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.     

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.     

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.     

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.     

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.     

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.     

Yield reduction in wheat in relation to leaf necrosis caused by Septoria tritici

In 1980, field inoculations withSeptoria tritici were performed on winter wheat cv. Okapi. Leaf necrosis progress curves were established; they were sigmoid and could be transformed into logit lines. There was a great difference in the slope of the logit lines between inoculated plots and non-inoculated controls. The mid-time values varied according to inoculum dosage and post-inoculation humidity treatment. Yields (kg ha^?1) and average grain weights (mg grain^?1) differed significantly between inoculated and non-inoculated plots, but seldom among inoculation treatments. Decrease of average grain weight completely explained yield loss, which was 878 kg ha^?1 or 12% of control (7045 kg ha^?1). This crop loss is considerable for a cultivar which is not particularly susceptible. A good correlation was found between the amount of necrosis at development stages DC=75 and 77 and yield depression. The integral of necrosis over time gave a high correlation with loss of kernel weight.     

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.     

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 predictive model for early-warning of Septoria leaf blotch on winter wheat

Disease–weather relationships influencing Septoria leaf blotch (SLB) preceding growth stage (GS) 31 were identified using data from 12 sites in the UK covering 8 years. Based on these relationships, an early-warning predictive model for SLB on winter wheat was formulated to predict the occurrence of a damaging epidemic (defined as disease severity of 5% or > 5% on the top three leaf layers). The final model was based on accumulated rain > 3 mm in the 80-day period preceding GS 31 (roughly from early-February to the end of April) and accumulated minimum temperature with a 0°C base in the 50-day period starting from 120 days preceding GS 31 (approximately January and February). The model was validated on an independent data set on which the prediction accuracy was influenced by cultivar resistance. Over all observations, the model had a true positive proportion of 0.61, a true negative proportion of 0.73, a sensitivity of 0.83, and a specificity of 0.18. True negative proportion increased to 0.85 for resistant cultivars and decreased to 0.50 for susceptible cultivars. Potential fungicide savings are most likely to be made with resistant cultivars, but such benefits would need to be identified with an in-depth evaluation.     

填闲作物腐解过程及其对后茬冬小麦产量的影响

研究了渭北旱塬地区不同填闲作物（长武怀豆（S）、黑麦草（R）及两者1∶1混合（M））翻压和氮肥水平（0、60、120 kg N·hm^-2）双因素处理下,填闲作物的腐解规律、碳氮释放动态及对后茬冬小麦产量的影响,并对腐解速率与Olsen模型进行了拟合。结果表明：各填闲作物翻压后腐解规律及碳氮释放特征均表现为“前期快-中期慢-后期加快”,填闲作物腐解规律符合Olsen模型,在第276天各处理累积腐解率均达70%以上。在第0~35天,同一施氮处理下,累积腐解率和腐解速率均表现为S>M>R（P<0.05）;第35天,S、M和R各处理干物质累积腐解率分别达到61.9%、55.5%和47.5%;在0~35 d,施氮对S、M的腐解影响不显著,对R影响显著,35 d后氮肥效应逐渐减弱;填闲作物的腐解同时伴随其碳、氮的快速释放,在第21天,S、M和R碳氮残留率分别达到40%、50%和60%左右。平均来看,S的碳氮释放速率显著高于R,与M无显著差异。与裸地对照相比,翻压填闲作物能够显著提高后茬冬小麦产量,其籽粒产量增加10%~35% （P<0.05）,其中翻压长武怀豆低氮处理和混合翻压低氮处理效果最佳。     

Laboratory studies on benzimidazole resistance in Septoria tritici1

A technique is described for testing isolates of Septoria tritici from winter wheat for resistance to benzimidazole fungicides. Secondary spores from 23 isolates were tested on Czapek Dox V-8 agar amended with benomyl at 1–10 μg ml. Twenty-one isolates were recovered from eight crops in England in 1984 and two (PBI isolates) were obtained in 1973. Thirteen isolates, including both PBI isolates, were sensitive to benomyl at 1 μg ml^?1 and nine were resistant at 10 μg ml^?1. The remaining isolate had a low proportion (1:3.7 x 10⁴) of resistant spores. The minimum inhibitory concentration for sensitive isolates was 0.2–0.4 μg ml ¹ benomyl and for resistant isolates was more than 1000 μg ml ¹. Benomyl-resistant isolates were cross-resistant to carbendazim, thiabendazole and thiophanate-methyl, but not to 12 other fungicides with different modes of action. The implications of these findings are discussed.     

Assessment of drought and its impact on winter wheat yield in the Chinese Loess Plateau

Drought has pronounced and immediate impacts on agricultural production, especially in semi-arid and arid rainfed agricultural regions. Quantification of drought and its impact on crop yield is essential to agricultural water resource management and food security. We investigated drought and its impact on winter wheat (Triticum aestivum L.) yield in the Chinese Loess Plateau from 2001 to 2015. Specifically, we performed a varimax rotated principal component analysis on drought severity index (DSI) separately for four winter wheat growth periods: pre-sowing growth period (PG), early growth period (EG), middle growth period (MG), and late growth period (LG), resulting in three major subregional DSI dynamics for each growth period. The county-level projections of these major dynamics were then used to evaluate the growth period-specific impacts of DSI on winter wheat yields by using multiple linear regression analysis. Our results showed that the growth period-specific subregions had different major DSI dynamics. During PG, the northwestern area exhibited a rapid wetting trend, while small areas in the south showed a slight drying trend. The remaining subregions fluctuated between dryness and wetness. During EG, the northeastern and western areas exhibited a mild wetting trend. The remaining subregions did not display clear wetting or drying trends. During MG, the eastern and southwestern areas showed slight drying and wetting trends, respectively. The subregions scattered in the north and south had a significant wetting trend. During LG, large areas in the east and west exhibited wetting trends, whereas small parts in south-central area had a slight drying trend. Most counties in the north showed significant and slight wetting trends during PG, EG, and LG, whereas a few southwestern counties exhibited significant drying trends during PG and MG. Our analysis identified close and positive relationships between yields and DSI during LG, and revealed that almost all of the counties were vulnerable to drought. Similar but less strong relationships existed for MG, in which northeastern and eastern counties were more drought-vulnerable than other counties. In contrast, a few drought-sensitive counties were mainly located in the southwestern and eastern areas during PG, and in the northeastern corner of the study region during EG. Overall, our study dissociated growth period-specific and spatial location-specific impacts of drought on winter wheat yield, and might contribute to a better understanding of monitoring and early warning of yield loss.     

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.     

Influence of take-all epidemics on winter wheat yield formation and yield loss

ABSTRACT The effects of take-all epidemics on winter wheat yield formation were determined, and disease-yield relationships were established to assess the agronomic efficacy and economic benefits of control methods. Epidemics were generated in naturally infested fields by varying cropping season, crop order in the rotation, and experimental fungicide seed treatment. Disease incidence and severity were assessed from tillering to flowering. Yield components were measured at harvest. Models simulating the formation of the yield components in the absence of limiting factors were used to estimate the losses caused by take-all. Losses were predicted by the disease level at a specific time or the area under the disease progress curve, reflecting accumulation during a specific period. Losses of grain number per square meter and 1,000-grain weight were linked to cumulative disease incidence between the beginning of stem elongation and flowering, and disease incidence at midstem elongation, respectively. Yield losses were accounted for by both cumulative disease incidence between sowing and flowering, and disease incidence at midstem elongation. Results confirm the importance of nitrogen fertilization in reducing the impact of take-all on wheat.     

Inoculum density — host response relationships of spring wheat cultivars to infection by Septoria tritici

Eightinoculumdensities (ID), increasing in geometric progression from 0.1×10⁵ to 12.8×10⁵ conidia ml^–1 ofSeptoria tritici were sprayed onto the leaves of five cultivars ofTriticum aestivum, differing in resistance toS. tritici. Host response to infection byS. tritici, measured as either thelatencyperiod (LPD) orpercentage ofleafarea covered bylesions with pycnidia (PLAL), was significantly linearly correlated with log₁₀ID (r=0.84 to 0.96, with 6 degrees of freedom). As the ID increased from 0.2×10⁵ to 12.8×10⁵ conidia ml^–1, LPD decreased by 1 to 2 days for the susceptible and resistant cultivars and by 3 to 5 days for the moderately susceptible cultivars. Within the same ID range, PLAL increased from 73% to 90% for the susceptible cultivar and 2 to 12% for the resistant cultivar. The ID₅₀ value (the number of conidia required for 50% PLAL) calculated from the regression equations relating log₁₀ID to PLAL for each cultivar, ranged from 6.42×10³ conidia ml^–1 for the susceptible cultivar to 7.13×10¹³ conidia ml^–1 for the resistant cultivar. Estimates of the number of conidia ml^–1 required to initiate a lesion (obtained by substituting 1% PLAL into the regression equations relating log₁₀ ID to PLAL) ranged from 16 conidia ml^–1 for the susceptible cultivar to 1.0×10⁴ conidia ml^–1 for the resistant cultivar. Using the change of PLAL with ID, the restriction of lesion development for each cultivar could be partitioned into that due to host resistance and that due to interaction between lesions. Log₁₀LPD was significantly correlated negatively with PLAL. As a general rule when screening cultivars for resistance toS. tritici, it is advisable to inoculate them with several inoculum densities and to determine the latency period.Samenvatting Sporensuspensies vanSeptoria tritici die in dichtheid (ID) varieerden volgens een geometrische reeks van 0,1 tot 12,8×10⁵ sporen.ml^–1, werden gebruikt bij de inoculatie van kiemplanten van vijf cultivars vanTriticum aestivum die verschilden in resistentie tegenS. tritici.De reactie van de waardplanten werd gemeten aan de latentie periode (LPD) en aan het percentage bladoppervlak dat ingenomen werd door vlekken met pycniden (PLAL). Beide grootheden toonden een significante lineaire correlatie met log₁₀ID (r=0,84 tot 0,96 bij 6 vrijheidsgraden). Bij een toename van ID van 0,2×10⁵ tot 12,8×10⁵ sporen.ml^–1 nam de lengte van de latentie periode af met 1 tot 2 dagen bij de vatbare en de resistente cultivars, en met 3 tot 5 dagen bij de matig vatbare cultivars (Fig. 2).In hetzelfde traject van inoculumdichtheid nam de PLAL toe van 73 tot 90% bij vatbare cultivars, en van 2 tot 12% bij de resistente cultivars (Fig. 3). De ID₅₀-waarde, het aantal sporen dat nodig is om 50% aantasting te verkrijgen, werd berekend uit de regressievergelijkingen die voor iedere cultivar het verband aangeven tussen log₁₀ ID en het aantastingspercentage (PLAL). Deze waarde varieerde van 6,42×10³ sporen per ml voor vatbare tot 7,13×10³ sporen per ml voor de resistente cultivars (Tabel 1). Schattingen voor het aantal sporen dat nodig is om een vlek te doen ontstaan werden verkregen door de 1% aantastingswaarde te substitueren in de eerder genoemde regressievergelijkingen. Deze waarden varieerden van 16 sporen.ml^–1 voor de vatbare cultivar tot 10000 sporen.ml^–1 voor de resistente cultivar (Tabel 1). Via de toename van de aantastingsgraad (PLAL) per eenheid stijging in de inoculumdichtheid kon de remming op de vlekontwikkeling opgesplitst worden in een gedeelte dat toegeschreven werd aan de resistentie van de plant en een gedeelte dat te wijten was aan de interactie tussen de vlekken (Fig. 4 en 5; Tabel 2).Log₁₀ LPD was negatief gecorreleerd met PLAL (Fig. 6). Bij het toetsen van cultivars op resistentie tegenS. tritici zouden sporensuspensies van verschillende dichtheden gebruikt moeten worden en zou de latentie periode, evenals de aantastingsgraad bepaald moeten worden.