Crop Science 41:1268-1275 (2001)
© 2001 Crop Science Society of America
CELL BIOLOGY & MOLECULAR GENETICS
Identification of Molecular Markers Associated with Adult Plant Resistance to Powdery Mildew in Common Wheat Cultivar Massey
Sixin Liub,
C. A. Griffey*,a and
M. A. Saghai Maroofa
a Dep. of Crop and Soil Environmental Sciences, Virginia Polytechnic Institute and State Univ., Blacksburg, VA 24061
b Dep. of Agronomy and Plant Genetics, 411 Borlaug Hall, Univ. of Minnesota, St. Paul, MN 55108
* Corresponding author (cgriffey{at}vt.edu)
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ABSTRACT
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Powdery mildew, caused by Blumeria graminis (DC.) E.O. Speer f. sp. tritici Em. Marchal (syn. Erysiphe graminis f. sp. tritici), is one of the major diseases of wheat (Triticum aestivum L.) worldwide. Race-specific resistance has been extensively used in wheat breeding programs, even though it is ephemeral. Adult plant resistance (APR) to powdery mildew is more durable than race-specific resistance. The APR to powdery mildew in winter wheat cultivar Massey has remained effective since its release in 1981. A cross was made between Massey and a powdery mildew susceptible cultivar Becker. Powdery mildew severity on F-2 leaves (the second leaf below the flag leaf) of 180 F2:3 lines was rated under natural disease pressure in the field. Among 213 RFLP and 139 microsatellite markers surveyed, 88 (41%) and 90 (65%) markers, respectively, detected polymorphism between Becker and Massey. Bulked segregant analysis (BSA) was used to facilitate the identification of molecular markers associated with APR to powdery mildew. Three quantitative trait loci (QTLs), designated as QPm.vt-1B, QPm.vt-2A, and QPm.vt-2B, were identified with interval mapping. They are located on wheat chromosomes 1B, 2A, and 2B, and, respectively, explained 17, 29, and 11% of the total variation of F2:3 lines for powdery mildew resistance. The three QTLs associated with APR to powdery mildew were derived from Massey, and displayed additive gene action. QPm.vt-2B also fits a recessive model for APR to powdery mildew. In a multi-QTL model, the three QTLs explained 50% of the total variation of F2:3 lines for APR to powdery mildew. The presence of the three QTLs was confirmed with 97 recombinant inbred (RI) lines, tested for APR to powdery mildew under natural powdery mildew pressure over 3 yr (F5:6–F7:8 generation). The molecular markers identified in this study have potential for use in marker-assisted selection and pyramiding of genes for resistance to powdery mildew.
Abbreviations: APR, adult plant resistance • BSA, bulked segregant analysis • cM, centimorgan • RFLP, restriction fragment length polymorphism • QTL, quantitative trait loci • LOD, log likelihood ratio • RI, recombinant inbred
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INTRODUCTION
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POWDERY MILDEW is one of the major diseases of wheat in the world. It is of economic importance especially in areas with maritime or semicontinental climate (Bennett, 1984). Yield losses ranging from 13 to 34% have been reported (Griffey et al., 1993; Leath and Bowen, 1989; Johnson et al., 1979). Resistant cultivars offer an effective, economical, and environmentally safe way to control powdery mildew. There are two types of resistance to powdery mildew. One is called race-specific resistance, which is effective for some isolates of powdery mildew, but ineffective for others. Race-specific resistance genes are expressed in seedlings and throughout the vegetative cycle of wheat. Currently, 33 alleles at 24 loci have been reported for resistance to powdery mildew (McIntosh et al., 1998; Shi et al., 1998). Even though race-specific resistance has been extensively used in wheat breeding programs, selection pressure exerted by cultivars with race-specific resistance genes results in the rapid build-up of pathotypes with matching virulence genes. Subsequently, race-specific resistance loses its effectiveness when confronted by pathotypes with matching virulence genes.
Another type of resistance to powdery mildew is called adult plant resistance (APR), which retards infection, growth and reproduction of the pathogen in adult plants but not in seedlings. It is also called "slow mildewing" (Shaner, 1973) and "partial resistance" (Hautea et al., 1987). This type of resistance can be identified in cultivars with defeated race-specific genes or lacking known race-specific resistance genes. APR to powdery mildew is more durable than race-specific resistance. For example, APR in wheat cultivar Knox and its derivatives remained effective against powdery mildew infection during the 20 yr in which these cultivars were grown commercially (Shaner, 1973). Massey, a derivative of ‘Knox62’, was released from Virginia Polytechnic Institute and State University in 1981 (Starling et al., 1984), and still has effective powdery mildew resistance in adult plants.
To improve the efficiency of wheat breeding for APR to powdery mildew, it is essential to understand the genetic basis of APR. Being a quantitative trait, APR to powdery mildew in wheat is more complex than race-specific resistance (Shaner and Finney, 1975). Using monosomic analysis, Chae and Fischbeck (1979) reported that 14 chromosomes were involved in APR to powdery mildew in the cultivar Diplomat. Both qualitative and quantitative genetic studies indicated that APR to powdery mildew was governed by two to three genes, with moderate to high heritability, in four cultivars, Knox 62, Massey, Redcoat, and Houser (Griffey and Das, 1994; Das and Griffey, 1994b). Hautea et al. (1987) reported that additive gene effects were the most important for APR to powdery mildew in four spring wheat crosses. Das and Griffey (1995) also reported that additive gene effects were predominant, but nonadditive effects also were significant for APR to powdery mildew.
The lack of extensive genetic information hinders the development of effective wheat breeding strategies for APR to powdery mildew. The objectives of this study were to identify molecular markers associated with APR to powdery mildew in common wheat cultivar Massey, to localize the QTLs to wheat chromosomes, to study the gene action of APR to powdery mildew, and to validate the QTLs with RI lines.
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MATERIALS AND METHODS
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Plant Materials
A cross was made between common wheat cultivars Becker and Massey. Seedlings of Massey are susceptible to the prevalent pathotypes of powdery mildew found in Virginia, yet resistance is expressed in the adult plant stage (Starling et al., 1984). Becker does not have any known genes for powdery mildew resistance and is susceptible to powdery mildew in both seedling and adult plant stages. Leaf tissue of 180 F2:3 lines (30–40 plants per F2:3 line) was used for DNA extraction.
Powdery Mildew Assessment
From the cross between Becker and Massey, 180 F2:3 lines with 50 seeds per line were space-planted at Warsaw, VA, in October 1994. A row of each parent was planted every 20 rows. Several rows of Becker were planted around the F2:3 population to ensure ample powdery mildew inoculum. Average powdery mildew severity on F-2 leaves (the second leaf below the flag leaf) of each F2:3 plant was assessed under natural disease pressure by the James disease assessment key (James, 1971). Disease severity of F2:3 plants was rated and was based on the percentage (0–50%) of leaf area covered by powdery mildew. The F2:3 plants were rated as 50% when the F-2 leaves had maximum coverage by powdery mildew. A score of 0% was given for plants without powdery mildew on F-2 leaves. Disease severity of the individual plants in a F2:3 row were averaged to obtain mean mildew severity for each F2:3 line.
A single head of each plant from putative nonsegregating resistant and nonsegregating susceptible F2:3 lines was harvested. Twenty randomly selected heads from each putative non-segregating F2:3 line were planted at Warsaw, VA, in October 1995, to test the homogeneity for reaction to powdery mildew of F3:4 lines in the adult plant stage. The powdery mildew reaction of F3:4 lines, rated under natural powdery mildew pressure, was used to select the best F2:3 lines for bulked segregant analysis.
RFLP Analysis
A total of 213 clones including WG (wheat genomic), BCD (barley cDNA), CDO (oat cDNA), ABC (barley cDNA), KSU (wheat genomic), and RZ (rice cDNA) clones was used to survey polymorphism between Becker and Massey. Four restriction enzymes (EcoRI, EcoRV, DraI, and HindIII) were used to digest the genomic DNA. DNA isolation and RFLP procedures were as described by Saghai Maroof et al. (1984)( 1996).
Microsatellite Analysis
A total of 139 wheat microsatellite primer pairs from published papers (Röder et al., 1998; Stephenson et al., 1998; Ma et al., 1996) was used to survey the polymorphism between Becker and Massey. PCR reactions were performed in a total volume of 10 µL in a thermal cycler (Perkin-Elmer, Norwalk, CT). After an initial denaturing step for 3 min at 94°C, 32 cycles were performed with 0.5 min at 94°C, 0.5 min at either 47, 55, or 60°C (depending on the primer pair), and 1 min at 68°C, followed by a final extension step of 7 min at 68°C. The reaction mixture and polyacrylamide gel electrophoresis were as described by Saghai Maroof et al. (1994).
Bulked Segregant Analysis
Two DNA bulks were constructed by mixing equal amount of DNA from four non-segregating resistant and from four non-segregating susceptible F2:3 lines, respectively. The two DNA bulks were included in the polymorphism survey to eliminate from consideration those polymorphic markers that were less likely to be associated with APR to powdery mildew. Markers that showed a similar pattern of polymorphism between the parents and between the two bulks were considered as putative resistance-related markers. These markers were used to genotype the 180 F2:3 lines to test for their association with APR to powdery mildew. According to published wheat and barley genetic linkage maps (http://wheat.pw.usda.gov/; verified March 6, 2001), additional polymorphic markers in the vicinity of the putative resistance-related markers were used to genotype the F2:3 lines.
Validation of Molecular Markers Associated with APR to Powdery Mildew with RI lINES
Recombinant inbred lines were used to verify QTLs associated with APR to powdery mildew. Only markers close to QTLs identified in the F2:3 generation and located on chromosomes 1B, 2A, and 2B (see results) were used to genotype the RI lines.
Seeds from a single head of each of 97 F2 plants, randomly selected among 200 F2 plants from the cross Becker/Massey, were planted in individual head rows in field plots at Warsaw, VA, in October 1993. A row of each parent was also planted every 10 rows. A randomly selected single head from each row was harvested and planted in a head row in each consecutive year. By the 1998–1999 growing season, 97 F7:8 RI lines were obtained.
Disease data were collected from F5:6, F6:7, and F7:8 head rows in 1997, 1998, and 1999, respectively, and from the two parents planted alternately every 10 rows each year at Warsaw, VA. Average disease severity on F-2 leaves of each head row was assessed as previously described for the F2:3 generation under natural pressure of powdery mildew. Disease data collected on RI lines in each generation over a 3-yr period were used to validate the QTLs identified in the F2:3 generation and to estimate heritability of APR to powdery mildew in Massey. Heritability was calculated by the variance component method (Fehr, 1987).
After random single head selection, the F6:7 lines were bulk harvested for each row. Forty seeds of each F6:7 line were planted in the greenhouse and the leaf tissue was collected and used for DNA extraction. Molecular markers associated with APR to powdery mildew in the F2:3 generation were used to genotype the RI lines. RI lines with a heterozygous pattern for a given marker were scored as missing data for the respective marker.
Statistical Analysis
Software JMP IN 3.1 (Sall and Lehman, 1996) was used for statistical analyses. Because of the skewed distribution of disease severity among F2:3 lines (see results), log10 transformed data were used in all statistical analyses. One-way ANOVA was conducted to determine significant (P < 0.05) association between putative resistance-related markers and APR to powdery mildew. Chi-square tests were used to test Mendelian segregation ratio (1:2:1) for codominant markers.
Interval Mapping
Genetic linkage maps were constructed by MAPMAKER 3.0b (Lander et al., 1987). A threshold log likelihood ratio (LOD) of 3.0 was used to group markers into linkage groups. Centimorgan (cM) values were calculated by the Haldane mapping function (Haldane, 1919). Linkage groups were scanned for the presence of QTLs at a LOD threshold of 3.0 at every 2.0-cM interval by MAPMAKER/QTL 1.1b with a free model (Lincoln et al., 1992; Paterson et al., 1988). The QTLs were designated according to the guidelines for nomenclature of quantitative trait loci in wheat (McIntosh et al., 1998). Pm and vt (Virginia Tech) were used for trait designator and laboratory designator, respectively. Gene action was tested by fitting QTLs to dominant, recessive and additive genetic models. One LOD score less than the free model indicated that a constrained model was unlikely.
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RESULTS
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QTLs for APR to Powdery Mildew in F2:3 Generation
Among the 180 F2:3 lines derived from the cross Becker/Massey, five lines were verified as nonsegregating resistant and four lines as nonsegregating susceptible to powdery mildew on the basis of disease data of the F3:4 lines. All other F2:3 lines segregated for powdery mildew resistance. The observed segregation pattern fit a 1:62:1 genotypic ratio (P = 0.32), indicating the possibility of three genes conferring APR to powdery mildew in Massey. The distribution of powdery mildew severity of F2:3 lines (Fig. 1) was skewed toward lower disease severity and deviated significantly from a normal distribution. The continuous distribution indicates that more than one gene likely controls APR to powdery mildew in this population.
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Fig. 1. Distribution of powdery mildew severity of F2:3 lines derived from Becker/Massey cross. The powdery mildew severity of the two parents Becker and Massey is indicated by arrows.
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Among 213 RFLP markers, 88 (41.3%) identified polymorphism between Becker and Massey. The microsatellite markers showed much higher polymorphism between the two parents. Ninety (64.7%) markers detected polymorphism among 139 microsatellites surveyed. In total, 178 (50.6%) markers were polymorphic between Becker and Massey.
Putative resistance-related markers identified via bulked segregant analysis were used initially to genotype the F2:3 lines. Low P-values from one-way ANOVA, performed to assess the relationship between the putative resistance-related markers and APR to powdery mildew, indicate that the markers listed in Table 1 are associated with APR to powdery mildew.
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Table 1. One-way ANOVA for putative powdery mildew resistance-related markers in the Becker/Massey F2:3 population.
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On the basis of the identified putative resistance-related markers, additional markers were chosen from the known wheat and barley genetic linkage maps (http://wheat.pw.usda.gov/) and used to genotype the F2:3 lines. A total of 58 markers were used to genotype the F2:3 lines, and genetic linkage maps were constructed by MAPMAKER 3.0. Because most of the microsatellite markers are genome specific, on the basis of the published wheat microsatellite genetic map (Röder et al., 1998), the putative resistance-related markers were located on chromosomes 1B, 2A, and 2B as shown in Fig. 2.
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Fig. 2. The likelihood plots of QTLs associated with APR to powdery mildew in Becker/Massey F2:3 wheat population. The arrows indicate the most likely positions of the QTLs. The horizontal dashed lines at LOD 3.0 represent the minimum LOD required for significance.
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The likelihood plots (Fig. 2) of interval mapping indicate that chromosome 2A is very important for APR to powdery mildew. The QTL on chromosome 2A, designated as QPm.vt-2A, has the highest LOD score (9.23) among the QTLs detected. QPm.vt-2A is located at the microsatellite marker interval, GWM304a-GWM312, and is 12.0 cM away from GWM304a. It explained 29% of the total variation of the F2:3 lines (Table 2). The resistance conferred by QPm.vt-2A was derived from Massey. Both the dominant (LOD = 4.47) and recessive models (LOD = 7.82) were deemed unlikely. This QTL fits an additive model with a LOD of 8.65.
The QTL detected on chromosome 2B was designated as QPm.vt-2B. This QTL peaked at the RFLP marker WG338 with a LOD score of 4.34 (Table 2). QPm.vt-2B explained 11% of the total variation of F2:3 lines. Only the dominant mode was deemed unlikely. The LOD scores for recessive and additive models were 4.30 and 3.64, respectively, suggesting recessive or additive gene effects for this QTL. Most of the markers located on chromosome 2B did not fit the 1:2:1 ratio for codominant markers (Table 3). Segregation distortions favoring the Massey allele for each marker were observed. Because of segregation distortion, the genetic distance is significantly reduced when compared with published genetic maps of wheat (Röder et al., 1998). Also, the order of the markers shows some difference from the published maps. Devos et al. (1993) reported similar results for the genetic map of chromosome 2B in the cross ‘Timgalen’ x RL4137. The T. timopheevi (Zhuk.) Zhuk. chromosome segment introgressed into chromosome 2B in Timgalen was preferentially transmitted. Recombination was also greatly reduced in that segment.
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Table 3. Chi-square test for Mendelian segregation ratio (1:2:1) of markers located on chromosome 2B in the cross Becker/Massey.
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The third QTL, designated as QPm.vt-1B, is located on chromosome 1B. The log-likelihood plot peaked at the interval between microsatellite marker GWM259 and RFLP marker WG241 with a LOD score of 5.25 (Table 2). QPm.vt-1B was 8.0 cM from microsatellite marker GWM259 and explained 17% of the total variation of F2:3 lines. QPm.vt-1B was derived from Massey and fits an additive model with a LOD of 5.24. Both dominant and recessive models were rejected as being likely. The three QTLs explain 50% of the total variation of F2:3 lines in a multi-QTL model.
Validation of QTLs for APR to Powdery Mildew in RI Lines
The average powdery mildew severity of RI lines was 6, 12, and 7% in 1997, 1998, and 1999, respectively. Higher disease pressure resulted in higher average powdery mildew severity in 1998. The disease data over 3 yr were correlated highly, and the coefficients of correlation were as follows: 1997 and 1998 (r = 0.76, P < 0.001); 1997 and 1999 (r = 0.81, P < 0.001); 1998 and 1999 (r = 0.81, P < 0.001). Distribution of average disease severity over 3 yr was continuous and skewed toward lower disease severity (data not shown), which suggested quantitative inheritance of resistance. Heritability of APR to powdery mildew was 0.75.
The critical markers associated with APR to powdery mildew in F2:3 lines were used to genotype the RI lines. One-way ANOVA for each marker was conducted to determine its effect on APR to powdery mildew (Table 4). Results indicated that all the markers had significant (P < 0.05) effects on mean disease severity over 3 yr. The marker GWM304a located on chromosomes 2A had the largest effect on APR to powdery mildew. Except for marker PSP3100, the remaining markers in Table 4 were significantly associated with disease data of each generation. Marker PSP3100 located on chromosome 1B had a slightly higher P-value (0.0826) for disease data of the F7:8 lines. Overall, the single marker analysis of RI lines confirmed the presence of the three QTLs for APR to powdery mildew identified in the F2:3 generation.
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Table 4. One-way ANOVA to validate molecular markers associated with APR to powdery mildew in Becker/Massey RI lines.
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Twenty-nine polymorphic markers located on chromosomes 1B, 2A, and 2B were mapped with the RI lines. Likelihood plots of QTLs associated with APR to powdery mildew using RI means across 3 yr (F5:6 – F7:8 generations) are presented in Fig. 3. In terms of marker order and genetic distance between markers, the maps were similar to the maps obtained from F2:3 data (Fig. 2.).
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Fig. 3. The likelihood plots of QTLs associated with APR to powdery mildew for Becker/Massey RI lines. The arrows point to the most likely positions of the QTLs. The horizontal dashed lines at LOD 3.0 represent the LOD threshold.
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Consistent with F2:3 data, QPm.vt-2A on chromosome 2A had the highest LOD score (4.65) among the three QTLs (Fig. 3). The peak of LOD score was at the interval between microsatellite markers GWM304a and GWM312, and was 6 cM away from marker GWM304a. QPm.vt-2A explained 26% of the phenotypic variation of RI lines. Even though the peaks of likelihood plots for chromosome 2B and 1B were slightly lower than LOD threshold 3.0, the consistent trends of likelihood plots between RI lines and F2:3 lines indicate the presence of QPm.vt-2B and QPm.vt-1B. The LOD score peaked at the interval between RFLP markers KSUD22 and WG338 for QPm.vt-2B. The peak was 4 cM away from marker KSUD22 with a LOD score of 2.89. This QTL accounted for 15% of the phenotypic variation of RI lines. QPm.vt-1B had a relatively low LOD score of 2.52 and explained 15% of the phenotypic variation of RI lines. The peak of the LOD score was 4 cM from microsatellite marker PSP3100. In a multi-QTL model, the three QTLs explained 44% of the phenotypic variation of RI lines.
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DISCUSSION
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Genetics of APR to Powdery Mildew in Wheat
The results of this study are in agreement with the classical genetic studies of APR to powdery mildew in Massey. Griffey and Das (1994) reported that two to three genes control APR to powdery mildew in Massey. In the current study, three QTLs for APR to powdery mildew were detected in the Becker/Massey F2:3 mapping population. The three QTLs were located on the chromosomes 1B, 2A, and 2B, respectively. Not only the number of genes, but also the gene action is in agreement with previous studies. The predominance of additive genetic effects for APR to powdery mildew has been previously reported (Hautea et al., 1987; Das and Griffey, 1994a, 1995). In the present study, all three QTLs had significant additive effects on APR to powdery mildew. Das and Griffey (1995) reported that susceptibility was dominant in the cross Becker/Massey. Similarly, we found that QPm.vt-2B is recessive for APR to powdery mildew in the same cross.
Recently, Keller et al. (1999) reported QTLs for APR to powdery mildew in a segregating wheat/spelt (Triticum spelta L.) population. With the method of composite interval mapping, 18 QTLs were detected, explaining 77% of the phenotypic variance in a simultaneous fit model. Only two QTLs with major effects were consistent over all five environments. It is interesting to note that the QTL on chromosome 2A identified by Keller et al. (1999) is in a similar genomic region as QPm.vt-2A identified in the present study. However, the number of QTLs is quite different from the current study. This may be explained by the fact that the parents of the two mapping populations are quite different. Different methods used to detect QTLs could also attribute to differences in the number of QTLs reported in the two studies. Any minor QTLs were unlikely to be detected with BSA in the current study. Also, fewer markers were mapped in the current study, which could account for fewer QTLs being detected. Furthermore, the difference may be due to the different experimental environments in the two studies. However, Keller et al. (1999) found only 3 of 18 QTLs that showed significant QTL X environment interaction. Likewise, Griffey and Das (1994) and Hautea et al. (1987) did not observe a significant effect of environment on APR to powdery mildew.
On the basis of molecular markers associated with QPm.vt-1B, this QTL is likely located on the long arm of chromosome 1B. So, it is unlikely to be located in homoeologous regions with powdery mildew resistance genes Pm3 and Pm24, which are located on 1AS (McIntosh and Bennett, 1978) and 1DS (Huang et al., 2000), respectively. Gene Pm28 is located on chromosome 1B (Peusha et al., 2000), genes Pm10 and Pm22 are both located on chromosome 1D (Peusha et al., 1996; Tosa et al., 1987), and gene Pm25 is located on 1A (Shi et al., 1998). Because of the limited information on the precise chromosomal position of these genes, it is not possible to compare them with QPm.vt-1B for orthology at this time. Ma et al. (1994) reported that Pm4a located on chromosome 2AL was 1.5 cM from RFLP marker BCD292, which is also in the vicinity of QPm.vt-2B. It is speculated that Pm4 and QPm.vt-2B may be orthologous in homoeologous regions. On the basis of seedling powdery mildew reaction of Massey with isolates of known virulence, there is no indication that Massey carries gene Pm6 (Griffey and Leath, 1998, personal communication), which is located on chromosome 2B (Nyquist, 1963). QPm.vt-2A and QPm.vt-2B are not likely located in homoeologous regions, as determined on the basis of LOD score comparisons between GWM526b and GWM382a on chromosome 2A and that between GWM526a and GWM382b on chromosome 2B (Fig. 2).
Application of Microsatellite Markers in Wheat Genetic Mapping
Microsatellite markers are highly polymorphic in wheat (Röder et al., 1998). Among 139 microsatellite markers, 90 (64.7%) were polymorphic between Becker and Massey. In contrast to microsatellite markers, only 41.3% of RFLP markers were polymorphic for the same parents. Without microsatellite markers, it would be difficult to identify QTLs by means of a mapping population derived from an intraspecific cross as in this study. For example, 213 RFLP markers were tested for polymorphism between Becker and Massey. Only 88 (41.3%) were polymorphic, and none of them mapped to the vicinity of QPm.vt-2A. In other words, it was almost impossible to detect QPm.vt-2A without microsatellite markers in this mapping population.
Microsatellites are not only highly polymorphic, but most of them are also genome-specific in wheat (Röder et al., 1998) and amplify only a single locus from one of the three genomes. This characteristic is very helpful to assign QTLs to specific chromosomes based on linkage with microsatellite markers. In this study, QTLs associated with APR to powdery mildew were located to wheat chromosomes 1B, 2A, and 2B, on the basis of their linkage with genome-specific microsatellite markers.
Marker Assisted-Selection for Resistance to Powdery Mildew
Even though APR to powdery mildew is more durable than race-specific resistance, selection of APR to powdery mildew is a time-consuming process involving extensive and precise quantitative measurements (Gustafson and Shaner, 1982). To demonstrate the possibility of marker-assisted selection for APR to powdery mildew, the RI lines were grouped according to genotype for the three QTLs, represented by markers GWM304a, KSUD22 and PSP3100, respectively. The RI lines with Massey alleles at all three loci had a mean disease severity of 3.4%, whereas the RI lines with Becker alleles at all three loci had a mean disease severity of 22.3%. These values are similar to those of the corresponding parents. Therefore, the molecular markers, especially the microsatellite markers, associated with the three QTLs have potential for use in marker-assisted selection for APR to powdery mildew.
On the basis of phenotype only, it is difficult to identify plants with both APR and race-specific resistance to powdery mildew. Fortunately, molecular markers linked to genes Pm1 (Ma et al., 1994; Hartl et al., 1995, 1999; Hu et al., 1997), Pm2 (Ma et al., 1994; Hartl et al., 1995), Pm3 (Hartl et al., 1993; Ma et al., 1994), Pm4 (Ma et al., 1994; Hartl et al., 1999), Pm12 (Jia et al., 1996), Pm13 (Donini et al., 1995; Cenci et al., 1999), Pm21 (Qi et al., 1996), and Pm25 (Shi et al., 1998) have been reported. With marker-assisted selection, it is also possible to select plants with both APR and race-specific resistance to powdery mildew.
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ACKNOWLEDGMENTS
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The authors thank Dr. M. K. Das for assistance with powdery mildew evaluation and Dr. R. M. Biyashev for technical support of molecular marker analysis. Also, thanks go to Drs. M. S. Röder and M. D. Gale for providing some of the microsatellite primers for this study. We are grateful to the USDA-ARS Western Regional Research Center for providing some of the RFLP markers for this project.
Received for publication September 5, 2000.
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ABSTRACT
INTRODUCTION
