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.     

Defining the resistance risk of the new powdery mildew fungicide quinoxyfen

The new powdery mildew fungicide quinoxyfen belongs to the novel quinoline class of chemistry. Although its biochemical mode of action is unknown, quinoxyfen does not act in the same way as other cereal fungicides. It is a systemic protectant which inhibits the early stages of mildew infection on a wide range of crops, and provides season-long protection from a single early-season spray applied around GS 31. The base-line sensitivity profile of quinoxyfen was defined for barley powdery mildew (Erysiphe graminis f.sp. hordei) from over 340 field isolates collected from different parts of the UK from 1991 onwards. Sensitivities ranged from <0·0001→0·16 mg litre^-1 with a mean of 0·003 mg litre^-1. Current work is extending the base-line sensitivity studies to wheat powdery mildew (E. graminis f.sp. tritici), and includes isolates from European trials, but so far this new data set has shown no differences from barley powdery mildew. Quinoxyfen-resistant mutants were generated in the laboratory, and some similar resistant strains were obtained from treated field crops. These laboratory and field strains were always defective, in some way, for sporulation and, curiously, all required the presence of quinoxyfen for survival in culture. Attempts to generate resistant mutants that sporulated normally were unsuccessful. These studies suggested that the resistance risk for quinoxyfen is low. The recommended anti-resistance strategy accompanying introduction of quinoxyfen avoids seed treatments and late-season applications. Instead, a single early (GS 31) treatment using either pre-formulated mixtures or alternating with a fungicide with different mode of action is recommended. This strategy will be supported by continued monitoring of wheat and barley powdery mildew. ©1997 SCI     

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.     

Physcion,a natural anthraquinone derivative,enhances the gene expression of leaf‐specific thionin of barley against Blumeria graminis

BACKGROUND: Physcion is a key active ingredient of the ethanol extract from roots of Chinese rhubarb (Rheum officinale Baill.) that has been commercialised in China for controlling powdery mildews. The biological mechanism of action of physcion against the barley powdery mildew pathogen was studied using bioassay and microarray methods. RESULTS: Bioassay indicated that physcion did not directly affect conidial germination of Blumeria graminis Speer f. sp. hordei Marchal, but significantly inhibited conidial germination in vivo. Challenge inoculation indicated that physcion induced localised resistance rather than systemic resistance against powdery mildew. Gene expression profiling of physcion‐treated barley leaves detected four upregulated and five downregulated genes (ratio ≥ 2.0 and P‐value < 0.05) by using an Affymetrix Barley GeneChip. The five upregulated probe sequences blasted to the same barley leaf‐specific thionin gene, with significant changes varying from 4.26 to 19.91‐fold. All downregulated genes were defence‐related, linked to peroxidase, oxalate oxidase, bsi1 protein and a pathogenesis‐related protein. These changes varied from ? 2.34 to ? 2.96. Quantitative real‐time PCR data confirmed that physcion enhanced the gene expression of leaf‐specific thionin of barley. CONCLUSION: Results indicated that physcion controls powdery mildew mainly through changing the expression of defence‐related genes, and especially enhancing expression of leaf‐specific thionin in barley leaves. Copyright © 2010 Society of Chemical Industry     

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.     

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.     

Rationally Designed Guanidine and Amidine Fungicides

A previous investigation established that compounds containing a guanidinium or amidinium grouping are effective inhibitors of sterol Δ⁸–Δ⁷ isomerase and/or Δ¹⁴ reductase activity in plant pathogenic fungi. A binding model for known fungicidal inhibitors of this enzyme has now been used to rationally design further guanidinium or amidinium inhibitors. Three novel classes of chemistry were investigated. The results of biochemical testing against ergosterol biosynthesis in Ustilago maydis (DC) Corda and of in-vivo testing for fungicidal activity against Erysiphe graminis DC f. sp. hordei Marchal (powdery mildew of barley), do much to support the binding model, and compounds with significant fungicidal activity have been found. © 1997 SCI.     

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.     

In-vivo evaluation of adjuvants for more effective control of celery leaf-spot (Septoria apiicola) and powdery mildew (Erysiphe graminis) of wheat with fungicides

The effects of seven adjuvants (at 0, 0.5, 1.0 and 2.0 g litre^?1) on the efficacies of four fungicides al 0.5 g litre^?1 were studied in the laboratory for the control of leaf-spot in celery (caused by Septoria apiicola) and powdery mildew on winter wheat (caused by Erysiphe graminis). The most effective fungicides for controlling leaf-spot were: tebuconazole + triadimenol = flutriafol > mancozeb + oxadixyl > prochloraz. However, addition of adjuvant to the fungicides gave a modified pattern of effectiveness. The efficacy of flutriafol was strongly enhanced by addition of all adjuvants, but those of prochloraz and mancozeb+oxadixyl only partially so. The tested adjuvants were mineral oil + surfactant, a polymer/alkoxylated alkyl ether blend, an ethoxylated alkylphenol, an ethoxylated hexitan ester blend, an ethoxylated nonylphenol and an alkylpolysaccharide- based adjuvant mixture. However, the addition of adjuvants to tebuconazole + triadimenol had a negative effect. Of all the adjuvants tested, the nonylphenol ethoxylate and a mixture of mineral oil /surfactant and alkylpolysaccharides gave the highest efficacy with the fungicides, while the mineral oil/surfactant and the alkylpolysaccharides alone were less effective. There was a positive relationship between high concentrations of adjuvants and their effectiveness, but there were some exceptions. The most effective fungicides for control of powdery mildew in wheat were prochloraz, mancozeb + oxadixyl and tebuconazole + triadimenol. There was a linear relationship between the high efficacy of the fungicide and the concentration of adjuvants to control powdery mildew in wheat. The highest concentration of adjuvant (2-0 g litre^?1) gave the highest efficacy for the fungicides.     

Structure-activity relationships in fungitoxicants related to triforine and chloraniformethan. Part IV: Evaluation of the predictive power of the Hansch analysis by the preparation and testing of further chloraniformethan analogues

Seven chloraniformethan analogues, which were predicted, on the basis of the quantitative Hansch analysis performed previously, to be as active as the parent compound against barley powdery mildew (Erysiphe graminis), were prepared. Their activity as leaf sprays against E. graminis was much lower than that calculated using the regression equations, indicating that the Hansch analysis is not a suitable method for predicting fungitoxicity in this chloraniformethan series of compounds.     

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.     

Laboratory and greenhouse evaluation of a new systemic fungicide,N,N'-bis-(1-formamido-2,2,2-trichloroethyl)-piperazine (CELA W 524)

The new systemic fungicide N,N'-bis-(1-formamido-2,2,2-trichloroethyl)-piperazine (CELA W 524) was shown to display a moderate to distinct fungitoxic activity in vitro towards several pathogenic and non-pathogenic fungi. Depending on the inert ingredients present², the available formulations proved to be either rather phytotoxic or virtually non-phytotoxic. Pre-infectional spraying with the non-phytotoxic formulation provided complete protection of barley, bean, cucumber, pea and tomato plants against barley powdery mildew, bean rust, cucumber powdery mildew and cucumber scab, pea powdery mildew and tomato leaf mould, respectively. some suppression of disease symptoms —although only at high concentrations of CELA W 524 — was observed in the case of leaf spot in pea plants. Upon post-infectional treatment disease control was less pronounced, although powdery mildew diseases and tomato leaf mould were effectively suppressed. When applied via the roots CELA W 524 proved to be systemically active, successfully protecting barley plants against powdery mildew, and cucumber plants against powdery mildew and cucumber scab.Samenvatting Het nieuwe systemische fungicide CELA W 524 (C. H. Boehringer Sohn, Ingelheim am Rhein, Duitsland) bleek een matige tot duidelijke fungitoxische werking in vitro te vertonen tegenover verschillende pathogene en niet-pathogene schimmels. Eén van de beschikbare formuleringen bleek vrij sterk fytotoxisch, de andere was nagenoeg niet fytotoxisch. Bespuiting vóór inoculatie met de niet-fytotoxische formulering resulteerde in volledige bescherming van gerst, bonen, komkommers, erwten en tomaten tegen respectievelijk gerstemeeldauw, boneroest, komkommermeeldauw en vruchtvuur, erwtemeeldauw en bladvlekkenziekte bij tomaat. Enige onderdrukking van ziektesymptomen trad ook op bij erwten, geïnoculeerd metAscochyta pisi, tenminste, wanneer hoge concentraties van CELA W 524 werden gebruikt. Bij bespuiting na inoculatie was het effect geringer, hoewel meeldauwziekten en bladvlekkenziekte bij tomaat toch doeltreffend bestreden werden. Toegediend via de wortels bleek CELA W 524 systemisch actief; het beschermde aldus gerst tegen meeldauw en komkommers tegen meeldauw en vruchtvuur.     

Synthesis of six heterocyclic multifunctional alkylating agents and evaluation of their activity againstPyrenophora avenae andBlumeria graminis f.sp.hordei

Six heterocyclic alkylating agents were synthesized and examined for activity against the oat stripe pathogenPyrenophora avenae on agar plates and against the barley powdery mildew fungusBlumeria graminis f.sp.hordei on barley seedlings. Radial growth ofP. avenae was not significantly affected by any of the compounds, but four of them,α,α-bis[4,7-bis(2-chloroethyl)-1,4,7-triazacyclononane]-para-xylene[3], 1,4,8,11-tetra(2-chloroethyl)-1,4,8,11-tetraazacyclotetradecane[4], 8,11-bis(2-chloroethyl)-1,4,8,11-tetraaza-5,7-oxocyclotetradecane[5] and 7,16-bis(2-chloroethyl-1,4,10,13-tetraoxa-7,16-diazaoctadecane[6], gave significant reductions in biomass ofP. avenae grown in liquid culture and in powdery mildew infection on barley when used at 25μM. Polyamine biosynthesis was examined by following the incorporation of labeled ornithine into polyamines inP. avenae. The four compounds3–6 which reduced mildew infection all reduced the flux of label through to the polyamines inP. avenae. Whether the reductions in mildew infection caused by these compounds is related to reduced formation of polyamines is not known and awaits investigation. http://www.phytoparasitica.org posting May 22, 2005.     

甘肃小麦白粉病抗源材料的筛选及抗病基因库的组建

2002-2005年,对收集到的30份已知抗白粉病基因载体品种在甘肃省的不同生态区进行了抗病性监测,结果表明:Pm1、Pm3 a、Pm3 b、Pm3 c、Pm3 f、Pm4 a、Pm5、Pm6、Pm7、Pm8、Pm17在田间抗病性丧失,失去利用价值;Pm2、Pm19、Pm4+8、Pm4、Pm5+6、Pm13在田间抗病性较低,不宜单独作为亲本利用;Pm1+2+9、Pm2+6、Pm2+mld、Pm2+talent、Pm4+2 x、Pm4 b、Pm4 b+5、Pm20、Pm21、Pmxbd在田间抗性表现良好,在今后的育种工作中应充分加以利用。同时经过4年抗病性监测,从省内外2 638份小麦品种(系)材料中筛选出了抗病性强、综合农艺性状优良的92R137、贵农21等优异材料10余份,初步组建了抗白粉病基因库。文中还对抗病基因现状和利用及今后如何避免由于抗源单一化带来的白粉病流行进行了初步探讨。     

Ectopic expression of barley constitutively activated ROPs supports susceptibility to powdery mildew and bacterial wildfire in tobacco

ROPs (also called RACs) are RHO-like monomeric G-proteins of plants, well-known as molecular switches in plant signal transduction processes, which are involved in plant development and a variety of biotic and abiotic stress responses. The barley (Hordeum vulgare) ROPs RACB, RAC1 and RAC3 have been shown to be involved in cellular growth, polarity and in susceptibility to the biotrophic barley powdery mildew fungus Blumeria graminis f.sp. hordei. We produced transgenic tobacco (Nicotiana tabacum) plants expressing constitutively activated (CA) mutants of the barley ROPs RACB and RAC3 to monitor the impact of heterologous ROP expression on cell polarity and disease susceptibility of tobacco. CA HvROPs influenced leaf texture, disturbed root hair polarity and induced cell expansion in tobacco. Both barley ROPs induced super-susceptibility to the compatible powdery mildew fungus Golovinomyces cichoracearum but only CA HvRAC3 induced super-susceptibility to the bacterial leaf pathogen Pseudomonas syringae pv. tabaci. Data suggest involvements of ROPs in tobacco cell expansion, polar growth and in response to bacterial and fungal leaf pathogens.     

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.     

Wax matters: absence of very‐long‐chain aldehydes from the leaf cuticular wax of the glossy11 mutant of maize compromises the prepenetration processes of Blumeria graminis

Conidial germination and differentiation, the so‐called prepenetration processes, of the barley powdery mildew fungus (Blumeria graminis f.sp. hordei) are triggered in vitro by very‐long‐chain aldehydes, minor constituents of barley leaf wax. However, until now it has not been demonstrated that these cuticle‐derived molecules also play a significant role in the initiation and promotion of the fungal prepenetration processes in vivo, on the surface of a living plant leaf. In the maize (Zea mays) wax mutant glossy11, which is completely devoid of cuticular very‐long‐chain aldehydes, germination and appressorial differentiation of B. graminis were strongly impeded. Spraying the mutant leaf surface with aldehyde‐containing wild‐type wax or pure n‐hexacosanal (C₂₆‐aldehyde) fully restored fungal prepenetration, whereas maize wild‐type leaf surfaces coated with n‐docosanoic acid exhibited reduced conidial germination rates of 23%, and only 5% of the conidia differentiated infection structures. In vitro studies were performed to further corroborate the extensive prevention of fungal germination and differentiation in response to artificial surfaces coated with aldehyde‐deficient maize wax. Because of its phenotype affecting the B. graminis prepenetration processes, the glossy11 mutation of maize may become a valuable molecular target and genetic tool that could provide a means of developing basal powdery mildew resistance in the globally important crops wheat and barley.     

Aromatic Substances Inhibiting Wheat Powdery Mildew Produced by a Fungus Detected with a New Screening Method for Phylloplane Fungi

Microorganisms isolated from wheat leaf surfaces were screened for inhibition of wheat powdery mildew. A new screening method, in which wheat leaves were inoculated with Blumeria graminis f. sp. tritici and incubated with the cultured microorganisms under non-contact conditions, was developed in the present study. Using this method, 10 phylloplane fungi that inhibited wheat powdery mildew were selected from 408 microorganisms isolated from wheat leaf surfaces. Among these 10 strains, a fungus designated as Kyu-W63 had an especially strong inhibitory effect. Kyu-W63 produced white colonies without spores when cultivated on PDA. Kyu-W63 had a strong aromatic odor when being cultured. Wheat powdery mildew was suppressed even though a membrane filter with a pore size of 0.45 μm was placed between the mycelial colony and wheat leaf segment. However, when activated charcoal was introduced, Kyu-W63 did not inhibit growth of B. graminis. It was presumed that volatile substances were involved in the inhibitory effect of Kyu-W63. GC-MS analysis was used to identify two substances produced by Kyu-W63 with molecular weights of 164 and 166. Kyu-W63 also inhibited the in vitro growth of four plant pathogenic fungi other than B. graminis. Received 19 September 2001/ Accepted in revised form 7 February 2002     

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.     

Induced accessibility and enhanced inaccessibility at the cellular level in barley coleoptiles. XIV. Evidence for elicitor(s) and suppressor(s) of host inaccessibility from Erysiphe graminis

The release of elicitors and suppressors by Erysiphe graminis, the powdery mildew pathogen of barley, was investigated by microscopy in combination with micromanipulation. The elicitors enhance inaccessibility whereas the suppressors prevent the action of the elicitors. Conidia were deposited onto barley coleoptiles and incubated for intervals that varied from 1-8 h. These conidia were termed the inducer conidia since they determined whether their presence would induce the cells of the host to become inaccessible to subsequent inoculations with the fungus. At specified intervals after inoculation the conidial germlings were removed from cells with a micromanipulator. The coleoptiles were then incubated for an additional 8 h after which, a new germling of the fungus was transferred to the same cell from which the inducer germling had been removed. This new germling was called the challenge germling since it was used to determine if it could challenge the host cell to express either induced accessibility or enhanced inaccessibility. The ability of these challenge germlings to penetrate the barley host cell was then assessed after they had been incubated on the cell for an additional 19 h. This allowed the determination of whether inaccessibility had been enhanced in the host. Inaccessibility was enhanced in the host only when the inducer conidia were incubated on host cells for more than 7 h. Scanning electron microscopy revealed that these challenge germlings did not penetrate into the host cell. Thus, enhanced inaccessibility had occurred. The results indicate that the E. graminis germling released a material that enhanced the inaccessibility of the barley host cells. We refer to this material as an elicitor. The transfer of a challenge germling to a coleoptile was made at various times after the removal of the inducer germling from the tissue. This allowed us to determine that more than 2 h is required for the enhancement of inaccessibility after the removal of the inducer germling from the tissue. If a germling, either the same or a different germling, was left on the host cell continuously, then enhanced inaccessibility did not occur. Rather, susceptibility occurred. These results suggest that the E. graminis germling releases a material that suppresses inaccessibility. We refer to this material as a suppressor. Thus, the results indicate that E. graminis conidia release an elicitor that enhances inaccessibility of barley cells and that they also release a suppressor that prevents enhanced inaccessibility in the barley cell.