The Nature of the Resistance of Oats to the Take-all Fungus: II. INHIBITION OF GROWTH AND RESPIRATION OF OPHIOBOLUS GRAMINIS SACC. AND OTHER FUNGI BY A CONSTITUENT OF OAT SAP

Growth of Ophiobohu gramimt and of O. gramims var. avenae isinhibited by concentrations of 3·3-4·0µg./ml.,and respiration by concentrations of 55µg./nil. of a partiallypurified substance from oat-leaf sap. The two varieties appearto be equally sensitive. The filtrate of boiled sap is inhibitorybut here dilution of the sap permits better growth of isolatesof var. avenae. Sap from oat roots is inhibitory to O.graminisonly, and fractionation of the sap shows that the inhibitorcan be masked by a growth stimulant. Inhibition of growth andrespiration can be reduced by glutathione and ascorbic acid,particularly if the inhibitor and reducing agent are previouslyincubated together for a few hours, suggesting that the inhibitoris inactivated on reduction. The capacity of var. avenae toovercome inhibition in the favourable medium provided by thecrude sap more readily than can the type variety is suggestedas the cause of the slight differential activity of the filtrateof leaf sap and the full differential activity of the root sap.Susceptibility of oats to var. avenae would thus be due to conditionsenabling the fungus to overcome toxicity rather than to an absenceof toxicity. Activity of the inhibitor against growth and respiration ofa number of fungi and a few other organisms has been tested.Bacteria and oat and barley roots are not affected but abouthalf of the fungi tested are inhibited although none is as sensitiveas O. gramims. No members of the fungi imperfecti tested aresensitive.     

The Nature of the Resistance of Oats to the Take-all Fungus

The two varieties of the take-all fungus, Ophiobolus graminisand 0. graminis var. Avenae, show a differential reaction tosap extracted from oats; the type variety, which is incapableof causing a lasting infection of oats in vivo, is inhibitedby the sap in vitro, whereas var. Avenae, pathogenic to oats,can grow in the sap. The inhibitory action of the sap is notdue to a lack of food material required by the fungus, but toa specific substance or toxin. The method used for assayingtoxicity is described. The toxin is thermostable, moderately stable on storage, virtuallyinsoluble in non-polar solvents, and soluble in acetone, water,and methanol. Methanol extraction of the ether-washed residueof the filtrate from boiled sap from oat leaves results in asemi-purified substance which, when added to 2 per cent. Yeastrelsolution, reduces growth of 0. graminis to half that of thecontrol at a concentration of to 10 p.p.m. The inhibitor is produced in considerable quantity in the leavesand stems of oats and in smaller quantity in the roots. It appearsto accumulate mainly during the period of active growth andto be less active, or present in smaller concentration, in adultplants. It can be detected in the seminal roots throughout theirexistence, although during the first 3 weeks of growth 0. graminiscan invade the cells of the cortex. It is present in greaterquantity in the crown roots, which are never penetrated. A similar inhibitor of 0. graminis can be extracted from Arrhenatherumelatius, but not from other grasses which, both in the fieldand in pot experiments, appear to be equally resistant.     

Failure of in vitro growth inhibition by cysteine to differentiate between Australian Gaeumannomyces graminis isolates pathogenic and non-pathogenic to oats

Radial growth of oat and non oat-attacking Australian isolates of Gaeumannomyces graminis was greatly inhibited by increasing concentration of DL-cysteine in basal medium agar, and growth was completely inhibited by cysteine concentrations of 3 μM. As a group, isolates of G. graminis var. tritici (both oat and non oat-attacking forms) were more inhibited than isolates of G.graminis var.avenae at 1 μM cysteine, but differences did not occur at other concentrations. Isolates of a lobed-hyphodiate fungus similar to G. graminis var. graminis were more tolerant of cysteine than other isolates. The findings indicate that in vitro inhibition of Australian G. graminis isolates by cysteine is not useful for differentiation between oat and non oat-attacking types, and is unlikely to be fundamentally related to the ability of isolates to attack oats.     

Detoxification of Benzoxazolinone Allelochemicals from Wheat by Gaeumannomyces graminis var. tritici, G. graminis var. graminis, G. graminis var. avenae, and Fusarium culmorum

The ability of phytopathogenic fungi to overcome the chemical defense barriers of their host plants is of great importance for fungal pathogenicity. We studied the role of cyclic hydroxamic acids and their related benzoxazolinones in plant interactions with pathogenic fungi. We identified species-dependent differences in the abilities of Gaeumannomyces graminis var. tritici, Gaeumannomyces graminis var. graminis, Gaeumannomyces graminis var. avenae, and Fusarium culmorum to detoxify these allelochemicals of gramineous plants. The G. graminis var. graminis isolate degraded benzoxazolin-2(3H)-one (BOA) and 6-methoxy-benzoxazolin-2(3H)-one (MBOA) more efficiently than did G. graminis var. tritici and G. graminis var. avenae. F. culmorum degraded BOA but not MBOA. N-(2-Hydroxyphenyl)-malonamic acid and N-(2-hydroxy-4-methoxyphenyl)-malonamic acid were the primary G. graminis var. graminis and G. graminis var. tritici metabolites of BOA and MBOA, respectively, as well as of the related cyclic hydroxamic acids. 2-Amino-3H-phenoxazin-3-one was identified as an additional G. graminis var. tritici metabolite of BOA. No metabolite accumulation was detected for G. graminis var. avenae and F. culmorum by high-pressure liquid chromatography. The mycelial growth of the pathogenic fungi was inhibited more by BOA and MBOA than by their related fungal metabolites. The tolerance of Gaeumannomyces spp. for benzoxazolinone compounds is correlated with their detoxification ability. The ability of Gaeumannomyces isolates to cause root rot symptoms in wheat (cultivars Rektor and Astron) parallels their potential to degrade wheat allelochemicals to nontoxic compounds.     

Effect of osmotic potential on the growth of Gaeumannomyces graminis and Phialophora spp.

The linear growth of 10 isolates each of Gaeumannomyces graminis var. graminis, G. graminis var. tritici and Phialophora graminicola and five isolates each of G. graminis var. avenae and a lobed-hyphopodiate Phialophora sp. was studied on osmotically adjusted agar at 20 °C. While most isolates of G. graminis var. avenae ceased growing at osmotic potentials of -60 bars (1 bar = 10⁵ Pa), six out of 10 isolates of G. graminis var. tritici grew at that potential. The growth of all isolates of G. graminis var. tritici and var. avenae ceased at -70 bars. In contrast, four out of 10 isolates of P. graminicola grew at -70 bars, but all stopped growing at -80 bars. Most of the isolates of G. graminis var. graminis and the lobed-hyphopodiate Phialophora sp. grew at -70 bars while three out of 10 isolates of G. graminis var. graminis and one out of five isolates of the lobed-hyphopodiate Phialophora sp. were capable of growth at -80 bars. None of the fungi grew at -90 bars. Detailed studies of the growth of two or three isolates each of the five fungi at 10, 20, 30 and 35 °C were carried out on osmotic agar controlled by the addition of either sodium chloride or potassium chloride. In general, similar reductions in growth occurred with decreasing osmotic potential regardless of the solute used. At 10 and 20 °C., all three isolates of P. graminicola showed optimal growth at about -5 bars while the other fungi grew fastest at -1²middot; bars. At 30 °C., one isolate of the lobed hyphopodiate Phialophora sp. and two isolates each of P. graminicola, G. graminis var. tritici and G. graminis var. avenae grew optimally at osmotic potentials of -10 to -15 bars. The other isolate of the Phialophora sp. and two isolates of G. graminis var. graminis studied grew optimally at the highest potential (-1·2 bars). However, at 35 °C the last three fungi exhibited optimal growth at osmotic potentials of-10 to -20 bars. The ecological significance of these results is discussed in relation to cross-protection against the take-all fungi by the avirulent fungi.     

Pathogenicity of ‘take-all’ fungus to oats: Its relationship to the concentration and detoxification of the four avenacins

Data for inhibition of the growth of Gaeumannomyces graminis var. tritici (Ggt) and var. avenae (Gga), Phialophora radicicola and Fusarium avenaceum, caused by avenacins, are presented. The avenacins found in all oat species examined are sufficient in quantity to totally suppress growth of wheat ‘take-all’ (Ggt), even old roots containing 25 μg/g (fr. wt). Fungal variants that can also attack oats [var. avenae (Gga)] show considerable variations in their tolerance to avenacin A-1, ec₅₀ values being 5–80 μg/ml. Nevertheless, all Gga isolates maintained some growth at avenacin A-1 concentrations as high as 200 μg/ml and it is this ability to grow, albeit slowly, at high concentrations that is the critical difference between Gga and Ggt strains. The pathogenicity towards oats of a range of isolates of Gga is related to the fungicidal activity of avenacins. Gga pathogenicity is shown to increase with poor nutrition of the oat hosts (poor illumination, lack of minerals). Fungal detoxification of avenacins produces mono-deglucosylavenacin A-1, bis-deglucosylavenacin A-1 and, in one case, tris-deglycosylavenacin A-1. Ggt strains left avenacin A-1 almost unaffected giving only traces of mono-deglucosyl product. Gga strains bring about mono- and bis-deglucosylation whilst Fusarium avenaceum causes mainly bis-deglucosylation. Mono-deglucosylavenacin is shown to be less inhibitory to Gga than is avenacin A-1, whilst the bis-deglucosyl compound is still less inhibitory.     

Origin and Inheritance of Group I Introns in 26S rRNA Genes of Gaeumannomyces graminis

Studies of the distribution of the three group I introns (intron A, intron T, and intron AT) in the 26S rDNA of Gaeumannomyces graminis had suggested that they were transferred to a common ancestor of G. graminis var. avenae and var. tritici after it had branched off from var. graminis. Intron AT and intron A exhibited vertical inheritance and coevolved in concert with their hosts. Intron loss could occur after its acquisition. Loss of any one of the three introns could occur in var. tritici whereas only loss of intron T had been found in the majority of var. avenae isolates. The existence of isolates of var. tritici and var. avenae with three introns suggested that intron loss could be reversed by intron acquisition and that the whole process is a dynamic one. This process of intron acquisition and intron loss reached different equilibrium points for different varieties and subgroups, which explained the irregular distribution of these introns in G. graminis. Each of the three group I introns was more closely related to other intron sequences that share the same insertion point in the 26S rDNA than to each other. These introns in distantly related organisms appeared to have a common ancestry. This system had provided a good model for studies on both the lateral transfer and common ancestry of group I introns in the 26S rRNA genes. Received: 17 May 1996 / Accepted: 14 January 1997     

Pathogenic and molecular variation support the presence of genetically distinct clonal lineages in Australian populations of Puccinia graminis f. sp. avenae

Previous studies of the causal agent of stem rust of oats (Puccinia graminis f. sp. avenae; P. g. avenae) in Australia have demonstrated a high level of pathogenic variability. In this work, the pathotypic structure of the Australian P. g. avenae population in 1999 was investigated, as well as the pathotypic and genetic diversity of a collection of 26 Australian isolates representing a 25-year period (1971–1996). In the 1999 sample, 16 races belonging to six international standard races were identified from 97 isolates, with standard race 94 predominant in all regions. Race 94+Pg-13,Pg-Sa,Pg-a, detected in southern New South Wales (sNSW) and northern New South Wales (nNSW), was virulent on all of the differential genotypes used. Detailed analyses of pathogenicity and AFLP variability among 26 isolates collected from 1971–1996 revealed that isolates of standard race 94 collected in 1999 were genetically distinct from other Australian races of P. g. avenae. This evidence, along with data from annual pathogenicity surveys, suggests that the group to which standard race 94 belongs appeared during the late 1980s, and that it increased in frequency to dominate P. g. avenae pathogen populations throughout Australia from 1992 onward. The existence of groups of P. g. avenae isolates in Australia that differ in pathogenicity and AFLP phenotype suggests that current populations have evolved from a number of isolates of the fungus that differ in their genetic backgrounds, which may have originated from independent introductions or from asexual hybridisational events.     

The effects of formalin, nabam, irrigation and nitrogen on Heterodera avenae Woll., Ophiobolus graminis Sacc. and the growth of spring wheat

In 1964, nabam (sodium ethylene bisdithiocarbamate), water and three amounts of nitrogen fertilizer were applied to spring wheat on soil treated and untreated with formalin. The experiment lasted for 3 years during which there were eight different formalin, no–formalin sequences. The nabam and irrigation treatments were discontinued when it was found they did not affect the principal pathogens present, the cereal cyst nematode Heterodera avenae and the take–all fungus Ophiobolus graminis. Formalin increased grain and straw yields in the year in which it was applied but led to increased H. avenae populations which adversely affected the succeeding crop. Formalin controlled O. graminis in the year it was applied except on land treated for the first time in 1966. H. avenae seemed to be the main check to growth until about June, and O. graminis later. At the end of the experiment, total grain yields and nematode soil populations were greatest in the plots treated with formalin each year and least in those never treated with formalin. Yield loss from either O. graminis or H. avenae alone could not be assessed because formalin usually controlled both during the season in which it was applied and both were present in untreated plots. However, in 1965, some comparisons of the effects of each pathogen were possible when one occurred in the presence of differing amounts of the other. A doubling of total grain yield over 3 years was accompanied by an eightfold increase in H. avenae in sequences of continuous formalin or formalin 1964 and 1965, whereas yield increases caused by extra nitrogen were not matched by such a big increase in H. avenae. This suggests that formalin might be affecting H. avenae through factors other than increased plant size and vigour, which in themselves would tend to encourage larger nematode populations. In the absence of formalin, H. avenae soil populations either fell or failed to increase.     

Methoxyhydroquinone, a Growth Inhibitor of Ophiobolus graminis, in Leaves of Oat Seedlings

A methanol extract of leaves of oat seedlings grown in sand cultures in the dark contained a compound which inhibited the growth of Ophiobolus graminis. The inhibitory factor was isolated and proved to be present in the plant as methoxyhydroquinone glucoside. The glucoside was readily hydrolysed to the corresponding aglucone. The methoxyhydroquinone, or possibly its oxydation product, methoxy-P-benzoquinone, was inhibitory to both Ophiobolus graminis var. graminis and Ophiobolus graminis var. avenae, whereas Fusarmm oxysporum var. lycopcrsici was not affected. Synthetic methoxyhydroquinone at 80 mg/l gave a 100% inhibition of Ophiobolus graminis var. graminis. After being exposed to 80 mg/l of the inhibitor for 24 h the mycelium was unable to initiate growth when transferred to a fresh nutrient solution. Only extracts from young leaves showed inhibitory activity, extracts from mature leaves giving no inhibition. The hydroquinone, or its glucoside, was not detected in roots of young seedlings, where avenacin was the only antifungal compound present.     

Fungal communities and health status of roots of winter wheat cultivated after oats and oats mixed with other crops

Health status of winter wheat roots and thecomposition of wheat root fungi were studiedover 1996-1999 following the cultivation ofoats in a pure stand and mixed with otherplants as forecrops. The infection of wheatroots by >Gaeumannomyces graminis wasobserved to be largely dependent on the kind offorecrop; the best being oats in a pure stand,and then oats with pea or lupin mixtures. Inthe emergence and shooting phases, saprophyticfungi were dominant, while in the stage of harddough stage mainly pathogenic fungi, especially>G. graminis were common. The pathogenicfungi were mostly represented by >G.graminis and >Fusarium spp., while >Rhizoctonia spp. were much less frequent.The composition of the fungal communitydepended considerably on the forecrop anddevelopment phase of the plant. The kind offorecrop significantly affected the frequencyof infection by >G. graminis. The highestnumber of isolates was obtained from wheat rootsof crops grown after a mixture of oats andbarley.     

Control of Ophiobolus patch in Agrostis turf using avirulent fungi and take-all suppressive soils in pot experiments

The incorporation of avirulent fungi such as Gaeumannomyces graminis var. graminis, an avirulent isolate of G. graminis var. tritici, a Phialophora sp. with lobed hyphopodia synonymous with Phialophora radiciola var. radicicola sensu Deacon and P. radicicola var. graminicola at the time of seeding Agrostis turf in pots of sterilised soil completely controlled Ophiobolus patch disease. The addition of a 5 mm layer of take-all suppressive (TAS) soils, artifically developed by the repeated addition of live mycelium of the varieties avenae, tritici and graminis of G. graminis to soil, controlled the disease to a lesser extent. However, a 20 mm layer of a TAS soil developed from live mycelium of G. g. avenae almost completely suppressed the disease. A survey of 66 golf and bowling greens in four states of Australia showed that P. r. graminicola was the most prevalent avirulent fungus.     

The effect of barley yellow dwarf virus on field populations of the cereal aphids, Sitobion avenae and Metopolophium dirhodum

The numbers of cereal aphids, especially Metopolophium dirhodum in 1979, and Sitobion avenae in 1980, were significantly increased on BYDV infected wheat and oats in 1979, and wheat, barley and oats in 1980. The differences were probably caused by attraction of alates of each species to virus infected plants which had changed colour as a result of their infection. Significantly more alates of M. dirhodum were found on virus infected oats in 1979, and of S. avenae on oats and barley in 1980, although not on wheat in either year. probably because the colour contrast in wheat was less intense than in the other crops. Flight chamber experiments with alates of both species confirmed their visual attraction to virus-infected leaves. The interaction between virus, vector and host plants is discussed with reference to the ecology of virus spread.     

A comparative study of Phialophora radicicola, an avirulent fungal root parasite of grasses and cereals

The biology and infection-behaviour of a typical isolate of Phialophora radicicola Cain have been compared with those of a representative isolate of Ophiobolus graminis (Sacc.) Sacc. Both species can utilize a nitrate source of nitrogen and both require thiamine and biotin for growth on inorganic nitro-gen; P. radicicola, but not O. graminis, was able to synthesize biotin when grown on asparagine as a nitrogen source. The pH range for good growth of P. radicicola in nutrient solution was narrower than that for O. graminis, and its growth rate on agar was only one-third. P. radicicola was the more active decomposer of cellulose, and its cellulolysis adequacy index was I.66 as com-pared with a value of 0.33 for 0. graminis. In agreement with prediction from Garrett's (I966) hypothesis on the cellulolysis adequacy index, saprophytic survival of P. radicicola in wheat straw was shortened by additional soil nitrogen, which prolongs survival of O. graminis.P. radicicola was found to spread ectotrophically over the roots of wheat, oats and barley by runner hyphae indistinguishable from those of O. graminis, but cortical infection caused no necrosis and no discernible check to growth of the infected cereals, nor any significant decrease in grain yield of inoculated wheat grown to maturity. Pre-existing infection of wheat roots by P. radicicola retarded spread of infection by O. graminis; inoculation of several grass species with P. radicicola reduced the extent of infection by O. graminis of wheat following the grasses.     

Formae speciales of cereal powdery mildew: close or distant relatives?

Powdery mildew is an important disease of cereals, affecting both grain yield and end‐use quality. The causal agent of powdery mildew on cereals, Blumeria graminis, has been classified into eight formae speciales (ff.spp.), infecting crops and wild grasses. Advances in research on host specificity and resistance, and on pathogen phylogeny and origins, have brought aspects of the subspecific classification system of B. graminis into ff.spp. into question, because it is based on adaptation to certain hosts rather than strict host specialization. Cereals therefore cannot be considered as typical non‐hosts to non‐adapted ff.spp. We introduce the term ‘non‐adapted resistance’ of cereals to inappropriate ff.spp. of B. graminis, which involves both pathogen‐associated molecular pattern‐triggered immunity (PTI) and effector‐triggered immunity (ETI). There is no clear distinction between the mechanisms of resistance to adapted and non‐adapted ff.spp. Molecular evolutionary data suggest that the taxonomic grouping of B. graminis into different ff.spp. is not consistent with the phylogeny of the fungus. Imprecise estimates of mutation rates and the lack of genetic variation in introduced populations may explain the uncertainty with regard to divergence times, in the Miocene or Holocene epochs, of ff.spp. of B. graminis which infect cereal crop species. We propose that most evidence favours divergence in the Holocene, during the course of early agriculture. We also propose that the forma specialis concept should be retained for B. graminis pathogenic on cultivated cereals to include clades of the fungus which are strongly specialized to these hosts, i.e. ff.spp. hordei, secalis and tritici, as well as avenae from cultivated A. sativa, and that the forma specialis concept should no longer be applied to B. graminis from most wild grasses.     

The invasion and development of the cereal cyst-nematode, Heterodera avenae in the roots of autumn-and spring-sown cereals

Observations were made at 2 or 4 wk intervals from December to harvest on all stages of Heterodera avenae in winter oats growing on infested land. Second-stage larvae were present in all soil samples except on 5 and 20 July. Invasion and development of larvae was slow during winter. The nodal and seminal roots of winter oats were both heavily invaded by the nematode; larvae which invaded seminal roots tended to become male whereas those in nodal roots tended to become female. There was a small second invasion in August. Females were first observed on the roots of winter oats on 17 May, 214 days after the crop was sown and 62 days after the first fourth-stage larva was observed. The nodal roots of spring barley contained few H. avenae larvae whereas these roots were heavily invaded in winter wheat and oats. In spring barley the nodal roots were developing in June and July when few second-stage larvae were in the soil whereas in winter oats and wheat the nodal roots were growing rapidly in April when larvae were most numerous, and so were heavily invaded.     

Time-course of Biosynthesis of Phenolics and Lignin in Roots of Wheat Genotypes Differing in Manganese Efficiency and Resistance to Take-all Fungus

Inhibition of lignin biosynthesis in Triticum aestivum L. rootsby Mn deficiency has been suggested as the mechanism of reducedresistance of Mn-deficient wheat roots to infection by the take-allfungus (Gaeumannomyces graminis var. tritici). This study evaluatedphenolics and lignin accumulation in roots of wheat genotypesdiffering in Mn efficiency (measured as growth and yield inMn-deficient soils) and take-all resistance. Seedlings of theMn-inefficient, take-all sensitive genotype Bayonet and theMn-efficient, more take-all resistant genotype C8MM were grownin nutrient solution without added Mn for 18 d and then transferredto a Mn-deficient sandy soil fertilized with Mn at 0 or 30 mgkg^-1. Both genotypes had Mn-deficient roots and shoots at thetime of transfer to the soil. Roots of both genotypes were inoculatedwith the take-all fungus 0, 1, 3 and 7 d after transfer. Twenty-fourhours after inoculation, take-all fungus penetrated the rootstele of take-all sensitive Bayonet but not of more resistantC8MM wheat. Rates of phenolics and lignin accumulation in rootsdeclined steadily during growth in soil for up to 8 d, werehigher in mature, fully differentiated parts of the root systemcompared to distal, younger root tissue, and were higher inBayonet than in C8MM. Manganese fertilization did not significantlyinfluence rates of phenolics and lignin accumulation but reduceddepth of radial penetration by hyphae in both genotypes. Therate of phenolics accumulation was positively (r = 0·91to 0·96) correlated with the rate of lignin accumulation.Mn-efficient C8MM had a higher rate of lignin accumulation perunit of phenolics than Mn-inefficient Bayonet over a wide rangeof phenolics synthesis rates. From this we suggest that C8MMhas a more efficient mechanism for conversion of phenolics tolignin, the trait which appears related to higher take-all resistanceof this genotype.Copyright 1994, 1999 Academic Press Gaeumannomyces graminis var. tritici, lignin, manganese, phenolics, resistance, roots, Triticum aestivum     

Reduction of Take-all Inoculum by Rotation with Lupins, Oats or Field Peas

The feasibility of use of lupins, oats and field peas as alternative rotation crops to reduce inoculum of the take-all fungus (Gaeumannomyces graminis var. tritici) (under Western Australian field conditions) and disease in following wheat was investigated with a one year field trial, the soil from which was used in two succeeding pot experiments. The possible mechanisms of reduction of inoculum and disease by these crops were examined testing the soil for pathogen and disease suppression. Rotation with lupins or oats for two seasons reduced (P <0.05) inoculum of the take-all fungus and lupins, oats or field peas reduced (P <0.05) disease in following wheat. Lupins alone reduced inoculum and disease, (P <0.1) after one season. No apparent suppression of the pathogen in the absence of host plants was recorded after one season of rotation, but after two seasons, lupins, oats or field peas all suppressed (P <0.02) growth of the pathogen within soil. However only field pea soil suppressed take-all in comparison with the wheat control. Although after two seasons all rotation crops were effective in reducing inoculum and disease the mechanisms of reduction appear to differ between the rotation crops used in this study.     

The genetics of resistance to powdery mildew in cultivated oats (Avena sativa L.): current status of major genes

The genetics of resistance to powdery mildew caused by Blumeria graminis f. sp. avenae of four cultivated oats was studied using monosomic analysis. Cultivar ‘Bruno’ carries a gene (Pm6) that shows a recessive mode of inheritance and is located on chromosome 10D. Cultivar ‘Jumbo’ possesses a dominant resistance gene (Pm1) on chromosome 1C. In cultivar ‘Rollo’, in addition to the gene Pm3 on chromosome 17A, a second dominant resistance gene (Pm8) was identified and assigned to chromosome 4C. In breeding line APR 122, resistance was conditioned by a dominant resistance gene (Pm7) that was allocated to chromosome 13A. Genetic maps established for resistance genes Pm1, Pm6 and Pm7 employing amplified fragment length polymorphism (AFLP) markers indicated that these genes are independent of each other, supporting the results from monosomic analysis.     

Intracellular Localization of the Systemic Fungicide Triadimenol in Phytopathogenic Fungi

The intracellular localization of the radioactively labelled fungicide (³H)triadimenol A in the in vitro grown sporidia of Ustilago avenae and in the in vivo cultured powdery mildew (Erysiphe graminis f. sp. hordet) on barley (Hordeum vulgare) is described. The specimens were prepared by low temperature techniques: shock freezing, freeze substitution and embedding in Spurr's low viscosity resin. The localization of the fungicide was achieved by means of conventional electron microscopic autoradiography. The available experimental data allow a first qualitative analysis of the distribution of silver grains on freeze substituted sporidia of U. avenae and the infection structures of Erysiphe graminis f. sp. hordei. Concerning U. avenae the fungicide is detected preferentially over the vacuoles, the cytoplasm, and the cell walls after a six month exposure. The host pathogen system powdery mildew on barley exhibits an accumulation of silver grains in the host cell wall adjacent to the infection site and the papillae whereas decisively fewer grains occur inside the haustoria. Apart from this general localization pattern the haustoria show ultrastructural changes caused by the fungicide treatment: vesiculation and collapse of the sheath membrane as well as a diffuse appearance of the haustorial cytoplasm. Around the haustoria an aggregation of host cytoplasm material is observed.