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Dep. of Plant Biology and Pathology, Rutgers University, 59 Dudley Rd., Foran Hall, New Brunswick, NJ 08901-8520
* Corresponding author (bonos{at}aesop.rutgers.edu).
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: NJAES, New Jersey Agricultural Experiment Station
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Jia and coworkers (2000) found a direct interaction between a resistance gene in rice and an avirulence gene in P. oryzae indicating a gene-for-gene resistance response. The pathogen exists as a mixture of pathogenic races; thus, host resistance genes remain effective for only a few years before the pathogen population shifts to new virulent races (Bonman et al., 1992; Zhu et al., 2000). This has led researchers to identify durable (quantitative) resistance mechanisms to more successfully prevent disease outbreaks in rice (Ahn and Ou, 1982; Cho et al., 1998). Chen et al. (2003) have studied quantitative disease resistance to rice blast using a recombinant inbred line population in rice and a barley (Hordeum vulgare L.) double haploid population. They found quantitative trait loci associated with resistance in both populations and suggested that there may be synteny in the quantitative trait loci (QTL) of blast resistance between barley and rice genomes. The similarity in QTL between rice and barley indicates that similar resistance mechanisms may be employed by different grass species, including perennial ryegrass. Results of Curley et al. (2003) support this hypothesis.
Since the first report of gray leaf spot disease on perennial ryegrass turf in 1991, the disease has spread from the transition zone, through the Mid-Atlantic states and north to New England (Schumann, 1999), as well as west through Indiana (Harmon et al., 2000) and Illinois (Pedersen, 2000). Additional outbreaks of gray leaf spot have been reported in California (Uddin et al., 2002). To date, two major epiphytotics have been reported on perennial ryegrass. The first major outbreak occurred in the Mid-Atlantic and North-Central regions of the country in 1995, whereas the second outbreak developed in the Mid-Atlantic, North-Central, Northeast, and Mid-Continent regions (Dernoeden, 1996). Ten to 50% of unprotected fairways or roughs were destroyed on affected golf courses, with a few golf courses reporting complete destruction (Dernoeden, 1996).
Fungicides such as azoxystrobin {methyl (E)-2-{2[6-(2-cyanophenoxy)pyrimidin-4-yloxy]phenyl}-3-methoxyacrylate}, thiophanate-methyl [dimethyl 4,4'-(o-phenylene)bis(3-thioallophanate)], and propiconazole [cis-trans-1-[2-(2,4-dichlorophenyl)-4-propyl-1,3-dioxolan-2-ylmethyl]-1H-1,2,4-triazole] are effectively used to control gray leaf spot (Clarke and Vaiciunas, 2001; Vincelli, 2000); however, fungicidal treatment can be very expensive. Even if fungicides are used to control the disease on fairways, untreated roughs can serve as primary sources of inoculum for future outbreaks (Uddin, 1999). In addition, fungicide resistant isolates of P. grisea that are resistant to Qo (strobilurin-like) fungicides have been identified (Vincelli and Dixon, 2002). Additionally, certain cultural practices are known to influence the development of gray leaf spot. In particular, nitrogen source and rate (Uddin et al., 2001; Clarke and Vaiciunas, 2001), mowing heights (Clarke and Vaiciunas, 2001; Williams et al., 2001), and clipping removal were found to affect disease severity (Vaiciunas and Clarke, 1998). Unfortunately, cultural practices alone usually do not provide satisfactory control of gray leaf spot in perennial ryegrass when disease pressure is moderate or high (Vincelli, 2000). Host resistance, if commercially available, would limit damage caused by this disease, while reducing the need for costly fungicide applications.
Although genetic resistance is potentially one of the most promising strategies for the control of gray leaf spot on perennial ryegrass, the germplasm base for perennial ryegrass cultivars is extremely narrow (Thorogood, 2003) and very little resistance has been observed in commercially available cultivars (Vaiciunas and Clarke, 1998). This illustrates the importance of maintaining adequate genetic diversity in perennial ryegrass to reduce rapid devastation from a single plant pathogen, such as P. grisea. Hoffman and Hamblin (2000a)(2000b) found that some perennial ryegrass collections from Canada, Japan, New Zealand, and Europe showed resistance to gray leaf spot. Trevathan et al. (1994) also found that some annual ryegrass introductions from Europe showed resistance to gray leaf spot.
Since 1996, the turfgrass breeding program at the New Jersey Agricultural Experiment Station (NJAES) has been involved in the collection of perennial ryegrass germplasm from many countries in Europe to improve and diversify the narrow germplasm base of this species. Germplasm from these collections are incorporated through a modified backcrossing program into the existing perennial ryegrass germplasm present at the NJAES. Gray leaf spot resistance identified in collections from Europe, as well as the very limited amount in existing North American germplasm, can be incorporated into the species via population improvement techniques. This should result in the development of cultivars of perennial ryegrass with improved resistance to this devastating disease.
Cultivar development involves selection of parents from a common germplasm pool of perennial ryegrass. Plants are selected with similar characteristics, intercrossed, and their progeny are tested in turf plot evaluations. The best performing single-plant progeny turf plots are selected and used as the germplasm pool for the next cycle of selection. It is important for breeding programs to identify appropriate germplasm and selection procedures to optimize selection for important turf characteristics including gray leaf spot resistance. The objectives of the study were to: (i) evaluate cultivars, experimental selections, and single-plant progenies of perennial ryegrass for resistance to gray leaf spot, (ii) develop populations from selected parents to determine improvements in the next generation, and (iii) determine heritability and the response to selection for gray leaf spot resistance in perennial ryegrass.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In both years, a natural outbreak of gray leaf spot occurred approximately 4 wk after seeding and lasted for approximately 4 to 5 wk. The severity of gray leaf spot present in each plot was assessed twice each year as the percentage of diseased plants in each 0.9- by 1.5-m plot.
During the fall and winter months of 2000, 89 single-plant progeny turf plots exhibiting superior resistance to gray leaf spot compared with commercial cultivars and other experimental selections were identified. These 89 plots were selected from 679 single-plant progenies from the 12 families evaluated in turf plots in the 2000 experiment (Fig. 1) . The plants were selected on the basis of disease resistance in combination with other important turfgrass characteristics such as vigor, dark green color, fine leaf texture, and increased shoot density. The plants that produced the seed of each of the 89 plots were clonally replicated and placed together in six isolated crossing blocks and two isolated spaced-plant nurseries, which were interpollinated in the spring of 2001. The plants selected for the isolated crossing blocks and nurseries were selected based on similar maturity, color, and growth habit and may not have included all 89 resistant parents. The single-plant progenies from each crossing block, the composite (a blend of equal amounts of seed from all progeny in each crossing block), and open-pollinated plants from the two nurseries were seeded into 338 turf plots in the 2001 experiment. Additionally, 72 single-plant progenies from two families from the same unselected NJAES gene pool of plants utilized in the original experiment in 2000 were included in the 2001 experiment. This unselected population was included to compare to the eight families developed from the 89 parents intentionally selected for gray leaf spot resistance.
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µ or R = µo – µ = ß(µs – µ) = ßS, where µ is the mean phenotypic value of the parent population before selection (679 single-plant progenies in 2000, 72 single-plant progenies in 2001) (Fig. 1 and 2)
, µs = the mean phenotypic value of selected parents (89 selected parents in 2000 (Fig. 1), µo is the mean phenotypic value of the offspring of the selected parents (338 single-plant progenies from the intercrossing of 89 selected parents) (Fig. 2), and ß = R/S = h2 (or realized heritability) (Falconer and Mackay, 1996).
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= a +b1
i +b22i; where Var(oi) is the phenotypic variance within the ith sibship, and i is the midparental value for this sibship. Sibship means can replace midparent values when the latter are unknown. A significant value of b2 is generally accepted as an indication of a major gene. Only families developed in isolated crossing blocks were used in this equation (Table 3).
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Most cultivars and selections recovered rapidly following the onset of colder temperatures (data not shown). These families will need to be evaluated under additional and higher intensity gray leaf spot epiphytotics to determine whether these sources will maintain resistance.
The 89 plants selected from the 2000 trial trace back to 36 different maternal sources. Fifty percent of these plants trace back to plant collections from Eastern Europe, where perennial ryegrass is indigenous. Hoffman and Hamblin (2000a) and Trevathan et al. (1994) found that collections of perennial and annual ryegrass from Europe were among the most resistant to gray leaf spot. The identification of new gray leaf spot resistant sources, in the current study, verifies the importance of collecting perennial ryegrass germplasm from multiple geographic locations. Collections from the center of origin, such as Eastern Europe, are likely a source of resistance due to the greater genetic diversity that exists in those locations.
Heritability and Response to Selection
Selection for a particular trait can result in a number of different outcomes. A gradual shift in the offspring population mean from the base population could occur if the trait has a low heritability, is controlled by many genes and/or is largely affected by the environment (Allard, 1960). A rapid shift in the offspring population mean from the base population could occur if the trait has a high heritability and may be controlled by a few genes with large effects (Allard, 1960). Yet another response is little or no change in the offspring population mean indicating a poor selection method or that the trait is not under genetic control (Allard, 1960). Broad-sense heritability estimated from REML variance components from the Proc MIXED analysis of entries replicated in both years was 0.92 (Table 1). The lack of significant genotype x environment interaction (Table 1) and high heritability estimate indicate that the phenotype of this trait (gray leaf spot resistance) is under strong genetic control.
To confirm this, we compared the selected and unselected populations. Both of the unselected populations in 2000 and 2001 were developed from the same perennial ryegrass germplasm pool present at NJAES and not selected for gray leaf spot resistance. Similar results were observed for the unselected NJAES germplasm pool in 2000 (53% disease) and 2001 (54% disease) (Fig. 1 and 2). Additionally, similar results were observed for the selected parents and intercrossed offspring. The response of the 89 selected single-plant progeny plots evaluated in 2000 had a mean percent disease = 31%, which was equivalent to the response of the offspring of the inter-crossed 89 selected parents evaluated in 2001 (32% disease) (Fig. 1 and 2). These data further support the idea that gray leaf spot resistance has a strong genetic component, but also proves that selection for resistance is highly effective.
Realized heritability was calculated from these data. The realized heritability (h2) or the efficiency of the response to selection (Lynch and Walsh, 1998) is defined as the response expressed as a proportion of the selection differential (R/S) (Falconer and Mackay, 1996). It is important to identify the efficiency of selection, to effectively develop gray leaf spot resistant cultivars of perennial ryegrass. The selection differential (S) was 22 which means that the selected plots showed 22% less gray leaf spot disease than the average of the population evaluated in 2000 (Fig. 1). S was equal to the Response to selection (R = 22) indicating a realized heritability estimate equivalent to 1.0 and a very high efficiency of selection. The realized heritability estimate, as with all other heritability estimates, is based on certain assumptions that have not been accounted for in this experiment. Inbreeding depression, environmental trends and drift are included in the response and may affect heritability estimates. In addition, realized heritability after the first generation, may underestimate the heritability of the base population (Falconer and Mackay, 1996). The importance of the data reported here lies in the fact that gray leaf spot resistance in perennial ryegrass seems to have a strong genetic component and that the type of selection utilized in this study was extremely effective in improving gray leaf spot resistance in subsequent generations. This will be beneficial for the future development of disease resistance cultivars.
The efficient response to selection can be explained by considering the breeding and selection methods used in this study. Plants are typically selected on the basis of phenotype for the determination of heritability. Selection of plants in this study, was based on genotypic evaluation using progeny tests in turf plots. The perennial ryegrass plants were open-pollinated with a heterogeneous collection of pollen of known origin. Performance of the open-pollinated single-plant progenies was then compared in turf plot evaluations. This progeny test essentially evaluates the combining ability (although not specifically tested) of different mother plants compared with the common pollen source (Poehlman and Sleper, 1995) and is a measure of the plant's ability to transmit favorable alleles or combinations of alleles to the progeny (Allard, 1999). Plants with good progeny performance were selected for gray leaf spot resistance in 2000, intercrossed and the resulting progeny were evaluated in 2001. The high selection efficiency observed indicated that parent selection based on this type of progeny test was a good selection method to predict the ability of the parents to transmit favorable alleles and improve gray leaf spot resistance in the next generation.
Although, similar results were observed in the second cycle of selection in 2002 (data not shown), it will be important to follow the response to selection for a number of generations. One common response observed in maize (Zea mays L.) is a rapid gain in the first few generations, followed by a protracted period of slow progress (Allard, 1960). The typical interpretation of this type of response is that the rapid progress in early generations is associated with increases in the frequency of a small number of important genes. The subsequent slow progress that follows is presumably associated with further small changes in the frequency of major genes and slow increases in the frequency of minor genes (Allard, 1960). The selection for these quasiqualitative characters causes a shift in the mean of the population toward the selected trait. A shift in the population mean toward resistance to gray leaf spot was observed in this study (Fig. 1 and 2). Selected families also had smaller variances than unselected families (Table 3). This is presumably because selection has increased the frequency of major genes to near fixation and hence reduced the genetic variability in the population (Allard, 1960). These current data support this type of response pattern. To determine if a highly heterozygous species such as perennial ryegrass exhibits the same response as a more controlled, well-studied, model crop such as maize, evaluation of future generations will be necessary. This will hopefully provide a clear indication of the type of response pattern associated with the selection for gray leaf spot resistance in perennial ryegrass.
Detection of Major Genes
Rice blast disease resistance involves both quantitative and qualitative inheritance (Ahn and Ou, 1982; Cho et al., 1998; Jia et al., 2000; Talbot and Foster, 2001). It is unknown whether gray leaf spot resistance in this particular population is a result of qualitative or quantitative inheritance. Since it is possible that a small number of genes with dominant or partially dominant effects could be affecting gray leaf spot resistance in perennial ryegrass, the detection of major gene effects was determined. Solving the equation proposed by Fain (1978) and later described by Lynch and Walsh (1998) resulted in a P value of 0.001 for the quadratic b2 term. These results indicate that a major gene was segregating when all families (both selected and unselected for resistance to gray leaf spot) were evaluated (Table 3). This is directly due to variance differences between the populations and indicates that a gene is segregating in some populations and not in others. Since resistance is absent or rare in U.S. populations it is possible that a major, dominant gene is present in only the resistant populations (containing mostly European germplasm).
Future research should utilize the extensive work conducted in rice, which has identified major genes and genes with quantitative resistance to rice blast disease (Talbot and Foster, 2001; Chen et al., 2003). Molecular analysis of the current perennial ryegrass populations and utilization of rice molecular markers may identify the location of those genes on the perennial ryegrass genome and the function of those resistance genes.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Further studies are being conducted to verify if the results found in this study are due to a small number of dominant or partially dominant genes and/or other types of gene interactions (i.e., quantitative effects). Because of the rapid mutation rate of the P. grisea fungus, the success of cultivars with improved prolonged resistance will also depend on the identification of durable quantitative resistance.
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Received for publication July 2, 2002.
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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This article has been cited by other articles:
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  Y. Han, S. A. Bonos, B. B. Clarke, and W. A. Meyer Inheritance of Resistance to Gray Leaf Spot Disease in Perennial Ryegrass Crop Sci., March 27, 2006; 46(3): 1143 - 1148. [Abstract] [Full Text] [PDF]
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) of 11 perennial ryegrass entries evaluated in both 2000 and 2001 experiments.