Physiological Aspects Related to Tolerance of Spring Wheat Cultivars to Septoria tritici Blotch

ABSTRACT The susceptible wheat cultivar Miriam exhibited tolerance under severe infection of Septoria tritici blotch (STB). Nethouse and greenhouse trials confirmed former field results in which losses in grain weight of 'Miriam' wheat due to STB infection were significantly lower than those of the susceptible cultivar Barkai, under equivalent severity and the same disease progress curve. Several physiological mechanisms that may explain this tolerance of 'Miriam' wheat were studied. A comparison between protected and infected plants proved that carbohydrate reserves in the culms and other vegetative plant parts did not account for the lower losses in grain weight of 'Miriam'. Each tiller was shown to be independent in its supply of carbohydrates to its grains, and no import from secondary tillers was observed. Differences in the ratio between grain weight and vegetative biomass could not explain the sustained grain filling of infected plants of 'Miriam'. The daily balance of CO(2) exchange of the ears was negative, since carbon fixation by the spike in the light was more than counterbalanced by night time spike respiration. Radioisotope studies revealed that mature, infected 'Miriam' plants maintained as large a percentage of the carbohydrates fixed at the vegetative stage and early grain filling as healthy plants. On the other hand, under the same conditions, infected 'Barkai' plants lost a larger fraction of these carbohydrates. The rate of carbon fixation per unit of chlorophyll and per residual green leaf area of infected 'Miriam' was higher than in healthy plants. It is proposed that this enhancement of photosynthesis in residual green tissue of infected plants of the tolerant cultivar Miriam compensates for the loss of photosynthesizing tissue due to STB.     

Partial Resistance to Septoria Tritici Blotch (Mycosphaerella graminicola) in Wheat Cultivars Arina and Riband

ABSTRACT Partial resistance to Septoria tritici blotch (STB) and its inheritance were investigated in a doubled-haploid population of a cross between cvs. Arina and Riband. The former has good partial resistance whereas the latter is susceptible. In adult plant trials in polytunnels, STB disease scores were negatively correlated with heading date. Resistance was not specific to any of the three fungal isolates used in these tests. A quantitative trait locus (QTL) for partial resistance to STB was identified in Riband on chromosome 6B and is named QStb.psr-6B-1. No QTL controlling a major part of the Arina resistance was identified, suggesting that its resistance may be dispersed and polygenic. There was no correlation between the lines' mean disease scores at the seedling and adult stages, implying that partial resistance to STB is developmentally regulated. Seedling resistance to the isolate IPO323 was isolate-specific and controlled by a single gene in Arina, probably allelic with the Stb6 gene in cv. Flame that confers resistance to the same isolate. The implications of these results for wheat breeding programs are discussed.     

Wheat Leaf Rust Uredospore Production on Adult Plants: Influence of Leaf Nitrogen Content and Septoria tritici Blotch

ABSTRACT Leaf rust uredospore production and lesion size were measured on flag leaves of adult wheat plants in a glasshouse for different lesion densities. We estimated the spore weight produced per square centimeter of infected leaf, per lesion, and per unit of sporulating area. Three levels of fertilization were applied to the plants to obtain different nitrogen content for the inoculated leaves. In a fourth treatment, we evaluated the effect of Septoria tritici blotch on leaf rust uredospore production. The nitrogen and carbon content of the spores was unaffected or marginally affected by lesion density, host leaf nitrogen content, or the presence of Mycosphaerella graminicola on the same leaf. In leaves with a low-nitrogen content, spore production per lesion was reduced, but lesion size was unaffected. A threshold effect of leaf nitrogen content in spore production was however, evident, since production was similar in the medium- and high-fertilizer treatments. In leaves inoculated with M. graminicola and Puccinia triticina, the rust lesions were smaller and produced fewer spores. The relationships among rust lesion density, lesion size, and uredospore production were fitted to a model. We determined that the density effect on spore production resulted mainly from a reduction in lesion size, the spore production per unit of sporulating surface being largely independent of lesion density. These results are consistent with those obtained previously on wheat seedlings. The main difference was that the sporulation period lasted longer in adult leaves.     

Identification and Molecular Mapping of a Gene in Wheat Conferring Resistance to Mycosphaerella graminicola

ABSTRACT Septoria tritici leaf blotch (STB), caused by the ascomycete Mycosphaerella graminicola (anamorph Septoria tritici), is an economically important disease of wheat. Breeding for resistance to STB is the most effective means to control this disease and can be facilitated through the use of molecular markers. However, molecular markers linked to most genes for resistance to STB are not yet available. This study was conducted to test for resistance in the parents of a standard wheat mapping population and to map any resistance genes identified. The population consisted of 130 F(10) recombinant-inbred lines (RILs) from a cross between the synthetic hexaploid wheat W7984 and cv. Opata 85. Genetic analysis indicated that a single major gene controls resistance to M. graminicola in this population. This putative resistance gene is now designated Stb8 and was mapped with respect to amplified fragment length polymorphism (AFLP) and microsatellite markers. An AFLP marker, EcoRI-ACG/MseI-CAG5, was linked in repulsion with the resistance gene at a distance of approximately 5.3 centimorgans (cM). Two flanking microsatellite markers, Xgwm146 and Xgwm577, were linked to the Stb8 gene on the long arm of wheat chromosome 7B at distances of 3.5 and 5.3 cM, respectively. The microsatellite markers identified in this study have potential for use in marker-assisted selection in breeding programs and for pyramiding of Stb8 with other genes for resistance to M. graminicola in wheat.     

Quantitative Trait Loci Analysis and Mapping of Seedling Resistance to Stagonospora nodorum Leaf Blotch in Wheat

ABSTRACT Stagonospora nodorum leaf blotch is an economically important foliar disease in the major wheat-growing areas of the world. In related work, we identified a host-selective toxin (HST) produced by the S. nodorum isolate Sn2000 and determined the chromosomal location of the host gene (Snn1) conditioning sensitivity to the toxin using the International Triticeae Mapping Initiative mapping population and cytogenetic stocks. In this study, we used the same plant materials to identify quantitative trait loci (QTL) associated with resistance to fungal inoculations of Sn2000 and investigate the role of the toxin in causing disease. Disease reactions were scored at 5, 7, and 10 days postinoculation to evaluate changes in the degree of effectiveness of individual QTL. A major QTL was identified on the short arm of chromosome 1B, which coincided with the snn1 toxin-insensitivity gene. This locus explained 58% of the phenotypic variation for the 5-day reading but decreased to 27% for the 10-day reading, indicating that the toxin is most effective in the early stages of the interaction. In addition, relatively minor QTL were identified on chromosomes 3AS, 3DL, 4AL, 4BL, 5DL, 6AL, and 7BL, but not all minor QTL were significant for all readings and their effects varied. Multiple regression models explained from 68% of the phenotypic variation for the 5-day reading to 36% for the 10-day reading. The Chinese Spring nullisomic 1B tetrasomic 1D line and the Chinese Spring-Triticum dicoccoides disomic 1B chromosome substitution line, which were insensitive to SnTox1, were more resistant to the fungus than the rest of the nullisomictetrasomic and disomic chromosome substitution lines. Our results indicate that the toxin produced by isolate Sn2000 is a major virulence factor.     

Elucidation of Septoria tritici x Wheat Interactions Using GUS-Expressing Isolates

ABSTRACT Isolate ISR398 of Septoria tritici (which produces none to few pycnidia on the wheat cv. Seri 82 and high coverage on cv. Shafir) and isolate ISR8036 (which is virulent on both cultivars) were genetically cotrans-formed using the selectable marker gene hph, which confers resistance to hygromycin B (hygB), and the reporter gene uidA, encoding beta-glucuronidase (GUS). Most of the genetically transformed isolates (98.8%) produced similar pycnidial coverage on seedlings of 'Seri 82' and 'Shafir' as the two wild-type isolates. Southern analysis of 25 randomly selected hygB(R)GUS(+) transformants probed with the uidA sequence revealed multiple insertion sites. GUS activity was determined fluorimetrically by measuring the conversion of 4-methylumbelliferyl beta-D-glucuronide (MUG) to 4-methylumbelliferone (MU). The high GUS-expressing transformants 398D97 and 8036E27 were used to elucidate fungal development within inoculated leaf tissue by using GUS activity to estimate the fungal proteins content in planta. Increase in fungal biomass was recorded in 'Shafir' inoculated with the GUS-expressing transformants 398D97 and 8036E27 following a 12-day latent period. A 15-day latent period was recorded in 'Seri 82' inoculated with 8036E27, whereas an 18-day latent period was recorded on 'Seri 82' inoculated with 398D97 and the two mixtures 398D97 + ISR8036 and ISR398 + 8036E27. The rate of fungal development and the estimated level of fungal proteins at the pycnidia maturation stage was high in leaves of 'Shafir' and moderate to low on 'Seri 82', even in cases in which no significant differences were recorded in pycnidial coverage. An endogenous capacity to hydrolyze beta-1,4-D-glucuronidase was recorded in leaves inoculated with wild-type isolates. The latent periods in MU production of the uidA-expressing transformants mimicked those recorded for the wild-type isolates. However, at all stages, the levels of MU produced in wheat inoculated with wild-type isolates were markedly lower than those produced by GUS-expressing transformants. The mode of interaction (compatible or incompatible) determined the onset of the induction, rate, and level of enzyme production.     

Studies on the Interaction Between Septoria tritici and Stagonospora nodorum in Wheat

Interactions between Stagonospora nodorum and Septoria tritici were studied. Results from a detached glume experiment indicated that the interaction may be isolate-dependent, as it was shown that the interaction between the two pathogens may be beneficial or antagonistic depending on the isolate of each pathogen present. The number of spores produced by both pathogens was significantly greater when an aggressive isolate of S. tritici was mixed with a non-aggressive isolate of S. nodorum, whereas the number of spores produced by both pathogens was significantly less when two non-aggressive isolates were mixed. There was a significant reduction in disease level when S. tritici was applied prior to S. nodorum, compared to vice versa in the growth chamber. Results from growth chamber and field studies showed that S. nodorum produced significantly more spores when both pathogens were present together. It is concluded that S. tritici has a stimulatory effect on spore production by S. nodorum. However, there was a reduction of S. tritici spores observed in the dual inoculation treatments, suggesting that S. nodorum inhibits S. tritici.     

Identification and Molecular Mapping of a Gene Conferring Resistance to Pyrenophora tritici-repentis Race 3 in Tetraploid Wheat

ABSTRACT Race 3 of the fungus Pyrenophora tritici-repentis, causal agent of tan spot, induces differential symptoms in tetraploid and hexaploid wheat, causing necrosis and chlorosis, respectively. This study was conducted to examine the genetic control of resistance to necrosis induced by P. tritici-repentis race 3 and to map resistance genes identified in tetraploid wheat (Triticum turgidum). A mapping population of recombinant inbred lines (RILs) was developed from a cross between the resistant genotype T. tur-gidum no. 283 (PI 352519) and the susceptible durum cv. Coulter. Based on the reactions of the Langdon-T. dicoccoides (LDN[DIC]) disomic substitution lines, chromosomal location of the resistance genes was determined and further molecular mapping of the resistance genes for race 3 was conducted in 80 RILs of the cross T. turgidum no. 283/Coulter. Plants were inoculated at the two-leaf stage and disease reaction was assessed 8 days after inoculation based on lesion type. Disease reaction of the LDN(DIC) lines and molecular mapping on the T. turgidum no. 283/Coulter population indicated that the gene, designated tsn2, conditioning resistance to race 3 is located on the long arm of chromosome 3B. Genetic analysis of the F(2) generation and of the F(4:5) and F(6:7) families indicated that a single recessive gene controlled resistance to necrosis induced by race 3 in the cross studied.     

Resistance stability to Septoria tritici blotch and comparison of screening methods for ranking STB disease

Journal of Plant Diseases and Protection - Septoria tritici blotch (STB), caused by the ascomycete fungus Zymoseptoria tritici (formerly known as Mycosphaerella graminicola), is a devastating...     

A Gene-for-Gene Relationship Between Wheat and Mycosphaerella graminicola, the Septoria Tritici Blotch Pathogen

ABSTRACT Specific resistances to isolates of the ascomycete fungus Mycosphaerella graminicola, which causes Septoria tritici blotch of wheat, have been detected in many cultivars. Cvs. Flame and Hereward, which have specific resistance to the isolate IPO323, were crossed with the susceptible cv. Longbow. The results of tests on F1 and F2 progeny indicated that a single semidominant gene controls resistance to IPO323 in each of the resistant cultivars. This was confirmed in F3 families of Flame x Longbow, which were either homozygous resistant, homozygous susceptible, or segregating in tests with IPO323 but were uniformly susceptible to another isolate, IPO94269. None of 100 F2 progeny of Flame x Hereward were susceptible to IPO323, indicating that the resistance genes in these two cultivars are the same, closely linked, or allelic. The resistance gene in cv. Flame was mapped to the short arm of chromosome 3A using microsatellite markers and was named Stb6. Fifty-nine progeny of a cross between IPO323 and IPO94269 were used in complementary genetic analysis of the pathogen to test a gene-for-gene relationship between Stb6 and the avirulence gene in IPO323. Avirulence to cvs. Flame, Hereward, Shafir, Bezostaya 1, and Vivant and the breeding line NSL92-5719 cosegregated, and the ratio of virulent to avirulent was close to 1:1, suggesting that these wheat lines may all recognize the same avirulence gene and may all have Stb6. Together, these data provide the first demonstration that isolate-specific resistance of wheat to Septoria tritici blotch follows a gene-for-gene relationship.     

Genotypic Diversity of the Wheat Leaf Blotch Pathogen Mycosphaerella Graminicola (anamorph) Septoria Tritici in Germany

The population structure and genotypic diversity of Mycosphaerella graminicola from six natural field populations in Germany were studied with molecular markers. To reveal the potential effects of plant host resistance on the pathogen population, hierarchical samples were taken from susceptible and resistant cultivars. A total of 203 single spore isolates was subjected to molecular marker analysis using the amplified fragment length polymorphism technique (AFLP). Among the 203 isolates analyzed, 142 different multilocus haplotypes (MLH) were identified revealing a high degree of genotypic diversity of the M. graminicola population. On average, a F _ST value of 0.04 was found, indicating a low genetic differentiation with only 4% of the genetic variation between the local populations but leaving 96% of the genetic variation within the populations. According to the low F _ST value, a high migration rate of Nm 12 was found. The observed high within-population diversity, and the significant migration between populations, prevented genetic isolation and differentiation of putative geographically separated populations. Furthermore, plant host resistance had no obvious effect on the population structure and diversity of M. graminicola. Genotypic variability can be attributed to sexual recombination which appears to have a considerably larger influence on the population structure. Gene flow on this scale could have significant implications for plant breeding and fungicide spraying programmes.     

Alternative Respiration: a Biochemical Mechanism of Resistance to Azoxystrobin (ICIA 5504) in Septoria tritici

The mechanism of resistance to ICIA 5504 (azoxystrobin) in a Septoria tritici mutant raised in the laboratory has been investigated. This mutant was approximately 10 times less sensitive than the wild-type strain in in-vitro tests towards spore germination or fungal growth. Glucose oxidation in whole cells was inhibited in the wild type (80% inhibition at 0·1 μg ml^-1), whereas in the resistant mutant, oxygen uptake was stimulated (50% stimulation at 1·0 μg ml^-1). Respiration of the wild-type strain was inhibited by antimycin A and cyanide but not that of the mutant. These results indicate the existence of an efficient alternative respiratory pathway in the mutant, which was inhibited by the addition of 2 mM salicylhydroxamate (SHAM). Using mitochondria, antimycin A and ICIA 5504 did not completely inhibit NADH oxidation in either strain. Addition of SHAM inhibited part of the antimycin- and ICIA 5504-insensitive oxygen uptake only in mutant mitochondria. For complete inhibition of oxygen reduction, SHAM and cyanide need to be present. Thus, three systems of electron transfer from exogenous NADH to oxygen are present in S. tritici mitochondria: the cytochrome pathway which is sensitive to ICIA 5504 and antimycin A inhibition in both strains, the system of NADH-cytochrome c reductase which bypasses the methoxyacrylate inhibition at the cytochrome bc₁ complex, and the alternative oxidase which is inhibited by SHAM, and which is partially functioning only in mitochondria isolated from the ICIA 5504-resistant mutant. When the S. tritici isolates were tested for their in-vivo sensitivity to ICIA 5504 on wheat, the resistant strain was controlled better than the wild type. This indicates that the decreased ATP formation by the alternative pathway of respiration was inadequate for efficient parasitic growth on the host. © 1997 SCI.     

PCR-based Assays to Assess Wheat Varietal Resistance to Blotch (Septoria Tritici and Stagonospora Nodorum) and Rust (Puccinia Striiformis and Puccinia Recondita) Diseases

A multiplex Polymerase Chain Reaction (PCR) assay was developed to detect and quantify four fungal foliar pathogens in wheat. For Septoria tritici (leaf blotch) and Stagonospora nodorum (leaf and glume blotch), the -tubulin gene was used as the target region. Diagnostic targets for Puccinia striiformis (stripe or yellow rust) and P. recondita (brown rust) were obtained from PCR products amplified with -tubulin primer sequences. Final primer sets were designed and selected after being tested against several fungi, and against DNA of infected and healthy wheat leaves. For detection of the four pathogens, PCR products of different sizes were amplified simultaneously, whereas no products were generated from wheat DNA or other non-target fungi tested. The presence of each of the diseases was wheat tissue- and cultivar specific. Using real-time PCR measurements with the fluorescent dye SYBR Green I, PCR-amplified products could be quantified individually, by reference to a standard curve generated by adding known amounts of target DNA. Infection levels for each of the diseases were measured in the flag leaf of 19 cultivars at Growth Stage (GS) 60–64 in both 1998 and 1999. The infection levels for the cultivars were ranked, and showed, with a few exceptions, a good correlation with the NIAB Recommended List for winter wheat, which is based on visual assessment of symptoms. With PCR, the presence of the different pathogens was accurately diagnosed and quantification of pre-symptomatic infection levels was possible. Although sampling and DNA detection methods need further optimisation, the results show that multiplex PCR and quantitative real-time PCR assays can be used in resistance screening to measure the interaction between different pathogens and their hosts at different growth stages, and in specific tissues. This should enable an earlier identification of specific resistance mechanisms in both early-stage breeding material and field trials.     

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

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

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.