Components of partial resistance to Septoria nodorum in winter wheat

Components of partial resistance to Septoria nodorum were investigated in 10 cultivars of winter wheat having similar field resistances. The components measured were infection frequency, latent period, size, shape and rate of growth of lesions, spore production and its rate of increase. Latent period was found to be lognormally distributed. Some of the components of resistance were found to be significantly different between cultivars. Cluster analysis also showed that cultivars could be distinguished on the basis of their components of resistance. Principal components analysis indicated that resistance could be broken down into four underlying factors, three of which could be readily interpreted. The measurements of the components of resistance were combined in a model, the r-index, based on Van der Plank's r. The amount of variation between cultivars cast some doubt on the predictive value of the index but all the cultivar values were well within the range bounded by two ‘synthetic’ cultivars made up of combinations of either the most resistant or the most susceptible components. It is considered that the r-index has potential in screening for field resistance. The possibility of incorporating the most resistant-type components into one cultivar is discussed. The use of cultivar mixtures containing cultivars having similar field resistances is also discussed in the light of the variability found in this study.     

Components of partial resistance of barley to Rhynchosporium secalis: use of seedling tests to predict field resistance

Components of partial resistance [disease incidence (DI), infection frequency (IF), latent period (LP), spores per lesion (SPL)] were assessed on glasshouse-grown barley seedlings following inoculation with spore suspensions of Rhynchosporium secalis at growth stage 12 (Zadoks, Chang & Konzak, 1974). Four experiments were carried out at different times during 1988. Three spring barley cultivars [two from Cyprus (cvs Kantara and Athenais) and one from the UK (cv. Triumph)] were used in the first three experiments. In the fourth experiment eight additional UK cultivars with NIAB resistance ratings ranging from 3 to 9 were used. Two races of R. secalis were used in the first three experiments and three in the fourth. The three cultivars, Kantara, Athenais and Triumph, were examined in all four experiments and significant differences detected for virtually all components of partial resistance in each. Differences, however, were often small and ranking of cultivars varied in different experiments. The greater susceptibility of cv. Kantara compared to cv. Athenais, observed under field conditions in Cyprus, would not be anticipated from the small differences in components of partial resistance observed in these experiments, but, for these cultivars, the possibility of a marked genotype x environment interaction cannot be discounted. Mean values for the components of partial resistance differed in the four experiments. LP was correlated with mean glasshouse temperature from inoculation to the onset of sporulation but differences in IF and SPL were not correlated with temperature. For these components, light quality and/or duration appeared to be more important. Overall, there were no differences between races but significant race X cultivar interactions were observed in two experiments. In the fourth experiment, examining 11 cultivars, there were significant differences between cultivars for all components of partial resistance. IF and LP were correlated but neither of these components was correlated with SPL indicating independent control of this latter component. Both IF and LP were correlated with field performance (NIAB ratings) but there was no correlation with SPL. However, combining IF with mean values of SPL restricted to the 5 days following the end of the LP, produced the best correlation (r= 0.92) with NIAB ratings. Problems of assessing components of partial resistance and possible means of improving assessments are discussed.     

Endophytic Fungi in Four Winter Wheat Cultivars (Triticum aestivum L.) Differing in Resistance Against Stagonospora nodorum (Berk.) Cast. & Germ. =Septoria nodorum (Berk.) Berk.

Four winter wheat cultivars with different levels of resistance to Septoria nodorum were investigated at four locations during two vegetation periods. Forty plants per cultivar and site were collected at random at seven defined growth stages from crop emergence to harvest. Samples from roots, culms, leaves, glumes and kernels were examined for the occurrence of endophytic fungi after surface sterilization. 83% of the 26944 isolates sporulated and were assigned to 213 species. The most frequent were: Septoria nodorum (20.1%), Alternaria tenuissima (9.8%), Epicoccum purpurascens (9.1%), Idriella bolleyi (6.9%), Fusarium graminearum (5.3%), Fusarium culmorum (4.0%), Cladosporium oxysporum (3.7%), Didymella exitialis (3.1%), Fusarium nivale (2.8%) and Rhizoctonia solani (2.1%). Each species occurred preferentially in one or more plant organs. A factorial analysis of variance showed that plant organ, sampling site, vegetation, period and cultivar in decreasing order of importance influenced the quantitative and qualitative composition of the fungal populations. No relationship between endophytic fungi was found to be constantly antagonistic or mutualistic. Septoria nodorum was isolated mainly from culms. The number of S. nodorum isolates differed significantly between cultivars in culms and glumes but not in flag leaves. The results are discussed in relation to resistance breeding and the effect endophytic fungi, might have on yield.     

In vitro screening for resistance against Septoria nodorum blotch in wheat

This study was carried out to develop an in vitro test for the identification of genotypes resistant to Septoria nodorum blotch. The basis for this project was a previous study in which a crude extract of S. nodorum was used as a selective agent (Keller et al. 1994). It was possible to distinguish resistant and susceptible cultivars in an in vitro test with zygotic embryos. In our project we wanted to test whether this in vitro test can also be used to detect resistant and susceptible genotypes in early segregating populations. Specific crosses between eight winter wheat lines showing contrasting resistance reaction for S. nodorum blotch on leaves and ears were made. The resistance level of both leaf and ear was evaluated after artificial inoculation in the field for the parental lines, the F₁ progenies, as well as for segregating F₃ and F₄ populations. In addition, this plant material was tested in vitro using methods similar to those described by Keller et al. (1994), i.e. culturing immature zygotic embryos and mature seeds on selective media. A good agreement between in vitro screening and field resistance on the ear was found for the parental lines, the F₁ and F₄ generation but not for the F₃ generations. This leads to the conclusion that the in vitro screening might be integrated into wheat breeding programs. Populations showing a high susceptibility to the pathogen metabolites in vitro could be discarded. Another promising implementation for wheat breeding would be the screening of advanced breeding material or candidate partners in a crossing program for resistance on the ear. However, the in vitro screening is not precise enough to select single plants in early segregating populations. Received: 18 January 1999 / Accepted: 30 April 1999     

Identification of molecular markers for resistance to Septoria nodorum blotch in durum wheat

The development of Septoria nodorum blotch-resistant cultivars has become a high priority objective for durum wheat breeding programs. Marker-assisted selection enables breeders to improve selection efficiency. In order to develop markers for resistance to Septoria nodorum blotch, a set of F₅ recombinant inbred lines, derived from the crosses Sceptre/3–6, Sceptre/S9–10 and Sceptre/S12–1, was developed based on the F₂-derived family method. Two RAPD markers, designated UBC521₆₅₀ and RC37₅₁₀, were detected by bulked segregant analysis and located approximately 15 and 13.1 centiMorgans (cM) from the resistance gene snbTM, respectively. A SCAR marker was also successfully developed for marker-assisted selection in breeding programs based on the sequence of the RAPD marker UBC521₆₅₀. This is the first report of DNA-based markers linked to resistance for Septoria nodorum blotch in durum wheat. Received: 8 March 2000 / Accepted: 25 June 2000     

Components of resistance to Cercospora arachidicola and Cercosporidium personatum in groundnuts*

The reactions of four groundnut cultivars to inoculation with two leaf spot pathogens (Cercospora arachidicola and Cercospodirium personatum) were investigated. Resistant cultivars showed longer latent periods, reduced sporulation and less defoliation; however, none of the cultivars was resistant to both pathogens. Numbers of lesions, time to leaflet loss and the percentage of spores which produced lesions were influenced by the concentration of C. personatum inoculum applied to the leaves and there were significant cultivar differences in lesion numbers at the lowest inoculum level. Only the first two characters were affected by applied spore concentration in the study with C. arachidicola. Generally it was found that lower inoculum levels gave a better separation of cultivars. The level of resistance was similar to that found in the slow resting reaction of cereal crops and should be useful in a groundnut breeding programme.     

Components of Resistance to Early Blight in Four Potato Cultivars: Effect of Leaf Position

Components of early blight resistance were quantified in leaves of different ages in four potato cultivars. The components of resistance: incubation period (IP), lesion number (LN), early blight severity, lesion expansion rate (LER), latent period (LP) and spore production by lesion area (SPLA), were evaluated separately in the lower, middle and upper leaves of four potato cultivars. Plants of cultivar Aracy (resistant), Delta (moderately resistant), Desirée (susceptible) and Bintje (susceptible) were inoculated with an Alternaria solani isolate at the beginning of the flowering stage. Disease severity varied in different plant parts. In all cultivars, regardless of resistance, the smallest values of LN, and severity were recorded on the upper leaves, suggesting that young tissues are less susceptible. In cultivar Aracy, the IP was long, with small values of LN and LER and consequently, low values of early blight severity in all leaf positions were recorded. Although IP was long in cultivar Aracy, no differences between the moderately resistant cultivar Delta and the susceptible cultivars Bintje and Desirée could be detected for this component. The IP was only influenced by leaf position in cultivar Aracy. Clear differences in resistance levels among cultivars could be detected regarding LN, severity and LER. However, neither LP nor SPLA were associated with resistance level of cultivars or with leaf position. Analyses according to plant part suggest that evaluations on leaves of the middle third part are most suitable for screening for early blight resistance in potato.     

A bioassay for predicting the resistance of wheat leaves to Septoria nodorum

Plants of the winter wheat cv. Bounty were inoculated with Septoria nodorum and then assayed for ergosterol. A detached leaf technique was used in which leaf segments were incubated on agar containing benzimidazole. Four levels of a conidial suspension inoculum were applied using a measured droplet technique. Ergosterol in plant extracts was measured using HPLC and its identity confirmed by co-injection with pure ergosterol as an internal standard. The extracts were also assayed by silica-gel thin layer chromatography with the sterol being visualised with rhodamine; the presence of ergosterol in the material from the two higher disease level treatments was confirmed by analysing the eluted spots in a spectrophotometer. Additional confirmation of ergosterol was obtained using gas-liquid chromatography comparing a mixture of known plant sterols with a sample from the diseased leaf tissue. The ergosterol assay was found to be very sensitive and offers a high level of reproducibility. It would therefore appear that such assays could be of value in breeding programmes when it is necessary to screen wheat cultivars for their reaction to S. nodorum or other fungal pathogens.     

The reaction of wheat genotypes to Septoria tritici

Seedling leaves, flag leaves, culms and heads of wheat genotypes were evaluated under glasshouse and field conditions for reaction to Septoria tritici. Parameters used for resistance screening were the incubation and latent periods, disease and sporulation levels, and 1000 grain weight. Significant correlations were obtained between reactions of different plant organs and between field and glasshouse tests; significant interspecific and intraspecific differences occurred for all screening parameters. Triticum aestivum varieties showed a wide range of disease reaction ranging from the high resistance of Elite Lepeuple, Maris Dove, Maris Ensign, Chalk and Tommy to the high susceptibility of Rothwell Sprite, Sovereign, Cardinal, Maris Ranger and Maris Templar.     

Components of Partial Disease Resistance in Wheat Detected in a Detached Leaf Assay Inoculated with Microdochium majus using First, Second and Third Expanding Seedling Leaves

The use of first, second and third expanding seedling leaves of wheat (L1, L2 and L3 respectively), inoculated with conidial suspensions of Microdochium majus (syn. Microdochium nivale var. majus) in a detached leaf assay, for detecting components of partial disease resistance (PDR) was investigated across a range of wheat cultivars. Incubation periods (period from inoculation to first appearance of symptoms; a dull grey–green water‐soaked lesion) and latent periods (period from inoculation to the first appearance of sporodochia) were longest and lesions smallest on L3. The expression of PDR components on L2 was intermediate to those on L1 and L3. The longer latent periods on L3 typically occurred after leaf senescence contrasting with latent periods on L1 and L2 where sporulation most frequently occurred on relatively green leaf tissue. Cultivar differences in the first appearance of symptoms, incubation period, which occurred before any leaf senescence was observed, correlated significantly across all leaf positions. Similarly cultivar differences in latent period were correlated for L1 and L2. However, latent periods on L3, which were the least consistent between cultivars across experiments, were not correlated with those of L1 or L2 in any experiment. The results indicate that due to the delay in sporulation until after leaf senescence, observations on latent period in L3 are less representative of what occurs in the whole plant where infection of living tissue is of greatest interest. This work indicates that the selection of the first or second expanding leaf of wheat is optimal for the use in the detached leaf assay using M. majus for studying components of PDR.     

Path-coefficient analyses and genetic parameters of the components of field resistance of potatoes to late blight

Variation, genetic parameters, interrelationships and phenotypic and genetic path analyses for components of field resistance of potatoes to Phytophthora infestuns were studied using detached leaves from 16 potato cultivars. Inter-genotypic variability was significant for the components and the Area Under Disease Progress Curve (AUDPC). The resistant cultivars generally had a longer latent period and lower lesion size and spore production than the susceptible cultivars. The correlations between AUDPC and infection efficiency, and between AUDPC and spore density were not significant, but latent period, lesion size and sporulation did correlate significantly with AUDPC. Genetic and phenotypic path-coefficient analyses indicated lesion size to be the most important component of field resistance. The genetic correlation coefficients between the AUDPC and infection efficiency, latent period and spore density arose mainly because of their indirect effects on AUDPC via lesion size. Lesion size and AUDPC had a high genetic coefficient of variation, heritability and genetic advance (genetic gain).     

Aggressiveness variation in Plasmopara halstedii and its alternation with non-race-specific resistance in sunflower

Aggressiveness variation and its alternation with non-race specific resistance in sunflower were studied in 19 Plasmopara halstedii isolates belonging to several races. Regarding aggressiveness criteria, percentage infection, latent period, sporulation density and dwarfing, on two sunflower inbred lines showing different levels of non-race specific resistance resistance FU and BT, there were significant differences in aggressiveness for P. halstedii isolates. The index of aggressiveness varied between 9.4 and 31.4. The inbred line BT, rather susceptible in the field, showed a higher percentage infection, a higher sporulation density, a shorter latent period and less reduced hypocotyl length than inbred line FU, which showed a greater resistance in the field. Percentage infection on FU was 1.4% less than BT, latent period on BT was 12.4% less than FU, sporulation density on FU was 22.3% less than BT and reduced hypocotyl length on BT was 15.3% less than FU. Consequently, it seems that the criteria as latent period, sporulation density and reduction of hypocotyl length could be used to measure non-race specific resistance in sunflower to P. halstedii under controlled conditions.     

Sporulation of Septoria nodorurn (and S. tritici) on spring wheat cvs Kolibri and Maris Butler in relation to growth stage, plant part and time of season

Sporulation of Septoria nodorum was assessed on inoculated spring wheat cvs Kolibri and Maris Butler. The time from inoculation to first production of spores (latent period) was similar over a range of constant temperatures (12–18°C) but was shorter for Kolibri at 6°C and at 24°C. Spore production was greatest during the period of stem extension and, for equal amounts of disease, was twice as great on Kolibri as on Maris Butler. Spore production on leaves was much more intense with Septoria tritici than S. nodorum for equal amounts of disease, and was less intense on heads than on leaves for both pathogens. Inoculation of plants resulted in significant losses in yield. Mans Butler consistently out-performed Kolibri on each component of yield. Kolibri was particular affected: 1000-grain weights were reduced at each growth stage tested especially by S. nodorum, grain numbers and yield/head were reduced particularly following inoculation at heading. Spore production of S. nodorum during the period of stem extension increased to a peak in early July and then declined but no reliable relationship with monitored weather accounted for this seasonal trend.     

Disease progress of non-specialised fungal pathogens in intraspecific mixed stands of cereal cultivars. II. Field experiments

Epidemic development of Septoria nodorum was examined in pure stands and mixtures of two spring wheat cultivars, Kolibri and Maris Butler. In mixtures, disease development was reduced almost to that of the more resistant pure stand (Maris Butler). Disease levels, however, were low and no significant differences in grain yield per head were demonstrated. Disease development was further investigated for the host-pathogen combinations winter wheat/S. nodorum and winter barley/Rhynchosporium secalis. With both combinations the mixed stands again reduced disease levels to almost that of the more resistant pure stands. In general, mixed stands were more effective against R. secalis, although, with S. nodorum, disease levels were low. The yield response in mixed stands differed for the host cultivars and was not significantly changed by the presence of disease. The complexity of analysing such situations and the implications of these findings for mixtures of cultivars differing in resistance to non-specialised pathogens are discussed.     

Inheritance of components of partial resistance of barley to Rhynchosporium secalis with particular reference to race specificity

Six spring barley cultivars with no known genes for resistance to specific virulences but varying in partial resistance to Rhynchosporium secalis, were crossed in all combinations (6 × 6 diallel including reciprocals). In addition to seeds from naturally selfed plants, seeds of all parent cultivars were also produced by artificial selfing (emasculation followed by pollination using pollen from the same cultivar). This ensured comparability between seeds of parents and F₁. Both sets of parents, F₁ and F₂ families were grown in the field as single spaced plants and inoculated at Zadoks growth stage 49 with spore suspensions (2 × 10⁶ spores ml^-1) of three races (pathotypes) of R. secalis (Zadoks, Chang & Konzak, 1974). Components of partial resistance, incubation period (ICP), infection frequency (IF) and spore production per lesion (SP/L) were assessed on each plant. There were highly significant differences for all three components of partial resistance in both sets of parent cultivars but rank order in both sets was similar as evidenced by correlation coefficients, r= 0.96 for ICP and IF and r= 0.87 for SP/L. All three components of partial resistance were strongly correlated with NIAB (National Institute of Agricultural Botany, Cambridge, UK) resistance ratings. Means of F₁ and F₂ families were correlated with mid-parent values for ICP and IF but not SP/L. No difference in aggressiveness was found between races but for each component of partial resistance there was a significant interaction between race and parent cultivar (artificial selfs) and, for IF and ICP, a significant interaction between race and F₁ family. There was no evidence of interaction between parent (natural selfs) and race nor between race and F₂ family. Examination of genetic control of resistance showed evidence of strong additive effects (combining ability) in both F₁ and F₂ for ICP and IF but not for SP/L. There was no evidence for maternal or reciprocal differences, but there was evidence for dominance effects although their nature differed between components of partial resistance and between F₁ and F₂ generations. In the F₁, but not the F₂ generation, several elements of dominance (direction, distribution of dominant genes between parent cultivars, specific combining ability) showed for ICP or IF (but not SP/L) significant interaction with race.     

Effects of alien cytoplasm substitution on the response of wheat cultivars to Septoria nodorum

The effects of alien cytoplasm substitution on the response of wheat to Septoria nodorum were studied, using alloplasmic series of two cultivars, Chris and Selkirk. In general, cytoplasmic substitution caused unidirectional effects on Septoria-response, alloplasmic lines of both cultivars expressing lower levels of partial resistance (in leaf and head tissue) but higher levels of yield tolerance than the corresponding euplasmic line. The reduced resistance in alloplasmics was closely associated with reduced incubation periods of Septoria infection in both leaf and head tissue. Cytoplasmic substitution resulted in increased yield tolerance to Septoria-infection in both the non-tolerant Selkirk and the relatively tolerant Chris. Unlike their effects on partial resistance, specific cytoplasms exerted similar effects on tolerance in the two parental cultivars, several cytoplasms of the D plasmatype being particularly effective in increasing Septoria-tolevance. The potential for the development of Septoria-toterant cultivars by the incorporation of alien cytoplasms is discussed, in view of the observed neutral effects of D plasmatype cytoplasms on other agronomic traits.     

Sources of resistance and susceptibility to Septoria tritici blotch of wheat

An association genetics analysis was conducted to investigate the genetics of resistance to Septoria tritici blotch, caused by the fungus Zymoseptoria tritici (alternatively Mycosphaerella graminicola), in cultivars and breeding lines of wheat (Triticum aestivum) used in the UK between 1860 and 2000. The population was tested with Diversity Array Technology (DArT) and simple‐sequence repeat (SSR or microsatellite) markers. The lines formed a single population with no evidence for subdivision, because there were several common ancestors of large parts of the pedigree. Quantitative trait loci (QTLs) controlling Septoria resistance were postulated on 11 chromosomes, but 38% of variation was not explained by the identified QTLs. Calculation of best linear unbiased predictions (BLUPs) identified lineages of spring and winter wheat carrying different alleles for resistance and susceptibility. Abundant variation in Septoria resistance may be exploited by crossing well‐adapted cultivars in different lineages to achieve transgressive segregation and thus breed for potentially durable quantitative resistance, whereas phenotypic selection for polygenic quantitative resistance should be effective in breeding cultivars with increased resistance. The most potent allele reducing susceptibility to Septoria, on chromosome arm 6AL, was associated with reduced leaf size. Genes which increase susceptibility to Septoria may have been introduced inadvertently into UK wheat breeding programmes from cultivars used to increase yield, rust resistance and eyespot resistance between the 1950s and 1980s. This indicates the need to consider trade‐offs in plant breeding when numerous traits are important and to be cautious about the use of non‐adapted germplasm.     

Disease spread of non-specialised fungal pathogens from inoculated point sources in intraspecific mixed stands of cereal cultivars

Spread of Septoria nodorum from inoculated point sources was examined in pure stands and mixtures of two spring wheat cultivars Kolibri and Maris Butler. Gradients in disease were observed soon after inoculation; the presence of the more resistant cultivar (Maris Butler) in the mixtures retarded the outward spread of disease compared with the susceptible pure stand (Kolibri). Regression analysis suggested that gradients in incidence were influenced by nearness to the source whereas gradients in severity were not. Spread of disease was also examined in pure stands and mixtures for the host/pathogen combinations, winter wheat (cvs Maris Huntsman and Maris Ranger)/S. nodorum and winter barley (cvs Maris Otter and Hoppel)/Rhynchosporium secalis. In contrast to the spring wheat experiment, no gradients were observed; explanations for the uniform distribution of disease were (a) extensive spread prior to the period of assessment, (b) no physical barrier to dispersal due to the juvenile growth stage of the crop and (c) exhaustion of the point-source.     

Assessing cultivar resistance to Sitodiplosis mosellana (Géhin) (Diptera: Cecidomyiidae) using a phenotyping method under semi‐field conditions

The orange wheat blossom midge, Sitodiplosis mosellana (Géhin), can significantly reduce wheat yield. Growing resistant wheat cultivars is an effective way of managing this pest. The assessment of cultivar resistance in field trials is difficult because of unequal pressure of S. mosellana caused by differences in cultivar heading dates relative to the flight period of S. mosellana adult females and huge variations of egg laying conditions from 1 day to another. To overcome these hurdles and to expose all cultivars homogeneously to the pest, an assessment method of cultivar resistance was developed under semi‐field conditions. In 2015, the resistance of 64 winter wheat cultivars to S. mosellana was assessed. Few or no larvae developed in the ears of resistant cultivars, but in susceptible cultivars, large numbers of larvae developed. Seventeen cultivars proved to be resistant, whereas 47 were susceptible. The identification of new resistant cultivars offers more opportunities to manage S. mosellana. The phenotyping method is easy, cheap, efficient and reliable. It can be used to guide the breeding of new resistant wheat cultivars. Using specific midge populations, this method could also be used in research on new resistance mechanisms in winter wheat or in other cereal species.     

A Possible Role for Ferrous Complexes in Fungal Disease Suppression: Glume Blotch and Net Blotch of Cereals

Germination of spores of Septoria nodorum and Pyrenophora teres was inhibited and germ-tube growth in germinated spores was reduced b y ferrous ions complexed with a number of cheiating agents. No such inhibition was observed with ferric complexes and none of the chelating agents in the desferri form was toxic to the fungi. The germination mechanism in spores suspended in ferrous-2,3-dihydroxybenzoic acid (Fe¹¹-DHBA) for 24 h could not subsequently be released to any great extent by incubation with ethylenediamjne-di (o-hydroxyphenyl-acetic acid) (EDDHA). Lesion development by the fungi in the presence of ferrous complexes on detached leaves of host plants was almost totally suppressed, but compounds which preferentially chelate ferric ions, used in the desferri form significantly stimulated lesion development by S. nodorum on wheat leaves. Cermination, appressorium formation and lesion development on leaves of host plants were also significantly reduced by Fe¹¹-DHBA when plants were sprayed to run-off up to 5 days prior to inoculation. Disease development and subsequent 1000 grain weight loss were reduced by approximately 50 percent in wheat and barley plants when the flag leaves were treated with Fe¹¹-DHBA (5 × 10^?4 M) prior to inoculation with S. nodorum and P. teres respectively, compared with inoculated, untreated plants.