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Published online 27 March 2006
Published in Crop Sci 46:1143-1148 (2006)
© 2006 Crop Science Society of America
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TURFGRASS SCIENCE

Inheritance of Resistance to Gray Leaf Spot Disease in Perennial Ryegrass

Yuanhong Hana,*, Stacy A. Bonosb, Bruce B. Clarkeb and William A. Meyerb

a Forage Division, Noble Foundation, 2510 Sam Noble Pky., Ardmore, OK 73401
b Dep. of Plant Biology and Pathology, Rutgers University, 59 Dudley Rd., Foran Hall, New Brunswick, NJ 08901-8520

* Corresponding author (yhan{at}noble.org)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Gray leaf spot disease, caused by Pyricularia oryzae Cavara, is an important disease in perennial ryegrass (Lolium perenne L.) turf. Host resistance is an ideal and promising approach to disease control. In this study, two diallel crosses involving six parents and eight parents, respectively, of perennial ryegrass were established to investigate the inheritance of gray leaf spot resistance. Parents and progenies were evaluated for gray leaf spot resistance in growth chamber experiments where they were inoculated with a mixture of five pathogen isolates. A field experiment was conducted on the progenies of one diallel cross. Effects of both general combining ability (GCA) and specific combining ability (SCA) were significant in both growth chamber and field tests. However, the GCA variance accounted for the major portion of the total genotypic variance. Narrow-sense heritability calculated by midparent–offspring regression ranged from 0.57 to 0.76, indicating additive gene effects were the major genetic component in control of gray leaf spot. Estimates of minimum number of genes ranged from 2.1 to 4.4, suggesting resistance to gray leaf spot was controlled by a small number of genes. All the results suggested that a breeding program basing on recurrent selection should be effective to improve the resistance to gray leaf spot in perennial ryegrass.

Abbreviations: GCA, general combining ability • R, resistant • S, susceptible • SCA, specific combining ability


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
GRAY LEAF SPOT has rapidly become one of the most important diseases of perennial ryegrass (Lolium perenne L.) in United States since its identification (Dernoeden, personal communications, 1986; Landschoot and Hoyland, 1992). The causual agent, P. oryzae Cavara [teleomorph Magnaporthe oryzae B. Couch, previously known as Magnaporthe grisea (Hebert) Barr], is a widely distributed filamentous ascomycete infecting more than 50 grasses including rice (Oryza sativa L.), wheat (Triticum aestivum L.), annual ryegrass (Lolium multiflorum Lam.), St. Augustinegrass (Stenotaphrum secundatum Walt), and tall fescue (Festuca arundinacea Schreb.) (Sprague, 1950; Talbot, 1995; Trevathan, 1982; Uddin et al., 1998). The devastating nature of this disease has seriously restrained the use of perennial ryegrass. In the mid-Atlantic region, 10 to 50% of perennial ryegrass was killed by P. oryzae in the infected fairways in 1995; whereas in some extreme cases, fairways were completely destroyed (Dernoeden, 1996). Given all the desirable characteristics of perennial ryegrass as a major cool-season turfgrass species, turf managers and golf course superintendents have to consider the risk of gray leaf spot and the expense of a fungicide program to prevent this disease, especially for fairways (Vincelli, 2000; Uddin et al., 2003). Many golf courses have converted their perennial ryegrass fairways to other species to avoid the threat of gray leaf spot (Turner, 2000). To take advantage of the many excellent traits of perennial ryegrass, new cultivars resistant to gray leaf spot need to be developed.

When gray leaf spot was first identified (Landschoot and Hoyland, 1992), most commercial cultivars were extremely susceptible. Efforts were made to identify resistant germplasm. Hoffman and Hamblin (2000) identified some resistant plants in their collections from Canada, Japan, New Zealand, and Europe. Bonos and coworkers (2004) identified new sources of resistant germplasm from both European collection and existing North American germplasm. Based on the performance of selected parents and their progenies in field trails, they found that gray leaf spot resistance in perennial ryegrass was strongly controlled by genetic factors. The estimation of broad-sense heritability was very high (0.92). Broad-sense heritability, however, does not differentiate additive gene effects from nonadditive (dominance or epistasis) effects. For an open-pollinated species like perennial ryegrass, the additive component is the most useful in a phenotypic recurrent selection breeding program. Narrow-sense heritability is defined as the proportion of resistance that is controlled by additive gene effects (Poehlman and Sleper, 1995). It gives a more accurate estimate of the gain from selection. Wang et al. (1989) obtained very low narrow-sense heritability estimates when they studied the inheritance of partial rice blast resistance in rice cultivar IR36. The narrow-sense heritability of gray leaf spot in perennial ryegrass has not been investigated.

Research has shown that both qualitative (complete) resistance and quantitative (partial) resistance were involved in rice blast resistance (Ahn and Ou, 1982; Bonman and Mackill, 1988; Inukai et al., 1994). Qualitative resistance is controlled by single gene in the host and a single gene in the pathogen, described as a gene-for-gene relationship (Flor, 1971). It can break down easily and is not stable across many pathogen races. Quantitative resistance is the cumulative effects of multiple genes and has a broad spectrum of resistance to many races of a pathogen. Many qualitative resistant genes in rice have been identified and isolated (Pan et al., 1996; Inukai et al., 1994); however, the understanding of quantitative resistance in rice blast disease is still not well understood (Wang et al., 1989). There are reports about the quantitative nature of gray leaf spot resistance in tall fescue (Hoffman and Hamblin, 2001; Tredway et al., 2003). Whether resistance to gray leaf spot in perennial ryegrass is qualitative or quantitative has not been fully elucidated.

Diallel crosses are traditionally employed to evaluate the general and specific combining abilities of parent plants (Griffing, 1956). Information on combining abilities is not only helpful to determine the potential contribution of resistant plants when used as parents in a breeding program, but it can also be used to estimate the proportion of genetic components (additive, nonadditive, and maternal effects) controlling the resistance. Narrow-sense heritability can also be calculated using data from both parents and progenies.

To investigate the genetic properties of gray leaf spot resistance in perennial ryegrass, we initiated a project in 2001 utilizing a diallel cross design. Our objectives of this study were to (i) estimate the general and specific combining abilities of various parents, (ii) determine the importance of additive and nonadditive gene actions in gray leaf spot resistance, (iii) calculate narrow-sense heritability for gray leaf spot resistance and (iv) estimate the effective number of genes involved in gray leaf spot resistance.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Diallel Crosses
Two separate diallel crosses were made in the spring of 2001 and 2003, one in each year. Six parents were used in the 2001 diallel cross (15 F1 single crosses with their reciprocals) and eight parents in the 2003 diallel (28 F1 single crosses and their reciprocals, Table 1). Parent plants were selected from the germplasm collection at the New Jersey Agricultural Experiment Station (NJAES). Selection of parent plants was based on the level of gray leaf spot resistance observed in previous studies, which was indicated by the performances of their progenies in field trials (Table 1).


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  		Table 1. Characterization of perennial ryegrass parents used in two diallel crosses for gray leaf spot (GLS) resistance and their general combining ability (GCA) of resistance in growth chamber and field experiments.

 
Every parent plant was increased by vegetative propagation using clonal tillers. Clones were planted in the field before winter to fulfill the vernalization requirement. Crosses were made in a greenhouse facility. Each single cross was isolated in its own room to prevent contamination. One plant of each parent was used in each single cross in the 2001 diallel cross. However, harvested seeds were not enough for replicated field trials. In the 2003 diallel crosses, three clones of each parent were used in each single cross to increase the seed yield. Reciprocal crosses in both diallel crosses were harvested separately.

Growth Chamber Tests
Growth chamber tests were conducted using artificial inoculation for each diallel cross. Both F1 progenies and the parents of one diallel cross were tested together for gray leaf spot resistance in a walk-in growth chamber (Environmental Growth Chambers, Chagrin Falls, OH) using the following procedures. Harvested seeds from each single cross were germinated in the greenhouse. Clones of parent plants were kept in the greenhouse in plastic pots (24.4 cm in diameter, Zarn Inc., Chicago, IL). They were cut to a height of 5 cm to induce new tillers. Two weeks after germination, young seedlings of the F1 progenies were transplanted into flats (48-cell flats for the 2001 diallel, 72-cell flats for the 2003 diallel) (Summit Plastic Company, Tallmadge, OH) along with the tillers (about same size as the F1 seedlings) from parents. Experiments used a randomized complete block design. Each flat was considered a replication. Twenty-one flats of plants were used in each experiment of the 2001 diallel. Each flat contained one F1 progeny seedling from each of the 15 single crosses and their reciprocals and one clonal tiller from each parent. Twenty flats of plants were used in each experiment of the 2003 diallel. Each flat contained one F1 progeny seedling from each of the 28 single crosses and their reciprocals, and one clonal tiller from each parent. F1 seedlings and parental tillers were arranged randomly into each flat. Extra cells were filled with similar seedlings to keep the conditions uniform, but they were not used for data collection. Edge effect was not considered in the arrangement of seedlings. Two experiments were conducted for each diallel.

Inoculation was conducted when the F1 seedlings were 6 wk old following the procedure of growth chamber Method 2 described by Han et al. (2003). Five isolates of P. oryzae (ANJ-01, RLVA, TFGGA, RSKY2, and RHF2NJ-1) were used to make the inoculum. The concentration of conidia suspension was approximately 50 000 conidia mL–1. Each flat of plants was sprayed with 100 mL conidia suspension. Ten days after inoculation, plants were evaluated for gray leaf spot resistance by estimating percentage of healthy leaf tissue per plant, which can be easily compared to the resistance performance of turf plots in field test.

Field Experiments
Field experiments were established on 15 Aug. 2003 for the 2003 diallel cross. Due to the insufficiency of seeds of certain crosses, a subset of six parent diallel without reciprocals was used. Seeds from each single cross were seeded into 0.9 by 1.5 m plots (18 g m–2). Plots were arranged in a randomized complete block design with three replications. This field trail was inoculated by a naturally occurring gray leaf spot epidemic about 1 mo after seeding. Plots were rated for gray leaf spot resistance by estimating the percentage of nondiseased turf area per plot.

Estimation of Gene Number
Three populations were used to estimate the minimum number of genes involved in gray leaf spot resistance. Each population was established from one single cross made in 2002 (Table 1). Ten F1 plants from the single cross were used to make a polycross block for F2 seed production. Harvested F2 seeds were germinated in the greenhouse. Twenty F2 seedlings, 20 F1 tillers, and two tillers from each parent were transferred into one 48-cell flat. For each population, 10 flats of plants were tested in the growth chamber using inoculation techniques previously described. Plants were evaluated for gray leaf spot resistance by estimating percentage of healthy leaf tissue per plant 10 d after inoculation.

Statistical Analysis
Combining Abilities
Data from the two diallel crossing experiments were subjected to analysis of variance with Griffing's (1956) diallel Method 3 (F1s and reciprocals included, but not parents), Model 1 (fixed effects). The general linear model for the analysis was

Formula
where Xijk = observed gray leaf spot resistance (percentage of apparently healthy tissue) of ijth cross in the kth block, u = population mean, gi = general combining ability (GCA) effect of ith parent, gj = GCA effect of jth parent, sij = specific combining ability (SCA) effect for ijth cross, rij = reciprocal effect for ijth cross, bk = effect of the kth block, eijk = residual effect. Data of the two growth chamber tests of the same diallel were combined for analyzing using the DIALLEL-SAS program of Zhang and Kang (1997).

Narrow-Sense Heritability Estimates
Narrow-sense heritability of gray leaf spot resistance was calculated using midparent–offspring regression for each growth chamber test. Means of the ratings of F1 progenies of each single cross were regressed against the average rating of their two parents. The slopes were taken as the estimates of narrow-sense heritability (Falconer, 1989).

Number of Effective Genes
Effective number of genes controlling gray leaf spot resistance was estimated using the following formula (Wright, 1968).

Formula
Where n = number of genes, P1 = mean resistance of parent 1, P2 = mean resistance of parent 2, {sigma}2F2 = variance of F2 population, {sigma}2E = environmental variance within family. The assumptions of this formula include: all the genes in control of the trait are unlinked; they affect the trait equally in size and direction; and there is no dominance or epistasis effects involved.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Combining Abilities
Analysis of variance was performed on two diallel crosses separately with combined data from two growth chamber tests or the field test. The calculated mean squares are given in Tables 2 and 3. Highly significant variance components were observed for F1 genotypes in all the analyses, indicating that gray leaf spot resistance is strongly controlled by the genotype of host plant. In the analysis of components within the genotypic variance, general combining ability effects were highly significant in every analysis (P ≤ 0.01), indicating that parents of different genotypes contributed differently to resistance in hybrid combinations. Specific combining ability effects were also highly significant (P ≤ 0.01) in all tests, indicating the involvement of nonadditive (dominance and/or epistasis) gene effects in gray leaf spot resistance. To determine the relative importance of general and specific combining abilities in determining the progeny performance, the proportions of sums of squares of GCA, SCA, and the reciprocal effect were calculated similar to Cisar et al. (1982) and the results are shown in Table 4. Through all the analyses, GCA effects accounted for 80 to 86% of the total genotypic variance, whereas SCA effects accounted for only 7 to 17%. The high GCA ratios suggested that gray leaf spot resistance was mainly controlled by additive gene effects and the resistance level of progenies can be accurately predicted based on the GCA of their parents. Also, it suggested that a breeding program using genotypic recurrent selection should be able to improve gray leaf spot resistance very effectively. The importance of SCA effects should not be completely dismissed. Certain gene or genes with dominant or epistatic effects are involved in the control of gray leaf spot resistance. However, these genes do not have a major effect on the final level of resistance of the plant and will not affect the outcome of a breeding program greatly. Progenies from each single cross of each diallel were classified as resistant (rating ≥ 60) and susceptible (rating < 60) groups. Obtained ratios failed to fit a one or two gene model (data not shown). This confirms that additive effect is more important than dominant effect in gray leaf spot resistance.


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  		Table 2. Analysis of variance for gray leaf spot resistance in two diallel crosses of perennial ryegrass made in 2001 and 2003 using combined data from two growth chamber experiments.

 

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  		Table 3. Analysis of variance for gray leaf spot resistance in a diallel crosses of perennial ryegrass made in 2003 using data from field test.

 

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  		Table 4. Percentage of entry sums of squares accounted for by general combing ability (GCA), specific combining ability (SCA), and reciprocal effect for gray leaf spot resistance in two diallel crosses of perennial ryegrass using data from growth chamber and field tests.

 
The GCA effect of each parent was calculated and compared with its own level of resistance to see how efficiently a parent can contribute resistance to its offspring (Table 1). Parents with significant positive value of GCA would contribute gray leaf spot resistance to their progenies, whereas parents with significant negative GCA value would pass susceptibility to their progenies. All the susceptible parents in the two diallel crosses had significant negative GCA values. Most resistant parents had significant positive GCA values with some exceptions. In the 2001 diallel cross DCM-34 showed a high level of resistance but a nonsignificant GCA value, suggesting the resistance in DCM-34 is partially rendered by dominant or epistatic gene effects and cannot be efficiently transmitted to its progenies. The GCA values were much larger in field test than in growth chamber tests. This was caused by the difference between rating from individual plants and rating from turf plots. A turf plot that consisted of progenies from two resistant parents had dense canopies, which masked some of the diseased tissue. When it was rated on the level of turf plot, it received a higher rating than the average rating of all the individuals within the plot. All the parents with high positive GCA would be good choices for use in breeding programs for gray leaf spot resistance improvement.

Significant SCA effects suggested that the progeny resistance levels of certain parent combinations were significantly higher or lower than the predictions based on parents' GCA. In the 2001 diallel cross, eight of the 15 pairs of parent combinations showed significant SCA effects (Table 5).The combination of two resistant parents DCM-34 and S99–51 had a highly significant negative SCA effect on resistance (–4.64). The significantly beneficial combination of two resistant parents included S99–51 and GLE-35, and RNS-18 and GLE-35. In the 2003 diallel cross, analysis across the two growth chamber tests showed that only six pairs of the total 28 pairs of parents had significant SCA effects (Table 6). One pair of resistant parent combination (GLE-20 and A00–206) had significant positive SCA effects and gained more resistance than what can be predicted from their GCA effects. Nonadditive gene actions are the causes of significant SCA values. Genes with dominant or epistatic effect must have been present and interacted in the parent pairs with significant SCA value.


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  		Table 5. Estimates of specific combining ability (SCA) effects in a diallel cross of six perennial ryegrass parents made in 2001 using combined data from two growth chamber tests.{dagger}

 

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  		Table 6. Estimates of specific combining ability (SCA) effects in a diallel cross of eight perennial ryegrass parents made in 2003 using combined data from two growth chamber tests.{dagger}

 
Highly significant reciprocal and maternal effects were observed in a growth chamber test of the 2003 diallel, but not in the 2001 diallel (Table 2). Reciprocal effect is associated with cytoplasmic inheritance from the female parent, however, accidental self-pollinating could be one of the reasons for the significant reciprocal and maternal effects (Dudley, 1963) observed in these crosses. The 2003 diallel cross was made with three clones from each parent in each single cross to make enough seed for turf plot evaluations. This may have increased the incidence of self-pollination. Further study is needed to investigate whether cytoplasmic inheritance is involved in gray leaf spot resistance. However, through all the analyses, sums of squares from reciprocal effect accounted for 3 to 7% of the total genotypic sums of squares (Table 4). Therefore, we consider that reciprocal effects were not large enough to significantly influence the estimate of GCA and SCA effects.

Heritability
Narrow-sense heritability of gray leaf spot resistance was estimated using midparent–offspring regression for each growth chamber test of all three sets of diallel crosses. The values range from 0.57 to 0.76 (Table 7). In the study of Bonos and coworkers (2004), broad-sense heritability of gray leaf spot was estimated as 0.92. Broad-sense heritability is higher than narrow-sense heritability because it doesn't exclude nonadditive gene effects from genotypic variance. The lower narrow-sense heritability obtained from this study suggested the presence of these nonadditive effects. Another reason that could have contributed to the relative low narrow-sense heritability was that the parent data were obtained using clonal tillers from old plants. Gray leaf spot is more severe on young seedlings than on mature plants (Landschoot and Hoyland, 1992). Although we cut the plant low to encourage the formation of young tillers, the selected tillers still were likely more tolerant to gray leaf spot than the seedlings of progenies. This could have caused a downward bias on the estimates of narrow-sense heritability.


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  		Table 7. Narrow-sense heritability of gray leaf spot resistance calculated using midparent–offspring regression method in two diallel crosses of perennial ryegrass using data of growth chamber tests.

 
Since narrow-sense heritability is the reflection of the percentage of additive gene effects, which are the gene effects that actually respond to selection, it provides more useful information to breeders. The moderate to high estimates of narrow-sense heritability in this study suggested that improving gray leaf spot resistance in perennial ryegrass would be very effective through a genotypic recurrent selection breeding program. The results were consistent with the combining abilities, which also indicated gray leaf spot resistance was mainly controlled by additive gene effects in this set of perennial ryegrass plants.

Number of Genes
Although estimates of gene number involved in resistance could be inaccurate given the many assumptions, it provides a general idea about whether a large or small number of genes control a trait in question. In this study, estimated minimum number of genes involved in gray leaf spot resistance ranged from 2.1 to 4.4 (Table 8). Since nonadditive factors were detected in the combining ability study, the estimates of gene number could have either upward or downward bias. Assumedly, there could be more than three to five genes involved in gray leaf spot resistance in these crosses. Bonos et al. (2004) reported that a major gene for gray leaf spot resistance was segregating in the resistant perennial ryegrass populations. Since nonadditive effect only accounts for a small portion of the total genetic variance, it is not likely that such a major gene with dominant effect exists in the parents used in this study. However, it is possible that one of the genes with additive effect contributed a much larger portion than the others to the resistance. More research is warranted on this subject, especially by using molecular markers to locate the genes on the perennial ryegrass genome and evaluate their functions.


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  		Table 8. Estimation of minimum number of genes (n) affecting gray leaf spot (GLS) resistance in perennial ryegrass, calculated from generation means of the populations of three crosses with {sigma}2E as environmental variance.

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
From this study, we concluded that both additive and nonadditive gene effects are involved in gray leaf spot resistance in the perennial ryegrass plants used in two diallel crosses. However, additive gene effects were the major factor in gray leaf spot resistance evaluated in these perennial ryegrass plants. The resistance level of progenies could be predicted accurately on the basis of GCA of their parents. The values of narrow-sense heritability were moderate to high. Resistance was mainly quantitatively inherited and controlled by a small number of genes in an additive manner. However, it is possible that some genes are more important than the others for gray leaf spot resistance. This study agrees with the former study (Bonos et al., 2004) in that resistance could be rapidly improved through a genotypic recurrent selection program. The quantitative nature suggests that the resistance will be stable against new pathogen isolates. However, due to the fact that gray leaf spot is a relatively new disease, P. oryzae population may have not fully exhibited its potential of variability on perennial ryegrass as on rice. The stability of the resistance needs to be further tested.

Received for publication July 25, 2005.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





This Article
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Right arrow Crop Genetics

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