Hydrolytic Enzymes of Ungerminated and Germinated Conidia of Erysiphe graminis DC f. sp. hordei Marchal

Activities of hydrolytic enzymes (esterase, lipase, phosphatase, protease, 1,3-β-glucanase, chitinase and amylase) were demonstrated in washings and extracts of ungerminated and germinated conidia of Erysiphe graminis f. sp. hordei , Marchal. No cellulase and only very little xylanase activity was detected. Considerable amounts, 65% of total conidia proteins can be extracted by shaking the conidia when the extraction medium contained non-ionic detergent only. Different extraction procedures were used to determine enzyme distribution between exo- and endoconidial compartments and to evaluate the enzyme extraction efficiency from conidia.
The possible physiological roles of the hydrolytic enzymes in the ungerminated as well as in the germinated conidia were discussed.     

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.     

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.     

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.     

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.     

Further Electron Microscopical Studies of Induced Resistance of Barley against Erysiphe graminis f. sp. hordei

Barley leaves were treated with a resistance inducer and inoculated with Erysiphe graminis two days later. 68 hours after inoculation many infection sites with cytoplasmic aggregations with increased number of golgi apparatus and increased endoplasmic reticulum could be observed. The extrahaustorial membrane invaginated and some free pieces of membrane appeared in the matrix. The haustoria vacuolized partially and curly filamentous electron dense material occurred. These changes took place after protective as well as after curative treatment. Due to cytochemical studies the vacuolar electron dense material in the vacuoles of the haustoria contained polymers of mannose or glucose.     

Origin and Proportions of the Components of Composite Populations of Erysiphe graminis f. sp. hordei

Comparisons of the pathotype composition in random samples of barley mildew populations collected in 1985 and 1986 in several European regions were made in an attempt to quantify the influence of migration on population structures. In each of the situations studied (northern France 1986 and eastern Lower Austria 1985 and 1986), evidence was obtained from influx of spores from at least two remote areas. Depending on year and location, 48 to 87 % of the resulting composite samples consisted of ‘foreign-type’ isolates. Implication of long-distance dispersal for studies on population dynamics and for control strategies relative to mildews and other wind-spread pathogens are discussed.     

Quantitative Studies of Resistance Induced By Avirulent Cultures of Erysiphe graminis f. sp. hordei in Barley

Quantification of resistance induced by avirulent cultures of Erysiphe graminis f. sp. hordei against virulent cultures in barley was attempted. Four mildew cultures and 4 barley varieties with known genes of virulence and resistance respectively were used. Pre, post and simultaneous inoculation of leaves was done with avirulent and virulent cultures. Pre-inoculation with avirulent cultures induced resistance in the host such that the pustule number and spore production by later inoculation of virulent cultures was reduced significantly. Once induced, such resistance was active up to 8 days. There was some indication of induced susceptibility if the inducing culture was characterized by medium virulence. Increase of inceulum density of the avirulent (inducer) culture increased the amount of induced resistance Further studies of the phenomenon of induced resistance are needed in relation to possible applications for disease control through inoculations. varietal mixtures and multilines.