Resistance to multiple leaf spot diseases in wheat

Tan spot (TS), Stagonospora nodorum blotch (SNB), and Septoria tritici blotch (STB) are three major leaf spot diseases of wheat worldwide. Host plant resistance (HPR) is one of the main components in the management of these diseases in wheat. The objective of this study was to identify new sources of resistance to TS (races 1 and 5), SNB, and STB. A total of 164 wheat genotypes developed by the International Maize and Wheat Improvement Center (CIMMYT), Mexico were individually evaluated for TS, SNB and STB in spring and fall of 2006 in the greenhouse. Two experiments were conducted in a randomized complete block design with three replicates. Each replicate consisted of 164 wheat genotypes planted in cones with three seedlings/genotype in each cone and disease reaction was assessed for each race or pathogen at the two- to three-leaf stage. Based on the disease reactions, three wheat genotypes were resistant to both TS and SNB, while 13 genotypes were resistant to TS and STB. Similarly, 13 genotypes were resistant to both SNB and STB. In addition, four wheat genotypes were highly resistant to TS, SNB, and STB. These results suggest that the resistant genotypes identified in this study possess high levels of resistance to multiple leaf spot diseases and could be valuable sources for wheat improvement programs.     

Resistance to Rhynchosporium secalis in six Nordic barley genotypes

Six six-row Nordic spring barley genotypes (Hordeum vulgare L.) were assessed in the field in Finland (1994 and 1995) for resistance to Rhynchosporium secalis (Oud.) J.J. Davis, the causal pathogen of scald, in artificially inoculated plots. The barleys were known not to contain major genes for resistance to scald and the purpose of these experiments was to identify quantitative differences in resistance to scald which might be exploitable in a breeding programme. Disease development was monitored, grain yield and yield components were recorded, and these data were compared with measurements taken from plants in plots kept free of disease. Data, averaged over both years, for disease development on the uppermost three leaves — areas under disease progress curves, terminal severity and apparent infection rates — indicated that ‘Verner’, ‘Pohto’ and ‘Pokko’ were symptomatically significantly more resistant to scald than ‘Arve’, ‘Loviisa’ and Jo 1599. Grain yields, thousand-grain weights, test weights and proportions of plump grains were all significantly reduced in plots inoculated with scald compared with those kept free of disease; ‘Verner’ appeared to be the most useful genotype for use in crossing programmes to improve scald resistance in Finnish barleys.     

Mature plant resistance of barley to barley leaf rust,another type of resistance

Summary Seedling and flag leaves of three barley cultivars were simultaneously inoculated with urediospores of barley leaf rust, race 1-2-1, and incubated at the same greenhouse bench. The inoculated leaves were harvested before urediospore formation was initiated. The volume of a large number of colonies was estimated by measuring colony area and colony depth by embedding the colony containing leaf segments into paraffin for microtome cutting. The colony volume was considerably smaller in flag leaves than in seedling leaves even in the extremely susceptible cultivar. On average the difference was about tenfold.     

Race-specific effects in major genic and polygenic resistance of barley to barley leaf rust in the field: identification and distinction

Summary Six partially resistant spring barley cultivars were exposed to four barley leaf rust (Puccinia hordei) races in the field and in the greenhouse. The 24 cultivar-race combinations were tested in field plots of 1.5×1.5 m² in two replications over two years. To reduce the interplot exchange of urediospores each plot was surrounded by winter rye.The level of barley leaf rust varied among cultivars, races and years. In both years the variance for cultivar-race interactions was highly significant and originating largely from the cultivar-race combinations Berac-22. Armelle-22, Armelle-A and Tyra-A. The Berac-22 interaction was towards higher, the other three interactions towards a lower level of barley leaf rust. The reduced rust levels of these three combinations were not due to interactions between the partial resistance of these cultivars and the aggressiveness of the races but to major genes for hypersensitivity not effective to the races 1-2-1 and F, common in Western Europe, but effective against the rare races 22 and A. This was revealed in the greenhouse experiments where all combinations had a susceptible infection type except Armelle-22, Armelle-A and Tyra-A, which showed low infection types in both the seedling and adult plant stages. The urediosori present in the field plots of these three combinations apparently arose from spores derived from other plots; this interplot interchange suggesting partial resistance.The interaction of Berac with race 22 truly was a small race-specific effect within the polygenic, partial resistance of barley to barley leaf rust like the one reported before between Julia and race 18.     

Localization of a resistance gene and identification of sources of resistance to barley leaf stripe

In order to determine more precisely the location of the barley leaf stripe gene, called the ‘Vada-resistance gene’, on barley chromosome 2, 63 chromosome-doubled barley lines were tested. Using data on known chromosome 2 genetic markers, the ‘Vada-resistance gene’ was estimated to be located between the markers MSU21 and Xris45b, and at a distance of about 20% recombination from the powdery mildew resistance gene MILa. We suggest that the ‘Vada-resistance gene’ is designated Rdg1a and that all former leaf stripe resistance gene designations should be rejected. To identify possible new sources of resistance, 11 barley cultivars/lines known to possess leaf stripe resistance and originating from different parts of the world, were tested with one Danish and two Syrian isolates of the leaf stripe fungus. Three apparently genetically different sources of race-specific resistance were found. The ‘Vada-resistance’ in the cultivar ‘Golf was effective against seven out of eight isolates’ populations of the leaf stripe fungus differing in geographical origin.     

Pleiotropic association of infection frequency and latent period of two barley cultivars partially resistant to barley leaf rust

Summary Cebada Capa, carrying four to six minor genes for a longer latent period (LP), was crossed to L94 and Vada, carrying no and five to six minor genes for a longer LP respectively. Of each of 68 F3-lines the infection frequency (IF) and the LP of ten just-heading plants were assessed. There appeared to be a strong association between IF and LP, whereby the relationship between IF and LP of both crosses could be described by a single linear regression equation. The data strongly suggest that the genes for increased LP pleiotropically decrease the IF. The possibility of a close linkage between genes for reduced IF and genes for increased LP, although unlikely, could not be excluded.     

Accumulating polygenes for partial resistance in barley to barley leaf rust,Puccinia hordei. II. Field evaluation

Summary Eight lines from the cross between Vada and Cebada Capa with long to very long latent periods and four barley cultivars representing the known range of partial resistance to barley leaf rust, caused by Puccinia hordei, were evaluated in the field for partial resistance and in the greenhouse for the latent period (LP) in the young flag leaf.Each of the 12 entries was sown (15-4-1983) on a plot of 1.0 m². There were four replicates. To reduce interplot interference the plots were separated from each other by 4.0 m of spring rye. The number of urediosori per tiller was evaluated at 27-6, 4-7, 12-7 and ten days after heading. The LP was measured on 10 to 15 plants per entry in 1982 and on 10 plants in 1983.The levels of partial resistance varied greatly. The difference in number of sori per tiller between the most susceptible cultivar, Akka, and the most resistant cultivar, Vada, was about 50 times. Between Akka and the most resistant line this was approximately 5000 times. The LP's varied similarly. Vada had a LP 64% longer than that of Akka, the LP of line 26-6-11 was 15% longer. The range of partial resistance has been extended more than twofold.The correlation coefficients between LP and the level of barley leaf rust, expressed in transformed scale units, varied from -0.99 for the first sampling date to -0.97 for the third sampling date. Sampling the same development stage, ten days after heading, did not improve the r-value (r=–0.98). The LP evaluated in the young flag leaf is shown to be a very reliable criterion for partial resistance in the barley-Puccinia hordei pathosystem.Earliness tends to be associated with susceptibility. The correlation of days to heading with LP was 0.63, and with the level of barley leaf rust in the field 0.64.     

Rapid phenotyping of adult plant resistance in barley (Hordeum vulgare) to leaf rust under controlled conditions

Breeding for adult plant resistance (APR) is currently impeded by the low frequency of annual field‐based testing and variable environmental conditions. We developed and implemented a greenhouse‐based methodology for the rapid phenotyping of APR to leaf rust in barley to improve the efficacy of gene discovery and cloning. We assessed the effects of temperature (18 and 23°C) and growth stage (1–5 weeks) on the expression of APR in the greenhouse using 28 barley genotypes with both known and uncharacterized APR. All lines were susceptible in week 1, while lines carrying Rph20 and several with uncharacterized resistance expressed resistance as early as week 2. In contrast, lines lacking Rph20 and carrying either Rph23 and/or Rph24 expressed resistance from week 4. Resistant phenotypes were clearest at 18°C. A subset of 16 of the 28 lines were assessed for leaf rust across multiple national and international field sites. The greenhouse screening data reported in this study were highly correlated to most of the field sites, indicating that they provide comparable data on APR phenotypes for screening purposes.     

Accumulating polygenes for partial resistance in barley to barley leaf rust,Puccinia hordei. I. Selection for increased latent periods

Summary The barley cultivar Cebaba Capa was crossed to the cultivar L94, which is assumed to carry no genes for increased latent periods, and Vada, which is assumed to carry five to six minor genes for a longer latent period (LP). In the F2 selection was carried out for short and long LP's in the young flag leaves to Puccinia hordei in both crosses. In the F3, F4 and F5 the selection for short as well as for long LP continued by selecting the extreme plants in the extreme lines, a typical pedigree selection approach.The LP's are given relative to those of L94, set at 100 and of Vada, set at 185. From the cross with L94 homogeneous lines were obtained with relative LP's of 100 and of 220. From the cross with Vada the extreme lines had LP's of 135 and around or even beyond 300.Cebaba Capa is thought to carry four to six minor genes with an average gene effect slightly larger than those of the five to six minor genes in Vada. From the four to six minor genes one or two may be identical to or closely linked with minor genes of Vada, the others appeared to be different. In the lines with LP's of close to 300 or even more the number of minor genes accumulated is thought to be in the order of eight or nine. These gene number estimates are based on independent assortment. If linkage occurs the number of genes involved may be larger.Because of the high correlation between LP in the young flag leaf and the partial resistance in the field the selected lines are assumed to have a partial resistance to barley leaf rust far beyond that of Vada, which represents almost the highest level of partial resistance in European cultivars.     

Molecular mapping of QTLs for non-parasitic leaf spot resistance and comparison of half-sib DH populations in spring barley

Quantitative trait loci (QTLs) for resistance against non-parasitic leaf spots (NPLS) were first characterized in a spring barley double haploid population derived from the cross IPZ 24727/Barke (Behn et al., 2004). The aim of the present study was to identify QTLs for NPLS resistance in the half-sibling DH population IPZ 24727/Krona and to compare them with the QTLs of the population IPZ 24727/Barke. An anther culture-derived doubled haploid population of 536 DH lines was developed from the cross IPZ 24727 (resistant)/Krona (susceptible). Field trials were performed over two years in two replications, scoring NPLS and agronomic traits that might interact with NPLS. A molecular linkage map of 1035 cM was constructed based on AFLPs, SSRs and the mlo marker. QTL analyses for NPLS identified three QTLs that accounted for 30% of the phenotypic variation. For comparison of the QTLs from each DH population, a consensus map was generated comprising 277 markers with a length of 1199 cM. In both populations, the QTLs for NPLS mapped to chromosomes 1H, 4H and 7H. A common QTL with a great effect in both populations and over all environments was localized at the mlo locus on chromosome 4H, indicating that the mlo powdery mildew resistance locus has a considerable effect on NPLS susceptibility. The steps necessary to validate the QTLs and to improve the NPLS resistance by breeding were discussed.     

Pathogenic variation in, sources of, and breeding for resistance to Phaeoisariopsis griseola causing angular leaf spot in common bean

If we are to breed common bean (Phaseolus vulgaris L.) for durable resistance to diseases, we must understand pathogenic variation and find sources of resistance. Our first objective was to determine the patterns of pathogenic variation found among isolates of Phaeoisariopsis griseola (PG), the fungus that causes angular leaf spot (ALS) in common bean. We characterized 433 PG isolates from 11 Latin American and 10 African countries, using differential cultivars, isozymes, and/or random amplified polymorphic DNA (RAPD) markers. We also systematically screened, for ALS resistance, common bean accessions from the world collection held at CIAT, and assessed the progress so far made in breeding for resistance to ALS. Despite their great diversity within and between countries on both continents, the PG isolates were classified into two major groups: Andean, and Middle American. Although each group had internal differences for virulence, and biochemical and molecular characteristics, the ‘Andean’ PG isolates were more virulent on common beans of Andean origin, than on those of Middle American origin, thus, suggesting a host-pathogen co-evolution. The ‘Middle American’ PG isolates, although more virulent on common beans from Middle America, also attacked Andean beans, thus, exhibiting a much broader virulence spectrum. To find sources of resistance, we tested 22,832 common bean accessions against naturally occurring PG isolates in the field at CIAT's Experiment Station, Quilichao, Colombia, between 1985 and 1992. The resulting 123 intermediate (scores of 4 to 6) and resistant (scores of 1 to 3) accessions were then tested in the greenhouse against selected 14 PG isolates of diverse origins. Nineteen accessions were intermediate or resistant to at least 13 of 14 PG isolates. Similarly, of 13,219 bred lines tested in the field between 1978 and 1996, 89 were intermediate or resistant. Of these, 33 bred lines proved intermediate or resistant to at least eight of nine PG isolates to which they were challenged in the greenhouse. We suggest that, to breed for durable resistance to ALS, common bean populations should be developed from crosses between Andean and Middle American gene pools. The populations should then be systematically evaluated and selected against the broadest range of the most virulent PG isolates of diverse evolutionary origins. This revised version was published online in August 2006 with corrections to the Cover Date.     

Inheritance of partial resistance to spot blotch in barley

Diallel crosses (without reciprocals) were made among 10 different barley genotypes with genetic variability for spot blotch resistance. Forty‐five F₁ hybrids and their parents were assessed for their combining abilities for the disease resistance. Three experiments, two in a growth chamber on detached leaf and seedlings tests and one in the field on adult plant stages, were undertaken using a randomized complete block design with five replicates. A mixed conidial suspension of nine virulent isolates of the pathogen was used for inoculation. Statistical analysis showed genetic variability for spot blotch resistance. Results showed that the cultivar Banteng, the Ethiopian line CI‐5791 and the Syrian line 79‐SIO‐9 had partial resistance in all experiments. General combining ability was significant, with either positive or negative values. Resistant genotypes show favourable GCA‐effects, and they could therefore be successfully used for breeding purposes.     

Resistance to powdery mildew in a doubled haploid barley population and its association with marker loci

The genetic basis of resistance to powdery mildew (Erysiphe graminis DC. f.sp. hordei Marchal) was analyzed using doubled haploid barley (Hordeum vulgare L.) lines from the cross Harrington/TR306. Based on infection types observed after inoculation with defined single-conidium isolates, the lines were classified into four groups. The observed phenotypic ratio fit a two-locus model. The two putative loci were mapped relative to molecular markers. One coincided with the previously mapped dMlg locus on chromosome 4. Based on the observed infection types, Harrington carries the Mlg resistance allele, and TR306 carries a second locus on chromosome 7 (5H); this was tentatively designated Ml(TR). It is the first reported race-specific powdery mildew resistance gene located on that chromosome. These two loci were also detected by simple interval mapping of disease severity data from naturally infected field plots. Composite interval mapping with the first two resistance loci as co-factors detected an additional locus on chromosome 6, with a minor effect on resistance. Finally, superimposing the race-specific classification onto the field data provided evidence for a minor-effect locus on chromosome 7 (5H). The Mlg locus had the largest effect, the Ml(TR) locus had an intermediate effect and the other two loci had very small effects. This study demonstrates the effectiveness of an integrated approach to identifying and mapping resistance loci using classification data from inoculated experiments and quantitative data from field experiments. This revised version was published online in July 2006 with corrections to the Cover Date.     

Resistance to Helminthosporium leaf blight and agronomic performance of spring wheat genotypes of diverse origins

Helminthosporium leaf blight (HLB), caused by a complex of Cochliobolus sativus (Ito & Kurib.) Drechsler ex Dastur and Pyrenophora tritici-repentis Died, is a serious disease of wheat (Triticum aestivum L.) in the warm lowlands of South Asia. Wheat cultivars grown in the area are either susceptible to HLB or possess low levels of resistance to it. A replicated field study was conducted in 1999 and 2000 at two sites in Nepal to determine the level of HLB resistance and other desirable traits in 60 wheat genotypes of diverse origin. The test genotypes were planted in main strips divided into two strips one of which was sprayed four times with Tilt (a.i. propiconazole) @ 125 g of a.i. ha^–1. Four readings of HLB were recorded to calculate the area under the disease progress curve (AUDPC). Other traits under investigation included biomass yield (BY), grain yield (GY), 1000-kernel weight (TKW), harvest index (HI), days to heading (DH) and maturity (DM), plant height (PHT), and effective tiller number (ETN). Wheat genotypes differed significantly for all traits. Mean AUDPC values ranged from 45 to 1268. A few exotic genotypes were highly resistant to HLB. Losses in GY due to HLB ranged from 2 to 26%, and TKW was reduced by up to 33%. A few genotypes showed HLB tolerance, i.e., relatively smaller GY and TKW reductions despite high levels of HLB. In general, medium to late maturity and higher levels of HLB resistance and low to high GY and TKW characterized genotypes exotic to South Asia. Biplot analysis identified several genotypes that were HLB-resistant and agronomically superior. Results suggest it is possible to improve HLB resistance of local wheat cultivars based on selective breeding using this pool of germplasm.     

Responses of wheat and barley genotypes to toxic concentrations of soil boron

Summary The growth and yield of seven wheat and two barley cultivars or lines, previously found to show different degrees of boron tolerance under field conditions, were compared in a pot experiment at a range of soil boron treatments. Soil treatments ranged up to 150 mg/kg applied B. Extractable B in soils ranged up to 103 mg/kg.At the highest B treatment seedling emergence was delayed, but the percentage emergence was not reduced. The degree of boron toxicity symptom expression varied between the wheat cultivars and lines, with the two most tolerant, Halberd and (Wq^*KP)^*WmH)/6/12, displaying the least symptoms.The concentration of boron applied to the soil which produced a significant depression of growth and yield varied between cultivars. For example, the yield of (Wq^*KP)^*WmH)/6/12 was not affected at the 100 mg/kg applied boron treatment, while the grain yield for (Wl^*MMC)/W1/10 was significantly reduced at the 25 mg/kg treatment.There was a linear increase in boron concentration in tillers at the boot-stage with increasing concentration of boron in the soil. The most boron tolerant genotypes had the lowest tissue boron concentrations in each of the treatments. Halberd and (Wq^*KP)^*WmH)/6/12 had approximately half the boron concentrations of the more sensitive genotypes at the 25 and 50 mg/kg treatments. Differential tolerance of boron within the tissue was also observed. Both Stirling and (Wl^*MMC)/W1/10 had significantly reduced total dry matter and grain yields at the 25 mg/kg treatment, while the concentrations of boron in boot stage tillers at this treatment were 118 and 100 mg/kg, respectively. On the other hand, Halberd and (Wq^*KP)^*WmH)/6/12 had tissue boron concentrations of 144 and 131 mg/kg, respectively, at the 50 mg/kg treatment but yield was unaffected.The relative responses in the pot experiment, for wheat, were in close agreement with field results. Halberd and (Wq^*KP)^*WmH)/6/12 had the highest grain yields, with the lowest concentrations of boron in the grain when grown under high boron conditions in the field. In pots these two genotypes proved to be the most tolerant of boron. For barley the advantage in grain yield in the field, expressed by WI-2584 compared with Stirling, was not repeated in pots. WI-2584 was, however, more tolerant than Stirling on the basis of total dry matter production.The results show that useful variation in boron tolerance exists among wheat, and that breeding should be able to provide cultivars tolerant to high levels of boron.     

Hordeum bulbosum-a new source of disease resistance-transfer of resistance to leaf rust and mosaic viruses from H. bulbosum into winter barley

An outline is given of results for the transfer of new resistances against leaf rust and barley mosaic viruses from Hordeum bulbosum into winter barley. Progenies from backcrosses of barley cultivars with H. bulbosum hybrids were tested both in conventional breeding trials and in additional tests under controlled conditions. Resistance to both pathogens proved to be stable and of good heritability, with differences occurring which depended on the combinations used. Lines with resistance to all leaf rust and mosaic viruses tested, or to either one, were selected. Both resistances segregated independently.     

The genetics of seedling resistance to leaf rust,Puccinia hordeiOtth. in some spring barley cultivars

Summary The three Dutch isolates studied carried virulence genes against the resistance genes Pa, Pa-2, Pa-4 and Pa-5, substantiating the widespread occurrence of these virulences in Western Europe as reported by others. The cultivars Cebada Capa, La Estanzuela, Gondar and Dabat carry the same dominant to semi-dominant gene, which is also found in Forrajera Klein and H2212. It is proposed to designate this gene Pa-7. This gene segregated independently from the Pa-3 gene in Rika x (Baladi x Rika) substantiating the data of Johnson (1968), but disagreeing with those of Roane & Starling (1970). EP 75 seems to carry a single dominant or semi-dominant gene conferring an intermediate resistance to the isolates used. This gene, tentatively designated as Pa-z, is different from the Pa, Pa-2, Pa-4, Pa-5 and Pa-7 genes. The intermediate resistance of Monte Christo is probably based on gene(s) different from EP 75.     

Tolerance of spring barley cultivars to leaf rust,Puccinia hordei

Summary Fifteen spring barley cultivars were evaluated in two years for their tolerance to leaf rust, Puccinia hordei. The consistency between the results obtained in the two experiments was rather poor. The most tolerant cultivars produced low seed yields, the least tolerant ones high seed yields. A strongly negative relationship existed between harves-index and tolerance.     

Characterization of greenbug resistance in barley

The greenbug [Schizaphis graminum (Rondani)] is an extremely damaging pest of barley (Hordeum vulgare L.), particularly in the southern Great Plains of the USA. Winter barley targeted for production in this region should incorporate resistance to greenbug in the form of the resistance gene Rsg1a (in ‘Post 90′) or Rsg2b (in PI 426756). This study was conducted to fully characterize the resistance profile of these two genes against important greenbug biotypes, and to determine which of the two resistance genes is most effective in protecting barley from the greenbug. Eight barley and four wheat cultivars and germplasms were challenged with six greenbug biotypes and damage ratings were recorded for each combination. In five of the six tests, ‘Post 90’ was significantly more resistant than PI 426756 to greenbug feeding damage. Based on the results presented here, we conclude that the resistance gene, Rsg1a, in ‘Post 90’ is the better choice for use in breeding programmes and will provide better protection than Rsg2b against the greenbug.