for barley and its pathogens, the wild barley–powdery mildew pathosystem co-evolves resulting in many specific resistances in the host as well as corresponding virulences in the pathogen. Many specific resistances have been used in European breeding programmes and a centre of pathogen diversity has arisen also especially in central Europe. This short review briefly summarizes the use of host resistances derived from wild barley and land races including the durable resistance gene mlo. The use of powdery mildew pathogenicity for studying new and unknown specific resistances and for identifying resistances in commercial varieties is described. However, highly heterogeneous wild barley is also characterized as a valuable source of minor genes for powdery mildew resistance. These might be exploited by barley breeders especially for winter barley improvement where the non-specific resistance gene mlo cannot be used.     

Temporary partial breakdown of mlo-resistance in spring barley by sudden relief of soil water-stress under field conditions: the effects of genetic background and mlo allele

This study, carried out under field conditions, assessed the extent to which temporary breakdown of mlo- resistance, following relief of water-stress, was determined by genetic background and mlo allele. Commercial barley cultivars expressing the mlo gene for resistance to powdery mildew ( Erysiphe graminis ( Blumeria graminis ) f.sp. hordei ) were tested as well as doubled haploid progeny from spring barley genotypes, a proportion of which were sown in the field in two successive years. Plants were protected from natural rain by a mobile rain shelter and either watered by trickle-irrigation or allowed to dry. Percentage mildew infection resulting from natural inoculum was recorded and the doubled haploid genotypes were classified as resistant, intermediate or susceptible on the basis of their control (watered) treatment scores. In each of the three designated classes, particular genotypes developed infection levels, following relief of water-stress, that were higher than those observed on the well-watered controls. This was found not to be related to the mlo allele as there was no significant difference between the increases observed on resistant plants carrying mlo9 and resistant plants carrying mlo11 . Differences in the degree of breakdown of resistance were attributed to genetic background rather than to the specific mlo allele.     

Initiation of appressorial germ tube differentiation and appressorial hooking: distinct morphological events regulated by cAMP signalling in Blumeria graminis f.sp. hordei

Appressorial differentiation by conidia of the barley powdery mildew fungus, Blumeria graminis f.sp. hordei, is dependent on perception of multiple leaf-derived signals. Recently, we have demonstrated that cAMP signalling and PKA play an important, but complex, role during early B. graminis conidial development. Here, we demonstrate that a rise in cAMP levels correlates with conidial differentiation on the barley leaf surface. No change in cAMP levels is observed when conidia fail to differentiate on a non-inductive surface. Moreover, the cAMP levels appear to both increase and decrease during conidial development on barley leaves, suggesting that appressorial differentiation requires differential activation of the cAMP-signalling pathway. In addition, we have dissected the time periods over which cAMP, the cAMP analogue 8-Br-cAMP, and the PKA inhibitor H89, are able to affect conidial differentiation. This reveals that H89 is only active prior to 4 h post-inoculation, corresponding to the initiation of appressorial germ tube development, whereas only cAMP and 8-Br-cAMP inhibit the later process of appressorial hooking, at 4–8 h post-inoculation. Thus, we provide data that supports a model in which cAMP signalling is required to be active to trigger initiation of appressorial germ tube development and inactive to allow subsequent appressorial germ tube hooking.     

What types of powdery mildew can infect wheat-barley introgression lines?

This work is a detailed study of the infection of fungal biotrophic pathogens causing powdery mildew diseases on introgression lines originating from the intergeneric hybridisation between wheat and barley (Triticum aestivum L. × Hordeum vulgare L.). Powdery mildew fungi are among the most widespread biotrophic pathogens of plants also and infect dicot and monocot species. Most powdery mildew species are strictly host specific. They colonize only a narrow range of species or one particular host species. The intergeneric hybridisation between wheat and barley could result in expansions of host ranges of the barley powdery mildew. Our experiments covered natural infections in the field and artificial infections under greenhouse conditions. Formae speciales of powdery mildew were identified on the basis of the sequencing results of ribosomal internal transcribed spacer sequences (rDNA-ITS). We identified Blumeria graminis f.sp. tritici isolate 14 (HM484334) on the wheat parent and all wheat-barley introgression lines and B. g. f. sp. hordei isolate MUMH1723 (AB 273556) on the barley parent, respectively. The wheat-barley introgression lines were inoculated with barley powdery mildew under greenhouse conditions. According to our results the added barley chromosomes (or segments) do not cause host range expansion of barley powdery mildew.     

Effect of Film-forming Polymers on Infection of Barley with the Powdery Mildew Fungus, Blumeria graminis f. sp. hordei

The effects of three film-forming compounds, Ethokem, Bond and Vapor Gard, on infection of barley by the powdery mildew fungus Blumeria graminis f. sp. hordei were examined in glasshouse and field experiments. The three compounds provided significant control of powdery mildew infection when applied as pre- or post-inoculation treatments in the glasshouse. Such treatment had no effect on plant growth. Bond and Vapor Gard reduced the germination of conidia of B. graminis by 78% and 85% respectively, and reduced the subsequent formation of appressoria (73% and 85% respectively) and haustoria (75% and 79% respectively). The three compounds were less effective in field experiments, although they provided significant control of mildew infection and had no impact on plant growth and grain yield.     

Paecilomyces farinosus destroys powdery mildew colonies in detached leaf cultures but not on whole plants

Since 2001, several isolates of Blumeria graminis, the causal agent of cereal powdery mildew, maintained on detached leaves at the John Innes Centre, Norwich, UK, have spontaneously become infected with an unknown filamentous fungus whose mycelia have quickly overgrown the powdery mildew colonies and destroyed them completely. A total of five isolates of the contaminant were obtained and identified as Paecilomyces farinosus based on morphological characteristics and rDNA ITS sequence data. To determine whether these P. farinosus isolates can be considered as biocontrol agents (BCAs) of powdery mildews, we studied the interactions between P. farinosus and the following four powdery mildew species: B. graminis f.sp. hordei infecting barley, Oidium neolycopersici infecting tomato, Golovinomyces orontii infecting tobacco and Podosphaera fusca infecting cucumber. The powdery mildew colonies of all these four powdery mildew species were quickly destroyed by P. farinosus in leaf cultures but neither conidial suspensions nor cell-free culture filtrates of P. farinosus isolates could suppress the spread of powdery mildew infections on diseased barley, tomato, tobacco or cucumber plants in the greenhouse. It is concluded that P. farinosus cannot be considered as a promising BCA of powdery mildew infections although it can destroy powdery mildew colonies in detached leaf cultures and can be a menace during the maintenance of such cultures of cereal, apple, cucurbit and tomato powdery mildew isolates.     

Nitrogen-induced changes in colony density and spore production of Erysiphe graminis f.sp. hordei on seedlings of six spring barley cultivars

The influence of increasing nitrogen supply (30, 60, 120 and 240 mg N per pot) on susceptibility was studied on seedlings of six cultivars of spring barley inoculated with virulent isolates of powdery mildew. The colony density (CD) measured as colonies per cm² was significantly increased with increasing application of nitrogen on all cultivars, and a significant interaction was found between N and cultivar. The different reactions of the cultivars could not be ascribed to lack of N uptake. In general, increasing N application enhanced the sporulation capacity of colonies (CSC) irrespective of increased CD and the cumulative production of spores per cm² leaf (CSCM) increased strongly with N application in all cultivars. No interaction between N and cultivar was found for the latter component. The increase in CSCM closely corresponded with the increase in N content and fresh weight of uninoculated leaves. No interaction between N treatment and powdery mildew isolates was found for infection efficiency and spore production per colony, when tested on one cultivar. The N-induced changes in infection and sporulation can explain the main part of the increasing effect of N fertilization on powdery mildew development in the field. The results indicate that it may be possible to breed for or select barley cultivars with low N impact on powdery mildew development.     

Comparison of resistance-inducing abilities of virulent and avirulent races of Erysiphe graminis f.sp. hordei and a race of Erysiphe graminis f.sp. tritici in barley

Resistance to powdery mildew was induced in barley by preinoculation with virulent and avirulent races of barley powdery mildew ( Erysiphe graminis f.sp. hordei ), and with a race of wheat powdery mildew ( E. graminis f.sp. tritici ). Four inducer densities were tested in 13 different induction periods between 1 and 24 h. Generally, the resistance induced by barley powdery mildew increased up to 10-12 h of induction and was maintained in longer induction periods. The inducing abilities of virulent and avirulent races could not be distinguished up to 10-12 h of induction, after which the inducing ability of avirulent races increased significantly in relation to virulent races. Wheat powdery mildew was able to induce more resistance than barley powdery mildew in induction periods up to 8 h. In a single inoculation procedure the number of haustoria developing from virulent barley powdery mildew decreased as inoculum density increased. The effect was ascribed to induction of resistance. This reduction of infection efficiency in the compatible interaction was compared to induced resistance. However, the inoculum density needed for 50% resistance induction in the double inoculation procedure was approximately 40 times higher than the inoculum density needed for 50% reduction in infection efficiency in the single inoculation procedure.     

Hydroxypyrimidine fungicides inhibit adenosine deaminase in barley powdery mildew

Adenine and adenosine are metabolized by the adenine salvage pathway during primary infection of barley powdery mildew, Erysiphe graminis f.sp. hordei. Operation of this pathway was affected by the hydroxypyrimidine fungicide, ethirimol. Adenosine deaminase, ADAase, which was detected in mildew conidia and infected plants, but not in healthy barley, was the only enzyme in this pathway inhibited by the fungicide in in vitro assays. This feature of the mildew enzyme was unusual, and correlates with the specificity of hydroxypyrimidines which act against powdery mildews only. Other properties of this enzyme were similar to ADAase from other sources. In structure/activity studies with dimethirimol analogs, poor fungicidal activity was often associated with failure to inhibit ADAase, especially when assayed during appressoria formation. Purine derivatives were much less specific, and their mode of action against powdery mildew is probably different. Ethirimol resistance was not related to changes in ADAase, nor was the fungicide altered to an inactive metabolite. It is concluded that ADAase is one site of hydroxypyrimidine action.     

Evidence that ethirimol may interfere with adenine metabolism during primary infection of barley powdery mildew

Ethirimol, a hydroxypyrimidine fungicide active against powdery mildews only, inhibited the formation of appressoria during primary infection of barley powdery mildew, Erysiphe graminis f.sp hordei. It also affected other stages of mildew development. Several adenine analogs had similar effects and ethirimol-resistant mildew strains were generally cross-resistant to these. Adenine and adenosine reduced the fungitoxicity of ethirimol. During the formation of appressoria [³H]adenine was incorporated into RNA but [¹⁴C]glycine was not, suggesting that purine biosynthesis did not occur. Ethirimol inhibited this RNA synthesis and it is concluded that the fungicide may interfere with adenine metabolism at some site subsequent to its synthesis.     

Effects of Diffuse Colonization of Grape Berries by Uncinula necator on Bunch Rots, Berry Microflora, and Juice and Wine Quality

ABSTRACT Production of grape (principally cultivars of Vitis vinifera) for high-quality wines requires a high level of suppression of powdery mildew (Uncinula necator syn. Erysiphe necator). Severe infection of either fruit or foliage has well-documented and deleterious effects upon crop and wine quality. We found that berries nearly immune to infection by U. necator due to the development of ontogenic resistance may still support diffuse and inconspicuous mildew colonies when inoculated approximately 3 weeks post-bloom. Fruit with diffuse mildew colonies appear to be healthy and free of powdery mildew in late-season vineyard assessments with the naked eye. Nonetheless, presence of these colonies on berries was associated with (i) elevated populations of spoilage microorganisms; (ii) increased evolution of volatile ethyl acetate, acetic acid, and ethanol; (iii) increased infestation by insects known to be attracted to the aforementioned volatiles; (iv) increased rotting by Botrytis cinerea; and (v) increased frequency of perceived defects in wines prepared from fruit supporting diffuse powdery mildew colonies. Prevention of diffuse infection requires extending fungicidal protection until fruit are fully resistant to infection. Despite a perceived lack of improvement in disease control due to the insidious nature of diffuse powdery mildew, potential deleterious effects upon crop and wine quality thereby would be avoided.     

The role of jasmonic acid signalling in wheat (Triticum aestivum L.) powdery mildew resistance reaction

Jasmonic acid (JA) signalling plays an important role in plant resistance to pathogens. Previously, JA has been found to play a role in induced disease resistance to necrotrophic pathogens in various plant species, but current researches showed that JA also enhanced resistance to biotrophic pathogens. However, its role in wheat (Triticum aestivum L.) powdery mildew (Blumeria graminis f. sp. tritici, Bgt) resistance reaction is largely unknown. To settle this issue, several typical powdery mildew resistant and susceptible wheat varieties were employed. The sensitivity to exogenous methyl jasmonate (MeJA) to wheat powdery mildew resistance, the concentration fluctuation of endogenous JAs after Bgt inoculation, and the expression profiles of nine pathogenesis-related protein genes (PR genes) after MeJA and Bgt treatments were studied systematically. Exogenous MeJA significantly enhanced the powdery mildew resistance of the susceptible varieties. After inoculation with Bgt, endogenous JAs accumulated rapidly, reached the maxima at 2 to 5 h post-inoculation (hpi), then decreased rapidly, and the concentration was almost the same as that of un-inoculated control at 96 hpi. The expression levels of the nine PRs were measured by real time quantitative RT-PCR (qRT-PCR) at different time points after MeJA application or Bgt inoculation respectively. The MeJA and Bgt strongly activated PR1, PR2, PR3, PR4, PR5, PR9, PR10 and Ta-JA2, but almost didn’t affect Ta-GLP2a. The induced powdery mildew resistance was positively correlated with the activated PR genes. JA plays a positive role in defence against Bgt. JA is a signalling molecule in wheat powdery mildew resistance and future manipulation of this pathway may improve powdery mildew resistance in wheat breeding.     

Interaction between powdery mildew and barley with mlo5 mildew resistance

Powdery mildew infection of barley with the mlo5 barley powdery mildew resistance gene was examined, using near-isogenic barley lines, with and without mlo5 resistance, and two near-isogenic powdery mildew isolates, HL3/5 and GE3 with high (virulent) or low (avirulent) penetration efficiency on the resistant barley line. In all isolate–host combinations (except GE3 on the resistant barley line), frequency of haustorium formation increased significantly from zero at 11 h after inoculation to a maximum by 13 h, and there was no subsequent increase up to 24 h. In the susceptible barley line, 27% of appressoria from both isolates formed haustoria. Although this was significantly higher than the frequency of haustorium formation (18%) of HL3/5 on the resistant barley line, HL3/5 was much more successful than GE3 (frequency of haustorium formation less than 1%). The fact that HL3/5 did not possess a generally higher ability to penetrate successfully to form haustoria on the susceptible barley line, indicates that HL3/5 did not overcome the mlo5 resistance by being generally more vigorous. In the resistant barley line, papillae were larger than in the susceptible line; however, both isolates were associated with papillae of the same diameter at the time of penetration. We suggest that the mlo5 resistant barley line confers two different forms of resistance: isolate-specific and isolate-nonspecific.     

Cytological events in tomato leaves inoculated with conidia of <Emphasis Type="Italic">Blumeria graminis</Emphasis> f. sp. <Emphasis Type="Italic">hordei</Emphasis> and <Emphasis Type="Italic">Oidium neolycopersici</Emphasis> KTP-01

Leaves of tomato and barley were inoculated with conidia of Blumeria graminis f. sp. hordei race 1 (R1) or Oidium neolycopersici (KTP-01) to observe cytological responses in search of resistance to powdery mildew. Both conidia formed appressoria at similar rates on tomato or barley leaves, indicating that no resistance was expressed during the prepenetration stage of these fungi. On R1-inoculated tomato leaves, appressoria penetrated the papillae, but subsequent haustorium formation was inhibited by hypersensitive necrosis in the invaded epidermal cells. On the other hand, KTP-01 (pathogenic to tomato leaves) successfully developed functional haustoria in epidermal cells to elongate secondary hyphae, although the hyphal elongation from some conidia was later suppressed by delayed hypersensitive necrosis in some haustorium-harboring epidermal cells. Thus, the present study indicated that the resistance of tomato to powdery mildew fungi was associated with a hypersensitive response in invaded epidermal cells but not the prevention of fungal penetration through host papilla.     

Cytology of infection of apple leaves by Diplocarpon mali

Diplocarpon mali, the causal agent of Marssonina leaf blotch of apple, causes severe defoliation during the growing season. Little information is available on the mode of infection and the infection process. In this study, the infection strategies of D. mali in apple leaves were investigated using fluorescence and electron microscopy. Conidia attached to leaf surface apparently by mucilage and germinated on both sides of leaves 6 h post-inoculation (hpi). The pathogen penetrated the cuticle by infection pegs formed either in germ tubes or appressoria in 6 hpi, and then formed haustoria in host epidermal and mesophyll cells accompanied by extension of subcuticular and intercellular hyphae. Five days post-inoculation (dpi), the intracellular hyphae were observed. At the same time, the subcuticular hyphal strands (SHS) were produced as a means for fast expansion and reproduction. About 7 dpi, acervuli formed on inoculated leaves. This was the first observation that D. mali formed haustoria and SHS as infection strategies. Our results suggest that D. mali may behave like a hemibiotroph, which can use both biotrophic and necrotrophic strategies to establish infections on apple leaves.     

A transient expression system to assay putative antifungal genes on powdery mildew infected barley leaves

A transient expression system has been established to assay the regulation and impact of putative antifungal proteins at the single cell level in barley (Hordeum vulgareL.) epidermal leaf cells infected with powdery mildew (Erysiphe graminisf.sp.hordei).The system combines biolistic transformation with direct comparison of the fungal attack in transfected and wild type cells. Many epidermal cells showed detectable expression of marker genes including a novel improved version of green fluorescent protein (GFP). The marker was detectable for at least 10 days during which individual host cells supported an unaffected asexual life-cycle of the pathogen.     

大麦白粉病菌遗传学研究进展

大麦白粉病是由布氏白粉菌属大麦专化型活体寄生菌Blumeria graminis f.sp.hordei（Bgh）引起的真菌病害,在全球大麦种植区普遍发生,危害日趋严重。大麦白粉病菌与寄主之间存在着“基因对基因”的关系,分化为不同的生理小种或致病型。由于病原菌基因突变、重组和流动以及寄主的选择作用,大麦Bgh种群毒性、致病型频率和分布不断发生变化。随着分子生物学技术飞速发展,应用分子标记已对30多个Bgh无毒基因位点进行了连锁作图分析,已克隆了Bgh无毒基因AVRk1和AVRa10,Bgh全基因组测序现已完成。文章综述了大麦白粉病菌的侵染循环、遗传分化及其无毒基因的定位、克隆和致病机制研究进展,并探讨了基于病原菌毒性进化和基因组解码信息获得持久控制大麦白粉病的有效手段。     

苦瓜白粉病病原菌和生理小种的鉴定及苦瓜对白粉病的抗性遗传分析

为明确海南省苦瓜白粉病的病原菌、生理小种及苦瓜对白粉病的抗性遗传规律,结合形态学鉴定和分子鉴定解析白粉病菌及生理小种种类,通过显微镜观察白粉病菌侵染过程,并应用主基因+多基因混合遗传模型分析法探讨苦瓜对白粉病的主要抗性遗传规律。结果表明:采集自海南省6个市(县)的苦瓜白粉病病原菌均为单囊壳白粉菌Sphaerotheca fuliginea,属生理小种2F,该菌在侵染苦瓜叶片时有4个关键时期:接种后4 h为分生孢子萌发高峰期,8 h为附着孢形成高峰期,16～24 h为次生菌丝形成高峰期,5 d为分生孢子梗形成高峰期。将其接种于苦瓜抗、感品系,对白粉病的抗性符合2对加性-显性-上位性主基因+加性-显性多基因模型,主基因和多基因共同控制苦瓜对白粉病的抗性,其中以主基因遗传为主,且会受到环境变异的影响。根据苦瓜抗性遗传规律,F2代主基因遗传率最高,受环境影响最小,在苦瓜的白粉病抗性育种中,以早期世代F2代作为有效选择世代。研究表明白粉病菌侵染叶片的前2 d是白粉病防治的最佳时期,所以在白粉病易发的物候期,可将防治时间提前1～2 d。     

大麦白粉菌种群毒性监测及抗性材料鉴定

2005和2006年从我国冬大麦区采集和分离大麦白粉菌单胞菌株729个,利用Pallas近等基因系进行致病型鉴定和群体毒性频率分析.同时,利用分离得到的不同致病型菌株,通过抗谱分析的方法鉴定了328份大麦品种(系)的白粉病抗性和抗病基因.结果显示:大麦白粉菌群体对抗病基因Mlal Mla(A12)、Mla3、Mla6 Mla14、Mla7 Mla(No3)、Mla7 Ml(Lg2)、Mla9 Mlk、Mla9、Mlal3 MlaRu3、Mlpl、Mlg(Cp)和mlo5的毒性频率为0;对Mla12 MlaEm2、Mla7 Mlk、Mlat Mla8、Mla10MlaDu2和Mlk1的毒性频率很低,分别为0.1%、0.4%、0.9%、2.8%和4.2%.两年共鉴定出不同的致病型21个,致病型000、001和003在两个年度皆为优势致病型.所鉴定的328份材料绝大多数感病,仅37份抗病材料,能明确推导出抗白粉病基因的品种(品系)很少,这些品种(品系)含有的抗白粉病基因为Mr(Bw)Mla8、Mlg、Mira Mla8、Mla9 Mla1、Mla Mla(A12)和mlo5.     

Mining wild barley for powdery mildew resistance

Powdery mildew, caused by Blumeria graminis f. sp. hordei (Bgh), is a worldwide disease problem on barley (Hordeum vulgare) with potentially severe impact on yield. Historically, resistance genes have been identified chiefly from cultivated lines and landraces; however, wild barley (H. vulgare subsp. spontaneum) accessions have proven to be extraordinarily rich sources of powdery mildew resistance. This study describes the characterization of a collection of 316 wild barley accessions, known as the Wild Barley Diversity Collection (WBDC), for resistance to powdery mildew and the genetic location of powdery mildew resistance loci. The WBDC was phenotyped for reaction to 40 different Bgh isolates at the seedling stage and genotyped with 10 508 molecular markers. Accessions resistant to all 40 isolates of Bgh were not found; however, three accessions (WBDC 053, 085 and 089) exhibited resistance to 38 of the isolates. Gene postulation analyses revealed that many accessions, while resistant, contained none of the 12 genes present in the Pallas near‐isogenic lines Mla1, Mla3, Mla6, Mla7, Mla9, Mla12, Mla13, Mlk1, MlLa, Mlg, Mlat and Ml(Ru2), suggesting that the accessions carry novel genes or gene combinations. A genome‐wide association study of powdery mildew resistance in the WBDC identified 21 significant marker‐trait associations that resolved into 15 quantitative trait loci. Seven of these loci have not been previously associated with powdery mildew resistance. Taken together, these results demonstrate that the WBDC is a rich source of powdery mildew resistance, and provide genetic tools for incorporating the resistance into barley breeding programmes.