Influence of crop growth and structure on the risk of epidemics by Mycosphaerella graminicola (Septoria tritici) in winter wheat

Generally, it is recognized that inocula of Septoria tritici present on the basal leaves of winter wheat crops are spread towards the top of the canopy by splashy rainfall. This mechanism of inoculum dispersal is commonly accepted to be a key limit on disease progression. Therefore, attempts to forecast epidemics of S. tritici often quantify rainfall by some means, but largely ignore measurement of pathogen and host variables. In the present study, we show that new wheat leaves emerge initially at a height below established leaves that can contain sporulating lesions of S. tritici . This presents the possibility of horizontal inoculum transfer, even without splashy rainfall. The extent and duration of overlap between emergent and established leaves was found to differ considerably with cultivar and sowing date. Nitrogen application had little effect on overlap, because differences in crop phenology, e.g. leaf area and nodal length, were relative. However, estimates of raindrop penetration to the base of crop canopies suggested that vertical movement of inoculum is affected by nitrogen application. Crops receiving more nitrogen are denser, and therefore less rainfall reaches the base of the canopy. The interactions between crop and pathogen development are discussed with reference to the implications for predicting disease risk. In particular, cultivar traits that promote disease escape are quantified.     

Airborne inoculum as a major source of Septoria tritici (Mycosphaerella graminicola) infections in winter wheat crops in the UK

The pattern and extent of primary infection by Septoria tritici were compared over a period of 3 years in winter wheat grown at sites with differing histories and from seed stocks obtained in different countries, in the open, under airtight cover and in sterilized soil. Only the airtight cover altered the number of lesions found, substantially reducing it. Lesions were evenly distributed. Lesions were found throughout the autumn and occasionally in the winter and spring on wheat seedlings exposed in trays to the open air for periods of 1 week, then given good conditions for infection to occur. This was true even at a site 0.4 km distant from wheat residues. The results show that the main source of primary infection of winter wheat in these experiments was evenly dispersed and airborne. It probably consisted mainly of ascospores of Mycosphaerella graminicola.     

Occurrence of Mycosphaerella graminicola, teleomorph of Septoria tritici, on wheat debris in the UK

Ascocarps of Mycosphaerella graminicola regularly developed in debris of wheat crops from eastern England after natural weathering. Identity with the Septoria tritici anamorph and pathogenicity to wheat were demonstrated by isolation and inoculation. Ascospores were present from autumn onwards and were most frequent in December and January. They may contribute to the primary inoculum for epidemics on autumn-sown wheat.     

A detached seedling leaf technique to study resistance to Mycosphaerella graminicola (anamorph Septoria tritici) in wheat

A detached seedling leaf technique was developed to screen for resistance to septoria tritici blotch of wheat and to detect specific interactions between cultivars and isolates. Wheat seedlings were inoculated with spore suspensions of Mycosphaerella graminicola . Detached primary leaves were then placed in a clear plastic box such that their cut ends were sandwiched between layers of agar containing benzimidazole, with a gap below the middle of the leaves. Mean levels of disease were affected by light and temperature, and also by the concentration of benzimidazole, such that higher concentrations resulted in less disease. Second leaves were more susceptible than seedling primary leaves. However, none of these factors affected ranking of disease among cultivars or cultivar-by-isolate interactions. Kavkaz–K4500 1.6.a.4, Synthetic 6x and Triticum macha showed specific susceptibility and resistance to different isolates. The detached leaf technique could be a useful complement to field trials and an alternative to whole seedling assays in assessing cultivar resistance and investigating the genetics of the host–pathogen interaction.     

Interacting effects of interrupted humid periods and light on infection of wheat leaves by Mycosphaerella graminicola (Septoria tritici)

Septoria tritici conidia require a long period at 100%, humidity to infect wheat plants successfully. When periods of 100% relative humidity were interrupted by up to 48 h at 75% relative humidity, infection by S. tritici on winter wheat cvs Longbow and Avalon was only slightly reduced. Breaks at 50% relative humidity had larger effects, but still allowed infection to occur. Infection was reduced more by a break in humidity which began in the light, but previous and subsequent light regimes strongly influenced the final outcome in all cases. When a wet period was interrupted twice, the two dry breaks interacted strongly under some circumstances; in these cases the probability of infection was much greater than expected from multiplying the effects of the single breaks. Under other conditions two breaks had more nearly multiplicative effects, or interacted to reduce the probability of infection below that expected from the individual breaks. There was evidence that light reduced the probability of successful infection earls in the infection process, but stimulated it later. The data were consistent with infection occurring faster on cv. Longbow, but otherwise being affected in qualitatively similar ways on both cultivars. The complexity of the effects and their interactions suggests that it will not be possible to find a simple set of infection conditions to apply in the field. The data rule out a number of simple mathematical models of the infection process.     

The occurrence of Mycosphaerella graminicola, teleomorph of Septoria tritici in France

Pseudothecia of Mycosphaerella graminicola , the teleomorph of Septoria tritici , were observed on wheat debris in France for the first time during autumn 1994. The colonies and conidia, derived from ascospores isolated by micromanipulation, conform to previously published data.     

Factors affecting development of Septoria tritici in winter wheat and its effect on yield

Severe infection with Septoria tritici occurred in four of five experiments designed to create a series of different disease epidemics. These experiments successfully identified periods suitable for infection. They also indicated the effect of sprays, timed before and after these periods, on disease development and yield.
Analysis of disease progress and weather records suggested that critical conditions for initial development of S. tritici occurred during early May at four sites. Heavy rain giving at least 10 mm on 1 day or a total of 10 mm or more on 2 or 3 successive days occurred at all four sites prior to the appearance of symptoms on a particular leaf layer, though less heavy rain may suffice to splash inoculum onto upper leaves in shorter, immature canopies where stem elongation is incomplete. At the fifth site, infection occurred later in May and disease failed to develop to a significant degree. At all sites, the length of the incubation period on any of the top three leaves was found to be between 396 and 496 degree days.
Control of winter epidemics of S. tritici had little effect on yield, whereas spray sequences commencing later than growth stage (GS) 31 but immediately prior to the critical periods provided the best disease control and yield benefit. Regression models incorporating, as independent variables, area under the S. tritici disease progress curve for any of the top three leaves from their emergence (GS 32-37) satisfactorily explained yield loss at the four sites where disease was severe. Consideration of leaf 2 or leaf 3 alone accounted for more than 82% of the variance at each site and a yield loss from infection of leaf 2 related to thermal time is suggested as 0.00265% per C per day from the appearance of symptoms.     

Sensitivity of Mycosphaerella graminicola (anamorph: Septoria tritici) to DMI fungicides across Europe and impact on field performance

Since the occurrence and spread of resistance to quinone outside inhibitors (QoI) in Mycosphaerella graminicola in the early 2000s in Europe, demethylation inhibitors (DMIs) form the backbone for control of Septoria leaf blotch. European monitoring studies, carried out by various research institutes and DMI manufacturers, have shown a shift of the European M. graminicola population towards increased ED₅₀ values for DMI fungicides. Populations of M. graminicola consist of very heterogeneous isolates within a region, and even within a field, in terms of DMI sensitivity. Sensitivity to DMIs is influenced by the haplotype of CYP51, the target of DMIs. New CYP51‐haplotypes have emerged and the frequency of less sensitive haplotypes in Europe has increased in recent years. Studies with efflux transporter inhibitors showed that not only CYP51, but also enhanced efflux, may play a role in the DMI sensitivity response. Sensitivity studies with 5 DMIs registered for Septoria leaf blotch control indicated that sensitivity of isolates to the 5 DMIs is heterogeneous and the overall correlation of sensitivity to the different DMIs is poor. A key requirement for sustainable control and resistance management of Septoria leaf blotch is therefore the continued availability of different DMIs.     

The Occurrence of Mycosphaerella graminicola and its Anamorph Septoria tritici in Winter Wheat during the Growing Season

The disease septoria tritici blotch of wheat is initiated by ascospores of the teleomorph Mycosphaerella graminicola or pycnidiospores of the anamorph Septoria tritici. We report for the first time the presence of the teleomorph, M. graminicola, in Denmark. With the objective of elucidating the importance of the teleomorph for the development of septoria tritici blotch, data on the occurrence of fruit bodies of the anamorph (pycnidia) and the teleomorph (pseudothecia) stages were collected over three growing seasons. Pseudothecia were present in the springs, however, high numbers of pseudothecia compared to pycnidia were not observed until July, too late to influence the epidemic. On an individual leaf layer, pycnidia were observed well before pseudothecia. As the leaves aged, progressively higher proportions of fruit bodies were observed to be pseudothecia. The period from the appearance of pycnidia to detection of pseudothecia was estimated as 29–53 days. At harvest, high proportions of sporulating fruit bodies in the crop were pseudothecia, suggesting that the primary source of inoculum for new emerging wheat crops in autumn is likely to be ascospores.     

Partial Resistance to Septoria Tritici Blotch (Mycosphaerella graminicola) in Wheat Cultivars Arina and Riband

ABSTRACT Partial resistance to Septoria tritici blotch (STB) and its inheritance were investigated in a doubled-haploid population of a cross between cvs. Arina and Riband. The former has good partial resistance whereas the latter is susceptible. In adult plant trials in polytunnels, STB disease scores were negatively correlated with heading date. Resistance was not specific to any of the three fungal isolates used in these tests. A quantitative trait locus (QTL) for partial resistance to STB was identified in Riband on chromosome 6B and is named QStb.psr-6B-1. No QTL controlling a major part of the Arina resistance was identified, suggesting that its resistance may be dispersed and polygenic. There was no correlation between the lines' mean disease scores at the seedling and adult stages, implying that partial resistance to STB is developmentally regulated. Seedling resistance to the isolate IPO323 was isolate-specific and controlled by a single gene in Arina, probably allelic with the Stb6 gene in cv. Flame that confers resistance to the same isolate. The implications of these results for wheat breeding programs are discussed.     

Quantitative trait loci for adult-plant resistance to Mycosphaerella graminicola in two winter wheat populations

Septoria tritici blotch (STB) is one of the most important leaf spot diseases in wheat worldwide. The goal of this study was to detect chromosomal regions for adult-plant resistance in large winter wheat populations to STB. Inoculation by two isolates with virulence to Stb6 and Stb15, both present in the parents, was performed and STB severity was visually scored plotwise as percent coverage of flag leaves with pycnidia-bearing lesions. 'Florett'/'Biscay' and 'Tuareg'/'Biscay', each comprising a cross of a resistant and a susceptible cultivar, with population sizes of 316 and 269 F(7:8) recombinant inbred lines, respectively, were phenotyped across four and five environments and mapped with amplified fragment length polymorphism, diversity array technology, and simple sequence repeat markers covering polymorphic regions of ≈1,340 centimorgans. Phenotypic data revealed significant (P < 0.01) genotypic differentiation for STB, heading date, and plant height. Entry-mean heritabilities (h(2)) for STB were 0.73 for 'Florett'/'Biscay' and 0.38 for 'Tuareg'/'Biscay'. All correlations between STB and heading date as well as between STB and plant height were low (r = -0.13 to -0.20). In quantitative trait loci (QTL) analysis, nine and six QTL were found for STB ratings explaining, together, 55 and 51% of phenotypic variation in 'Florett'/'Biscay' and 'Tuareg'/'Biscay', respectively. Genotype-environment and QTL-environment interactions had a large impact. Two major QTL were detected consistently across environments on chromosomes 3B and 6D from 'Florett' and chromosomes 4B and 6B from 'Tuareg', each explaining 12 to 17% of normalized adjusted phenotypic variance. These results indicate that adult-plant resistance to STB in both mapping populations was of a quantitative nature.     

Spatial distributions of Septoria nodorum and S. tritici within crops of winter wheat

In some crops of winter wheat selected from a range monitored in Western Europe during 1981-83, Septoria nodovum and S . tritici were spatially very uniformly distributed from the beginning of the growing season onwards. A cube root transformation produced a constant variance in lesion numbers per leaf, similar for both pathogens at about 0.5. This permits the sample size needed for a given accuracy to be estimated. Counts of conidia washed from leaves by a standard procedure had a constant coefficient of variation, independent of disease level. Large samples would be necessary for accurate counts, particularly if leaves from different layers were examined separately. The pattern of lesion numbers on leaves was best described by a negative binomial distribution: this predicts an incidence-severity curve to which the data conformed closely. Hence incidence estimates can be used to estimate severity, which may be more economical of sampling effort. Correlations between lesion counts on different leaves of the same tiller were negative and highly significant for S. nodorum in 1982, but positive and significant for S. tritici in 1983. The causes of this difference are unknown.     

Effects of temperature, leaf wetness and cultivar on the latent period of Mycosphaerella graminicola on winter wheat

After inoculation of winter wheat cv. Longbow at a single time, lesions of M. graminicola were produced over a long interval starting 15–35 days after inoculation, dependent on temperature. There was no evidence that a single infection gave rise to more than one lesion. After the initial infection period at 100% relative humidity (r.h.), keeping leaves wet for c. 10 h per day did not shorten latent period on seedlings. Experiments in controlled-environment chambers demonstrated a minimum latent period at approximately 17°C Variation in the latent period of individual lesions was also minimum at this temperature. The latent period varied among the cultivars tested, cv. Longbow having the shortest, cv. Avalon having almost the longest. Field observations broadly confirmed the results of experiments in constant-environment chambers.     

Specificity of incomplete resistance to Mycosphaerella graminicola in wheat

We examined interactions between wheat (Triticum aestivum) and Mycosphaerella graminicola, causal agent of Septoria tritici blotch, to determine whether specific interactions occur between host and pathogen genotypes that could be involved in eroding resistance. The moderate resistance of the wheat cultivar Madsen has eroded significantly in the Willamette Valley of Oregon since its release in 1990. Foote is a replacement cultivar expressing moderate resistance and was released in 2000. Isolates of M. graminicola were collected from Foote and Madsen in 2004 and 2005 and tested on each cultivar in growth chamber and greenhouse experiments. There was a significant (P 相似文献   

Resistance of wheat to septoria tritici blotch (Mycosphaerella graminicola) and associations with plant ideotype and the 1BL−1RS translocation

Septoria tritici blotch (STB) is a major disease of wheat, reaching epidemic proportions in many parts of the world. In several studies, taller, later-maturing cultivars have had lower disease levels. This study was undertaken to investigate the genetic associations of natural field infection by STB with disease-escape mechanisms related to heading date and height components, mainly leaf spacing, in a population where height and maturity are not controlled by major genes. In field trials of a single seed-descent population of a cross between two nonsemi-dwarf cultivars, Apollo (with strong partial resistance to STB) and Thésée (susceptible), conducted over 3 years, there was a negative correlation between STB and heading date. There was no correlation between STB and distance from stem base to leaf 2; and there was an unexpected positive correlation between STB and distance from flag leaf to leaf 2, which contradicted the so-called 'ladder effect' postulated in STB epidemiology. No effect was detected of the presence of the 1BL−1RS translocation on STB levels. The largest single contributor to variation in STB levels was genetic variation between the progeny lines, and the narrow-sense heritability was 42%. These results suggest that breeders can select for STB resistance alongside optimal stature within the range of height which is adaptive in a particular environment.     

Pyraclostrobin reduces germ tube growth of QoI‐resistant Mycosphaerella graminicola pycnidiospores and the severity of septoria tritici blotch on winter wheat

The effect of the quinone outside inhibitors (QoI) azoxystrobin and pyraclostrobin on yields of winter wheat where QoI resistant Mycosphaerella graminicola isolates were dominant was investigated in field trials in 2006 and 2007. Pyraclostrobin significantly increased yields by 1·57 t ha^?1 in 2006 and 0·89 t ha^?1 in 2007 when compared to the untreated controls, while azoxystrobin only provided a significant increase of 1·28 t ha^?1 in 2006. These yield increases were associated with reduction in septoria tritici blotch (STB) development as determined by weekly disease assessments over a 7 week interval. The effect of pyraclostrobin on STB was studied in controlled environment experiments using wheat seedlings inoculated with individual M. graminicola isolates. Pyraclostrobin significantly reduced STB symptoms by up to 62%, whether applied 48 h pre‐ or post‐ inoculation with resistant M. graminicola isolates containing the cytochrome b mutation G143A. Extremely limited disease (<1%) was observed on similarly treated seedlings inoculated with an intermediately resistant isolate containing the cytochrome b mutation F129L, while no disease was observed on seedlings inoculated with a wild‐type isolate. Germination studies of pycnidiospores of M. graminicola on water agar amended with azoxystrobin or pyraclostrobin showed that neither fungicide inhibited germination of spores of resistant isolates containing the mutation G143A. However, pyraclostrobin significantly reduced germ tube length by up to 46% when compared with the untreated controls. Although the QoIs can no longer be relied upon to provide effective M. graminicola control, this study provides an insight into why QoIs still provide limited STB disease control and yield increases even in situations of high QoI resistance.     

Frequency of sexual recombination by Mycosphaerella graminicola in mild and severe epidemics

The importance of sexual recombination in determining fungal population structure cannot be inferred solely from the relative abundance of sexual and asexual spores and reproductive structures. To complement a previously reported study of proportions of Mycosphaerella graminicola ascocarps and pycnidia, we investigated the share of sexual recombinants among isolates randomly derived from the same field at the same time. Early in three successive growing seasons (those ending in 1998, 1999, and 2000), field plots of the susceptible winter wheat cultivar Stephens were inoculated with suspensions of two M. graminicola isolates that each had rare alleles at restriction fragment length polymorphism (RFLP) loci. Near harvest time, leaves were randomly sampled from the same plots, and a population of over 100 monopycnidial isolates was created for each year of the experiment. Natural populations were also sampled from noninoculated plots in the 1999 and 2000 seasons, in order to compare allele frequencies. Based on RFLP haplotypes and DNA fingerprints, isolates from the inoculated plots were categorized by both inspection and Bayesian methods as inoculant clones, recombinants, or immigrants. Inoculation in the 2000 season was delayed, and the recovery rate of inoculant types was just 1 to 2%. In 1998, a high-disease year, and 1999, a low-disease year, inoculants comprised 36 and 22 to 23% of end-of-season samples, respectively. In those 2 years, recombinants as a percentage of inoculant descendants (both sexual and asexual) were 35 and 32%, respectively. By comparison, the study of fruiting bodies had found 93 and 32% of M. graminicola fruiting bodies were ascocarps in 1998 and 1999, respectively. These findings support the hypothesis that sexual recombination makes a relatively consistent contribution to M. graminicola population structure, despite differences in epidemic severity and ascocarp proportions.     

Quantitative PCR monitoring of the effect of azoxystrobin treatments on Mycosphaerella graminicola epidemics in the field

Quantitative PCR and visual monitoring of Mycosphaerella graminicola epidemics were performed to investigate the effect of curative and preventative applications of azoxystrobin in wheat field crops. A non-systemic protectant and a systemic curative fungicide, chlorothalonil and epoxiconazole, respectively, were used as references. PCR diagnosis detected leaf infection by M graminicola 3 weeks before symptom appearance, thereby allowing a clear distinction between curative and preventative treatments. When applied 1 week after the beginning of infection, azoxystrobin curative activity was intermediate between chlorothalonil (low effect) and epoxiconazole. When applied preventatively, none of the fungicides completely prevented leaf infection. There was some indication that azoxystrobin preventative treatments may delay fungal DNA increase more than epoxiconazole at the beginning of leaf infection. Both curative and preventative treatments increased the time lapse between the earliest PCR detection and the measurement of a 10% necrotic leaf area. Azoxystrobin only slightly decreased the speed of necrotic area increase compared with epoxiconazole. Hence, azoxystrobin activity toward M graminicola mainly resides in lengthening the time lapse between the earliest PCR detection and the measurement of a 10% necrotic leaf area. Information generated in this way is useful for optimal positioning of azoxystrobin treatments on M graminicola.