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     

A Compromised Mlo Pathway Affects the Response of Barley to the Necrotrophic Fungus Bipolaris sorokiniana (Teleomorph: Cochliobolus sativus) and Its Toxins

ABSTRACT In search of new durable disease resistance traits in barley to control leaf spot blotch disease caused by the necrotrophic fungus Bipolaris sorokiniana (teleomorph: Cochliobolus sativus), we developed macroscopic and microscopic scales to judge spot blotch disease development on barley. Infection of barley was associated with cell wall penetration and accumulation of hydrogen peroxide. The latter appeared to take place in cell wall swellings under fungal penetration attempts as well as during cell death provoked by the necrotrophic pathogen. Additionally, we tested the influence of a compromised Mlo pathway that confers broad resistance against powdery mildew fungus (Blumeria graminis f. sp. hordei). Powdery mildew-resistant genotypes with mutations at the Mlo locus (mlo genotypes) showed a higher sensitivity to infiltration of toxic culture filtrate of Bipolaris sorokiniana as compared with wild-type barley. Mutants defective in Ror, a gene required for mlo-specified powdery mildew resistance, were also more sensitive to Bipolaris sorokiniana toxins than wild-type barley but showed less symptoms than mlo5 parents. Fungal culture filtrates induced an H2O2 burst in all mutants, whereas wild-type (Mlo) barley was less sensitive. The results support the hypothesis that the barley Mlo gene product functions as a suppresser of cell death. Therefore, a compromised Mlo pathway is effective for control of biotrophic powdery mildew fungus but not for necrotrophic Bipolaris sorokiniana. We discuss the problem of finding resistance traits that are effective against both biotrophic and necrotrophic pathogens with emphasis on the role of the anti-oxidative system of plant cells.     

Host range expansion in a powdery mildew fungus (<Emphasis Type="Italic">Golovinomyces</Emphasis> sp.) infecting <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>: <Emphasis Type="Italic">Torenia fournieri</Emphasis> as a new host

Since 2003, Torenia fournieri plants grown for experimental purposes were repeatedly infected by powdery mildew in a laboratory in Hungary. Based on morphological characteristics, the pathogen belonged to the mitosporic genus Oidium subgen. Reticuloidium, the anamorph stage of Golovinomyces. The rDNA ITS sequence was identical to that of two other powdery mildew fungi, infecting Arabidopsis and Veronica, respectively, in different parts of the world. According to a previous phylogenetic analysis of ITS and 28S rDNA sequences, those two powdery mildews belong to a recently evolved group of Golovinomyces characterized by multiple host range expansions during their evolution. Both the ITS sequence and the morphological data indicate that the powdery mildew anamorph infecting Torenia also belongs to this group. It is likely that the powdery mildew infections of the experimental T. fournieri plants, native to south-east Asia, were the result of a very recent host range expansion of a polyphagous Golovinomyces because (i) T. fournieri is absent from our region, except as an experimental plant grown in the laboratory, (ii) the powdery mildew fungus infecting this exotic plant belongs to a group of Golovinomyces where host range expansion is a frequent evolutionary scenario, (iii) cross-inoculation tests showed that this pathogen is also able to infect other plant species, notably A. thaliana and tobacco, and (iv) no Golovinomyces species are known to infect T. fournieri anywhere in the world. Although host range expansion has often been proposed as a common evolutionary process in the Erysiphales, and also in other biotrophic plant pathogens, this has not been clearly demonstrated in any case studies so far. To our knowledge, this is the first convincing case of a host range expansion event in the Erysiphales.     

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.     

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.     

Some studies on the systemic activity of the thiophanate fungicides in plants

Three closely related fungicides, thiophanate (NF 35), thiophanate methyl (NF 44) and NF 48 have been found to be active against a wide range of diseases including the powdery mildews of apple, cucumber and barley; apple and pear scab and grey mould of soft fruit. This paper deals with the preliminary experiments to compare their relative mobilities and biological persistence in crop plants, using the powdery mildews as indicator infections. NF 44 and NF 48 in particular showed a high degree of systemic activity by root uptake and persistence of action against barley and cucumber mildew but very little systemic activity against apple mildew on potted rootstocks. They have also displayed trans-laminar and localised movement towards the leaf apex in sprayed cucumber and barley leaves but movement was not so marked in sprayed apple leaves. NF 48 was also active as a seed dressing against loose smut of barley.     

Complex interplay of future climate levels of CO2, ozone and temperature on susceptibility to fungal diseases in barley

Barley (Hordeum vulgare) was grown in different climatic environments with elevated [CO₂] (700 vs 385 ppm), [O₃] (60/90 vs 20 ppb) and temperature (24/19 vs 19/12°C day/night) as single factors and in combinations, to evaluate the impact of these climatic factors on photosynthesis and susceptibility to powdery mildew and spot blotch disease. No significant increase in net CO₂ assimilation rate was observed in barley grown under elevated [CO₂] at ambient temperature. However, this rate was positively stimulated under elevated temperature together with a slightly higher potential quantum efficiency of PSII, both at ambient and elevated [CO₂], suggesting that photosynthesis was not limited by [CO₂] at ambient temperature. When growing under elevated temperature or [O₃], infection by the biotrophic powdery mildew fungus decreased, whereas disease symptoms and growth of the toxin‐secreting hemibiotrophic spot blotch fungus increased compared to ambient conditions, implying that climate‐induced changes in disease severity could be linked to the trophic lifestyle of the pathogens. Elevated [CO₂] decreased powdery mildew infection but had no effect on spot blotch disease compared to ambient condition. However, the effect of elevated [CO₂], [O₃] and temperature did not act in an additive manner when combined. This led to a surprising disease development in the combination treatments, where powdery mildew infection increased despite the individual reducing effect of the climatic factors, and spot blotch disease decreased despite the individual promoting effect of temperature and ozone, emphasizing the importance of conducting multifactorial experiments when evaluating the potential effects of climate change.     

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.     

Calcium ion promotes successful penetration of powdery mildew fungi into barley cells

To determine whether Ca²⁺ promotes powdery mildew penetration, Ca²⁺-treated barley coleoptiles were inoculated with conidia of pathogenic and nonpathogenic fungi. Penetration efficiency of the pathogenic powdery mildew Blumeria graminis was enhanced by Ca²⁺ treatment, but that of the necrotrophic pathogen Helminthosporium sp. remained unaffected. Similarly, when actin-dependent penetration resistance is suppressed with cytochalasin A, Ca²⁺ treatment specifically enhanced penetration of the nonpathogenic powdery mildew Erysiphe pisi but not that of other nonpathogens. Calmodulin inhibitors suppressed the promotive effect of Ca²⁺ on B. graminis penetration. These results suggest that barley powdery mildew specifically requires Ca²⁺ and calmodulin for penetration.     

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.     

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.     

Prior Inoculation with Non-pathogenic Fungi Induces Systemic Resistance to Powdery Mildew On Cucumber Plants

A spray inoculation of the first leaf of 2-leaf stage cucumber plants with a non-pathogenic isolate of Alternaria cucumarina or Cladosporium fulvum before a challenge inoculation with the pathogen Sphaerotheca fuliginea induced systemic resistance to powdery mildew on leaves 2–5. Systemic resistance was expressed by a significant (p < 0.05) reduction in the number of powdery mildew colonies produced on each leaf of the induced plants, as compared with water-sprayed plants. Systemic resistance was evident when a prior inoculation with each of the inducing fungi was administered 1, 3 or 6 days before the challenge inoculation with S. fuliginea. Increasing the inoculum concentration of A. cucumarina or C. fulvum enhanced the systemic protection and provided up to 71.6% or 80.0% reduction, respectively, in the number of colonies produced on upper leaves, relative to controls. Increasing the inoculum concentration of S. fuliginea used for challenge inoculation, increased the number of powdery mildew colonies produced on both induced and non-induced plants. Pre-treated plants, however, were still better protected than controls, indicating that the level of systemic protection was related to the S. fuliginea inoculum concentration. The induction of systemic resistance against powdery mildew by biotic agents, facilitates the development of a wide range of disease management tools.     

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     

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.     

Induction of Systemic Resistance in Cucumber against Several Diseases by Plant Growth-promoting Fungi: lignification and Superoxide Generation

Five fungal isolates (Trichoderma, Fusarium, Penicillium, Phoma and a sterile fungus) from zoysiagrass rhizosphere that promote plant growth were tested for their ability to induce systemic resistance in cucumber plants against Colletotrichum orbiculare. Roots of cucumber plants were treated with these fungal isolates using barley grain inocula (BGI), mycelial inocula (MI) or culture filtrate (CF). Most isolate/inoculum form combinations significantly reduced the disease except BGI of Trichoderma. These fungal isolates were also evaluated for induction of systemic resistance against bacterial angular leaf spot and Fusarium wilt by treatment with BGI. Penicillium, Phoma and the sterile fungus significantly reduced the disease incidence of bacterial angular leaf spot. Phoma and sterile fungus protected plants significantly against Fusarium wilt. Roots treated with CFs of these fungal isolates induced lignification at Colletotrichum penetration points indicating the presence of an elicitor in the CFs. The elicitor activity of CFs was evaluated by the chemiluminescence assay using tobacco callus and cucumber fruit disks. The CFs of all isolates elicited conspicuous superoxide generation. The chemiluminescence activity of the CF of Penicillium was extremely high, and its intensity was almost 100-fold higher than that of other isolates. The chemiluminescence activity was not lost following treatment with protease or autoclaving or after removal of lipid. The MW 12,000 dialyzed CF fraction was highly effective in eliciting chemiluminescence activity. Chemiluminescence emission from cucumber fruit disks treated with Penicillium was the same as that obtained from tobacco callus, except that the lipid fraction also showed a high activity. Both the MW 12,000 fraction and the lipid fraction induced lignification in the epidermal tissues of cucumber hypocotyls.     

Powdery mildew resistance in selections from moroccan barley landraces

Thirty barley landraces collected from Morocco in 1985 and 1989, and held in the Polish Gene Bank, IHAR, Radzików, Poland, were screened for resistance to powdery mildew. Fifteen tested landraces (50%) showed powdery mildew resistance reactions and 24 single plant lines were selected. Eighteen lines originating from 13 landraces were tested with 17 isolates of powdery mildew and another six lines originating from six landraces were tested with 23; the isolates were chosen according to their virulence spectra observed on the ‘Pallas’ isolines differential set. Three lines (E 1090-2-2, E 1110-3-2 and E 1077-1-1) showed resistance to all powdery mildew virulence genes prevalent in Europe. In 21 lines, unknown genes alone or in combination with specific ones were detected. Five different resistance alleles(Mlat, Mlal, Mla3, Mlg andMl(CP)) were postulated to be present in the tested lines, alone or in combination:Mlat was postulated to be present in nine (~38%) lines;Mlg andMl(CP) in two lines, andMla1 andMla3 in one tested line each. The use of newly identified sources of resistance in barley breeding as a means of controlling powdery mildew is discussed.     

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     

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.