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.     

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.     

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.     

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.     

Interactions between Erysiphe graminis and Septoria nodorum on wheat

Interactions between Erysiphe graminis f.sp. tritici and Septoria nodorum on wheat were studied in the greenhouse and in a 2-year field experiment using artificial inoculation. The integrated form of the logistic growth model dy/dt = ry (1 -y/K ), with infection rate r and final accumulated disease K , was fitted to the disease progress data. Septoria nodorum substantially reduced the disease severities of E. graminis , and caused significant reductions of at least 60% in final accumulated disease K of E. graminis. In the field trials, E. graminis increased the final accumulated disease K of S. nodorum. Owing to the extremely low severity of E. graminis , the increase of S. nodorum severity was small, and significance was given in one of the two years only, with an increase in K of roughly 30%. In the pot experiment, final accumulated disease K of S. nodorum remained unchanged, but there was a significant 30% increase in the infection rate r of S. nodorum. The difference between field and pot trials was explained by the climatic conditions in the greenhouse which excluded secondary infections of 5. nodorum , and which are important factors for disease progress in the field.     

Effects of treatments to perennial ryegrass on the development of Septoria spp. in a subsequent crop of winter wheat

An experiment started in spring 1979 tested the effects of different treatments to perennial ryegrass, established as pure stands or undersown in spring wheat, on the severity of septoria diseases on the grass and in a following crop of winter wheat, sown in autumn 1980. Other plots were fallowed in 1979 and 1980 before sowing to winter wheat, also in autumn 1980.
Septoria tritici was most severe in winter wheat after fallow and least severe in wheat that had been direct-drilled after ryegrass. These effects were attributed to differences in amounts of available nitrogen, and consequent differences in crop growth, rather than to any differences in primary inoculum.
Symptoms attributed to S. nodorum on the ryegrass were more common where the grass had been established under spring wheat than where it had been sown as a pure stand. They also tended to be more common where ryegrass had been inoculated with spores of S. nodorum than where it had been sprayed with captafol. Similar effects on symptoms attributed to Septoria spp. (mostly S. tritici ) on the wheat were apparent in July. The results support the conclusion that the greater severity of septoria diseases that often occurs on wheat after grass than after non-graminaceous breaks, is probably due in part to survival of the pathogens on grass and, in some circumstances, on debris remaining from a previous wheat crop. However, other factors are also likely to be involved.     

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.     

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.     

Yield Reduction in Wheat in Relation to Leaf Disease From Yellow (tan) Spot and Septoria Nodorum Blotch

Yellow or tan spot (caused by Pyrenophora tritici-repentis) and septoria nodorum blotch (caused by Phaeosphaeria nodorum) occur together and are a constraint to wheat yields in Australia. Recently, higher crop yields and lower fungicide costs have made fungicides an attractive management tool against these diseases. Yield-loss under different rates of progress of yellow spot and septoria nodorum blotch was examined in four experiments over three years to define the relationship between disease severity and yield. In these experiments, differences in disease were first promoted by inoculations either with P. tritici-repentis-infected stubble or aqueous spore suspensions of P. nodorum. Disease progress was further manipulated with foliar application of fungicide. The pattern of disease development varied in each year under the influence of different rainfall patterns. The inoculation and fungicide treatments produced differences in disease levels after flag leaf emergence. The infection of yellow spot or septoria nodorum blotch caused similar losses in grain yield, ranging from 18% to 31%. The infection by either disease on the flag or penultimate leaf provided a good indication of yield-loss. Disease severity on flag leaves during the milk stage of the crop or an integration of disease as area under the disease progress curve on the flag leaves based on thermal time explained more than 80% variance in yield in a simple regression model. The data provided information towards the development of disease management strategies for the control of septoria nodorum blotch and yellow spot.     

Effects of inoculum density,leaf wetness duration and nitrate concentration on the occurrence of lettuce leaf spot

In Ehime Prefecture, Japan, lettuce leaf spot (Septoria lactucae) caused huge losses in marketable lettuce yields. To explore potential measures to control disease outbreaks, the effects of inoculum density, leaf wetness duration and nitrate concentration on the development of leaf spot on lettuce (Lactuca sativa) were evaluated. Conidia were collected from diseased plants in an infested field by single-spore isolation and were used to inoculate potted lettuce plants with different conidial concentrations. Lesions developed on inoculated lettuce plants at inoculum concentrations from 10⁰ to 10⁶ conidia/ml. The disease was more severe when the inoculum exceeded 10² conidia/ml, and severity increased with increasing concentrations. Assessment of the relationship between disease development and the duration of postinoculation leaf wetness revealed that symptoms appeared when the inoculated plants remained wet for 12 h or longer. The number of lesions and total nitrogen content in the lettuce leaves both increased when nitrate was applied.     

Assessment of upward movement of rain splash using a fluorescent tracer method and its application to the epidemiology of cereal pathogens

To help understand inoculum transport in splash-dispersed cereal pathogens, the pattern and extent of upward movement of splash droplets produced by rain was measured using a 'splashmeter'. This comprised a cylinder of chromatography paper arranged vertically at the centre of an annular reservoir containing UV-fluorescent cellulose-binding dye. Natural rainfall from April to August 1985 was examined over grass and, latterly, in a wheat crop; artificial rain generated in a rain tower was used for controlled experiments. The decay in proportion of droplets reaching a given height corrected to equivalent receptor areas was exponential above 5–10 cm. The height at which equivalent areas of receptor were covered with dye was strongly dependent on the drop size spectrum of the incident rainfall, rising when larger drops were present. Presumably because of this, the results show that splash height could not be predicted from data on rainfall volume rate alone. Field observations suggested that a sudden outbreak of Septoria tritici lesions in a wheat crop could be related to unusually great upward transport of rain splash detected 3 weeks earlier by the splashmeter.     

Decreasing azole sensitivity of Z. tritici in Europe contributes to reduced and varying field efficacy

Journal of Plant Diseases and Protection - Septoria tritici blotch (STB; Zymoseptoria tritici) is the most important leaf disease of wheat in Northern and Western Europe. The problem of fungicide...     

Position of inoculum in the canopy affects the risk of septoria tritici blotch epidemics in winter wheat

The spread of septoria leaf blotch in wheat caused by Mycosphaerella graminicola is widely reported to depend on the occurrence of splashy rainfall. Previous studies have also implicated an important effect of canopy architecture on the risk of disease spread. This is because architecture affects the proximity of the yield-forming leaves to inoculum present on older diseased leaves within the crop. This study demonstrated that infection of the final three leaves of winter wheat could occur in the absence of splashy rainfall. For cvs Riband and Longbow, the final two leaf layers emerged at a position ≈ 8 cm below older leaves containing sporulating lesions. Under these conditions, infection of new leaves occurred in treatments that simulated dew and nonsplashy rainfall. These treatments resulted in disease incidences of 10–40% above the untreated control on the final two leaf layers. Within a season, the distance between diseased and healthy leaves during the period of stem extension varied substantially across a range of 30 cultivars. While the magnitude of these differences was not the same across seasons, the relative differences between cultivars were generally consistent, suggesting a strong genotype influence on lesion proximity. This study shows how knowledge of the distribution of lesion proximity within a crop can be used to estimate the risk of inoculum transfer for a given maximum splash height. A rapid crop-monitoring method for estimating the distribution of lesion proximity was developed and tested. Lesion proximity was manipulated by plant growth regulator (PGR) treatments. Significant increases in disease, between 14 and 62%, were observed on the upper canopy leaves of plants treated with PGR. The largest differences were observed in treatments where lesion proximity was most affected.     

Effects of ozone exposure and leaf age of wheat on infection processes of Septoria nodorum Berk.

The infection process of Septoria nodorum was studied on the flag and second leaves of wheat plants at the flag-leaf emergence growth stage (GS 39), after pre-inoculative fumigation with different doses of ozone (0, 80, 180 or 240 μg/m³) for 7 h daily over 7 days. On ozone-treated plants, formation of appressoria occurred earlier, leading to a greater number and density of appressoria and faster growth of superficial infection hyphae than on plants grown without ozone amendment. Additionally, on leaves treated with 180 and 240 μg/m³ ozone, the percentage of papillae per appressorium was only half that on control plants. The number of germ tubes per conidium was not significantly affected by any treatment. Effects of increasing leaf age on the infection process were similar to those obtained with ozone. These results provide further evidence for similarities in the physiological effects of ozone and leaf ageing, which support the hypothesis that disease alterations due to ozone are strongly determined by the senescing effects of the pollutant.     

Quantitative trait loci for seedling and adult plant resistance to Stagonospora nodorum in wheat

Stagonospora nodorum blotch (SNB) caused by Stagonospora nodorum is a severe disease of wheat (Triticum aestivum) in many areas of the world. S. nodorum affects both seedling and adult plants causing necrosis of leaf and glume tissue, inhibiting photosynthetic capabilities, and reducing grain yield. The aims of this study were to evaluate disease response of 280 doubled haploid (DH) individuals derived from a cross between resistant (6HRWSN125) and susceptible (WAWHT2074) genotypes, compare quantitative trait loci (QTL) for seedling and adult plant resistance in two consecutive years, and assess the contribution of QTL on grain weight. Flag leaves and glumes of individuals from the DH population were inoculated with mixed isolates of S. nodorum at similar maturity time to provide accurate disease evaluation independent of morphological traits and identify true resistance for QTL analysis. Fungicide protected and inoculated plots were used to measure relative grain weight (RGW) as a yield-related trait under pathogen infection. The lack of similar QTL and little or no correlation in disease scores indicate different genes control seedling and adult plant disease and independent genes control flag leaf and glume resistance. This study consistently identified a QTL on chromosome 2DL for flag leaf resistance (QSnl.daw-2D) and 4BL for glume resistance (QSng.daw-4B) from the resistant parent, 6HRWSN125, explaining 4 to 19% of the phenotypic variation at each locus. A total of 5 QTL for RGW were consistently detected, where two were in the same marker interval for QSnl.daw-2D and QSng.daw-4B indicating the contribution of these QTL to yield related traits. Therefore, RGW measurement in QTL analysis could be used as a reliable indicator of grain yield affected by S. nodorum infection.     

Adaptation of Septoria nodorum to wheat or barley on detached leaves

Single-spore cultures of wheat- and barley-adapted isolates of Septoria nodorum were serially passaged through detached leaves of wheat or barley. Rare instances of change in host adaptation were observed after passaging but associated changes in other characters, including heterokaryon compatibility, identified the changed isolates as contaminants. Inoculation of progressively more dilute spore suspensions to detached leaves revealed that some isolates were highly infective to the host to which they were adapted, with a high probability of a single spore initiating infection. Inocula deliberately contaminated with a small proportion of spores of opposite host adaptation revealed changes when passaged on wheat that mimicked the rare changes observed on passaging pure wheat- or barley-adapted isolates. These results suggest that adaptation to wheat or barley is a stable property of pure cultures of S. nodorum and that the occasional changes in host adaptation observed on passaging result from selection of contaminants.     

Effect of foliar diseases of wheat on the physiological processes affecting yield under semi-arid conditions

The effects of Septoria leaf blotch (caused by Mycosphaerella graminicola) and leaf rust (caused by Puccinia recondita ) on the physiological processes affecting yield were studied under semi-arid conditions in Israel. In the absence of water stress, photosynthetic activity decreased as the rate of disease development increased. Transpirational activity was reduced in leaves infected by Septoria leaf blotch, but intensified in leaves with slight leaf rust infections (0.5-5% severity). When leaf rust seventy exceeded 5%, the transpirational water loss declined. Both diseases were associated with a decrease in the maximum rate of grain-weight accumulation, but did not affect the duration of weight accumulation. An inverse linear relationship between disease and final grain weight was observed. Under conditions of water stress, photosynthetic activity of leaves slightly infected by Septoria leaf blotch (1-5"., severity) was greater than that of uninfected leaves. Severely infected leaves (" 25% severity) photosynthesized the least. Both diseases were associated with a decrease in the maximum rate of grain weight accumulation, but the duration of accumulation was longer in tillers with a slow rate of disease development than in uninfected tillers. The relationship between disease and final grain weight in these cases was parabolic; yield was increased at low levels of infection. Theoretical explanations and practical implications of these phenomena are presented and discussed.     

Studies on dispersal of Septoria tritici pycnidiospores in wheat–clover intercrops

Four experiments studied the effects of a clover understorey on pycnidiospore dispersal of Septoria tritici in a wheat–clover intercrop under simulated rain. Clover significantly reduced the dispersal of spores in a horizontal direction by 33% at a distance of 15 cm from a line inoculum source compared with a wheat monocrop. The clover also reduced the vertical movement of spores from infected leaves at the base of wheat plants by an average of 63% compared to the monocrop, and this suggests that the main movement of spores was from the base upwards. Splash experiments using blue colour marker showed the vertical decline of splash and the number of drops per cm² with height caught on paper adjacent to trays of clover was described by the exponential decline model. The effect of clover in reducing vertical splash approached an asymptote as the leaf area index of the understorey increased. Simulated rain-splash increased the level of disease on the flag leaf and, in one experiment, there was a significant interaction between rain-splash and clover in reducing the number of lesions on the flag leaf. The level of disease resulting from one splash event was low, indicating that subsequent pathogen multiplication is probably required to bring about high severities of disease.     

Leptosphaeria nodorum on wheat in Ireland

Symptomless leaves of young wheat crops collected in January and February produced, after surface-sterilization and incubation on an agar medium, perithecia typical of Leptosphaeria nodorum. Single-ascospore cultures produced pycnidia typical of Septoria nodorum, whose conidia caused necrotic lesions to develop when sprayed on wheat seedlings.     

Research on Septoria Leaf Blotch: Recent Advances1

A review of the recent advances in research on Septoria leaf blotch is given with particular emphasis on epidemiology and control strategies. The effect of straw infected with pycnidia of Septoria tritici or perithecia of its perfect stage Mycosphaerella graminicola , infected seeds and alternative hosts on the initiation of epidemics is discussed. Reference is made to the effect of plant stature on the vertical progress of the disease. The role of fungicide protection in the control of the disease is discussed. Resistance and tolerance to Septoria leaf blotch is dealt with in more detail, with reference to the author's own research, and with implications on breeding strategies.