Dispersal of Septoria nodorum Pycnidiospores by Simulated Rain and Wind

The influence of wind on the splash dispersal of Septoria nodorum pycnidiospores was studied in a raintower/wind tunnel complex with single drops or simulated rain falling on spore suspensions or infected stubble with windspeeds of 1.5 to 4 m/sec. When single drops fell on spore suspensions (depth 0.5 mm, concentration 7.8 × 10⁵ spores/ml) most of the spore-carrying droplets collected on fixed photographic film between 0–4 m downwind (windspeed 3 m/sec) were >200 μm in diameter. However, most spores were carried in droplets with diameter > 1000 μm, 70 % of which carried more than 100 spores. When simulated rain fell on infected stubble most of the spore-carrying droplets collected beyond 1 m downwind (windspeeds 1.4 and 4 m/sec) were <200 μm in diameter and none were >600 μm; most of these droplets carried only one spore. The distribution of splash droplets (with diameter >100 μm) deposited on chromatography paper showed a maximum at 40–50 cm upwind of the target but many more droplets were deposited 20–30 cm downwind, when single drops fell on a spore suspension (concentration 1.2 × 10⁵ spores/ ml) containing fluorescein dye with a windspeed of 2 m/sec; droplets were collected up to 3 m downwind but not more than 70 cm upwind. With a windspeed of 3 m/sec, numbers of sporecarrying droplets and spores collected on film decreased with increasing distance downwind; most were collected within 2 m of the target but some were found up to 4 m. When simulated rain fell on infected stubble, increasing the windspeed from 1.5 to 4 m/sec greatly increased the number of spores deposited more than 1 m downwind. At 1.5 m/sec none were collected beyond 2 m downwind, whereas at 4 m/sec some were collected at 4 m. A few air-borne S. nodorum spores were collected by suction samplers at a height of 40 cm at distances up to 10 m downwind of a target spore suspension on which simulated rain fell.     

Factors Affecting the Germination of Septoria nodorum Pycnidiospores

Most pycnidiospores of Septoria nodorum were released from pycnidia into water in 30 min, more than 50% being released in 10 min. The viability of pycnidiospores in aqueous suspension, assessed by germination on a selective agar medium, decreased more rapidly in daylight than in darkness but no spores germinated after 50 h. The presence of cirrhus extract stimulated germination of pycnidiospores in suspension.     

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.     

Incidence and effects of Septoria nodorum on wheat cultivars

Wheat cultivars were grown in two field trials in which half the plots were artificially inoculated with Septoria nodorum. There were differences between cultivars in the degree of 5. nodorum infection on flag leaves and on ears. Hybrid 46, Maris Huntsman and Maris Widgeon were among those with the least infection, and Maris Ranger and Maris Templar among those with the most. Severity of infection was positively correlated with both short stature and earliness of maturity. Infection on leaves was positively correlated with infection on ears. Inoculation reduced yield by decreasing two of its components, grain number per ear and 1000-grain weight. A very early maturing cultivar, Sterling, sustained no yield loss despite severe infection; for the remaining cultivars, yield loss was positively correlated with degree of infection.     

Influence of environmental conditions on spore dispersal and infection by Septoria nodorum

In field experiments using healthy trap plants it was found that pycniospores of S. nodorum were dispersed from diseased wheat plants whenever rain fell and occasionally in the absence of rain. Of two spring wheats tested, cv. Kolibri seemed a better ‘source’ plant and cv. Maris Butler a better ‘receptor’ when rainfall was light, but this difference was not apparent when rainfall was heavy. On 44 occasions, plants of Kolibri and M. Butler were also exposed to natural conditions immediately following artificial inoculation with S. nodorum. Infection of plants occurred on 10 occasions and was associated with the following minimum conditions: r.h, at inoculation >63%; and in the following 24 h, minimum temperature <6°C, at least 4 h with r.h. >90% and not more than 4 h with r.h. >60%.     

Variation in Metabolite Production by Septoria nodorum Isolates Adapted to Wheat or to Barley

The production of secondary metabolites by S. nodorum strains was examined in relation to their adaptation to wheat or to barley. Eleven strains of each type previously characterized were tested for the production in culture of (-)-(3R)-mellein (ochracin), (3R)-O-metbylmellein; (-)-(3R,4R)-4-hydroxyniellein, (-)-(3R,4S)-4-hydro-xymellein, mycophenolic acid, septorine, N-methoxy-septorine, N-methoxyseptorinol, and a new compound (=UN).
Mellein was produced by every strain. Both 4-hydro-xymellein isomers were yielded by all wheat-adapted strains and six barley-adapted strains. Most of the strains produced mycophenolic acid. On the other hand the pigmentation of culture filtrates of wheat-adapted strains was pale-yellow whereas the pigmentation of barley-adapted strains was grey-blue. The productions of O-methylmellein, septorine, N-metboxyseptorine and N-methoxyseptorinol were typical for wheat-adapted strains. The detection of UN was restricted to the filtrates of barley-adapted strains. If the pigmentation of filtrates and the production of septorines, UN and O-methyl-mellein are considered the strains fell into two groups. This classification is related to the host adaptation except for one barley-adapted strain that showed characteristics of wheat-adapted strains and one wheat-adapted strain which did not produce septorines.     

Antifungal compounds in winter wheat resistant and susceptible to Septoria nodorum

The content of benzoxazinone compounds in Septoria nodorum resistant and susceptible wheat cultivars fell with time, reaching an undetectable level 3–4 wk after germination. The compounds were absent from field-grown plants extracted at growth stages 2–3, 6–7, 10, 10.5 and 11. Additional antifungal compounds were detected in healthy and diseased field-grown plants of both cultivars at all growth stages, although qualitative and quantitative differences in inhibitory activity were small and their role in resistance remains open to discussion.     

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     

Inheritance of resistance to leaf and glume blotch caused by Septoria nodorum Berk. in winter wheat

Sixteen crosses between eight winter wheat cultivars were screened for resistance to Septoria nodorum leaf and glume blotch in the F₁ and F₄ generations using artificial inoculation in the field. The F₁ of most crosses showed dominance for susceptibility on both ear and leaf. The effects of general combining ability were of similar magnitude as the effects for specific combining ability. On the basis of the phenotypic difference of the parents, no prediction was possible about the amount and the direction of genetic variance in the segregating populations. The variation observed in this study both within and among the segregating populations suggests a quantitative inheritance pattern influencing the expression of the two traits. The components of variance between F₂ families within a population were as high as (for S. nodorum blotch on the ear) or higher (for S. nodorum blotch on the leaf) than those between populations. Therefore, strong selection within a few populations may be as effective to obtain new resistant genotypes as selection in a large number of populations. In almost all crosses, progenies were found that were more resistant than the better parent. Thus transgression breeding may be a tool to breed for higher levels of resistance to S. nodorum blotch. Highly resistant genotypes were found even in combination with two susceptible parents. The genetic source for Septoria resistance is probably broader than is generally assumed and could be used to improve S. nodorum resistance by combination breeding followed by strong selection in large populations. Received: 18 January / Accepted: 30 April 1999     

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.     

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     

Effects of inoculation with Septoria nodorum on yield components of spring wheat

Spring wheat, cv. Kolibri, was artificially inoculated with spore suspensions of Septoria nodorum, from the time of full expansion of the third leaf to the end of flowering and the effect on yield components recorded. Most components were reduced by increasing severity of infection and grain weight losses of 56 and 35% were produced on single-stemmed and tillering plants respectively, by the most severe infection. The effects on components of yield differed considerably between the main stems and the tillers and an increase in grain number on moderately infected tillers partly compensated for reductions in yield on the main stem.     

Salicylic and jasmonic acids in regulation of the proantioxidant state in wheat leaves infected by Septoria nodorum Berk

Influence of mediators of the signal systems of salicylic (SA) and jasmonic (JA) acids and their mixture on reactive oxygen species' (ROS) (superoxide radical O2*- and H2O2) generation and activity of oxidoreductases (oxalate oxidase, peroxidase and catalase) in leaves of wheat Triticum aestivum L. infected by Septoria leaf blotch pathogen Septoria nodorum Berk. has been studied. Presowing treatment of seeds by SA and JA decreased the development rate of fungus on wheat leaves. SA provided earlier inductive effect on production of O2*- and H2O2 compared with JA. The protective effect of the salicylic and jasmonic acids against Septoria leaf blotch pathogen was caused by activation of oxalate oxidase, induction of anion and cation peroxidases, and decrease of catalase activity. Ability of compounds to stimulate ROS in the plant tissues can be used as criteria for evaluation of immune-modulating activity of new substances for protection of the plants.     

Field techniques for assessing the reaction of winter wheat cultivars to Septoria nodorum

Methods of assessing the reaction of winter wheat cultivars to Septoria nodorum were compared in nine field trials. Plots were inoculated either by scattering naturally infected wheat debris or by spraying with a suspension of conidia at different growth stages, and were subsequently kept damp by several methods. Disease development was monitored by assessing the percentage cover of different organs by lesions, or by measuring the maximum height of lesions above ground. Major differences between cultivars were evident with all methods of inoculation and assessment, but time of assessment was critical for best differentiation. Inoculation with conidia soon after ear emergence, followed by assessment of lesions on flag leaves and ears, was a simple and effective method. Yield loss due to disease was measured by making comparisons either between inoculated and uninoculated plots, with various degrees of separation, or between inoculated plots with and without fungicidal treatment: the first of these methods was more effective in maintaining a disease differential. Yield loss was positively correlated with severity of disease and there was little evidence that cultivars differed in tolerance of disease. Disease was measured more easily and with less error than yield loss.     

Bacteria associated with Stagonospora (Septoria) nodorum increase pathogenicity of the fungus

In studies with a laboratory isolate of the fungal pathogen Stagonospora ( Septoria ) nodorum three different isolates of bacteria were closely associated with the fungus. Bacteria were also closely associated with fresh isolates of S. nodorum obtained from artificially and naturally infected field material. Although a range of bacteria was isolated, only one type of bacterium was found to be associated with each isolate of S. nodorum . In co-inoculation studies with pycnidiospores of the fungus on detached leaves, some of the bacterial isolates significantly increased the pathogenicity of the fungus, particularly Xanthomonas maltophilia , Sphingobacterium multivorum , Enterobacter agglomerans and Erwinia amylovora . Evidence is presented indicating that one of the ways that the 'helper bacteria' may assist in the establishment of infections is by the production of lipases that were not detected in germinating fungal spores.