Genetic variation in Puccinia graminis collected from oats, rye, and barberry

Puccinia graminis, the causal agent of stem rust, was collected from its alternate host barberry (Berberis spp.) and two different uredinial hosts, oats (Avena sativa) and rye (Secale cereale). The samples were analyzed using 11 polymorphic simple sequence repeat (SSR) markers. There were large differences between fungal populations on oats (P. graminis f. sp. avenae) and rye (P. graminis f. sp. secalis), and the genetic variation within the different formae speciales was also high. It was possible to distinguish between the two formae speciales on barberry. Additional genotypic groups not present in the field samples from oats and rye were also identified on barberry. Our results confirm the importance of barberry in maintaining the populations of P. graminis in Sweden and the importance of the sexual stage for the survival of the pathogen.     

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.     

Components of partial resistance to Erysiphe graminis in spring barley

The development of powdery mildew on seven spring barley varieties lacking effective major resistance factors was compared in a small-plot field trial. Differences were observed at later growth stages indicating partial resistance in some varieties. Latent period and sporulation capacity were examined on the fifth or sixth leaf of these varieties plus several known to be susceptible, using plants raised under spore-free conditions. Latent period was extended in most partially resistant varieties compared with susceptible controls, more so at 5°C than at 10°C. Cumulative total spore production per colony over the 16-day sporulation period differed markedly between varieties, the most resistant yielding one-third as many spores as the susceptible controls. There was evidence that these resistance components were correlated with each other and with previous estimates of infection frequency. The significance of this finding, in relation to selection for improved levels of partial resistance, is discussed.     

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.     

Virulence spectrum in populations of the barley powdery mildew pathogen, Erysiphe graminis f. sp. hordei in Tunisia and Morocco in 1992

The virulence of 57 single colonies of Erysiphe graminis f. sp. hordei from Tunisia and Morocco was investigated using the Pallas near-isogenic differential set. Virulence patterns in general were similar in both countries. Va8, Va10+Du2, V41/145, VLa and VRu2 appear to be common in the region. The resistance alleles Mla7 and Mla9, in combination with other resistances, were highly effective against the isolates tested. No virulence against mlo and Mla9 + k was detected in Tunisia. No virulence on Mla7 and Mla9 was detected in Morocco. The frequency of virulence against several resistances was significantly higher in Morocco than in Tunisia. On the other hand, virulence against other resistances was higher in Tunisia than in Morocco. Three isolates from Morocco infected mlo to an extent greater than previously described. All pathotypes were unique. An attempt is made to interpret the results by comparison with pathotype evolution in Europe.     

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.     

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.     

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.     

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.     

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.     

Interactions between Erysiphe graminis and Septoria nodorum on wheat

Interactions between Erysiphe graminis f.sp. tritici and Septoria nodorum on wheat were studied in the greenhouse and in a 2-year field experiment using artificial inoculation. The integrated form of the logistic growth model dy/dt = ry (1 -y/K ), with infection rate r and final accumulated disease K , was fitted to the disease progress data. Septoria nodorum substantially reduced the disease severities of E. graminis , and caused significant reductions of at least 60% in final accumulated disease K of E. graminis. In the field trials, E. graminis increased the final accumulated disease K of S. nodorum. Owing to the extremely low severity of E. graminis , the increase of S. nodorum severity was small, and significance was given in one of the two years only, with an increase in K of roughly 30%. In the pot experiment, final accumulated disease K of S. nodorum remained unchanged, but there was a significant 30% increase in the infection rate r of S. nodorum. The difference between field and pot trials was explained by the climatic conditions in the greenhouse which excluded secondary infections of 5. nodorum , and which are important factors for disease progress in the field.     

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.     

Virulence patterns in populations of Erysiphe graminis f.sp. hordei in Europe in 1986

A total of 3166 single colony progeny of Erysiphe graminis f.sp. hordei , constituting random samples of the aerial inoculum from 68 regions of Western Europe, were analysed in the laboratory for virulence to barley resistance genes Mlra, Mlg, Mla6+Mlg. MILa, Mla7+Mlk, Mlall, Mla9, MlaI3 + MlRu3. Mlal, Mla3 and mlo. No isolate virulent to mlo was detected. Distinct regional patterns were observed for the frequencies of most of the other virulence genes. They stressed the role of selection exerted by the cultivars grown, as well as the effect of wind dissemination of mildew populations. These two phenomena are particularly well exemplified by the data on early stages in the evolution of the newly emerging Val3 virulence in Denmark. In contrast with the preceding evolution, there was a considerable decrease of the frequency of a broadly distributed virulence ( Va6); the most probable reasons for this are outlined.     

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.     

Genetic variation in Danish populations of Erysiphe graminis f.sp. hordei: estimation of gene diversity and effective population size using RFLP data

Genetic variation of the barley powdery mildew fungus ( Erysiphe graminis f.sp. hordei ) was estimated in three Danish local populations. Genetic variation was estimated from the variation amongst clones of Egh , and was therefore an estimate of the maximum genetic variation in the local populations. The average gene diversity, Ĥ , was estimated as 0.84. The effective population size was estimated as: log₁₀ ( N^ _e ) = 0.64 − log₁₀(μ), or 4.4 × 10⁹, assuming a nucleotide mutation rate ( μ) of 10⁻⁹ per base per generation. There was no significant genetic differentiation between locations.     

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.     

The influence of the primary germ tube on infection of barley by Erysiphe graminis f. sp. hordei

In each of three separate genotype combinations of Erysiphe graminis f. sp. hordei and barley the probability of infection success by an appressorium was reduced by close proximity to a primary germ tube interacting with the same host cell, whether this germ tube was produced by the same or another conidium. Infection success of appressoria was unaffected by events in an adjacent cell, whether associated with other conidia or with the primary germ tube of the same conidium. This interaction between germ tubes and underlying host cells should be considered when mechanisms of resistance to E. graminis f. sp. hordei are investigated.     

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

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

Interactions between barban and protectants on maize, oats and barley

In glasshouse experiments 1,8-naphthalic anhydride (NA) as a seed dressing, and N,N-diallyl-2,2-dichloroacetamide (R25788), either as a seed dressing or mixed in the spray tank with the herbicide, protected maize (Zea mays L.) from subsequent barban damage. When applied after maize emergence, R25788 was not effective through the foliage but had to reach the soil to provide protection. Soil drench treatments of R25788 countered barban damage successfully if applied up to 7 days prior to spraying or up to one day after. Delay of application to 2 or 3 days after barban resulted in only partial protection. Neither NA nor R25788 completely protected barley (Hordeum vulgare L.) or oats (Avena saliva L.) from a subsequent barban spray. Interactions entre le barbane et des produils protecteurs sur le maïs, l'avoine el l'orge Dans des expériences faites en serre, I'anhydride 1,8-naphtalique (AN) employé pour l'enrobage des semences et le N,N-dial-lyl-2,2-dichloroacetamide (R 25788) egalement en enrobage oumélangé dans le réservoir du pulvérisateur avec I'herbicide, ont protégé le maïs (Zea mays L.) des dommages résultant d'un traitement au barbane. Lorsqu'il a été appliqué après la levée du maïs, le R 25788 ne s'est pas montré efficace par voie foliaire: il doit atteindre le sol pour assurer une protection. Les traitements du sol en pluie abondante avec le R 25788 ont supprime efficacement les dégCts du barbane s'ils ont été effectues au plus 7 jours avant le traitement ou 1 jour au plus apres. Lorsque l'application a été faite 2 ou 3 jours apres le traitement au barbane, la protection n'a été que partielle. Ni l'AN ni le R 25788 n'ont complètement protégé I'orge (Hordeum vulgare L.) ou I'avoine (Avena sativa L.) d'un traitement ultériéur avec le barbane. Interaktionen zwischen Barban und Schutzstoffen (Antidote) bei Mais. Hafer und Gerste In Gewächshausversuchen wurde Mais (Zea mays L.) vor einer Schädigung durch Barban geschützt, wenn 1,8-Naphthylsäure-anhydrid (NA) als Saatgutbeize und N,N-Diallyl-2,2-dichlor-acetamid (R 25788) entweder als Saatgutbeize oder als Tankmischung mit dem Herbizid angewendet wurden. Wenn R 25788 nach dem Auflaufen des Maises appliziert wurde, musste es um wirksam zu werden, auf den Boden gelangen; eine Wir-kung über die Blätter konnte nicht festgestellt werden. Durch die Anwendung von R 25788 im Giessverfahren trat kein Barbanschaden auf, vorausgesetzt, dass die Anwendung bis zu 7 Tage vor oder bis zu einen Tag nach der Spritzung erfolgte. Wurde die Applikation erst 2 oder 3 Tage nach der Barbansprit-zung durchgeführt, dann wurde nur ein teilweiser Schutz erzielt. Gerste (Hordeum vulgare L.) und Hafer (Avena saliva L.) wurden weder durch NA noch durch R 25788 völlig vor der schädigenden Wirkung einer darauffolgenden Barbanspritzung geschützt.