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.     

Patterns of development of Septoria nodorum and S. tritici in some winter wheat crops in Western Europe, 1981-83

In data collected at 19 sites in Western Europe during 1981-83. two patterns of development of Septoria nodorum and S. tritici on foliage of winter wheal were distinguished. In sudden outbreaks, lesions appeared simultaneously on the upper leaf layers of crops, usually after the end of stem extension; these outbreaks were ascribed to short, heavy rain storms in which pycnidiospore inoculum in basal leaves was elevated up to 60 cm through the crop canopy. Gradual epidemics were characterized by disease arising on successive leaf layers as they appeared during sustained periods of weather suitable for inoculum transport and infection.
The data indicate incubation periods of 2-4 weeks for S. nodorum and 3-5 weeks for S. tritici. it is suggested that a leaf layer cannot normally sustain more than one pathogen generation and that its infection arises from inoculum borne on leaves older than in the layer situated immediately below it. The potential level of disease in a crop may relate to the amount of inoculum present in spring. The proportions of disease caused by the two Septoria species varied greatly between sites and years, but the data provided no explanation.
It is concluded that a septoria forecast scheme needs to recognise the importance of sudden disease outbreaks and to include not only weather but also host growth and inoculum factors.     

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.     

小麦品种对小麦白粉病菌侵染信号传递初探

 通过测定抗病性不同的小麦品种在接种小麦白粉病菌后钙调素含量的变化发现其存在差异。如抗病品种钙调素含量高峰出现在接种后4 h,即寄主与白粉病菌接触阶段。而感病品种接种后24 h出现钙调素含量高峰。比较测定了不同的钙调素抑制剂对小麦品种抗白粉性的影响,发现10mmol/L TFP和5 mmol/L CPZ处理使抗病品种变为高度感病,50 mmol/L CPZ处理使小麦感染白粉菌后反应型下降,表明钙调素与小麦抗白粉性有关。     

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.     

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.     

Improved control of septoria tritici blotch in durum wheat using cultivar mixtures

Mixtures of cultivars with contrasting levels of resistance can suppress infectious diseases in wheat, as demonstrated in numerous field experiments. Most studies focus on airborne pathogens in bread wheat, while splash-dispersed pathogens have received less attention, and no studies have been conducted in durum wheat. We conducted a 2-year field experiment in Tunisia to evaluate the performance of cultivar mixtures with varying proportions of resistance (0%–100%) in controlling the polycyclic, splash-dispersed disease septoria tritici blotch (STB) in durum wheat. To measure STB severity, we used a high-throughput method based on digital image analysis of 3,074 infected leaves collected from 42 and 40 experimental plots during the first and second years, respectively. This allowed us to quantify pathogen reproduction on wheat leaves and to acquire a large data set that exceeds previous studies with respect to accuracy and precision. Our analyses show that introducing only 25% of a disease-resistant cultivar into a pure stand of a susceptible cultivar provides a substantial reduction of almost 50% in disease severity compared to the susceptible pure stand. However, incorporating two resistant cultivars instead of one did not further improve disease control, contrary to predictions of epidemiological theory. Susceptible cultivars can be agronomically superior to resistant cultivars or be better accepted by growers for other reasons. Hence, if mixtures with only a moderate proportion of the resistant cultivar provide a similar degree of disease control as resistant pure stands, as our analysis indicates, such mixtures are more likely to be accepted by growers.     

Magnaporthe oryzae conidia on basal wheat leaves as a potential source of wheat blast inoculum

Wheat blast caused by Magnaporthe oryzae Triticum causes significant losses on wheat during outbreak years in several South American countries. Despite reports of wheat blast leaf lesions on some wheat cultivars, the importance of inoculum originating from leaves in severely affected commercial fields is disputed. It is generally considered that leaf lesions and/or sporulation on leaves do not usually appear before the occurrence of spike blast in wheat. The purpose of this study was to (i) determine the occurrence of wheat blast on basal leaves, (ii) estimate the number of conidia produced on these leaves, and (iii) determine the impact of current fungicide application practices on inoculum produced from sporulating lesions on basal wheat leaves. Inoculations at the three‐leaf stage showed that certain cultivar and isolate combinations caused more disease on old wheat leaves than young expanding leaves. Under optimum conditions, M. oryzae had the potential to produce tens to hundreds of thousands of conidia on small amounts of wheat basal leaves. A mean of 1 669 000 conidia were produced on 1 g dry basal leaves of a highly susceptible cultivar under optimum conditions for sporulation. Conidia production on leaves coincided with spike emergence under both greenhouse and field conditions. When field studies were conducted under natural epidemic conditions, foliar fungicide applications reduced the amount of M. oryzae conidia on basal leaves by 62–77% compared to non‐sprayed controls. An earlier application of foliar fungicides might reduce inoculum if conidia from basal leaves contribute to wheat spike blast development.     

Resistance, epidemiology and sustainable management of Rhynchosporium secalis populations on barley

Rhynchosporium secalis is one of the most destructive pathogens of barley worldwide, causing yield decreases of up to 40% and reduced grain quality. Rhynchosporium is a polycyclic disease. Primary inoculum includes conidia produced on crop debris, infected seeds and possibly ascospores, although these have not yet been identified. Secondary disease spread is primarily by splash dispersal of conidia produced on infected leaves, which may be symptomless early in the growing season. Host resistance to R. secalis is mediated by both 'major' or host-specific genes (complete resistance) and 'minor' genes of smaller, generally additive effects (partial resistance). Crop growth stage and plant or canopy architecture can modify the expression of resistance. Resistance genes are distributed unevenly across the barley genome, with most being clustered on the short arms of chromosomes 1H, 3H, 6H and 7H, or in the centromeric region or on the long arm of chromosome 3H. Strategies used to manage rhynchosporium epidemics include cultivar resistance and fungicides, and also cultural practices such as crop rotation, cultivar mixtures and manipulation of sowing date, sowing rate or fertiliser rate. However, the high genetic variability of R. secalis can result in rapid adaptation of pathogen populations to render some of these control strategies ineffective when they are used alone. Sustainable control of rhynchosporium needs to integrate major-gene-mediated resistance, partial resistance and other strategies such as customized fungicide programmes, species or cultivar rotation, resistance gene deployment, clean seed and cultivar mixtures.     

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.     

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.     

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.     

Real-time PCR quantification of latent infection of wheat powdery mildew in the field

Wheat powdery mildew, caused by the fungal pathogen Blumeria graminis f. sp. tritici (Bgt), is a destructive wheat disease worldwide. The key issue for the disease forecast is to timely and accurately estimate and quantify the latent infection levels in volunteer seedlings where the pathogen over-winters or over-summers to serve as sources of initial inoculum of epidemics. To improve the conventional method, a real-time PCR assay had been established in this study to quantify latent infection level of wheat leaves. Artificially and naturally infected leaves in wheat fields at different geographical locations in China were collected and processed to determine the latent infection levels. Linear relationships between the molecular disease index (MDX) and the observed disease index (DX) were obtained from artificial inoculation experiments. Field experiments showed that the spatial distribution patterns of MDX matched well with those of DX in the most cases. This study demonstrated that the real-time PCR assay was a useful tool to rapidly and accurately quantify the latent infection levels of wheat powdery mildew and to efficiently estimate the initial inoculum potentials of epidemics in the fields.     

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.     

Early stages of septoria tritici blotch epidemics of winter wheat: build‐up,overseasoning, and release of primary inoculum

Septoria tritici blotch (STB), caused by Mycosphaerella graminicola, is the most prevalent disease of wheat worldwide. Primary inoculum and the early stages of STB epidemics are still not fully understood and deserve attention for improving management strategies. The inoculum build‐up and overseasoning involves various fungal structures (ascospores, pycnidiospores, mycelium) and plant material (wheat seeds, stubble and debris; wheat volunteers; other grasses). Their respective importance is assessed in this review. Among the mechanisms involved in the early stages of epidemics and in the year‐to‐year disease transmission, infection by ascospores wind‐dispersed from either distant or local infected wheat debris is the most significant. Nevertheless, infection by pycnidiospores splash‐dispersed either from neighbouring wheat debris or from senescent basal leaves has also been inferred from indirect evidence. Mycosphaerella graminicola has rarely been isolated from seeds so that infected seed, although suspected as a source of primary inoculum for a long time, is considered as an epidemiologically anecdotal source. Mycosphaerella graminicola can infect a few grasses other than wheat but the function of these grasses as alternative hosts in natural conditions remains unclear. Additionally, wheat volunteers are suspected to be sources of STB inoculum for new crops. This body of evidence is summarized in a spatio‐temporal representation of a STB epidemic aimed at highlighting the nature, sources and release of inoculum in the early stages of the epidemic.     

Protective effects of a wheat cultivar mixture against splash‐dispersed septoria tritici blotch epidemics

The use of cultivar mixtures to control foliar fungal diseases is well documented for windborne diseases, but remains controversial for splash‐dispersed diseases. To try to improve this strategy, a cultivar mixture was designed consisting of two wheat cultivars with contrasted resistance to Mycosphaerella graminicola , responsible for the rainborne disease septoria tritici blotch (STB), in a 1:3 susceptible:resistant ratio rather than the 1:1 ratio commonly used in previous studies. The impact of natural STB epidemics in this cultivar mixture was studied in field experiments over 4 years. Weekly assessments of the number of sporulating lesions, pycnidial leaf area and green leaf area were carried out on the susceptible cultivar. In years with sufficient STB pressure, disease impacts on the susceptible cultivar in the mixture were always significantly lower than in the pure stand (e.g. 42% reduction of pycnidial leaf area for the three upper leaves in 2008 and 41% in 2009). In years with low STB pressure (2010 and 2011), a reduction of disease impacts was also shown but was not always significant. After major rainfall events, the number of sporulating lesions observed on the susceptible cultivar after one latent period was reduced on average by 45% in the mixture compared to the pure stand. All the measurements showed that a susceptible cultivar was consistently protected, at least moderately, in a mixture under low to moderate STB pressure. Therefore, the results prove that the design of an efficient cultivar mixture can include the control of STB, among other foliar diseases.     

Dynamics of primary and secondary infection in take-all epidemics

ABSTRACT Using a combination of experimentation and mathematical modeling, the effects of initial (particulate) inoculum density on the dynamics of disease resulting from primary and secondary infection of wheat by the take-all fungus, Gaeumannomyces graminis var. tritici, were tested. A relatively high inoculum density generated a disease progress curve that rose monotonically toward an asymptote. Reducing the initial inoculum density resulted in a curve that initially was monotonic, rising to a plateau, but which increased sigmoidally to an asymptotic level of disease thereafter. Changes in the infectivity of particulate inoculum over time were examined in a separate experiment. Using a model that incorporated terms for primary and secondary infection, inoculum decay, and host growth, we showed that both disease progress curves were consistent with consecutive phases dominated, respectively, by primary and secondary infection. We examined the spread of disease from a low particulate inoculum density on seminal and adventitious root systems separately. Although seminal roots were affected by consecutive phases of primary and secondary infection, adventitious roots were affected only by secondary infection. We showed that the characteristic features of disease progress in controlled experiments were consistent with field data from crops of winter wheat. We concluded that there is an initial phase of primary infection by G. graminis var. tritici on winter wheat as seminal roots grow through the soil and encounter inoculum, but the rate of primary infection slows progressively as inoculum decays. After the initial phase, there is an acceleration in the rate of secondary infection on both seminal and adventitious roots that is stimulated by the increase in the availability of infected tissue as a source of inoculum and the availability of susceptible tissue for infection.     

The influence of nitrogen supply on the ability of wheat and potato to suppress Stellaria media growth and reproduction

Summary This paper tests the hypothesis that increased soil nitrogen supply reduces the growth of late-emerging weeds in wheat and potato by enhancing canopy leaf area development and thereby reducing the availability of light for weed growth. Two series of experiments were conducted: one in spring wheat (1997, 1999) with sown Stellaria media and one in potato (1998, 1999) with naturally emerged weeds, including S. media . For each crop, two cultivars were grown at three levels of nitrogen supply. In wheat, as in a monoculture of S. media , total dry weight and seed number of the weed increased with soil nitrogen supply, whereas in potato the opposite was found. Increased soil nitrogen supply increased the nitrogen uptake of S. media in wheat, despite the reduced light availability, indicating that S. media in wheat was limited by nitrogen. In potato, on the other hand, growth of S. media was limited by light availability, which decreased with increased soil nitrogen supply . We conclude that the differences in response of S. media in wheat and potato to additional nitrogen supply are attributable to the dual influence of soil nitrogen supply on light and nitrogen availability, which are mediated by the crops.     

铁春1号"小麦耐白粉病机制研究

 铁春1号小麦耐小麦白粉病的机制是:①光合补偿作用:"铁春1号"小麦旗叶的表现光合速率下降较小,而真正光合速率比健康植株高,表明侵染点间的绿色组织具有光合补偿作用,一定程度上弥补病原菌的破坏与消耗。②超氧化物歧化酶活性增强,能及时清除因病害胁迫产生的超氧自由基(O₂·),避免了O₂对细胞膜的伤害。"铁春1号"膜磷脂种类在染病后没有变化,而感病品种膜磷脂在染病后增加了2个组份。③贮存物质运转的补偿作用:"铁春1号"在病害胁迫下,叶、茎、叶鞘中的干物质,以及氮、磷等养分的运出率增大,通过充分利用贮存物质部分的补偿光合产物低之不足。最终使籽粒中氮、磷及碳水化合物不减少。     

PSR(Puccinia Striiformis Repeat)序列的基因组特异性和指纹遗传稳定性研究

 以PSR331S₃(Puccinia striiformis repeat)为探针,对感染小麦条锈菌P. striiformis f. sp. tritici模式菌系的叶片和常用繁殖寄主健康叶片总DNA进行Southern分析,结果显示PSR331S₃具有良好的指纹分辨力和基因组特异性。对系列单孢系的指纹分析表明,PSR位点在有丝分裂中是稳定遗传的,可以用于条锈菌的遗传分析。