The expression of partial resistance to Erysiphe graminis in spring barley

The development of powdery mildew on five barley varieties selected for partial resistance in the field and lacking identifiable, effective race-specific resistance factors was examined. Inoculation tests demonstrated that a reduction in the number of colonies was a major component of this resistance. Microscopic examination of segments from the fourth and fifth leaf revealed several stages at which colony development was arrested. The failure of individual penetrations to establish compatible colony growth was classified as being due either to hypersensitive or to non-hypersensitive resistance in host epidermal cells. Varying proportions of these two types of resistance mechanism were evident in all varieties examined, including the susceptible controls. In four resistant varieties, one or other mechanism predominated. The possibility of enhancing partial resistance by combining sources in which different mechanisms operate is discussed.     

The inheritance of mechanisms of partial resistance to Erysiphe graminis in spring barley

The response of epidermal cells to attempted penetration by Erysiphe graminis was examined in the fifth leaf of 13 barley lines selected for partial resistance. In leaf segments fixed and stained 72 h after inoculation cell reactions were classified as susceptible or as exhibiting non-hypersensitive or hypersensitive resistance. The proportion of cells with each type of reaction varied in a continuous manner among the lines and there were significant differences between lines in the levels of non-hypersensitive and hypersensitive resistance. Crosses were made between lines exhibiting high and low levels of each resistance mechanism. Fifty challenged epidermal cells per duplicate leaf segment were examined on 45-50 F2 plants derived from two such crosses. The frequency distribution of each of the three types of cell reaction in the fifth leaf was found to be continuous and, in general, normally distributed and in no case was there a significant difference between parent and progeny mean values, suggesting that the two resistance mechanisms were under the control of several genes showing predominantly additive gene action.     

Histology of infection by Erysiphe graminis f.sp. hordei in spring barley lines with various levels of partial resistance

The histology of resistance to infection by conidia of Erysiphe graminis f.sp. hordei has been studied in barley lines with various levels of partial resistance. Resistance reduced the proportion of conidia which formed haustoria and was expressed more strongly in fifth than in first formed leaves. Where attempted penetration from the first appressorial lobe failed, a second and sometimes a third lobe could be formed. Most penetrations were from first lobes, a few from second lobes and a negligible number from third lobes. Adult plant resistance decreased penetration success from both first and second lobes. An unidentified component of resistance appeared to impede the development of less vigorous conidia before they stimulated a host cell response. However, in all hosts the majority of failed penetration attempts stimulated a localized host response implicating this response in resistance. Some attempts caused hypersensitive death of epidermal cells in both resistant and susceptible hosts; the proportion of dead cells was higher in fifth than first leaves. Stomatal subsidiary cells were more susceptible to penetration than interstomatal cells and in resistant leaves the majority of successful penetrations were into subsidiary cells. The pathogen was also able to avoid resistance to penetration by establishing endophytic infection following entry of germ tubes through stomates.     

Adult plant resistance to powdery mildew (Erysiphe graminis)in three barley cultivars

The mechanism(s) of adult plant resistance (a.p.r.) to Erysiphe graminis f. sp. hordei were examined in two spring barley cultivars, Athos and Porthos, which possess similar identified mildew resistance genes and seedling resistance, but differential field resistance. These cultivars were compared with Golden Promise, a universally susceptible cultivar. Differential a.p.r. to a compatible race was detected as a reduction in the number and sporulation of colonies on later-formed Athos leaves. Golden Promise exhibited some a.p.r., but much less than either Athos or Porthos. Differential a.p.r. was expressed at several infection stages, viz. appressorial formation, penetration and sporulation. Epidermal hypersensitivity occurred infrequently in all leaf position/cultivar interactions. Papillae were associated with >90% of all penetration failures. The incidence of papillae in successful penetrations at 96 h was 20% on later-formed leaves of Athos compared to c. 60% on all other leaves.     

Utility of defeated resistance genes to powdery mildew,Erysiphe graminis f. sp. hordei,in spring barley variety mixtures

In 1980 and 1981 experiments have been performed to study the utility of defeated resistance genes to powdery mildew in spring barley variety mixtures. For this purpose the epidemic development of powdery mildew in pure stands and mixtures of four spring barley varieties was monitored. In three varieties the resistance was overcome several years ago. One variety is still resistant. Changes in frequencies of corresponding virulence genes were also studied.It was found that the variety mixtures slowed down the mildew epidemic only at the beginning of the season. Some reduction was lost later on since the pure stands reached their saturation level of infection at an earlier date than the more healthy mixtures. The level of reduction of infection depended on the composition of the mixtures and the number of components. Two-way mixtures appeared to be very risky due to the high frequency of corresponding combinations of virulence genes in the pathogen. Furthermore, two-way mixtures appeared to be unable to reduce the infection rate sufficiently when the infection pressure was high.In 1981 the frequencies of combinations of corresponding virulence genes increased very much in the Diva-Mazurka mixture but declined in the pure stands of both varieties.On the basis of these results it is concluded that the utility of defeated resistance genes involved in this study is very limited.The low number and the irregular distribution of the Dutch spring barley varieties over the different resistance groups prohibit the use of variety mixtures in the Netherlands.Samenvatting In 1980 en 1981 is onderzoek verricht over de bruikbaarheid van doorbroken resistentiegenen tegen meeldauw,Erysiphe graminis f. sp.hordei, in zomergerstrassenmengsels. Daartoe werd de epidemieontwikkeling in monocultures en mengsels van vier zomergerstrassen bestudeerd. Van drie rassen is de resistentie reeds verscheidene jaren doorbroken. Eén ras is nog resistent. De frequenties van compatibele meeldauwvirulentiegenen werden eveneens bepaald.Het bleek dat in vergelijking met de monocultures in de rassenmengsels de meeldauwepidemie alleen in het begin van het seizoen werd afgeremd. Later in het seizoen ging weer enige reductie in aantasting van de mengsels verloren doordat in de monocultures de aantasting eerder het verzadigingspunt bereikte dan in de minder aangetaste mengsels. De mate waarin de aantasting gereduceerd werd was afhankelijk van de aard en het aantal componenten in het mengsel. Twee-componentenmengsels bleken vrij riskant te zijn vanwege de aanwezigheid van combinaties van corresponderende virulentiegenen in de meeldauwpopulatie. Verder bleken tweecomponentenmengsels niet in staat de infectiesnelheid afdoende af te remmen wanneer de infectiedruk hoog werd.De frequentie van combinaties van corresponderende virulentiegenen nam sterk toe in het Diva-Mazurka-mengsel doch nam af in de monocultures van deze beide rassen.Gezien de verkregen resultaten lijkt de bruikbaarheid van de in dit onderzoek betrokken doorbroken resistentiegenen erg beperkt. Het geringe aantal zomergerstrassen op de rassenlijst en de ongelijke verdeling over de te onderscheiden resistentiegroepen belemmeren de toepassing van rassenmengsels in de Nederlandse akkerbouw.     

Components of partial resistance to Puccinia striiformis in barley

Substantial differences in the rates of mycelial growth and sporulation of different isolates of Puccinia striiformis were observed when seedling and upper leaves of five barley cultivars with susceptible reaction types were inoculated. The highest rate of mycelial growth was usually on cv. Sultan, followed by Proctor, Astrix, Senta and Zephyr in that order, but more spores were produced on Senta and Astrix than on Proctor, Zephyr or seedlings of Sultan. Host-specific differences between isolates were slight, but some isolates were more aggressive than others, with generally higher mycelial growth and sporulation rates and shorter latent periods. The effect of temperature on sporulation varied little between cultivars or isolates, but there was some evidence of high temperature resistance in Senta and Zephyr. Latent periods were longer on flag leaves than on seedling leaves, and spore production per unit area of leaf infected was lower on the flag leaf of all cultivars except Sultan. Low sporulation rates seemed due to adult plant resistance in Astrix and Senta, and overall resistance, more strongly expressed in adult plants than in seedlings, in Proctor and Zephyr. Sultan exhibited slight seedling resistance, but lacked the adult plant resistance of other cultivars.     

Selection on Erysiphe graminis in pure and mixed stands of barley

The response of populations of Erysiphe graminis f. sp. hordei to selection by pure and mixed stands of three spring barley cultivars was studied in two field trials. The range of virulence of the pathogen genotypes selected in mixed host stands was dependent on the relative fitness of each genotype over all hosts. Unnecessary virulences were rapidly selected against on some hosts, but were less deleterious or favoured on others. In general there was selection for widely adapted pathogen genotypes in mixed host populations but this selection for flexibility limited the abilitytions in the absolute size of the pathogen population in host mixtures reduced the absolute frequencies of pathogen genotypes with combined virulences in comparison with those in pure stands. It is argued that host mixtures are therefore unlikely to favour rapid pathogen evolution towards races which are both widely adapted and highly virulent on all component cultivars which they can infect his definition docs not conform with conventional usage in population genetics.     

Selection of pathotypes of Erysiphe graminis f.sp. hordei in pure and mixed stands of spring barley

Pathotypes of Erysiphe graminis f.sp. hordei were monitored at fortnightly intervals in pure and mixed stands of spring barley during the course of mildew epidemics in two field trials. Mixtures were composed of cultivars with Arabische (gene Mla12 ), Laevigatum ( Ml(La) ), and Monte Cristo ( Mla9 ) resistance, respectively. The three-way mixtures were either random or, in 1989, laid out as one-row mixtures (i.e., regularly alternating rows of different genotypes) or three-row mixtures (i.e., regularly alternating three-row strips of different genotypes), respectively. In 1990 only random mixtures and six-row mixtures were compared with pure stands. The virulence complexity (i.e., the average number of virulence factors per isolate with reference to Mla12, Ml(La) , and Mla9 ) was always maximal in the random mixtures. In 1989, linear regression of complexity on mildew generations gave significant b -values (slopes) of 0·049, 0·031, and 0·025 in the random mixture, one-row mixture, and three-row mixture, respectively; the b -value from pure stands was not significant. In 1990, another sampling technique allowed selection to be observed on each genotype in the mixtures separately. In the random mixture b -values were 0·048, 0·064 and 0·017 (not significant) on Mla12, Ml(La) , and Mla9 cultivars respectively. In six-row mixtures and in pure stands, there was no significant increase in complexity ( b > 0) on any of the mixture components. Although the frequency and relative fitness of complex pathotypes were higher in all types of mixtures than in pure stands, selection towards complex races was much less intense in row mixtures than in random mixtures in both field trials.     

Measuring the sterol content of barley leaves infected with powdery mildew as a means of assessing partial resistance to Erysiphe graminis f.sp. hordei

Barley leaf segments of several partially resistant cultivars were inoculated with different strains and combinations of strains of Erysiphe graminis. Infection level was assessed by counting colony number, by measuring sporulating area using an image analyser, and by extracting and quantifying sterols. Good correlations were obtained between data from all three methods. One cultivar contained less sterol than expected from the number of colonies present but some of the resistance in that cultivar was known to be due to reduced colony size. Image analysis was unable to discern the reduced colony size component of resistance. Scanning electron microscopy revealed that single colonies may originate from more than one conidial infection. Sterol analysis was the most representative method of comparing the reaction of diverse host genotypes to mildew infection.     

Molecular variation in populations of Erysiphe graminis on barley, oats and rye

DNA isolated from the formae speciales of Erysiphe graminis that grow on barley, wheat, rye and oats was studied using restriction endonucleases and DNA/DNA hybridization procedures. DNA fragments were purified by molecular cloning and a few containing repeated sequences were used to demonstrate the many variations in restriction fragments both within and between the four formae speciales. In an analysis of six single-colony isolates of the barley mildew pathogen collected from different UK sites in different years, more than a quarter of the fragments scored varied among isolates. One isolate, with an uncommon pathogenicity character, differed from the remainder in the distribution of DNA bands. Isolates of rye mildew were also distinct from one another but isolates of oat mildew from a population of similar size appeared to belong to a single clone.
It is concluded that the chromosomes of E. graminis contain many families of dispersed repeated sequences and that there may be extensive polymorphism for restriction endonuclease cleavage sites associated with these repeats. Such unselected polymorphisms could be useful in helping to understand and discriminate among the factors affecting population structure in the pathogen as it responds to different agricultural practices.     

Resistance of Erysiphe graminis on barley and wheat to sterol C-14-demethylation inhibitors1

Isolates of Erysiphe graminis f. sp. hordei and tritici with decreased sensitivity to triadimefon showed cross-resistance to other inhibitors of sterol C-14-demethylation, such as triadimenol, propiconazol, diclobutrazol, prochloraz and nuarimol. The isolates exhibited a moderate degree of resistance to these compounds. No cross-resistance was detected to tridemorph, fenpropimorph and pyrazophos. The resistant hordei isolates were more sensitive to ethirimol than the sensitive isolate. The competitive abilities of resistant hordei and tritici isolates were inferior to that of the sensitive isolates. In the presence of the fungicides no differences in germination, appressorium formation and penetration between the sensitive and resistant isolates were observed; 48 h after inoculation the sensitive isolate showed several morphological alterations and further fungal development was arrested. At four to five times higher doses of triadimefon, similar morphological alterations were detected in the resistant isolate. Low concentrations of triazole fungicides which slightly affected mycelium growth of both the sensitive and the resistant isolate of f.sp. hordei severely inhibited development of conidiophores of the sensitive isolate whereas that of the resistant isolate was hardly affected.     

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.     

The spread oi Erysiphe graminis f. sp. hordei in mixtures of barley varieties

A model is proposed of mechanisms which might affect the progress of Erysiphe graminis f. sp. hordei in mixtures of barley varieties. Results obtained from two field trials indicate that the efffect of mixtures may be panitioned into three categoriesof the influence of the reduced density of the susceptible plants, the barrier effect of the resistant plants, and the induced resistance due to the non-virulent pathogen biotypes. In the early stages of plant growth the lower density of susceptible plants accounted for most of the reduction in pathogen development in mixtures. As the epidemic progressed, the barrier and induced resistance effects increased in importance and the total mixture effect was at a maximum mid-way through epidemic development. Towards the end of the trials the overall mixture effect declined though the influence of induced resistance was at its maximum. The reasons for these changes and their implications for the use of host varietal mixtures in disease control are discussed.
Mixtures also protected the crop against a pathogen other than the target organism.     

Detection of components of partial resistance to mildew (Erysiphe graminis f.sp. hordei) incorporated into advanced breeding lines of barley using measurement of fungal cell wall sterol

Characters for partial resistance to mildew ( Erysiphe graminis f.sp. hordei ) derived from primitive barley lines were tested and found to have been transmitted to F9 progeny of crosses with the susceptible cultivar Golden Promise with varying degrees of efficiency, and putative transgressive segregation was observed. Derived sterol extracts from mildew cell walls were used, in conjunction with infection frequency data, to quantify the mildew present, and this sterol was characterized as being of MW 470, probably (3.beta)-ergosta-5,24(28)dienol (C31.H54.0.SI SILANE).     

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.     

Structure-activity relationships in fungitoxicants related to triforine and chloraniformethan. I. Synthesis and fungitoxicity to Erysiphe graminis on barley

Seventy analogues of chloraniformethan and triforine were prepared and tested for activity against barley powdery mildew caused by Erysiphe graminis. Structural variations, including replacement of the trichloromethyl, formamido, 3,4-dichloroanilino or piperazin-1,4-diyl groups, yielded information concerning the structural characteristics required for activity in leaf-spray or root-drench tests, and it appears that the XCHNHCO grouping (where X=substituted nitrogen, oxygen or sulphur) is a prerequisite for activity.     

The role of the abaxial leaf surface waxes of Lolium spp. in resistance to Erysiphe graminis

Field and glasshouse observations of Lolium spp. grasses indicated that the lower, abaxial, leaf surface was rarely infected by powdery mildew ( Erysiphe graminis ) even when the upper, adaxial, surface was densely colonized. Experiments showed that conidia of two strains of E. graminis , one from Lolium and one from Avena , germinated equally well on both surfaces of Lolium and Avena leaves, but that the subsequent growth and development of germlings was impaired on the lower surface of Lolium leaves, so that most formed only multiple short germ tubes or an abnormal long tube, and only c. 25% or fewer formed infection structures. This contributes to the apparent resistance of the lower Lolium leaf surface to powdery mildew and may help to explain why the disease is relatively unimportant in UK ryegrass crops, since infection structures develop at a high frequency on only 50% of the leaf area, i.e. the upper surface. Scanning electron microscopy showed that the epicuticular waxes on the lower Lolium leaf surface form amorphous sheets. This contrasts with the crystalline plate waxes seen on the upper surface of Lolium leaves and on both surfaces of oat leaves. However, when the lower Lolium leaf surface was washed with chloroform to remove epicuticular wax, normal germling and infection structure development was obtained on the wax-free surface. This suggests that the sheet waxes prevent the pathogen gaining access to features of the cuticular membrane which trigger normal germling development.     

Levels of resistance of Erysiphe graminis f.sp. hordei to the systemic fungicide triadimenol

A random sample of conidiospores ofErysiphe graminis f.sp.hordei was obtained from the atmosphere above Cambridge, England, by incubating them on barley seedlings exposed on three dates in 1985. The asexual progeny of each spore was tested for its response to a range of doses of a systemic triazole fungicide, triademenol. Principal components analysis of the data revealed that the majority of isolates had a distinct level of response to triadimenol, being resistant to triadimenol at the rate apoplied to seed commercially. The remaining isolates appeared either to be sensitive to the lowest dose of triadimenol used, or to have one of two intermediate levels of resistance. There was a significant increase in the frequency of isolates with higher levels of resistance to triadimenol during 1985. This is likely to have been a response to the continuing widespread use of demethylation inhibitor (DMI) fungicides by British farmers.     

Genetic control of the resistance of Erysiphe graminis f.sp. hordei to five triazole fungicides

The genetics of resistance to ergosterol demethylation inhibitor (DMI) fungicides of the triazole (conazole) group was examined in a cross between two isolates of the barley powdery mildew fungus, Erysiphe graminis (=  Blumeria graminis ) f.sp. hordei . One isolate, E1, was previously identified as being resistant to the triazole fungicide triadimenol, while the other, HL3/5, was sensitive. The 56 progeny tested were classified into two distinct groups, either being resistant to triadimenol, like E1, or sensitive, like HL3/5. The segregation ratio was not significantly different from 1:1, consistent with responses to triadimenol being controlled by a single gene. In further tests with cyproconazole, epoxiconazole, propiconazole and tebuconazole, all the progeny classified as resistant to triadimenol were also more resistant to each of these other triazole fungicides than were any of the triadimenol-sensitive progeny. This is consistent with the triadimenol resistance allele also conferring cross-resistance to the other triazoles. The ratio between the responses of the resistant and sensitive progeny (the resistance factor, RF) was greatest for triadimenol, followed by tebuconazole, propiconazole, epoxiconazole and cyproconazole, in that order. The RF for triadimenol was much greater when the fungicide was applied as a seed treatment than when it was sprayed. Five isolates, covering the five levels of responses to triadimenol identified previously in the UK population of E. graminis f.sp. hordei , were used as standards; a triadimenol-sensitive isolate and one with the lowest level of resistance were sensitive to all four of the other fungicides, while three isolates with higher levels of triadimenol-resistance were also more resistant to the other chemicals.