A region of barley chromosome 6H harbors multiple major genes associated with net type net blotch resistance

Net type net blotch (NTNB), caused by Pyrenophora teres f. teres Drechs., is prevalent in barley growing regions worldwide. A population of 118 doubled haploid (DH) lines developed from a cross between barley cultivars ‘Rika’ and ‘Kombar’ were used to evaluate resistance to NTNB due to their differential reaction to various isolates of P. teres f. teres. Rika was resistant to P. teres f. teres isolate 15A and susceptible to isolate 6A. Conversely, Kombar was resistant to 6A, but susceptible to 15A. A progeny isolate of a 15A × 6A cross identified as 15A × 6A#4 was virulent on both parental lines. The Rika/Kombar (RK) DH population was evaluated for disease reactions to the three isolates. Isolate 15A induced a resistant:susceptible ratio of 78:40 (R:S) whereas isolate 6A induced a resistant:susceptible ratio of 40:78. All but two lines had opposite disease reactions indicating two major resistance genes linked in repulsion. Progeny isolate 15A × 6A#4 showed a resistant:susceptible ratio of 1:117 with the one resistant line also being the single line that was resistant to both 15A and 6A. An RK F₂ population segregated in a 1:3 (R:S) ratio for both 15A and 6A indicating that resistance is recessive. Molecular markers were used to identify a region on chromosome 6H that harbors the two NTNB resistance genes. This work shows that multiple NTNB resistance genes exist at the locus on chromosome 6H, and the recombinant DH line harboring the resistance alleles from both parents will be useful for the development of NTNB-resistant barley germplasm.     

Biological Relationship of Meloidogyne hapla Populations to Alfalfa Cultivars

Greenhouse and growth chamber studies were established to determine if there are pathological and physiological differences among Meloidogyne hapla populations from California (CA), Nevada (NV), Utah (UT), and Wyoming (WY) on alfalfa cultivars classified as resistant or susceptible to root-knot nematodes. In the greenhouse, plant survival was not consistent with resistance classifications. While all highly resistant Nevada Synthetic germplasm (Nev Syn XX) plants survived inoculation with all nematode populations, two cultivars classified as moderately resistant (''Chief'' and ''Kingstar'') survived (P ≤ 0.05) inoculation with M. hapla populations better than did ''Lobo'' cultivar, which is classified as resistant. Plant growth of Nev Syn XX was suppressed by only the CA population, whereas growth of the other alfalfa cultivars classified as M. hapla resistant or moderately resistant was suppressed by all nematode populations. Excluding Nev Syn XX, all alfalfa cultivars were severely galled and susceptible to all nematode populations. Except for Nev Syn XX, reproduction did not differ among the nematode populations on alfalfa cultivars. Nev Syn XX was not as favorable a host to CA as were the other cultivars; but, it was a good host (reproductive factor [Rf] = 37). Temperature affected plant resistance; the UT and WY populations were more pathogenic at 15-25 C, and CA was more pathogenic at 30 C. Nev Syn XX was susceptible to all nematode populations, except for CA, at only 30 C, and all other alfalfa cultivars were susceptible to all nematode populations at all temperatures.     

Changes in Water Uptake in Light and Darkness of the Barley Cultivar ‘Professor Schiemann’ Infected with Powdery Mildew

In the investigated barley cultivar resistant toErysiphe graminis f. sp.hordei Marchal a temporary decrease in water uptake may be observed during the first days following infection in light. No marked differences could be detected in darkness. Starting from the stage of formation of fructification organs the ratio between water uptake in light and in darkness in the resistant cultivar is similar to that of the control, while the opposite is true in the susceptible cultivar where the ratio markedly decreases with the development of powdery mildew. The resistant cultivar responds more rapidly to the pathogen by changes in water uptake than the susceptible one.     

Quantification of Pyrenophora teres in infected barley leaves using real-time PCR

Net blotch is a barley foliar disease caused by two forms of Pyrenophora teres: Pyrenophora teres f. teres (PTT) and Pyrenophora teres f. maculata (PTM). To monitor and quantify their occurrence during the growing season, diagnostic system based on real-time PCR was developed. TaqMan MGB (Minor Groove Binder) primers and probes were designed that showed high specificity for each of the two forms of P. teres. As a host plant internal standard, TaqMan MGB primers and probe based on RacB gene sequence were designed. The method was optimised on pure fungal DNA and on plasmid standard dilutions. Quantification was accomplished by comparing Ct values of unknown samples with those obtained from plasmid standard dilutions. The assay detects down to five gene copies per reaction. It is able to produce reliable quantitative data over a range of six orders of magnitude. The developed assay was used to differentiate and quantify both forms of P. teres in infected barley leaves. Correlation R² = 0.52 was obtained between the Ct values and size of symptoms areas in early stage of infection. Application of the TaqMan MGB technology to leaf samples collected in 20 barley varieties in the region Kromeriz during the growing season of 2003 and 2004 revealed that P. teres f. teres predominated in these 2 years. The developed method is an important tool to quantify and monitor the dynamics of the two forms of P. teres during the growing season.     

Nematicides Enhance Growth and Yield of Rotylenchulus Reniformis Resistant Cotton Genotypes

Rotylenchulus reniformis resistant LONREN-1×FM966 breeding lines developed at Auburn University have demonstrated that the nematode resistance is accompanied by severe stunting, limited growth, and low yields. The objectives of this study were to evaluate the effects of applying nematicides to selected LONREN breeding lines on R. reniformis nematode populations, plant stunting, and yield. Four resistant breeding lines from the LONREN-1×FM966 cross, one susceptible line from the LONREN-1×FM966 cross, as well as LONREN-1, BARBREN-713, and the susceptible cultivar DP393 were evaluated with and without nematicides in the presence of R. reniformis. In the greenhouse, nematicides increased plant height across all genotypes compared with no nematicide. Rotylenchulus reniformis populations were 50% lower in the resistant lines compared with the susceptible lines at 45 days after planting (DAP). In microplot and field trials, the phenotypic stunting of all genotypes was reduced by aldicarb with increases in plant heights at 30 and 75 DAP. Increases in yields were evident across all genotypes treated with aldicarb. In all three trial environments, BARBREN-713 outperformed the LONREN-derived lines as well as ‘DP393’ in seed cotton yields, while having significantly lower R. reniformis egg densities than the susceptible genotypes.     

Symptom Severity, Yield, Seed Mottling and Seed Transmission of Soybean Mosaic Virus in Susceptible and Resistant Soybean: The Influence of Infection Stage and Growth Temperature

The effect of soybean mosaic virus (SMV) infection on symptom severity, yield, seed mottling and seed transmission in soybean in relation to the growth stage at infection and subsequent temperature was investigated using a susceptible (Harosoy), a moderately resistant (Evans) and a highly resistant (Merit) cultivar. Disease symptoms were more severe with early infection. A greater reduction in plant growth and seed yield, and higher percentages of mottled seeds and seed transmission of SMV also occurred with early infection. Virus titer was higher in younger plants than in older ones and also higher in plants infected at the ealier stage than at the later stage of growth. Merit (a highly resistant cultivar previously reported to be immune to seed mottling) inoculated at the early stage of plant growth resulted in infection and production of some mottled seeds. Temperature affected all parameters investigated. The effect of temperature was greater in the susceptible cultivar than in the resistant one. The optimal temperature for symptom severity, yield, seed mottling and seed transmission was 20 °C. Virus titer was highest at 30 °C in all three cultivars. Maturity of susceptible cultivar was delayed by infection.     

Necrotrophic effector‐triggered susceptibility (NETS) underlies the barley–Pyrenophora teres f. teres interaction specific to chromosome 6H

Barley net form net blotch (NFNB), caused by the necrotrophic fungus Pyrenophora teres f. teres, is a destructive foliar disease in barley‐growing regions worldwide. Little is known about the genetic and molecular basis of this pathosystem. Here, we identified a small secreted proteinaceous necrotrophic effector (NE), designated PttNE1, from intercellular wash fluids of the susceptible barley line Hector after inoculation with P. teres f. teres isolate 0–1. Using a barley recombinant inbred line (RIL) population developed from a cross between the sensitive/susceptible line Hector and the insensitive/resistant line NDB 112 (HN population), sensitivity to PttNE1, which we have named SPN1, mapped to a common resistance/susceptibility region on barley chromosome 6H. PttNE1–SPN1 interaction accounted for 31% of the disease variation when the HN population was inoculated with the 0–1 isolate. Strong accumulation of hydrogen peroxide and increased levels of electrolyte leakage were associated with the susceptible reaction, but not the resistant reaction. In addition, the HN RIL population was evaluated for its reactions to 10 geographically diverse P. teres f. teres isolates. Quantitative trait locus (QTL) mapping led to the identification of at least 10 genomic regions associated with disease, with chromosomes 3H and 6H harbouring major QTLs for resistance/susceptibility. SPN1 was associated with all the 6H QTLs, except one. Collectively, this information indicates that the barley–P. teres f. teres pathosystem follows, at least partially, an NE‐triggered susceptibility (NETS) model that has been described in other necrotrophic fungal disease systems, especially in the Dothideomycete class of fungi.     

Resistance to Rhynchosporium secalis in the winter barley cultivar Vulcan

The six-rowed winter barley cultivar Vulcan was found to be highly resistant to Rhynchosporium secalis in the field, although this resistance was not apparent in routine screening tests of barley genotypes carried out at the seedling stage. More precise seedling tests, using graded inoculum levels to derive dose-response curves, showed that cv. Vulcan had an ED₅₀ which was 4.8 times that of the susceptible, two-rowed cv. Maris Otter, and 4.1 times that of cv. Maris Puma. The inheritance of resistance was investigated at the adult plant stage in a cross with cv. Maris Otter. Although a clear-cut segregation of the F₃ progeny into resistant, segregating and susceptible phenotypes was not observed, the genetic variance of the F₃ family means was compatible with the hypothesis of monogenic control of resistance. Resistance was inherited independently of the factor for six-rowed head type.     

The evidence for abundance of QTLs for partial resistance to Puccinia hordei on the barley genome

Using AFLP markers, a linkage map was constructed based on a recombinant inbred population of barley derived from a cross between a leaf rust susceptible line, L94, and a partially resistant line, 116-5. The constructed map showed a similar marker distribution pattern as the L94 × Vada map. However, it contained more large gaps, and for some chromosome regions no markers were identified. These regions are most likely derived from L94 because 116-5 was selected from the progeny of a cross of L94 × cv. Cebada Capa. Five QTLs for partial resistance to isolate 1.2.1. were mapped on the L94 × 116-5 map. Three QTLs were effective in the seedling stage, jointly contributing 42% to the total phenotypic variance. Three QTLs were effective in the adult plant stage, collectively explaining 35% of the phenotypic variance. Evidence for two additional linked minor-effect QTLs effective in the adult plant stage was also uncovered. The major-effect QTL, Rphq3, was the only one that was effective in both developmental stages. Moreover, Rphq3, was also identified in the L94 × Vada population, being effective to two rust isolates. The other QTLs were detected in either of the two populations, providing evidence for the existence of many loci for partial resistance to leaf rust on the barley genome. To date, 13 QTLs for partial resistance have been mapped, therefore, a strategy of accumulating many resistance genes in a single cultivar, resulting in a high level of partial resistance, is feasible.     

Reproduction of Virulent Isolates of Meloidogyne incognita on Susceptible and Mi-resistant Tomato

The reproductive potential of natural and laboratory-selected Meloidogyne incognita isolates virulent against the tomato Mi resistance gene, all derived from a single egg-mass, were compared when the nematodes were inoculated on susceptible and resistant tomato. Fewer second-stage juveniles (P = 0.01) of the two virulent populations selected under laboratory conditions matured to females on the resistant tomato compared to the susceptible cultivar. In contrast, no differences were found between the number of egg masses produced on the resistant versus the susceptible tomato by the two natural virulent isolates. No clear general trends concerning the fecundity of the females could be inferred from the comparative analysis of the numbers of eggs per egg mass x tomato cultivar combination. These observations suggested that the genetic changes induced under environmentally controlled nematode growth might be different from those occurring in natural Mi-resistance breaking biotypes grown without environmental control.     

Parasitic Variability of Meloidogyne incognita Populations on Susceptible and Resistant Cotton

Root gall induction and egg production by the four recognized host races and two cytological races of Meloidogyne incognita were compared on cotton Gossypium hirsutum cvs. Deltapine 16 (root-knot susceptible) and Auburn 634 (highly resistant). The 12 nematode populations included in the study were from various parts of the world. No population increases occurred on the highly resistant cultivar. After 45 days, populations of host races 1 and 2 induced slight root galling on both cuhivars with only limited reproduction. Host race 4 populations induced moderate root galling with higher reproduction on Deltapine 16 than that of race 1 or race 2 populations. Host race 3 populations induced severe root galling with population density increases of 7-30-fold. In a complementary study, 24 cotton cultivars or breeding lines were compared for suitability as hosts for a typical population of M. incognita race 3. The poorest hosts, ''Aubnru 623,'' ''Auburn 634,'' and ''McNair 220,'' yielded fewer eggs after 45 days than were added initially. The best hosts - ''M-8.'' ''DES 24-8,'' ''McNair 235,'' and ''Coker 20l'' - yielded > 5 times as many eggs as were added initially.     

Molecular Analysis of Rhynchosporium secalis Population Collected from Barley Cultivars having Different Resistance Specificities

Molecular analysis was performed to detect genetic diversity in 106 Rhynchosporium secalis isolates collected from different regions of Canada using random amplified polymorphic DNA (RAPD) markers. The isolates collected from barley cultivars having different resistance specificity to R. secalis and grown in geographically distinct regions, exhibited reproducible variation for 2–3 polymorphic PCR products per decamer primer. Analysis of 1960 RAPD markers data obtained with five primers formed 5 groups with different genetic similarity. High genetic variation was observed in R. secalis isolates obtained from resistant and susceptible cultivars of barley. Isolates collected from susceptible cultivars showed a tendency to group together, whereas isolates from resistant cultivars were divergent. R. secalis isolates infecting different barley cultivars released as resistant to the barley scald formed a specific group with UPGMA, even though all these isolates were collected from the same epidemiological region. Analysis of 15 isolates collected from one resistant cultivar Duke formed three clusters with low bootstrap values indicating high genetic diversity among the isolates present on a single host cultivar.     

Investigations on Adult Plant Resistance of Barley Against Erysiphe graminis f.sp. hordei

Spring barley cultivars and lines were tested for 3 years in field studies for adult plant resistance against Erysiphe graminis f.sp. hordei. The cultivars Osiris and Asse were selected for further detailed cytological studies and compared with the susceptible cultivar Peruvian. Under controlled greenhouse conditions, the percentage of conidia that had formed a functional haustorium and secondary hyphae (infection efficiency) was reduced in fifth leaves of the adult plant resistant cultivars. On fifth and flag leaves of adult plant resistant cultivars, papillae were formed more frequently under primary germ tubes and appressoria, and fungal penetration was prevented more often than on the susceptible cultivar Peruvian. In ultrastructural studies various types of papillae were observed, but could not be strictly correlated with penetration success or failure of the fungus.     

Comparative virulence of Pyrenophora teres f. teres from Syria and Tunisia and screening for resistance sources in barley: implications for breeding

Aims: The aim of this study is to investigate the pathogenic diversity and virulence groups among Pyrenophora teres f. teres isolates, sampled from Syria and Tunisia, and to identify the most effective source of resistance in barley that could be used in breeding programmes to control net blotch in both countries. Methods and Results: One hundred and four isolates of P. teres f. teres were collected from barley in different agroecological zones of Tunisia and Syria. Their virulence was evaluated using 14 barley genotypes as differential hosts. The upgma clustering identified high pathogenic variability; the isolates were clustered onto 20 pathotypes that were sheltered under three virulence groups, with high, intermediate and low disease scores. According to susceptibility/resistance frequencies and mean disease ratings, CI05401 cultivar ranked as the best differential when inoculated with the Syrian isolates. However, CI09214 cultivar was classified as the best effective source of resistance in Tunisia. Conclusions: All P. teres f. teres isolates were differentially pathogenic. CI09214 and CI05401 cultivars were released as the most effective sources of resistance in Syria and Tunisia. Significance and Impact of the Study: National and international barley breeding programmes that seek to develop resistance against P. teres f. teres in barley should strongly benefit from this study. This resistance cannot be achieved without the proper knowledge of the pathogen virulence spectrum and the sources of host resistance.     

Barley 4H QTL confers NFNB resistance to a global set of <Emphasis Type="Italic">P. teres</Emphasis> f. <Emphasis Type="Italic">teres</Emphasis> isolates

Net form net blotch (NFNB), caused by Pyrenophora teres f. teres Drechs., is prevalent in barley-growing regions worldwide. A population of 132 recombinant inbred lines (RILs) developed from a cross of the barley varieties ‘Falcon’ and ‘Azhul’ were used to evaluate resistance to NFNB due to their differential reactions to isolates of P. teres f. teres from Australia, Canada, Japan, and the USA. Falcon is a six-rowed, hulless feed barley harboring resistance to NFNB, while Azhul is a six-rowed, hulless food barley with high levels of susceptibility to many P. teres f. teres isolates. Seedling disease resistance data were collected on seedlings of parents, RILs, and checks in a growth chamber. The population was genotyped using Illumina’s GoldenGate assay, and quantitative trait loci (QTL) were detected on chromosomes 2H, 3H, 4H, and 6H. We identified a single genetic region on barley chromosome 4H that provided varying levels of resistance to all P. teres f. teres isolates evaluated.     

Relationships Between the Population Density of Meloidogyne incognita and Growth of Tobacco

Seedlings of tobacco cultivars resistant (NC95) and susceptible (McNair 30) to Meloidogyne incognita were grown in 15-cm diameter clay pots containing steamed soil infested with 0, l, 2, 4, 8, 16, 32, and 64 eggs of M. incognita per 1.5 cm³ soil. Plants were maintained in the greenhouse for 3 weeks, and then transferred to the field for 12 weeks. Growth of tobacco was expressed separately as dry weight of leaves and as plant height. Least squares regression analysis showed that tobacco growth-nematode density interactions are in agreement with Seinhorst''s exponential model Y = m + (l-m) czp. Tobacco growth was not affected significantly as nematode density was increased from 0 to tolerance levels, which were approximately 2 and 1 eggs per 1.5 cm³ soil for the resistant and susceptible cultivars, respectively. As nematode density was increased beyond tolerance level, tobacco growth decreased sharply until a minimum yield was approached. The minimum leaf weights and plant heights of the resistant cultivar at the highest nematode density were greater than those of the susceptible cultivar.     

Effect of Host Plant Resistance on Haustorium Formation in Cereal Rust Fungi

Haustoria of Puccinia triticina (wheat leaf rust fungus) and P. hordei (barley leaf rust fungus) were isolated from susceptible and partially resistant wheat lines, and susceptible, hypersensitive and partially resistant barley lines. Haustoria were counted and measured. The size of haustoria was similar in the partially resistant and susceptible genotypes but haustoria were smaller in the hypersensitive barley line L94+Pa7. The number of haustoria was reduced in both partially and hypersensitive lines when compared with susceptible ones. Therefore it seems that the reduction in the number of haustoria is a consequence of the resistance that can be attributable either to early abortion of infection units or reduced colony growth. The reduction of the number of haustoria was more pronounced in the adult plant stage.     

Relative Virulence of Meloidogyne incognita Host Races on Soybean

Sensitivity and host efficiency of susceptible (''Lee 68'', ''Coker 156'') and resistant (''Bragg'', ''Centennial'', ''Forrest'', ''Lee 74'') soybean (Glycine max (L.) Merr.) cultivars for races of Meloidogyne incognita (Mi) were determined in greenhouse experiments. Eight Mi populations collected from the southeastern United States were utilized. All Mi races reproduced readily on Lee 68 and Lee 74 and moderately on Forrest and Bragg. Coker 156 exhibited resistance to races 1 and 2, and some race 3 populations, but was very susceptible to certain race 3 and 4 populations. Reproduction of all races was lowest on Centennial. Forrest and Centennial shoot growth was not significantly suppressed by any race. There were no distinct differences in virulence between races except for a race 3 population which reproduced readily on all cultivars, stunting their growth. Considerable variation in reproduction existed within races 1 and 3.     

<Emphasis Type="Italic">In vitro</Emphasis> selection and regeneration of chrysanthemum (<Emphasis Type="Italic">Dendranthema grandiflorum</Emphasis> Tzelev) plants resistant to culture filtrate of <Emphasis Type="Italic">Septoria obesa</Emphasis> Syd

Callus cultures derived from leaf segments of chrysanthemum cultivar ‘Snow Ball’ which was susceptible to Septoria obesa were successfully used for in vitro selection for resistance to this pathogenic fungus. Resistant cell lines were selected by culturing callus on growth medium containing various concentrations of S. obesa filtrate. Resistant calluses obtained after two cycles (30 d each cycle) of selection were used for plant regeneration. About 30% of the plants regenerated from the resistant calluses and 70–80% of the plants raised from cuttings had acquired considerable resistance against the pathogen in the field. No phenotypic variation was observed in the selected regenerates.     

Stripe Rust Resistance in Roegneria kamoji (Poaceae: Triticeae) and its Genetic Analysis

The Roegneria kamoji accession ZY 1007 was resistant to the mixed predominant races of Puccinia striiformis f.sp. tritici (Pst) in China based on field tests at adult‐plant stage. The seedling resistance evaluation of ZY 1007 showed that it was resistant to stripe rust physiological strains CYR29, CYR33 and PST‐V26, which were the predominant races of Pst in China. The female parent R. kamoji cv. Gansi No.1 (susceptible to Pst) was crossed with ZY 1007 (resistant to Pst). Parents, F₁ and F₂ populations were tested in a field inoculated with the mixed urediniospores. ZY 1007 and all the observed 11 F₁ hybrid plants were resistant, while plants of Gansi No.1 were susceptible. Among the 221 F₂ plants, 168 plants were resistant and 53 were susceptible, and the segregation of resistant and susceptible plants fits 3R:1S ratio (χ² = 0.074, P > 0.75). It confirmed that the resistance of stripe rust in ZY 1007 was controlled by a single dominant gene and temporarily designated as YrK1007.