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.     

Ultrastructural and Histochemical Studies on Mildew of Barley (Erysiphe graminis DC. f. sp. hordei Marchal)

Plants of the mildew susceptible barley cultivar Peruvian and the adult plant resistant cultivar Osiris were inoculated with Erysiphe graminis f. sp. hordei at the first and fifth leaf stages. Samples taken at 32 hr after inoculation were examined by electron microscopy to compare papillae associated either with penetration failure or with successful penetration of the fungus into the epidermal cell and haustorium formation. Four types of papillae with ultrastructural differences could, be classified. Although their definite association with fungal ingress or failure is not possible, our data suggest that papillae with larger, more compacted and amorphous or globular structures may be more effective as penetration barriers than others, with more or less uniform distribution of irregular, smaller electrondense structures.     

Increase of Complex Pathotypes of Erysiphe graminis f. sp. hordei in Two-Component Mixtures of Spring Barley Cultivars

Pathotypes of Erysiphe graminis f. sp. hordei were monitored at fortnightly intervals in pure stands of cultivars Golf with Laevigatum resistance [gene Ml(La)], Steina with Arabische resistance (Mla 12) and Harry with Monte Cristo resistance (Mla9), as well as in two two-component mixtures [Golf + Steina (G + S) and Golf + Harry (G + H)] during the course of mildew epidemics in replicated field trials. The number of colonies per main tiller was also recorded. Virulence complexity (i.e. the average number of virulence genes per isolate with respect to the two matching resistance genes of the mixed cultivars) was always higher in mixtures than in pure stands. Linear regression of complexity on the number of elapsed mildew generations (based on actual temperature data) gave significant b-values (slopes) of 0.039, 0.034, 0.022 and 0.018 on Golf in G + S, Steina in G + S, Golf in G + H and Harry in G + H, respectively. In pure stands, there, was no significant complexity increase on any cultivar. The fact that b-values were higher on either component in the G + S than in the G + H mixture indicates stronger selection for the complex, i.e. two-virulence, pathotypes in G + S. Complex pathotypes had a higher relative fitness than simple (one-virulence) pathotypes in both mixtures but not in the pure stands. The absolute frequencies of complex pathotypes as measured by the area under the colony frequency curve (AUCFC) were higher on all cultivars in mixtures than in pure stands, except on Steina.     

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.     

Virulence Gene Frequency Change in Erysiphe graminis f. sp. hordei Due to Selection by Non-Corresponding Barley Mildew Resistance Genes and Hitchhiking

Individual isolates (in total 4040) of Erysiphe graminis f. sp. hordei were collected between 1989 and 1991 in fields of spring barley varieties with resistance genes Mla9, Mla12 and Ml(La) in the local air spora, i.e. remote from barley fields, in Hesse, Germany. Their virulence pattern was determined on a 13-partite differential set. In the air spora. virulence complexity (the number of virulence genes per isolate) increased by 30% (4.2 vs. 5.5) between 1989 and 1991. This was mainly due to an increase of pathotypes with virulence genes Va7, Val3, Vk and V(La). In each mildew field population, frequencies of several non-corresponding virulence genes, e.g. V(La) on Mla9 variety, greatly increased during the period. Such change can be caused by hitchhiking selection, i.e. indirect selection resulting from asexual reproduction, and direct selection by non-corresponding resistance genes. To separate the two effects, the aerial barley mildew population of 1991 was sampled on cv. Pallas and near-isogenic lines of Pallas. Frequencies of non-corresponding virulence genes as observed in samples from the near-isogenic lines were compared to expected non-corresponding virulence frequencies which were obtained by subdividing the sample on Pallas into 12 subsamples, comprising all isolates virulent to line 1 through 12, respectively. Among 90 resistance/virulence gene combinations selection for non-cor-responding virulence was significant in five cases (e.g. Va9 on Mla7 host) and against non-corresponding virulence in nine cases (e.g. Vk on Ml(La) host). Hitchhiking selection was significant in eight cases. In 11 cases the two types of selection were significant in combination but not individually. It is suggested to monitor the stability of selection against non-corresponding virulence genes in race surveys and utilize it in diversified barley crops such as variety mixtures in order to retard the evolution of complex races.     

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.     

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.     

Recurrent Selection for Adaptation of Erysiphe graminis f. sp. hordei to Partial Resistance and the Effect of Environment on Expression of Partial Resistance of Barley

Five isolates of powdery mildew were subjected to recurrent selection on partially resistant barley cultivars and tested at intervals for adaptation to overcome partial resistance. A trend towards adaptation to the recurrent host was observed in infection frequency data, but biomass per colony, measured using sterol cell-wall determinations and enzyme linked immunosorbent assay, was not greatly affected. The expression of partial resistance was examined after seedling growth under two different environmental regimes. Significant interactions with both overall resistance expression and latent period were detected using infection frequency data, and the effects were both cultivar and isolate dependent. Variability between single colony derivatives of an isolate, which had previously shown adaptation to a partially resistant cultivar, was detected in infection frequency data indicating instability and a non-nuclear component to adaptation.     

The Effect of Infection by Erysiphe graminis f.sp. hordei on Protein Synthesis in Vivo in Leaves of Barley

The effect of infection by Erysiphe graminis f.sp. hordei onprotein synthesis in a susceptible cultivar of Hordeum vulgarehas been studied using in vivo labelling. Results have indicateda decline in protein synthesis in the chloroplasts at 1, 3 and5 days after inoculation and in the cytoplasm at 3 and 5 daysafter inoculation. The most extensive effect of the pathogenwas on the synthesis of a 32 Kd protein synthesised in the chloroplasts. (Received May 16, 1984; Accepted July 10, 1984)     

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.