Evaluation of a DNA probe for the quantitative detection of common bacterial blight in common bean and its application in a breeding program

Summary Breeding of Phaseolus vulgaris L. for resistance to common bacterial blight (CBB) can be done with visual evaluations of symptoms to distinguish broad resistance classes, but a more quantitative measure was needed for genetic studies of resistance. A novel method of evaluation was developed by quantifying Xanthomonas campestris pv. phaseoli (XCP) in bean leaf tissue infected with CBB using a ³²P-labeled probe and densitometric analysis of hybridization signals. Quantification of bacterial populations using the probe was highly correlated (r=0.98) with the number of colony forming units (CFU) from plate counts of the same leaf samples. The probe was used to follow XCP population dynamics on susceptible (BAT 41) and resistant (OAC 88-1) bean genotypes. OAC 88-1 supported a maximum XCP population which was approximately tenfold less than BAT 41. The probe was also used to study an F₂/F₃ population segregating for resistance. Narrow sense heritability estimates were less for resistance measured on the basis of bacterial populations (0.18–0.26) than on visual scores of symptoms (0.29–0.38). The anticipated response to selection for CBB resistance would be less based on bacterial numbers than based on symptom expression in this population. In breeding for resistance to CBB, selection based on visual symptoms combined with measurements of XCP populations using a DNA probe can be used to develop bean genotypes that are both resistant to symptom development and bacterial multiplication.Abbreviations CBB common bacterial blight - CFU colony forming units - XCP Xanthomonas campestris pv. phaseoli     

Methods for detecting the cassava bacterial blight pathogen: A practical approach for managing the disease

Cassava bacterial blight (CBB), caused by Xanthomonas axonopodis pv. manihotis (Xam), is a particularly destructive disease in South America and Africa. The movement of infected asymptomatic stems is a major means of pathogen dispersal as well as infected seeds. The success of a cassava-seed certification program depends on the availability of reliable tests to detect the pathogen in vegetative planting materials and true seeds. We report here the different methods that permitted to detect the pathogen in cassava tissues. A polymerase chain reaction (PCR) test was developed for this pathogen. The PCR assay worked well for pathogen detection in extracts from leaf and stem lesions and the minimum number of cells that could be detected ranged from 3 × 10² to 10⁴ CFU per ml. Nested-PCR worked well for Xam detection from naturally infected seeds. This technique was specific, sensitive, and rapid for detecting Xam in cassava true seeds. The highest detection level found was 1–2 viable cells per reaction. A dot-blot assay was developed by evaluating a 898 bp DNA fragment unique to Xam strains as a diagnostic DNA probe. The probe detected Xam strains in crude extracts of leaf and stem lesions, cassava fruits and sexual seeds that were naturally infected. Overall sensitivity of the dot-blot method was about10³CFU per reaction. The dot-blot hybridization technique can be easily used for culture indexing. A monoclonal antibody (MAb) was also used for an enzyme-linked immunosorbent assay (ELISA) and tested with various infected tissues. Overall sensitivity of the method was about10³CFU per reaction. This revised version was published online in July 2006 with corrections to the Cover Date.     

A major QTL for common bacterial blight resistance derives from the common bean great northern landrace cultivar Montana No.5

Knowledge of the evolutionary origin and sources of pest resistance genes will facilitate gene deployment and development of crop cultivars with durable resistance. Our objective was to determine the source of common bacterial blight (CBB) resistance in the common bean Great Northern Nebraska #1 (GN#1) and GN#1 Selection 27 (GN#1 Sel 27). Several great northern cultivars including GN#1, GN#1 Sel 27, and Montana No.5 (the female parent of the common x tepary bean interspecific population from which GN #1 and GN # 1 Sel 27 were derived) and known susceptible checks were evaluated for CBB reaction in field and greenhouse environments. These genotypes and CBB resistant and susceptible tepary bean including Tepary #4, the male parent and presumed contributor of CBB resistance toGN#1 and GN#1 Sel 27, were assayed for presence or absence of three SCAR markers tightly linked with independent QTLs conditioning CBB resistance. The parents and F₂ of Montana No. 5/GN #1 Sel 27 and Montana No.5/Othello(CBB susceptible) were screened for CBB reaction and SCAR markers. CBB resistance in Montana No.5 was comparable to that of GN#1 and GN#1 Sel27. The SAP6 SCAR marker present in GN#1 and GN#1 Sel 27 was also present in Montana No.5, and it co-segregated (R ² =35%) with the CBB resistance in the Montana No.5/Othello F₂ population. Although a few CBB resistant and susceptible transgressive segregants were found in the F₂ of MontanaNo.5/GN #1 Sel 27 and later confirmed by F₃ progeny tests, SAP6 SCAR marker was present in all progenies. None of the tepary bean specific CBB resistance-linked SCAR markers were present in GN#1, GN#1 Sel 27, or Montana No.5. A cluster analysis of 169 polymorphic PCR-based markers across three common bean and Tepary #4 indicated that GN#1, GN#1 Sel 27, and Montana No.5 were closely related, and not related at all with Tepary #4.Thus, these results clearly indicate Montana No.5, not Tepary #4, as the source of CBB resistance in GN#1 and GN#1 Sel 27. This revised version was published online in August 2006 with corrections to the Cover Date.     

Resistance to common bacterial blight in Phaseolus vulgaris L. recombinant inbred lines under natural infection of Xanthomonas axonopodis pv. phaseoli

Among the main causes of poor yield in common beans are fungal, viral and bacterial diseases. Common bacterial blight, caused by Xanthomonas axonopodis pv. phaseoli (Xap), is one of the major bacterial diseases leading to significant losses in Brazil. Chemical control is ineffective, therefore, the use of resistant varieties becomes an interesting alternative. The objective of the present work was to evaluate disease resistance under natural infection of the pathogen in 109 recombinant inbred lines (F₇) of P. vulgaris originated from the cross HAB-52 (susceptible — snapbean) × BAC-6 (resistant — common bean) in two different environments, as well as to calculate genetic parameters to assist in the selection of promising materials to be used in the CBB resistance breeding program. The data of the genetic parameters were compared to those calculated for the F₃ generation originated from the same cross. The heritability results for DI (disease index) and VI (variation index) in F₃ were 26.85% and 0.26, respectively, whereas in F₇ they were 91.77% and 1.36, respectively. These results demonstrate a potential to be explored for this advanced population, that in the future, along with other pathogen variability studies and tests in other environments, may provide more information regarding a more precise evaluation of promising genotypes to be used in common bean breeding programs aiming to obtain CBB resistant varieties. This revised version was published online in July 2006 with corrections to the Cover Date.     

Quantitative trait loci analysis in Theobroma cacao using molecular markers. Inheritance of polygenic resistance to Phytophthora palmivora in two related cacao populations

Two related segregating populations of Theobroma cacao L. were analysed for their resistance to Phytophthora palmivora. The first F₁ population was obtained by crossing two susceptible cacao clones of Catongo (a highly homozygous genotype) and Pound 12(a highly heterozygous genotype) and the second population was obtained by backcrossing a single F₁ tree with Catongo. The genetic maps obtained for each population were compared. The F₁ map includes 162 loci and the backcross has 140 loci. The two maps, F₁ and BC₁, exhibit high co-linear loci organization covering respectively, 772 and 944 cM.Phytophthora resistance was assessed by measuring the size increase of a lesion at five (DL5)and ten days (DL10) after pod inoculation. Six different QTL were detected in the F₁ and BC₁ populations. One QTL was found in both populations, and appeared to be a major component of disease resistance, and explaining nearly 48% of the phenotypic variance in the F₁ population. The absence of some yield QTL detection in the BC₁ in comparison with the F₁ population is due to the lack of transmission of the favouring alleles for these QTL from the single F₁ tree used for the backcross. The phenotypic variance explained by the action of the quantitative trait alleles indicated that genetic factors of both major and minor effects were involved in the control of the character studied. QTL conferring increased resistance to Phytophthorawere identified in both susceptible parents, suggesting the presence of transgressive traits and the possibility of selection in cacao. Pleiotropic and epistatic effects for the QTL were also detected. Finally, the use of marker assisted selection (MAS) in cacao breeding programs is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interactions between QTL SAP6 and SU91 on resistance to common bacterial blight in red kidney bean and pinto bean populations

Breeding efforts to improve resistance in dry bean to common bacterial blight (CBB) have focused on applying marker assisted selection strategies. We examined the interaction between two independent QTL (quantitative trait loci), SAP6 and SU91, on the expression of resistance to CBB in a pinto bean F₂ population and dark red kidney bean F₂ population. The disease reaction for each F₂ population was scored at several time points after inoculation. The dominant SCAR markers SAP6 and SU91 were simultaneously genotyped as codominant markers using a multiplex real-time PCR assay. For both populations, at each time point plants having genotypes of (su91 su91//SAP6 SAP6); (su91 su91//SAP6 sap6), and (su91 su91//sap6 sap6) were significantly more susceptible to CBB than plants with any of the six other possible genotypes (SU91 _//_ _). Only two examples were observed across both populations and all time points in which mean disease reactions were significantly different between any of the six (SU91 _//_ _) genotypes. No significant differences were observed between SU91 SU91 and SU91 su91 genotypes at any time point for either F₂ population. Similarly, no significant differences in CBB reaction were observed among the three SAP6 genotypes for plants that had the su 91/su91 genotype. The results indicate CBB resistance in these two populations is primarily conditioned by the presence of at least a single copy of the SU91 QTL. The effects of pathogen isolate and population on discerning the influence of QTL on CBB resistance are discussed.     

Interaction of common bacterial blight quantitative trait loci in a resistant inter‐cross population of common bean

Common bacterial blight (CBB), caused by Xanthomonas axonopodis pv. phaseoli, is an important disease of common bean (Phaseolus vulgaris L.). Genetic resistance is the most economically efficient, environmentally friendly and socially acceptable approach to control plant diseases including CBB. To examine the main and interaction effects of the previously identified CBB resistance quantitative trait loci (QTL) associated with markers BC420 (B) on Pv06, SAP6 (S) on Pv10 and PVctt001 (P) on Pv04, in presence of the major QTL associated with the marker SU91 (S_u) on Pv08, a resistant F_4:5 recombinant inbred line population of the reciprocal crosses between OAC Rex (bb/ss/PP/S_uS_u) and HR45 (BB/SS/pp/S_uS_u) was evaluated under artificial field inoculation in disease nurseries in 2009 and 2010. While, in presence of the CBB QTL on Pv08, the QTL on Pv06 of HR45 accounted for 37–46% of phenotypic variation in the field, the effect of QTL on Pv04 and Pv10 were not significant under field conditions, even in the absence of the Pv06 QTL. Broad‐sense heritability estimates of CBB resistance and the QTL associated with BC420 were high for CBB severity and the area under the disease progress curve, promoting the continued efforts in pyramiding the QTL on Pv06 and Pv08 in common background, which provide high levels of resistance.     

Correlated resistance of cassava to mosaic and bacterial blight diseases

Summary The two most serious diseases of cassava (Manihot esculenta Crantz) are cassava mosaic disease (CMD) and cassava bacterial blight (CBB) (Xanthomonas manihotis Starr). Clone 58308, derived from the third backcross of the interspecific cross of cassava (M. esculenta) x ceara rubber (M. glaziovii), showed a high level of resistance to both diseases. Crosses of 58308 with several other clones which varied from susceptible to moderately susceptible to both diseases gave progenies with a significant genotypic correlation between resistance to both diseases (r=0.90), apparently due to linkage. The heritabilities of resistance to the diseases were estimated at 50–70% for CMD and 25–65% for CBB. Resistance to both diseases is assumed to be polygenic. The correlated response to selection for CMD and for CBB was estimated.     

Effects of resistance genes,heat tolerance genes and cytoplasms on expression of resistance to Pseudomonas solanacearum (E.F. Smith) in potato

Summary Effects of resistance genes and heat tolerance genes on expression of resistance to bacterial wilt caused by Pseudomonas solanacearum were investigated in 30 F₁ progenies from parents with different levels of bacterial wilt resistance and heat tolerance. A race 1 and a race 3 isolate of the bacterium were used for inoculation under screenhouse conditions at two locations. Results obtained indicated that with reduction in levels of parental resistance, resistance in the F₁ progenies was also reduced. Under hot conditions, a reduced heat tolerance in the genetic background also resulted in lower levels of resistance expression. The effect of heat tolerance tended to diminish at lower temperatures leaving the effect of resistance genes more consistent. There existed a strong interaction between resistance genes and genes for heat tolerance. The nature of resistance to bacterial wilt in potato and implications for breeding for resistance are discussed.Effects of reciprocal crosses on expression of resistance to a race 1 isolate under hot screenhouse conditions, were studied in 5 sets of reciprocal F₁ progenies involving different resistant and susceptible parents. The reciprocal differences observed were not significant suggesting absence of cytoplasmic effects on expression of resistance.     

Gamete selection for improving physiological resistance to white mold in common bean

White mold (WM), caused by Sclerotinia sclerotiorum (Lib.) de Bary, is a widespread disease of dry and green bean (Phaseolus vulgaris L.) in North America. Gamete selection (GS) was effective to combine and pyramide resistant genes and quantitative trait loci (QTL) for common bacterial blight. Our objective was to determine the effectiveness of GS to introgress physiological resistance to white mold. Two inter-gene-pool double-cross populations were developed. Selection for WM resistance was practiced from F₁ to F₄. Thirteen selected F_1:5 breeding lines of each population and their four parents were evaluated. Two separate inoculations were made on each plant 1 week apart using a cut-stem method. The WM reaction was scored at 16, 23, and 33 days post inoculation (DPI) using a scale from 1 (no disease) to 9 (severely diseased or dead). In F₁, 52% of Pop I (USPT-WM-1/CORN 601//USPT-CBB-1/92BG-7) and 67% of Pop II (Chase/I9365-25//ABL 15/A 195) susceptible plants were discarded. In F_4, only 1.2% of families from Pop I, and 0.9% for Pop II, survived the selection process. An average of 20.5% gain in WM resistance was obtained for both populations in F₄. Four breeding lines of Pop I had significantly (P = 0.05) lower WM score (4.1–4.6) and four were equal (4.7–4.9) to the best WM-resistant parent 92BG-7 (4.9), while ten breeding lines of Pop II were equal (4.5–4.8) to the best WM-resistant parent A 195 (4.6). Thus, GS was effective for improving WM resistance in common bean.     

Diallel analysis of resistance to head blight caused by Fusarium culmorum in winter wheat

Summary Ten homozygous winter wheat genotypes representing different levels of resistance to Fusarium head blight were crossed in all possible combinations excluding reciprocals. Parents, F₁ and F₂ were inoculated with one pathogenic strain of Fusarium culmorum. Data for head blight, observed 21 days after first inoculation (OBS-2), and for the area under the disease progress curve, based on observations 14, 21 and 28 days after first inoculation (AUDPC), were analyzed. The contrast between parents and F₁ crosses indicated dommance effects of the resistance genes. Diallel analysis according to Griffing's Method 4, Model 1 showed significant general combining ability (GCA) effects for both F₁ and F₂; specific combining ability effects were not significant. With the exception of one genotype for which general performance for Fusarium resistance was not in agreement with its GCA, the resistance to F. culmorum was uniformly transmitted to all offspring, and the parents can be described in terms of GCA. It is suggested that in the progenies with one of the awned lines as parent, one resistance gene was linked with the gene coding for presence of awns, located on chromosome 4B. A single observation date, taken at the right time, was as effective in assessing resistance as the AUDPC.     

Genetic analysis of resistance to barley scald (Rhynchosporium secalis) in the Ethiopian line `Abyssinian' (CI668)

A doubled haploid barley (Hordeum vulgare L.) population from a cross between the cultivar `Ingrid' and the Ethiopian landrace `Abyssinian' was mapped by AFLP, RFLP, SSR and STS markers and tested for resistance to isolates`4004', `2', `16-6', `17', `22' and `WRS 1872' of Rhynchosporium secalis (Oudem.) J.J. Davis, the causal agent of leaf scald. Resistance tests were conducted on parents, DH-lines, a near-isogenic line of `Abyssinian' (NIL) into `Ingrid', and an F₂ population descended from the same F₁ plants as the DHs. The DH population segregated for at least two major R. secalis resistance QTL. All isolates tested identified a major QTL on chromosome 3 (3H) associated with R. secalis resistance, in a 4 cM support interval between the co-segregating markers Bmac0209/Falc666 and MWG680. The QTL was linked with the markers Falc666 (2.3 cM), YLM/ylp (0.3 cM), MWG680 (1.7 cM), cttaca2 (2.5 cM) and agtc17 (9.8 cM). The second QTL was located on chromosome 1 (7H).However, this QTL was only detected by one isolate and was located in an interval of 16 cM in the distal part of the chromosome. At this QTL the allele for improved scald resistance originated from the parent `Ingrid'. There were a number of minor QTL on chromosomes 2 (2H), 4 (4H) and 6 (6H) that were not repeatable either across replications or analysis methods. The importance of checking QTL-models by cross-validation is stressed. This revised version was published online in August 2006 with corrections to the Cover Date.     

The inheritance of resistance to head blight caused by Fusarium culmorum in winter wheat

Summary Crosses were made among ten winter wheat genotypes representing different levels of resistance to Fusarium head blight to obtain F₁ and F₂ generations. Parents, F₁ and F₂ were inoculated with one strain of Fusarium culmorum. Data on incidence of head blight 21 days after first inoculation were analyzed. Broad-sense heritabilities averaged 0.39 and ranged from 0.05 to 0.89 in the individual F₂ families. The joint-scaling test indicated that the inheritance of Fusarium head blight resistance was adequately described by the additive-dominance model, with additive gene action being the most important factor of resistance. With respect to the non-additive effects, dominance of resistance predominated over recessiveness. The number of segregating genes governing resistance in the studied populations was estimated to vary between one and six. It was demonstrated that resistance genes differed between parents and affected resistance differently.     

Monogenic resistance in tomato to Fusarium oxysporum f. sp. lycopersici race 3

Summary Resistance to fusarium wilt, incited by Fusarium oxysporum (Schlecht.) f. sp. lycopersici (Sacc.) Snyder & Hansen race 3 in tomato (Lycopersicon esculentum Mill.) was discovered in LA 716, a L. pennellii accession. A resistant BC₁F₃ breeding line, E427, was developed from LA 716. E427 was crossed with the susceptible cv. Suncoast and F₁, BCP₁, BCP₂ (to Fla 7155, a susceptible parent) F₂, F₃, and BCP₂S₁ seeds were obtained. Segregation for resistance following root dip inoculation over three experiments indicated a single dominant gene controlled resistance. Five of the 12 BCP₁S₁'s segregated more susceptible plants, whereas one of the 12 segregated more resistant plants than expected (P<0.05). Three of 23 F₃ lines segregated more susceptible plants than expected while 1 of the 23 had more resistant plants than expected (P<0.05). Segregation in all other lines fit expected ratios. Five of the 23 F₃'s were homozygous resistant which was an acceptable fit to expectations (P=0.1–0.5). The gene symbol I ₃ is proposed for resistance to race 3 of the wilt pathogen. Deviations from expected ratios in data reported here and for other breeding lines indicate an effect of modifier genes and/or incomplete penetrance. Plant age at inoculation and seed dormancy did not affect results.Florida Agricultural Experiment Station Journal Series No. 8101.     

Introgressing resistance to bacterial and viral diseases from the middle American to Andean common bean

The genetic base of cultivars within market classes of common bean (Phaseolus vulgaris L.) is narrow. Moreover, small- and medium-seeded Middle American cultivars often possess higher yield and resistance to abiotic and biotic stresses than their large-seeded Andean counterparts. Thus, for broadening the genetic base and breeding for higher yielding multiple stress resistant Andean cultivars use of inter-gene pool populations is essential. Our objective was to determine the feasibility of introgressing resistance to Been common mosaic virus (BCMV, a potyvirus), and the common [caused by Xanthomonas campestris pv. phaseoli (Xcp) and X. campestris pv. phaseoli var. fuscans (Xcpf)] and halo [caused by Pseudomonas syringae pv. phaseolicola (Psp)] bacterial blights from the Middle American to Andean bean, using gamete selection. Also, we investigated the relative importance of the use of a landrace cultivar versus elite breeding line as the last parent making maximum genetic contribution in multiple-parent inter-gene pool crosses for breeding for resistance to diseases. Two multiple-parent crosses, namely ZARA I = Wilkinson 2 /// ‘ICA Tundama’ / ‘Edmund’ // VAX 3 / PVA 773 and ZARA II = ‘Moradillo’ /// ICA Tundama / Edmund // VAX 3 / PVA 773 were made. From the F₁ to F₅ single plant selection was practiced for resistance to the common and halo bacterial blights in both populations at Valladolid, Spain. The parents and F₅-derived F₆ breeding lines were evaluated separately for BCMV, and common and halo bacterial blights in the greenhouse at Filer and Kimberly, Idaho in 2001. They were also evaluated for the two bacterial blights, growth habit, seed color and 100-seed weight at Valladolid in 2002. All 20 F₁ plants of ZARA I were resistant or intermediate to common and halo bacterial blights in the greenhouse, but their F₂ and subsequent families segregated for both bacterial blights. Segregation for resistant, intermediate, and susceptible plants for common bacterial blight occurred in the F₁ of ZARA II. Simple correlation coefficient for common bacterial blight between the F₁ and F₁-derived F₂ families was positive (r = 0.54 P < 0.05) for ZARA II. From the F₂ to F₅ the number of families resistant to both bacterial blights decreased in both populations. Only four of 20 F₁ plants in ZARA I resulted in seven F₆ breeding lines, and only one of 32 F₁ plants in ZARA II resulted in one F₆ breeding line resistant to the three diseases. None of the selected breeding lines had seed size as large as the largest Andean parent. The use of elite breeding line or cultivar as the last parent making maximum genetic contribution to the multiple-parent inter-gene pool crosses, relatively large population size in the F₁, and simultaneous selection for plant type, seed traits as well as resistance to diseases would be crucial for introgression and pyramiding of favorable alleles and quantitative trait loci (QTL) of interest between the Andean and Middle American beans.     

A single dominant gene for downy mildew resistance in broccoli

Downy mildew, incited by Peronospora parasitica (Pers.: Fr.) Fr., is a destructive disease of broccoli (Brassica oleraceaL., Italica Group). Resistant cultivars represent a desirable control method to provide a practical, environmentally benign, and long-term means of limiting damage from this disease. Doubled-haploid (DH) lines developed by us exhibit a high level of downy mildew resistance at the cotyledon stage. To determine the mode of inheritance for this resistance, a resistant DH line was crossed to a susceptible DH line to make an F₁, from which F₂ and backcross (BC) populations were developed. All populations were evaluated for response to artificial inoculation with P. parasitica at the cotyledon stage. All F₁ plants (including reciprocals) were as resistant as the resistant parent, indicating no maternal effect for this trait. F₂ populations segregated approximately 3resistant to 1 susceptible, BC populations using the resistant parent as the recurrent parent contained all resistant plants, and the BC to the susceptible parent segregated 1 resistant to 1 susceptible. These results indicate that resistance is controlled by a single dominant gene. This gene should be easily incorporated into F₁ hybrids and used commercially to prevent downy mildew at the cotyledon stage. This revised version was published online in August 2006 with corrections to the Cover Date.     

Identification of segregation-distortion-neutral alleles to improve pollen fertility of indica-japonica hybrids in rice (Oryza sativa L.)

The genetics of resistance to Ascochyta blight (Ascochyta fabae f. sp. fabae) was studied in two populations of faba bean (Vicia faba). Plants of a resistant population, ILB 752, and a susceptible one, NEB 463, were screened for their reaction to the pathogen and the results were quantified on a scale of 0–5. Crosses were made between plants both within and between accessions and the F₁ and F₂ generations assessed in a field trial 21 and 45 days after inoculation. Disease scores were greater at 45 days than at 21 days and they were not significantly affected by the presence of susceptible spreader rows in part of the trial. ILB 752 carried a major dominant gene conferring resistance while NEB 463 carried the recessive allele for susceptibility. Furthermore, a minority of plants of NEB 463 appeared to carry at least one pair of complementary recessive genes, also conferring resistance. Most of the plants of ILB 752 were homozygous for the dominant resistance gene and a few were heterozygous. Reciprocal crosses behaved identically, indicating the absence of maternal effects in the expression of Ascochyta blight resistance in faba beans. The results show that it is important to confirm the level of heterozygosity for the resistance genes in this partially outbreeding species before crossing is commenced. The major dominant gene for resistance, identified in ILB 752, has clear potential for use in breeding for Ascochyta blight resistance. The minor genes identified in NEB 463 also show the potential for accumulating resistance through mass selection. This revised version was published online in July 2006 with corrections to the Cover Date.     

QTL analysis and mapping of pre-harvest sprouting resistance in Sorghum

One of the most important agronomic problems in the production of sorghum [Sorghum bicolor (L.) Moench] in humid climates is pre-harvest sprouting (PHS). A molecular linkage map was developed using 112molecular markers in an F₂ mapping population derived from a cross between IS 9530 (high resistance to PHS) and Redland B2 (susceptible to PHS). Two year phenotypic data was obtained. By means of interval mapping analysis, two significant QTL were detected in two different linkage groups with LOD scores of 8.77and 4.39. Each of these two QTL individually explained approximately 53% of the phenotypic variance, but together, in a two-QTL model, they explained 83% of the phenotypic variance with a LOD score of 12.37.These results were corroborated by a one-way ANOVA in which the four flanking markers of the most likely QTL positions displayed highly significant values in theF-test, and significant variation in trait expression was associated with marker genotypic classes. The four markers with highest effect in the one-way ANOVA were also detected in the second year replication of the F₂ population, and significant genotype × environment interactions was observed. The putative relationship between PHS resistance in sorghum and the maize Vp1 gene is also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Molecular mapping and identification of QTL's associated to oat crown rust partial resistance

Summary Molecular mapping is a promising strategy for studying and understanding traits with complex genetic control, such as partial resistance to oat crown rust. The objectives of this research were to develop molecular maps from the progenies of the cross UFRGS7 (susceptible) × UFRGS910906 (partially resistant) and to identify QTLs (quantitative trait loci) associated to partial resistance to oat crown rust in two generations of that population.DNA of 86 genotypes of the F₂ and 90 genotypes of the F₆ UFRGS7 × UFRGS910906 population were used to generate AFLP markers. Molecular maps were constructed using Mapmaker Exp. 3.0 and QTLs for partial resistance to oat crown rust were identified with Mapmaker/QTL software. Five hundred and fifty seven markers in the F₂ and 243 markers in the F₆ generations were identified. The F₂ map integrated 250 markers in 37 linkage groups. The F₆ map integrated 86 markers in 17 linkage groups.Five QTLs were identified for partial resistance to oat crown rust in the F₂ generation and three QTLs in the F₆. The QTL identified on F₆ through the PaaaMctt340 AFLP marker showed consistency across two environments and two generations (F₄ and F₆), and appear to have potential for marker-assisted selection in oat.     

Genetics of Cyst Nematode Resistance in Soybean PIs 467312 and 507354

Soybean Cyst nematode (SCN) Heterodera glycines Ichinohe is the most serious pest of soybean [Glycine max (L.) Merr.] in the world and genetic resistance in soybean cultivars have been the most effective means of control. Nematode populations, however, are variable and have adapted to reproduce on resistant cultivars over time due mainly to the narrow genetic base of SCN resistance in G. max. The majority of the resistant cultivars trace to two soybean accessions. It is hoped that new sources of resistance might provide durable resistance. Soybean plant introductions PI 467312 and PI 507354, are unique because they provide resistance to several nematode populations, i.e. SCN HG types 0, 2.7, and 1.3.6.7 (corresponding to races 3, 5, and 14) and HG types 2.5.7, 0, and 2.7 (corresponding to races 1, 3, and 5), respectively. The genetic basis of SCN resistance in these PIs is not yet known. We have investigated the inheritance of resistance to SCN HG types 0, 2.7, and 1.3.6.7 (races 3, 5, and14) in PI467312 and the SCN resistance to SCN HG types 2.5.7 and 2.7 (races 1 and 5) in PI 507354. PI 467312 was crossed to ‘Marcus’, a susceptible cultivar to generate F₁ hybrids, 196 random F₂ individuals, and 196 F_2:3 families (designated as Pop 467). PI 507354 and the cultivar Hutcheson, susceptible to all known SCN races, were crossed to generate F₁ hybrids, 225 random F₂ individuals and 225 F_2:3 families (designated as Pop 507). The F_2:3 families from each cross were evaluated for responses to the specific SCN HG types in the greenhouse. Chi-square (χ²) analyses showed resistance from PI 467312 to HG types 2.7, and 1.3.6.7 (races 5 and 14) in Pop 467 were conditioned by one dominant and two recessive genes (Rhg rhg rhg) and resistance to HG type 0 (race 3) was controlled by three recessive genes (rhg rhg rhg). The 225 F_2:3 progenies in Pop 507 showed a segregation of 2:223 (R:S) for response to both HG types 2.5.7 and 2.7 (corresponding to races 1 and 5). The Chi-square analysis showed SCN resistance from PI 507354 fit a one dominant and 3 recessive gene model (Rhg rhg rhg rhg). This information will be useful to soybean breeders who use these sources to develop SCN resistant cultivars. The complex inheritance patterns determined for the two PIs are similar to the three and four gene models for other SCN resistance sources known to date.