Components of partial resistance to Leptosphaeria nodorum among seven soft red winter wheats

Summary Seven soft red winter wheat cultivars were evaluated for partial resistance to Leptosphaeria nodorum under field conditions. The results demonstrate that resistance is available among cultivars that are adapted to the southeastern U.S. and that resistance is long lasting. Resistant cultivars had longer incubation and latent periods, slower rates of lesion development, and reduced the level of sporulation of L. nodorum. Seedlings of susceptible cultivars sustained severe disease with dew periods as short as 48 hr in greenhouse tests. One resistant cultivar sustained little damage with dew period as long as 144 hr.Latent period was shortest on the second leaf below the flag leaf (F-2 leaf) of all cultivars and longest on the flag leaf which reflected the effect of microclimate and leaf age. The range in length of latent period on the flag, F-1, and F-2 leaves of resistant cultivars was less than that for susceptible cultivars. Differences between cultivars were greatest (up to 6.8 days) for the F-2 leaf. A delay in production of inoculum on the F-2 and lower leaves of resistant cultivars should delay infection of the flag leaf and spike.The greatest differentiation among cultivars for sporulation of L. nodorum was on upper leaves at Feekes growth stage 11.2. Oasis consistently had less sporulation than other cultivars at all sampling dates and leaf positions. The rate of disease progress up the plant (disease severity) and area under the disease progress curve were also least on Oasis.There were significant correlations among components of resistance and associated components suggesting that a single or interrelated mechanisms control expression of resistance. Overall, Oasis was the most resistant cultivar followed by Coker 762 and Coker 747. There cultivars have remained resistant for ten or more years. Stacy was intermediate in resistance and was the most variable of the seven cultivars for the components tested. Holley, Omega 78, and Florida 301 were highly susceptible by all criteria tested. Plant height was not a factor in resistance. The resistant cultivars are mostly later in maturity than the susceptible cultivars. In this study, crop maturity was similar under existing environmental conditions during the two seasons when most data were collected.     

Chromosome and chromosomal arm locations of genes for resistance to Septoria glume blotch in wheat cultivar Cotipora

Summary Septoria glume blotch, caused by Stagonospora nodorum, is an important disease of wheat (Triticum aestivum). Separate genetic mechanisms were found to control flag leaf and spike resistance. Genes for resistance to S. nodorum were located on different chromosomes in the few wheat cultivars studied. These studies only partially agree on the chromosome locations of gene in wheat for resistance to S. nodorum, and chromosomal arm locations of such genes are not known. The objectives of this study were to determine the chromosome and chromosomal arm locations of genes that significantly influence resistance to S. nodorum in wheat cultivar Cotipora. Monosomic analysis showed that flag leaf resistance was controlled by genes on chromosomes 3A, 4A, and 3B whereas the spike resistance was controlled by genes on chromosomes 3A, 4A, 7A, and 3B (P=0.01). Additionally, genes on chromosomes 6B and 5A influenced the susceptibility of the flag leaf and spike reactions, respectively (P=0.01). Telocentric analysis showed that genes on both arms of chromosome 3A, and the long arms of chromosomes 4A and 3B were involved in the flag leaf resistance whereas genes on both arms of chromosome 4A, the short arm of chromosome 3A, and the long arm of chromosome 3B conferred spike resistance.     

A quantitative trait locus on chromosome 5B controls resistance of <Emphasis Type="Italic">Triticum turgidum</Emphasis> (L.) var. <Emphasis Type="Italic">diccocoides</Emphasis> to Stagonospora nodorum blotch

Stagonospora nodorum blotch (SNB) is an important foliar disease of durum wheat (Triticum turgidum var. durum) worldwide. The combined effects of SNB and tan spot, considered as components of the leaf spotting disease complex, result in significant damage to wheat production in the northern Great Plains of North America. The main objective of this study was the genetic analysis of resistance to SNB caused by Phaeosphaeria nodorum in tetraploid wheat, and its association with tan spot caused by Pyrenophora tritici-repentis race 2. The 133 recombinant inbred chromosome lines (RICL) developed from the cross LDN/LDN(Dic-5B) were evaluated for SNB reaction at the seedling stage under greenhouse conditions. Molecular markers were used to map a quantitative trait locus (QTL) on chromosome 5B, explaining 37.6% of the phenotypic variation in SNB reaction. The location of the QTL was 8.8 cM distal to the tsn1 locus coding for resistance to P. tritici-repentis race 2. The presence of genes for resistance to both SNB and tan spot in close proximity in tetraploid wheat and the identification of molecular markers linked to these genes or QTLs will be useful for incorporating resistance to these diseases in wheat breeding programs.     

Response of wheat cultivars to infection by Stagonospora nodorum isolates/mixture on detached and intact seedling leaves

Stagonospora nodorum blotch (SNB) of wheat caused by Stagonospora nodorum (Berk.) Castellani & E.G. Germano is among the major foliar diseases of wheat worldwide. The response of four wheat cultivars for reaction to infection by isolates/mixture of S. nodorum was tested on detached and intact seedling leaves in the laboratory and glasshouse experiments, respectively. The four wheat cultivars tested significantly differed from each other in response to infection by the isolates/mixture of S. nodorum. Similarly, the aggressiveness of the four isolates/mixture of the pathogen on the four wheat cultivars was significantly different as demonstrated by the parameters tested in the two experiments. However, no significant (p ≤ 0.05) isolate x cultivar interaction was observed for all the parameters tested in the two experiments. Highly significant positive or negative correlations were observed between parameters, i.e., disease severity (DS), incubation period(IP), and size of necrotic lesions (SNL), tested on detached and intact seedling leaves, respectively. This suggests that use of the detached leaf technique has considerable promise for quick screening of wheat cultivars against SNB. This revised version was published online in August 2006 with corrections to the Cover Date.     

Environmental stability of partial resistance in spring wheat to wheat leaf rust

Summary Five spring wheat cultivars differing in partial resistance (PR) to wheat leaf rust were tested at Wageningen (the Netherlands) on a sandy and a clay site, El Batan (CIMMYT, Mexico) and Ponta Grossa (Brazil) over two years. The cultivars were Skalavatis 56, Little Club (both very susceptible), Westphal 12A, Akabozu and BH 1146 (all three with high levels of PR). The results showed that PR was expressed at all four locations in both years. The level of expression was influenced by the environment but the cultivar ranking was hardly affected. Selection for PR in the field can therefore be carried out over a wide range of environments.     

Comparison of resistance of triticale,wheat and spelt to septoria nodorum blotch at the seedling and adult plant stages

Summary On average, the cereal species studied were susceptible to septoria nodorum blotch (SNB), except for spring triticale on leaf and head and winter titicale on leaf, that appeared to be significantly more resistant, than the other ones.In all three species the SNB response of the adult plants was to a limited extent only predicted by the reaction on first leaf seedlings. In most cases it was impossible to predict the response to SNB of adult plants on the basis of seedling reaction. Correlations between the adult plant stage and the seedling stage, or detached seedling leaves, appears not to be sufficient for use in practical breeding work. A reversal of reaction to SNB was even found between the above growth stages in studied spring and winter wheat varieties.     

Selection for resistance to Septoria nodorum in wheat

Summary A population of 572 F₂ derived F₃ lines from six crosses were used to estimate parameters relevant to selection for resistance to Septoria nodorum of wheat. Lines were grown in disease free (fungicide sprayed) and inoculated microplots in 2 replications of a split-plot design in a single environment in 1977. Average yield reduction due to disease was approximately 50%; this was associated with an average septoria score of 50% on the flag leaf, an average septoria score of 42% on the head, and a reduction of 37% in seed weight. Low S. nodorum scores were correlated with late heading date, tall plant height, high grain yield, and high seed weight in diseased plots, and high seed weight % (seed weight in diseased plots expressed as a percentage of seed weight in fungicide sprayed plots).Restricted selection indexes were used to study the relative contributions of disease escape, true resistance, and tolerance to variability in grain yield in diseased plots, seed weight in diseased plots, and seed weight %. True resistance appeared to be the most important factor causing variation in grain yield in diseased plots and seed weight %. Tolerance and escape seemed to be more important for seed weight in diseased plots.Heritabilities of S. nodorum scores on the flag leaf and head were 63% and 52%, respectively. Leaf and head scores could be used most effectively as selection criteria to upgrade resistance in a population before harvest.Selection for high seed weight % slightly reduced yields in disease free plots, although yield in diseased plots and seed weight in diseased plots were increased. However, selection for increased yield or increased seed weight in diseased plots improved yield in disease free plots. It is suggested that direct selection for yield or seed weight in diseased plots is likely to achieve more desirable goals than selection for seed weight %.     

Resistance to septoria nodorum blotch in the Aegilops tauschii accession RL5271 is controlled by a single gene

Septoria nodorum blotch is the most important leaf disease of wheat in Western Australia. A potentially useful source of resistance has been identified in an accession of Aegilops tauschii. To study the genetics of resistance of this source a cross was made between the resistant Ae. tauschii accession, RL5271, and a susceptible accession, CPI110889. The resistant parent took significantly longer to develop symptoms, developed significantly fewer lesions and expressed significantly lower levels of disease than the susceptible parent. The F1 mean response for disease severity indicated there was no complete dominance. The F3 families were classified using three approaches. In the first approach the individual F3 plant response was used to classify the F3 families. In the second approach the F3 family means and standard errors were used to classify the F3 families. In the final approach Best Linear Unbiased Predictors of disease score and standard error for each F3 family derived from a REML analysis were used to classify the F3 families. The genotypic ratios generated by each of the approaches suggested that resistance is controlled by a single gene. The effectiveness of the resistance and its simple genetic control in the Ae. tauschii, accession RL5271 may be a useful resistance source for use in a bread wheat breeding program. This revised version was published online in July 2006 with corrections to the Cover Date.     

Resistance to Septoria nodorum blotch in several Triticum species

Summary Resistance to septoria nodorum blotch in Triticum monococcum, T. tauschii, T. timopheevii, T. dicoccum and T. durum was evaluated on plants at the three-leaf stage in greenhouse tests. A high frequency of resistant genotypes was found in T. monococcum, T. tauschii and T. timopheevii, but not in T. dicoccum and T. durum. The resistance of F₁ plants of crosses of resistant T. monococcum (PI 289599) and T. timopheevii (PI 290518) accessions with susceptible common wheat cv. Park and durum wheat cv. Wakooma, respectively, was evaluated on the basis of percentage leaf necrosis, lesion number, lesion size and incubation period. No dominance was found for long incubation period, but various dominance relationships occurred for low percentage leaf necrosis, low lesion number and small lesion size, depending on the cross. Multiple regression analysis showed that lesion number contributed more to percentage leaf necrosis than lesion size or incubation period. Resistance to septoria nodorum blotch was transferred successfully from T. timopheevii to cultivated durum wheat. Resistant BC₁F₇ lines, recovered from the T. timopheevii (PI 290518) × Wakooma cross, showed normal chromosome behaviour at meiosis (14 bivalents) and were self-fertile. However, an effective level of resistance was not recovered in lines derived from the other interspecific crosses.     

Race-specific aspects of partial resistance in wheat to wheat leaf rust,Puccinia recondita f.sp. tritici

Summary Partial resistance (PR) in wheat to wheat leaf rust (Puccinia recondita f.sp. tritici) is characterized by a slow epidemic build-up despite a susceptible infection type. Two greenhouse tests and two field tests, in which 11 spring wheat cultivars were exposed to five wheat leaf rust races, revealed some indication for race-specificity of PR.In the greenhouse, the expression of PR was highly dependent on the environment. Significant cultivar-race interactions in the first experiment were lost in the second experiment probably due to cultivar-environment and cultivar-race-environment interactions.In the polycyclic field tests several factors played a role in explaining the inconsistency of the cultivar-race interactions, such as differences in initial inoculum, genotypic differences in earliness, interplot interference or environmental conditions.One cultivar-race combination showed a significant but small interaction towards susceptibility in both field experiments. The interaction was probably too small to detect in the monocyclic greenhouse tests. The results do not conflict with the idea that a gene-for-gene relationship could exist between PR-genes in the host and genes in the pathogen.Some problems with regard to the selection of PR in wheat to wheat leaf rust are discussed.     

Partial resistance to wheat leaf rust in 18 spring wheat cultivars

Summary Eighteen spring wheat cultivars were tested in microfields and race nurseries for their partial resistance PR to wheat leaf rust under low and high disease pressure respectively. Large differences existed between the 18 cultivars, Skalavatis 56 being the most susceptible and Ponta Grossa 1 being the most resistant cultivar. Of the three epidemic parameters, disease severity (DS) at the time that the susceptible check was severely diseased and area under the transformed disease severity curve (AUTC) and the logistic growth rate (r), AUTC and DS were highly correlated. Both seemed to be reliable estimators of PR but DS should be preferred for economical reasons. The logistic growth rate seemed to be unsuitable as an estimator of partial resistance.High and low disease pressure gave similar cultivar ranking. PR can be screened and selected equally well in race nurseries with low space, low time and low cost input as in microfields with high space, time and cost input.Cultivar differences in development rate had a large impact on the cultivar differences for amount of disease and can therefore greatly bias the estimation of cultivar resistance. The resistance of early cultivars tended to be underestimated whereas the resistance of late cultivars tended to be overestimated. The effect of differences in developmental rate was most pronounced in the flag leaf. It is advisable to avoid the assessment of disease levels on the flag leaf only and to incorporate in the tests several susceptible and resistant checks that cover the range of development rates in the material to be selected, because otherwise selection for resistance will tend to select also for lateness.Regression of the epidemiological parameters on three components of partial resistance revealed that latency period (LP) is an important factor in determining the resistance observed in the field explaining on average 67% of the observed variation. Adding infection frequency (IF) and urediosorus size (US) to the linear model increased the proportion of the observed variation in the field explained by the components to 80%. This result supports the idea that the components of PR inherit independently, at least, in part.     

Generation mean analysis to identify and partition the components of genetic resistance to Septoria nodorum in wheat

Summary Foliar symptom severity of seedlings artificially inoculated with S. nodorum were used to idenify the type of gne action controlling resistance to this pathogen in the early generations of two wheat crosses. In both crosses a resistant spring wheat cultivar was crossed to a susceptible cultivar. Reciprocal crosses were included in the analysis to determine if the cytoplasm contributed in any significant degree to the level of resistance present.Results indicated that resistance was polygenic and that it could be explaned prinerpally by additive gene effects. Some differences in reciprocal crosses were evident, but a significant role for the eytoplasm in resistance is not indicated.Cooperative investigations of the Agricultural Research Service of the U. S. Department of Agriculture and the Montana Agricultural Experiment Station. Journal Paper No. 1210.     

Role of chromosome 3A in stomatal resistance of winter wheat

Summary Leaf stomatal resistance, through transpiration and photosynthesis control, constitutes a major factor of productivity and adaptation in wheat. The aim of the investigations reported here was to identify chromosomal effects on the expression of the maximum stomatal resistance, determined under optimum conditions of irradiance and water supply. Leaf stomatal resistance was measured, on wheat grown in pots under natural and well-watered conditions, using a LI-COR LI-6200 portable photosynthesis system under a saturating light>1400 mol m^-1 s^-2. Reciprocal sets of chromosome substitution lines between two hard red winter wheat cultivars, Wichita and Cheyenne, were used to identify the chromosomes involved in the expression of this trait. The two parental cultivars were significantly different for the parameter investigated. Chromosome 3A appeared to be involved in the expression of the stomatal resistance value under optimum conditions. Taking into account the relationships previously established between this parameter and some agronomic traits, chromosome 3A might be involved directly in productivity-determining processes or in the adaptation to water conditions, in wheat.     

The influence of disease complexes involving Leptosphaeria (Septoria) nodorum on detection of resistance to three leaf spot diseases in wheat

Summary In controlled inoculation studies with Septoria nodorum and Pyrenophora tritici-repentis, estimates of the relative proportion of each pathogen demonstrated differences in the responses of cultivars to pathogen mixtures that were not apparent from measurements of diseased leaf areas. Under field conditions estimates of the relative proportion of S. nodorum, P. tritici-repentis and S. tritici varied between field screening locations in Western Australian but also between lines within locations. Lines with known resistance to P. tritici-repentis and S. tritici, but susceptible to S. nodorum, could not be distinguished from susceptible lines on the basis of leaf area diseased or grain weight depression when S. nodorum was present in the disease complex. Such conditions, while suitable for the selection of combined resistance to these pathogens, were unsuitable for identifying resistance to individual pathogens. As symptoms were similar, the proportion of diseased leaf area sporulating with each pathogen provided a means of measuring the variation in disease development induced on lines varying in resistance. Knowledge of the components of disease and their relative importance were essential in understanding varietal response information under mixed infections of these leaf spot pathogens.     

Influence of race and post infection temperature on two components of partial resistance to wheat leaf rust in seedlings of wheat

Summary Components of partial resistance, infection frequency and latency period, were determined in 71 winter and spring wheat genotypes in the seedling stage, after infection with three races of leaf rust (Felix 3B, Clement B and Betuwe 85C) at three different day/night temperature regimes (24/18°C, 18/12°C and 12/6°C). The genotypes were split into two groups and two separate experiments were carried out. Five genotypes, SVP 84039, Akabozu, Banco, BH 1146 and Orso, conferred a low infection frequency and a long latency period and Westphal 12A a long latency period, indicating a relatively high level of partial resistance. The correlation coefficient between infection frequency and latency period was low. Race-specificity was not found. There was a significant temperature effect on the latency period. In the second experiment the temperature x genotype interaction was significant. Temperature-response functions of transformed data demonstrated that the latency periods of four relatively resistant genotypes, Westphal 12A, Banco, BH 1146 and Orso and of Sarno and Mirela were most sensitive to temperature. The range between the genotypes with the longest and the shortest latency period was highest at 12°C. Therefore, low temperature regimes are preferred to distinguish differences in level of partial resistance.     

Relationship of plant height and days to maturity with resistance to spot blotch in wheat

A total of 1,407 spring wheat (T. aestivum) lines of Indian and CIMMYT (International Maize and Wheat Improvement Centre, Mexico) origin were evaluated for plant height, days to maturity and resistance to spot blotch (caused by Bipolaris sorokiniana) during the 1994–95, 1995–96 and 1996–97 crop seasons. The frequency distribution of genotypes, based on disease score ignoring the growth stages, differed from the distribution in which disease score was assessed on a similar growth stage. Two crosses each,between `tall resistant × dwarf susceptible' and `late resistant × early susceptible' genotypes, were made. The evaluation of homozygous resistant lines in the F₃, F₄ and F₅ generations of both crosses showed a wide range of plant height and days to maturity. These lines showed significant differences for plant height and days to maturity but did not show a significant difference for AUDPC values of spot blotch. The correlation coefficients for AUDPC versus plant height or days to maturity were weak, i.e., – 0.336 and 0.061, respectively. Results indicated that resistance to spot blotch severity was independent of plant height and days to maturity in progenies from these crosses.     

Chromosome location of resistance to septoria leaf blotch and common bunt in wheat-barley addition lines

Septoria leaf blotch and common bunt are important diseases of wheat to which Hordeum vulgare is resistant. Addition lines of H. vulgare in wheat were utilized to determine which H. vulgare chromosomes carry resistance genes. Resistance to septoria leaf blotch was conferred by gene(s) present all over the barley genome, but more strongly by those located on chromosomes 7 and 4. Almost complete resistance to common bunt was conferred by gene(s) present in chromosomes 6 and a slight but significant level of resistance was conferred by chromosome 7. This revised version was published online in August 2006 with corrections to the Cover Date.     

Influence of development stage and host genotype on three components of partial resistance to leaf rust in spring wheat

Summary Latency period (LP), infection frequency (IF) and urediosorus size (US) of leaf rust were determined on primary leaves and young flag leaves of 18 spring wheat cultivars. A large growth stage effect and a large cultivar effect on all three components were observed. Partial resistance as measured by the three components was generally better expressed in the adult plant stage than in the seedling stage. Associated variation of the components was observed: long LP, low IF and small US tended to go together. The association was not complete, cultivars with clear deviations of this association for one of the components were found suggesting the existence of at least partly different genetic factors controlling the respective components. LP measured on flag leaves gave the most reliable results and, therefore, could best be used as a selection criterion in breeding programs for partial resistance.     

Genetics of resistance in the wheat cultivar chhoti lerma against stem rust race 122

Summary Genetics of rust resistance against stem rust race 122 in Chhoti Lerma was studied both by conventional and aneuploid analysis. Observations on F₁, F₂ and F₂ backcross progenies revealed the operation of two recessive genes, controlling resistance in Chhoti Lerma. Monosomic analysis confirmed the operation of two recessive genes conferring resistance to race 122 located on chromosomes 1D and 7D. A minor gene or modifier was also located on chromosome 1B. This was concluded from the fact that F₂ of mono's x Chhoti Lerma exhibited skewness in favour of resistant plants.     

Influence of post-infection temperature on three components of partial resistance in wheat to wheat leaf rust

Summary Three components of partial resistance (PR) were studied at three post-infection temperatures using seven spring wheat genotypes differing in level of PR and two different wheat leaf rust races. The components were latency period (LP), infection frequency (IF) and urediosorus size (US). The expression of LP was more sensitive to temperature than the expression of the other two components. LP-prolonging genes were better expressed at low temperatures than at high temperatures and cultivar differences tended to increase with decreasing temperature in both seedling and adult plant stages. The reaction of IF to temperature differed from that of LP and US, probably because IF is regulated by another mechanism than LP and US.It is recommended to perform PR-screening tests at low rather than at high temperatures. If temperatures are maintained at about 8–13°C (night-day), seedlings can be used to screen for PR instead of the more expensive adult plant tests.The effectiveness of PR in seedling stage at low temperatures suggests that the seedling stage may have epidemiological significance as the low temperatures (8–13°C) are relevant for seedlings in the field.