Seasonal changes in infection of barley seedlings by Erysiphe graminis f.sp. hordei

Infection of artificially inoculated barley seedlings, exposed out of doors, was variable but there was a seasonal trend. As temperature affects both incubation time and the proportion of successful infections, these two factors will have additive effects on disease development. Infections developed only slowly in winter but, in both 1972 and 1973, conditions became much more favourable to infection in late April or early May.     

Activity of triadimefon vapour against Erysiphe graminis f. sp. hordei

A series of experiments from 1978 to 1983 investigated the activity of triadimefon vapour against barley powdery mildew (Erysiphe graminis f. sp. hordei). In early experiments there was a pronounced protectant action when the dry product was used as the vapour source, and this protection persisted after test seedlings had been removed from the vicinity of the vapour source. In a glasshouse, sprayed seedlings remained sources of active vapour for at least 6 wk after spraying. In later experiments, effects of triadimefon vapour were much smaller, probably because the pathogen had become less sensitive to the fungicide. The dry product was no longer an effective vapour source but infection of test seedlings was still decreased if they were exposed to vapour emanating from impregnated absorbent paper or sprayed seedlings, presumably because the effective surface areas were much greater. Possible benefits and problems that arise from vapour action in glasshouse and field experiments are discussed.     

Seasonal changes in incubation time of Erysiphe graminis f.sp. hordei.

Incubation time of Erysiphe graminis f.sp. hordei varied seasonally, but between early May and late September it was always between 5 and 8 days. This is approximately the time of year when the 30-year weekly mean air temperature exceeds 10^oC. Amount of infection during summer was decreased by hot, dry weather. In winter, incubation times were more variable and long incubation times were usually associated with few infections. Variation in incubation time during the summer is unlikely to be of practical importance in determining the rate of epidemic development.     

Investigations on Adult Plant Resistance of Barley Against Erysiphe graminis f.sp. hordei

Spring barley cultivars and lines were tested for 3 years in field studies for adult plant resistance against Erysiphe graminis f.sp. hordei. The cultivars Osiris and Asse were selected for further detailed cytological studies and compared with the susceptible cultivar Peruvian. Under controlled greenhouse conditions, the percentage of conidia that had formed a functional haustorium and secondary hyphae (infection efficiency) was reduced in fifth leaves of the adult plant resistant cultivars. On fifth and flag leaves of adult plant resistant cultivars, papillae were formed more frequently under primary germ tubes and appressoria, and fungal penetration was prevented more often than on the susceptible cultivar Peruvian. In ultrastructural studies various types of papillae were observed, but could not be strictly correlated with penetration success or failure of the fungus.     

The influence of photoperiod on growth patterns of Erysiphe graminis f. sp. hordei

Detached seedling leaf segments of five spring barley genotypes were inoculated with an isolate of barley mildew to which they possessed different levels of resistance. Segments of each host genotype were then incubated under either continuous light or treatments with 2, 4, 8 or 16 h darkness per 24 h cycle. Macroscopic observation showed that the latent period of infection was reduced slightly in treatments with at least 4 h darkness/24 h. Yellowing of detached segments occurred fastest under continuous light and slowest under a 16 h dark/8 h light cycle. Microscope observation of segments fixed 4·5 days after inoculation showed that as the length of the dark period increased, so the number of haustoria formed per colony also increased. This increase in haustorial production appeared to be associated with an accentuation in the synchrony of production of the secondary and tertiary haustorial generations. Varietal differences in susceptibility were also more marked in segments incubated under short days. It is concluded that under the conditions of temperature and light intensity used, a 16 h dark/8 h light system is most desirable for quantifying mildew resistance because it allows confident identification of distinct haustorial generations, accentuates differences in varietal susceptibility and delays chlorophyll degradation in detached barley leaves infected with mildew.     

Studies on the tolerance of Erysiphe graminis f. sp. hordei to systemic fungicides

Successive sowings of glasshouse-grown barley plants were treated with either tridemorph, ethirimol or both fungicides, and inoculated with an initially fungicide-sensitive isolate of Erysiphe graminis f. sp. hordei. The time taken for symptoms to appear, compared with that for untreated plants, decreased with successive sowings. This was interpreted as evidence for the increase in the frequency of fungicide-tolerant propagules in the pathogen population. Effective mildew control was obtained by the use of either or both fungicides in trial plot and field crops. Seedling assays for tolerance to tridemorph and ethirimol showed that tolerance was more evident in treated than in untreated crops in June. Some mildew populations partially tolerant to one fungicide also showed reduced sensitivity to the other. By July a response intermediate between tolerant and sensitive was recorded on all the plots, probably indicating the mixing of fungal populations from adjacent plots. Larger numbers of tolerant isolates were found in random samples from treated than from untreated crops.     

Evidence for Resistance-inducing Compounds in Conidia of Erysiphe graminis f.sp. hordei

Preparations of Erysiphe graminis f.sp. hordei conidia were spray-applied to the first leaf of barley plants which were subsequently challenge inoculated with virulent powdery mildew. The powdery mildew reducing effect of the preparations was assessed by scoring the outcome of the challenge inoculation. Homogenates of ungerminated conidia, germinated conidia, and methanol-water extracts of germinated conidia reduced the number of powdery mildew colonies. Cell wall fragments from ungerminated conidia, germinated conidia, and conidial germination fluid obtained from conidia germinated in aqueous suspension did not reduce the number of powdery mildew colonies. Microsconical analysis of the infection course following challenge inoculation indicated that the powdery mildew reducing effect is due partly to induced resistance.     

Production of branches from unsuccessful primary germ tubes of conidia of Erysiphe graminis f. sp. tritici on wheat leaves

Confusion exists regarding nomenclature and possible significance of outgrowths produced by conidial primary germ tubes (PGT) of Erysiphe graminis f. sp. tritici Em. Marchai on wheat leaves. Production of these outgrowths has been taken as a criterion of successful host-parasite relationship. However, such outgrowths have been observed to have developed from 12.3 % of the unsuccessful PGT on a resistant wheat and 9.2 % of the unsuccessful PGT on a susceptible wheat.It is proposed that these outgrowths may be called PGT branches which may develop whether the penetration by PGT is successful or not. In case the PGT penetration is successful, then such a PGT branch may be called a secondary hypha. These branches from unsuccessful PGT may also produce host responses in the form of halos in the host cell-walls or may also penetrate the host cells successfully and produce apparently healthy (normal) haustoria. Both PGT branches and the secondary hyphae may be called functional when they produce either host responses in the form of halos or penetrate successfully and produce haustoria, or produce both halos and haustoria.     

Extrachromosomal plasmid-like DNA in the obligate parasitic fungus Erysiphe graminis f. sp. hordei

Summary The obligate parasitic fungus, Erysiphe graminis f. sp. hordei, was found to harbour plasmid-like extrachromosomal DNA. A 1.35-kb fragment of this 9kb plasmid was cloned into the pUC12 vector. No homology was detected to nuclear or mitochondrial DNA. As only about half of the 27 isolates examined contained plasmid-like DNA, this appears to be inessential for fungal survival. The plasmid is frequent in European isolates and is found in both newly collected isolates and in isolates kept under laboratory conditions for many years. No correlation between presence of plasmid and specific avirulence/virulence genes was found. The plasmid appear to be located in the mitochondria.     

Apoplastic pH signaling in barley leaves attacked by the powdery mildew fungus Blumeria graminis f. sp. hordei

To investigate apoplastic responses of barley (Hordeum vulgare L.) to the barley powdery mildew fungus Blumeria graminis f. sp. hordei, noninvasive microprobe techniques were employed. H(+)- and Ca(2+)-selective microprobes were inserted into open stomata of barley leaves inoculated with Blumeria graminis f. sp. hordei race A6 conidia. Resistance gene-mediated responses of barley genotype Ingrid (susceptible parent line) and the near-isogenic resistant Ingrid backcross lines (I-mlo5, I-Mla12, and I-Mlg) were continuously monitored from 20 min to 4 days after inoculation. The main events were categorized as short-term responses around 2 h after inoculation (hai), intermediate responses around 8 and 12 hai, and long-term responses starting between 21 and 24 hai. Short-term responses were rapid transient decreases of apoplastic H(+)- and Ca2+ activities that lasted minutes only. Kinetics were similar for all genotypes tested, and thus, these short-term responses were attributed as nonspecific first encounters of fungal surface material with the host plasma membrane. This is supported by the observation that a microinjected chitin oligomer (GlcNAc)8 yielded similar apoplastic alkalinization. Intermediate responses are trains of H+ (increase) spikes that, being different in susceptible Ingrid and penetration-resistant I-mlo5 (or I-Mlg), were interpreted as accompanying specific events of papillae formation. Long-term events were massive slow and long-lasting alkalinizations up to two pH units above control. Since these latter changes were only observed with near-isogenic hypersensitive reaction (HR)-mounting genotypes I-Mla12 and I-Mlg but not with I-mlo5 or, to a smaller extent, with susceptible Ingrid, both lacking significant rates of HR, they were rated as cell death specific. It is concluded that apoplastic pH changes are important indicators of host-pathogen interactions that correlate with both the different stages of fungal development and the different types of host defense response.     

RFLP mapping of three new loci for resistance genes to powdery mildew (Erysiphe graminis f. sp. hordei) in barley

Three new major, race-specific, resistance genes to powdery mildew (Erysiphe graminis f. sp. hordei) were identified in three barley lines, RS42-6*O, RS137-28*E, and HSY-78*A, derived from crosses with wild barley (Hordeum vulgare ssp. spontaneum). The resistance gene origining from wild barley in line RS42-6*O, showed a recessive mode of inheritance, whereas the other wild barley genes were (semi)-dominant. RFLP mapping of these three genes was performed in segregating F₂ populations. The recessive gene in line RS42-6*O, was localized on barley chromosome 1S (7HS), while the (semi)-dominant genes in lines RS137-28*E, and HSY-78*A, were localized on chromosomes 1L (7HL) and 7L (5HL), respectively. Closely linked RFLP clones mapped at distances between 2.6cM and 5.3 cM. Hitherto, specific loci for powdery mildew resistance in barley had not been located on these chromosomes. Furthermore, tests for linkage to the unlocalized resistance gene Mlp revealed free segregation. Therefore, these genes represent new loci and new designations are suggested: mlt (RS42-6*O), Mlf (RS137-28*E), and Mlj (HSY-78*A). Comparisons with mapped QTLs for mildew resistance were made and are discussed in the context of homoeology among the genomes of barley (H-vulgare), wheat (Triticum aestivum), and rye (Secale cereale). Duplications of RFLP bands detected in the neighbourhood of Mlf and mlt might indicate an evolutionary interrelationship to the Mla locus for mildew resistance.