Studies on the Interaction Between Septoria tritici and Stagonospora nodorum in Wheat

Interactions between Stagonospora nodorum and Septoria tritici were studied. Results from a detached glume experiment indicated that the interaction may be isolate-dependent, as it was shown that the interaction between the two pathogens may be beneficial or antagonistic depending on the isolate of each pathogen present. The number of spores produced by both pathogens was significantly greater when an aggressive isolate of S. tritici was mixed with a non-aggressive isolate of S. nodorum, whereas the number of spores produced by both pathogens was significantly less when two non-aggressive isolates were mixed. There was a significant reduction in disease level when S. tritici was applied prior to S. nodorum, compared to vice versa in the growth chamber. Results from growth chamber and field studies showed that S. nodorum produced significantly more spores when both pathogens were present together. It is concluded that S. tritici has a stimulatory effect on spore production by S. nodorum. However, there was a reduction of S. tritici spores observed in the dual inoculation treatments, suggesting that S. nodorum inhibits S. tritici.     

In vitro production of pycnidia by Septoria tritici

The in vitro production of pycnidia bySeptoria tritici was examined on six media reported to induce the formation of fruiting bodies. Among 26 freshly isolated cultures from various parts of the world, consistent differences in growth type were found which were only partially influenced by nutritional and environmental conditions. Cultures with yeast-like growth produced hardly any pycnidia or pseudopycnidia, while cultures with intermediate or mycelial growth types produced them frequently. Incubation in continuous darkness induced intermediate to mycelial growth types rather than yeast-like growth types in some cultures, and concomitantly the production of more pycnidia. Potato-dextrose agar induced intermediate to mycelial growth types and production of (pseudo)pycnidia more often than V8 agar and wheat leaf extract agar, which had previously been reported to be especially beneficial to (pseudo) pycnidium formation byS. tritici. Isolates with a consistently yeast-like growth type, producing (virtually) no fructifications under any of the experimental conditions, were slightly stimulated to form pseudopycnidia on water agar supplemented with sterile pieces of maize, wheat or carnation leaves.     

Elucidation of Septoria tritici x Wheat Interactions Using GUS-Expressing Isolates

ABSTRACT Isolate ISR398 of Septoria tritici (which produces none to few pycnidia on the wheat cv. Seri 82 and high coverage on cv. Shafir) and isolate ISR8036 (which is virulent on both cultivars) were genetically cotrans-formed using the selectable marker gene hph, which confers resistance to hygromycin B (hygB), and the reporter gene uidA, encoding beta-glucuronidase (GUS). Most of the genetically transformed isolates (98.8%) produced similar pycnidial coverage on seedlings of 'Seri 82' and 'Shafir' as the two wild-type isolates. Southern analysis of 25 randomly selected hygB(R)GUS(+) transformants probed with the uidA sequence revealed multiple insertion sites. GUS activity was determined fluorimetrically by measuring the conversion of 4-methylumbelliferyl beta-D-glucuronide (MUG) to 4-methylumbelliferone (MU). The high GUS-expressing transformants 398D97 and 8036E27 were used to elucidate fungal development within inoculated leaf tissue by using GUS activity to estimate the fungal proteins content in planta. Increase in fungal biomass was recorded in 'Shafir' inoculated with the GUS-expressing transformants 398D97 and 8036E27 following a 12-day latent period. A 15-day latent period was recorded in 'Seri 82' inoculated with 8036E27, whereas an 18-day latent period was recorded on 'Seri 82' inoculated with 398D97 and the two mixtures 398D97 + ISR8036 and ISR398 + 8036E27. The rate of fungal development and the estimated level of fungal proteins at the pycnidia maturation stage was high in leaves of 'Shafir' and moderate to low on 'Seri 82', even in cases in which no significant differences were recorded in pycnidial coverage. An endogenous capacity to hydrolyze beta-1,4-D-glucuronidase was recorded in leaves inoculated with wild-type isolates. The latent periods in MU production of the uidA-expressing transformants mimicked those recorded for the wild-type isolates. However, at all stages, the levels of MU produced in wheat inoculated with wild-type isolates were markedly lower than those produced by GUS-expressing transformants. The mode of interaction (compatible or incompatible) determined the onset of the induction, rate, and level of enzyme production.     

Weak selection by field sprays for flutriafol resistance in Septoria tritici

Change in sensitivity of populations of Septoria tritici resulting from single sprays of the sterol demethylation-inhibiting fungicide flutriafol was measured. Field trials were conducted over 3 years at two sites separated by about 15 km, on cvs Mercia and Riband at each site, in plots at least 10 × 12 m. Treatments included the full recommended rate and a reduced dose of flutriafol and a mixture of flutriafol with chlorothalanil. Sprays were applied at GS37; samples were taken just before spraying and as soon as lesions appeared on leaf 2, which had had no visible disease at the time of spraying. Epidemiological evidence suggests that sprays acted both curatively and as protectants. Significant shifts in sensitivity occurred after spraying in both water-sprayed and fungicide-sprayed plots; all plots became less sensitive. Disease severity after spraying clearly decreased with increasing fungicide concentration and with the use of a mixture of flutriafol and chorothalanil. However, after correction for the shifts in the water-sprayed plots, no dose produced a significant change in population sensitivity level, although sample sizes were large. There was no trend in sensitivity in the population over the 3 years of the experiment.     

Splash dispersal of Leptosphaeria maculans pycnidiospores and the spread of blackleg on oilseed rape

The fungus Leptosphaeria maculans causes blackleg (phoma stem canker), one of the most serious diseases of oilseed rape. The role of pycnidiospores produced during asexual reproduction is poorly documented and limits the understanding of the pathogen's population dynamics. The objectives of this study were to assess rain-splash dispersal of pycnidiospores of L. maculans from phoma leaf spots, and transmission of the disease from oilseed rape stubble carrying pycnidia. The work was conducted in still air with either a drop generator or a rain simulator. The impact of simulated incident drops on phoma leaf spots resulted in the dispersal of L. maculans pycnidiospores within splash droplets. Ninety per cent of the spores were collected within 14 cm of the source and a few were regularly observed up to 40 cm. Pycnidiospores produced on oilseed rape stubble and dispersed by simulated rain infected oilseed rape trap plants in a spatial pattern that matched the spatial dispersal of the pycnidiospores. In the field, rain-splash dispersal of pycnidiospores could increase the pathogen population and may enhance sexual reproduction by facilitating the mating of initially spatially separated isolates of opposite mating type.     

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.     

Host-pathogen relationship of geographically diverse isolates of Septoria tritici and wheat cultivars

Pathogenic variability of 14 Septoria tritici isolates from different locations in the USA (California, Oregon, and Texas) was determined on seedlings of two sets of geographically diverse wheat cultivars under greenhouse conditions. Significant isolate effects, cultivar effects, and isolate × cultivar interactions were found, and a substantial amount of variation was accounted for by the interaction terms compared with the main effects of isolate and cultivar. All isolates were pathogenic on the cultivars tested but the degree of virulence on the individual cultivars varied among isolates. Linear contrasts between all homologous combinations (isolate × cultivar combination of same geographic location) and all heterologous combinations (isolate × cultivar combination of different locations) indicated that homologous combinations produced significantly more disease than heterologous combinations. The results demonstrate location-specific adaptation of S. tritici. Implications of pathogenic variability and local adaptation in S. tritici are discussed.     

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.     

Physiological Aspects Related to Tolerance of Spring Wheat Cultivars to Septoria tritici Blotch

ABSTRACT The susceptible wheat cultivar Miriam exhibited tolerance under severe infection of Septoria tritici blotch (STB). Nethouse and greenhouse trials confirmed former field results in which losses in grain weight of 'Miriam' wheat due to STB infection were significantly lower than those of the susceptible cultivar Barkai, under equivalent severity and the same disease progress curve. Several physiological mechanisms that may explain this tolerance of 'Miriam' wheat were studied. A comparison between protected and infected plants proved that carbohydrate reserves in the culms and other vegetative plant parts did not account for the lower losses in grain weight of 'Miriam'. Each tiller was shown to be independent in its supply of carbohydrates to its grains, and no import from secondary tillers was observed. Differences in the ratio between grain weight and vegetative biomass could not explain the sustained grain filling of infected plants of 'Miriam'. The daily balance of CO(2) exchange of the ears was negative, since carbon fixation by the spike in the light was more than counterbalanced by night time spike respiration. Radioisotope studies revealed that mature, infected 'Miriam' plants maintained as large a percentage of the carbohydrates fixed at the vegetative stage and early grain filling as healthy plants. On the other hand, under the same conditions, infected 'Barkai' plants lost a larger fraction of these carbohydrates. The rate of carbon fixation per unit of chlorophyll and per residual green leaf area of infected 'Miriam' was higher than in healthy plants. It is proposed that this enhancement of photosynthesis in residual green tissue of infected plants of the tolerant cultivar Miriam compensates for the loss of photosynthesizing tissue due to STB.     

Fungitoxicity of plant growth regulators (PGRs) and PGR/fungicide mixtures in soak treatments to Septoria apiicola pycnidiospores

Mixtures of the plant growth regulators (PGRs) gibberellins A4 and A7 (GA 4/7) and ethephon (2-chloroethylphosphonic acid), applied as soak treatments at 20° C for 24 h, were toxic to pycnidiospores of Septoria apiicola on agar, apparently as a result of low pH of their solutions. Ultraviolet spectroscopy showed that the aqueous solubility of the fungicides benomyl and carbendazim increased significantly when buffered at low pH or mixed with aqueous solutions of GA 4/7 and ethephon. This resulted in greater quantities of ¹⁴C-labelled fungicides entering celery seeds imbibed in PGR/fungicide mixtures in comparison with fungicides alone.
A 24-h seed soak at 20° C in a mixture of aqueous benomyl, GA 4/7 and ethephon eliminated S. apiicola in infected celery seeds. A thiram soak was also effective but PGR mixtures alone did not completely eliminate the pathogen.     

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.     

Suppression of Septoria tritici and Puccinia recondita of wheat by an antibiotic-producing fluorescent pseudomonad

A fluorescent Pseudomonas sp. (strain LEC 1) isolated from soil suppressed Septoria tritici by 88% and Puccinia recondita by 98% when applied to wheat seedlings 3 h prior to inoculation with the pathogens. Fractionation and analysis of two inhibitory compounds from cultures of strain LEC 1 on thin-layer chromatography plates yielded the phenazine antibiotics 1-hydroxyphenazine (phOH) and chlorora-phin. 1-hydroxyphenazine reduced the growth of Pythium aphanidermatum, Fusarium oxysporum f.sp. vasinfectum, Sclerotinia sclerotiorum and S. rolfsii on a denned medium, with greatest activity against P. aphanidermatum and least against S. rolfsii. Application of 160 mg/l phOH to wheat seedlings reduced S. tritici by 61% and P. recondita by 75%. Lower concentrations of the antibiotic were less effective. Growth of 5. tritici in liquid medium was reduced by phOH at 5–20 mg/l and prevented at 40 mg/l. When phOH was added to the medium 48 h after inoculation (logarithmic growth phase), growth was interrupted but later recovered with phOH concentrations up to 40 mg/l; there was no recovery at 60 mg/l. Chlororaphin inhibited the growth of S. tritici on malt agar medium.     

Modeling of Relationships Between Weather and Septoria tritici Epidemics on Winter Wheat: A Critical Approach

ABSTRACT Two models for predicting Septoria tritici on winter wheat (cv. Riband) were developed using a program based on an iterative search of correlations between disease severity and weather. Data from four consecutive cropping seasons (1993/94 until 1996/97) at nine sites throughout England were used. A qualitative model predicted the presence or absence of Septoria tritici (at a 5% severity threshold within the top three leaf layers) using winter temperature (January/February) and wind speed to about the first node detectable growth stage. For sites above the disease threshold, a quantitative model predicted severity of Septoria tritici using rainfall during stem elongation. A test statistic was derived to test the validity of the iterative search used to obtain both models. This statistic was used in combination with bootstrap analyses in which the search program was rerun using weather data from previous years, therefore uncorrelated with the disease data, to investigate how likely correlations such as the ones found in our models would have been in the absence of genuine relationships.     

Factors determining the severity of epidemics of Mycosphaerella graminicola (Septoria tritici) on winter wheat in the UK

Epidemics of disease caused by Septoria tritici were studied in detail in 11 crops of winter wheat cv. Longbow over 4 years. Serious damage to the uppermost two leaf layers was caused by splash-borne infection from lower in the crop early in the life of the leaves, followed by one or rarely two cycles of multiplication within a leaf layer. Infection conditions rarely limited damage, even in a dry year; the timing and, to a lesser extent, amount of initial inoculum movement to an upper leaf layer was of greater importance. Timing of initial infection was determined by when rain splash occurred in relation to emergence of a leaf layer. Occurrence of infections soon after a leaf layer started to emerge allowed more time for multiplication of disease within that layer. These infections tended to be more severe because the leaves were closer to inoculum sources within the crop. Slight differences in phenology between locations explain why initially random disease distributions sometimes become aggregated. Early-sown crops are at greater risk because they mature more slowly, allowing more disease multiplication and better transfer between leaf layers.     

Resistance stability to Septoria tritici blotch and comparison of screening methods for ranking STB disease

Journal of Plant Diseases and Protection - Septoria tritici blotch (STB), caused by the ascomycete fungus Zymoseptoria tritici (formerly known as Mycosphaerella graminicola), is a devastating...     

Wheat Leaf Rust Uredospore Production on Adult Plants: Influence of Leaf Nitrogen Content and Septoria tritici Blotch

ABSTRACT Leaf rust uredospore production and lesion size were measured on flag leaves of adult wheat plants in a glasshouse for different lesion densities. We estimated the spore weight produced per square centimeter of infected leaf, per lesion, and per unit of sporulating area. Three levels of fertilization were applied to the plants to obtain different nitrogen content for the inoculated leaves. In a fourth treatment, we evaluated the effect of Septoria tritici blotch on leaf rust uredospore production. The nitrogen and carbon content of the spores was unaffected or marginally affected by lesion density, host leaf nitrogen content, or the presence of Mycosphaerella graminicola on the same leaf. In leaves with a low-nitrogen content, spore production per lesion was reduced, but lesion size was unaffected. A threshold effect of leaf nitrogen content in spore production was however, evident, since production was similar in the medium- and high-fertilizer treatments. In leaves inoculated with M. graminicola and Puccinia triticina, the rust lesions were smaller and produced fewer spores. The relationships among rust lesion density, lesion size, and uredospore production were fitted to a model. We determined that the density effect on spore production resulted mainly from a reduction in lesion size, the spore production per unit of sporulating surface being largely independent of lesion density. These results are consistent with those obtained previously on wheat seedlings. The main difference was that the sporulation period lasted longer in adult leaves.     

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.     

Inhibition of Septoria tritici and other phytopathogenic fungi and bacteria by Pseudomonas fluorescens and its antibiotics

Pseudomonas fluorescens strain PFM2, antagonistic to Septoria tritici , also inhibited growth of several other phytopathogenic bacteria and fungi in vitro. Three inhibitory compounds were extracted from liquid glycerol-peptone-phosphate medium in which strain PFM2 was grown for 4 weeks. The compound recovered in greatest quantity (70 mg/1) was identified as 2,4-diacetylphloroglucinol. The other two antibiotics, which were produced in small amounts (< 5 mg/l), were not identified. A similar pattern of inhibition against the different bacteria and fungi was exhibited by both 2,4-diacetylphloroglucinol and strain PFM2 when grown in a medium of the same composition as that in which the antibiotic was produced.