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TURFGRASS SCIENCE

Inheritance of Dollar Spot Resistance in Creeping Bentgrass

^a Dep. of Plant Biology and Pathology, Rutgers University, 59 Dudley Rd. Foran Hall, New Brunswick, NJ 08901-8520
^b Dep. of Dep. of Agronomy, University of Wisconsin-Madison, 1575 Linden Dr., Madison, WI 53706-1597

^* Corresponding author (bonos{at}aesop.rutgers.edu).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
The dollar spot disease incited by Sclerotinia homoeocarpa F.T. Bennet. is a common and destructive disease of both cool- and warm-season grasses throughout the world. Genetic resistance to dollar spot is an important control strategy; however, the genetic mechanism of dollar spot resistance in turfgrasses is unknown. The first objective of this study was to determine broad-sense heritability and predicted gain from selection of dollar spot resistance in creeping bentgrass (Agrostis stolonifera L.). This was completed through dollar spot disease evaluation of 265 randomly selected creeping bentgrass clones arranged in a randomized complete block design with six and five clonally propagated replicates in each of two locations evaluated over 2 yr. Five isolates of S. homoeocarpa were used to inoculate the field studies and applied at a rate of 1.75 g m^-2 of prepared inoculum. The second objective was to evaluate inheritance characteristics of dollar spot disease resistance. Major gene calculations, heterosis, maternal effects, chi square analysis of segregation ratios, and number of effective factors were determined through the evaluation of dollar spot disease resistance of progeny from controlled crosses between fixed resistant and susceptible bentgrass clones. These progenies along with parental clones were established in field trials and inoculated with dollar spot. The continuous population distribution of phenotypes for clones and progeny indicated that dollar spot resistance may be quantitatively inherited. Broad-sense heritability estimates (0.56 on a single plant basis and 0.90 on an 11-plant clonal mean basis) indicated replication increased selection efficiency and that improvement in dollar spot resistance in creeping bentgrass should be possible. A minimum of two to five effective factors depending on the cross may be associated with dollar spot resistance.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
DOLLAR SPOT DISEASE is one of the main disease problems on close-cut creeping bentgrass. More than 70% of fungicides used on golf courses are used for control of dollar spot, brown patch (Rhizoctonia solani Kühn.), and anthracnose [Colletotrichum graminicola (Ces.)G.W. Wils.] diseases (Meritz Marketing Research Inc., St. Louis, MO).

The most promising of the control strategies available is genetic resistance. Vincelli et al. (1997) documented that dollar spot disease varied over years because of environmental conditions and found that some cultivars of creeping bentgrass exhibited apparent partial resistance to dollar spot. Vargas (1994) states that although cultivars vary in their susceptibility, none are considered completely resistant to S. homoeocarpa. Warnke (1995) evaluated 31 cultivars of A. tenuis and A. stolonifera for resistance to S. homoeocarpa in a growth chamber. Ninety-six percent of the plants in his study showed little or no resistance to dollar spot under these conditions. It is evident from these studies that environmental conditions contribute to the variation in disease resistance reported.

Heritability is the proportion of the observed variation in a progeny that is inherited (Nyquist, 1991; Poehlman and Sleper, 1995, p. 71–75). Plant breeders are interested in heritability because characters with higher values can be improved more rapidly with less intensive evaluation than those with lower heritabilities (Nyquist, 1991). Broad-sense heritability estimates include all genetic effects (additive, dominance, and epistatic), while narrow-sense heritability estimates only additive variance (Poehlman and Sleper, 1995, p. 71–75). Narrow-sense heritability estimates are the most useful to the plant breeder because additive gene effects can be selected for in a breeding program; however, crosses are necessary and parent-progeny regression with F₂ populations are typically used to calculate the estimates. Broad-sense heritability can be determined from variance components among clonal replicates by an analysis of variance (Poehlman and Sleper, 1995, p. 71–75) and can include growing plants at more than one location and for more than 1 yr. Broad-sense heritability estimates can be useful in determining the selection efficiency and aid in the improvement of selection procedures, with the understanding that narrow-sense heritability estimates will most likely be somewhat lower.

Burton and Devane (1953) estimated broad-sense heritability for plant yield green weight, seed yield, greenness rating, and disease resistance in tall fescue (Festuca arundinacea Schreb.) from replicated clonal material. This method was designed to separate the variation observed in a segregating population into genetic, environmental, and genetic x environmental variance components (Burton and Devane, 1953). This method was utilized in this study to estimate, from replicated clonal material, the progress that might be expected in developing creeping bentgrass with improved dollar spot resistance.

Heritability has been estimated for a number of turfgrass traits, including turfgrass characteristics in bermudagrass (Cynodon spp.)(Wofford and Baltensperger, 1985), shoot water content of creeping bentgrass under soil dehydration and elevated temperatures (Lehman and Engelke, 1993), and rust (caused by Puccinia spp.) resistance in perennial ryegrass (Lolium perenne L.) (Reheul and Ghesquiere, 1996; Rose-Fricker et al., 1986). Broad-sense heritability of dollar spot resistance in Poa trivialis L. (Hurley and Funk, 1985) ranged from 0.54 to 0.90 for shaded field-grown plants and greenhouse-grown plants, respectively. Other than this study, there has been no research conducted on heritability estimates or inheritance characteristics of dollar spot resistance in any other turfgrass species.

The objectives of this study were twofold: (i) determine population distributions, broad-sense heritability, and predicted gain from selection of dollar spot resistance in creeping bentgrass from ANOVA components of disease resistance ratings from randomly selected creeping bentgrass clones replicated in two locations and evaluated over 2 yr; and (ii) evaluate inheritance characteristics of dollar spot disease resistance (major gene calculations, heterosis, maternal effects, segregation ratios, and number of effective factors) through disease resistance ratings of F₁ progeny from controlled crosses between fixed resistant and susceptible clones.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Objective 1: Heritability and Gain From Selection
Evaluation Trials
Originally, 500 clones of creeping bentgrass were collected from old golf courses in New Jersey, eastern New York, Illinois, and Arizona and randomly selected from ‘Crenshaw’ (10 clones). ‘L-93’ (10 clones) and ‘Penncross’ (3 clones). Single stolons from each clone were transplanted to a 6.4- by 6.4-cm cell filled with BX potting media (Pro-Mix HP, K.C. Shafer, York, PA). Each clone was separated equally into 11 vegetative replicates and maintained under greenhouse conditions for 6 mo at a height of 1.9 cm with a modified reel mower.

Two studies were established at the Plant Biology Research and Extension Farm in North Brunswick, NJ, on a Nixon loam (fine-loamy, mixed, mesic Typic Hapludult). One location was located on a plateau with a row of oak trees to the southern side (Site 1). The other location (200 m from the first) was approximately 4.5 m lower in elevation, bordered on the eastern side by a row of trees, and wood line on the southern side (Site 2). Each site was arranged in a randomized complete block design with six replications in Site 1 and five replications in Site 2. Plants were transplanted to the field in the spring of 1998, 38 cm apart. The area surrounding the plugs was seeded to ‘Jamestown II’ Chewing's fescue (Festuca rubra subsp. commutata Gaudin) at a rate of 34 g m^-2. Both sites received the same amount of fertilizer throughout both years of the study, a total of 11.7 g N m^-2 in 1998 as 4.8 g N m^-2 as (10-4.5-8.3) N-P-K on 18 May 1998 and 6.9 g N m^-2 as (12-1.8-6.6) on 16 Sep 1998, and a total of 19.6 g N m^-2 as (12-1.8-6.6) in the spring and summer of 1999 as 3.6 g N m^-2 on 20 Mar 1999; 3.6 g N m^-2 on 27 Mar 1999; 5.47 g N m^-2 on 26 May 1999; 2.4 g N m^-2 on 9 Jul 1999; and 4.4 g N m^-2 on 27 July 1999. Turf at Site 1 was maintained at a height of 1.3 cm while turf at Site 2 was maintained at a height 1.7 cm for the 1998 and 1999 growing season. Brown patch disease (Rhizoctonia solani Kühn) was prevented in both years with labeled rates of azoxystrobin [methyl(E)-2-{2-[6-(2-cyanophenoxy)pyrimidin-4-yloxy]phenyl}–3-methoxy acrylate] fungicide to reduce the complication of fungal competition.

Inoculation Method
Five isolates of S. homoeocarpa were used as inoculum in the field studies to simulate natural infection. Three isolates (H1, H2, and H3) were collected from the Plant Biology and Pathology Research and Extension Farm at North Brunswick, NJ; these were collected from a mixture of Poa annua L. and bentgrass, Crenshaw creeping bentgrass, and another bentgrass host. The other two isolates (A1 and A2) were collected from the Plant Biology and Pathology Research and Extension Farm at Adelphia, NJ; one from a perennial ryegrass (Lolium perenne L.) host, the other from a Kentucky bluegrass (Poa pratensis L.) host.

Isolates were grown separately on sterilized Kentucky bluegrass seed for inoculation. Two hundred grams of Kentucky bluegrass seed was autoclaved for 15 min. at 151°C. Seventy-five milliliters of dH₂O was added to the Kentucky bluegrass seed in an Erlenmeyer flask, and let sit overnight. One half of a Petri dish containing a single isolate was cut into 1- by 1-cm pieces and transferred to the flask. Isolates were grown separately in flasks for approximately 3 wk at room temperature. The inoculum was dried on newspaper for 3 d, and forced through a seed sieve (1.63 by 9.53 mm) (No. F Seedburo Equip. Co., Chicago, IL). Equal proportions of seed infested with each isolate was mixed and applied with a drop spreader at a rate of 1.75 g m^-2 on 24 June 1998. Light irrigation was applied to enhance fungal growth out of the Kentucky bluegrass seed onto the turfgrass leaves. Dollar spot symptoms appeared 2 wk after inoculation and plants were rated every week during the growing season (June through Oct) in 1998 and 1999 on a 1-to-9 scale. Nine represented 0 to 5% diseased turf, eight represented approximately 10% diseased turf, seven represented approximately 15 to 25% diseased turf, six represented approximately 30 to 40% diseased turf, five represented 40 to 50% diseased turf, four represented approximately 60 to 70% diseased turf, three represented approximately 75 to 85% diseased turf, two represented approximately 90% diseased turf, and one represented 95 to 100% diseased turf.

Statistical Analysis
All data were subjected to analysis of variance. Disease evaluation was conducted over different years and different locations using a split-plot in time model recommended by Steel et al. (1997). Plant mortality, caused by unknown factors (possibly including dollar spot), resulted in a large number of clones that were not present in the second year, at one location, or in a majority of replicates. These clones were removed from the analysis of variance, leaving a group of 265 clones balanced across locations and years.

Broad-sense heritability was determined by means of variance components calculated from expected means squares from the ANOVA. A random model for all effects was used because years were not chosen with respect to expected climatic conditions, locations were not chosen with respect to specific ecological properties (Gordon et al. 1972) and no information on disease resistance of bentgrass clones was known before the initiation of the study. Heritability was determined on a single-plant basis (Hsp) as well as a clonal-mean basis (Hc) to determine the most efficient selection method as follows:

where ²_c = the total genetic variance (for clones), ²_cy = clone x year variance, ²_cl = clone x location variance, ²_cr = clone x rep within location variance, ²_cyl = clone x year x location variance, and ²_e = experimental error (clone x year x rep within location) variance (Poehlman and Sleper, 1995, p. 71–75). Components in parenthesis = phenotypic variance (²_P).

where letters in the denominator refer to the number of replicates (9), locations (2), and years (2). The gain from selection (G_s) was determined following the procedure defined by Burton and Devane (1953) and Poehlman and Sleper (1995)(p. 71–75):

where i = the selection intensity, top 5% = 2.06, and Hsp or Hc refer to the heritability on a single-plant basis or a clonal-mean basis, respectively.

Objective 2: Inheritance Characteristics
Controlled Crosses
It is unknown whether a few genes with large effects or many genes with small effects control resistance to dollar spot disease in creeping bentgrass. In an attempt to begin to determine the mode of inheritance, crosses were made among and between plants expressing a high degree of resistance (mean dollar spot rating >7.5) and those expressing a high degree of susceptibility (mean dollar spot rating <2.5). Four fixed clones, two resistant (Penncross-2 and L93-10) and two susceptible (Crenshaw-5 and 7418-3) were selected from the replicated clonal trial described above and used as both male and female parents in the development of full-sib F₁ progeny. The parents, Penncross-2, L-93-10, and Crenshaw-5 were selected from the cultivars Penncross, L93, and Crenshaw, respectively; the parent 7418-3 was collected from Piping Rock Golf Course, Long Island, NY. These plants had not been testcrossed or inbred before their selection and are most likely heterozygous at many loci due to the cross-pollinated nature (Allard, 1999) of creeping bentgrass. All possible combinations of crosses were attempted (Susceptible x Susceptible, Resistant x Susceptible and Resistant x Resistant). Not all crosses were successful in 1999, so the crosses were repeated in 2000; however, resistant x resistant crosses were not successful either year. A full description of the controlled crosses is presented in Table 1 .

Seed was harvested from both plants in the cross, dried, and then treated with 0.2% (w/v) KNO₃ to induce germination. Individual seedlings were put in seedling flats described above and transplanted to the field on 16 Oct. 1999 or 17 Sep. 2000, along with at least three replicates of the parents used in the crosses. Plants were maintained as mowed spaced-plants at a height of approximately 2.5 cm with a rotary mower. Progeny were inoculated with three isolates of S. homoeocarpa (H1, H2, and A1) to simulate natural infection. Dollar spot disease was evaluated as described above in 1999 and 2000.

Statistical Analysis
The determination of major genes was determined by the equation proposed by Fain (1978) and described by Lynch and Walsh (1998):

where Var(o_i) is the phenotypic variance within the ith sibship, and _i is the midparental value for this sibship. A significant value of b₂ is taken as an indication of a major gene (Lynch and Walsh, 1998). Heterosis, the comparison between progeny and mid-parent means, and maternal effects, the comparison of progeny of reciprocal crosses, were evaluated for significance with the two-sample t-test (Kitchens, 1998). Progeny of the controlled crosses were classified as either resistant or susceptible based on dollar spot ratings, progeny with dollar spot ratings >7.5 were considered resistant and all others considered susceptible. Krupinsky and Berdahl (1982) and Yin et al. (1996) used similar methods to determine resistance vs. susceptibility. Chi square tests were used to evaluate goodness-of-fit of the observed segregation ratios to expected disomic segregation patterns (Table 2) . Elgin and Ostazeski (1985) and Tofte et al. (1991) have also utilized this method. Minimum number of effective factors (loci) was determined using the Castle-Wright formula (Poehlman and Sleper, 1995, p. 71–75) modified for environmental variation.

where P₁ and P₂ are the means of the parent clones, ²_E is the environmental variance among replicates of a parental clone pooled for all parental clones, and ²_F1 is the variance of the F₁ progeny.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

View larger version (50K): [in this window] [in a new window]   Fig. 1. Population distribution of creeping bentgrass clones in response to dollar spot disease averaged over 2 years and 2 locations.

Heritability is a measure of the efficiency of a particular selection system in its ability to separate genotypes. From Table 4, it is evident that utilizing clonal replication in a selection system increases the heritability. Heritability in the broad sense on parental clones evaluated over 2 yr and two locations on a single plant basis was 0.56, while introducing clonal replication increased the heritability to 0.90. Since dollar spot resistance is affected by environmental conditions and heritability based on a single plant basis was only 0.56, because of environmental effects it is possible that clones showing some levels of resistance may be rejected if only replicated once in each location. By increasing the replication of individual clones, environmental variation is reduced and it is more likely that clones showing consistent resistance over environments will be identified. The estimates of genetic variance and broad-sense heritability apply only to this population of creeping bentgrass plants and the particular environments sampled (Dudley and Moll, 1969). Since the genetic variance (²_c) may contain variance due to dominance and epistatic effects, these numbers must be realized as the maximum heritabilities expected when replicated clones are used for selection.

Yin et al. (1996) found similar results when evaluating red rot (caused by Colletotrichum falcatum Went.) resistance in sugarcane (interspecific hybrids of Saccharum). The broad-sense entry-mean heritability estimates from parental clones were moderate to high for all the disease traits. However, the estimates based a single-stalk analysis were nearly half the estimates based on multiple (3) stalk entry means.

Results of this study indicate that a substantial percent gain from selection of the top 5% of the population can be expected when phenotypic selection is used (42% on a single plant basis and 68% on an 11-plant mean basis) (Table 4). The top 5% of the population were chosen for the selection intensity because of the increased number of susceptible plants that would be selected if the intensity was reduced to 10%. It is also important to note that since broad-sense heritability estimates were used in the calculations for percent gain and contain nonadditive genetic effects, which are not utilized during recurrent selection, this would be the maximum percent gain expected from this population. Gain from selection increased by 27% when comparing a single plant vs. an 11-plant mean. The breeder would need to decide whether this increase in gain merits the increased effort, time, and space involved in replication and evaluation of larger number of replicates. Data from this study indicate that a selection method utilizing increased replication may be worthwhile to identify clones with consistent disease reactions over a number of environments.

The information contained here is important because it (i) indicates that there appeared to be adequate total genetic variation present to ensure the effectiveness of a selection program for improvements in dollar spot resistance in creeping bentgrass and (ii) it identified the most efficient selection method for improving dollar spot resistance in creeping bentgrass by comparing heritability estimates and predicated gain from selection of a single-plant vs. an 11-plant mean. Results from this study indicate that an efficient dollar spot selection program should include the selection of disease resistance based on replicated clones evaluated over a number of years and locations. This method could possibly be useful in selecting for dollar spot resistance in other cross-pollinated species.

Objective 2
Inheritance Characteristics
F₁ Progeny Distributions. Since broad-sense heritability estimates indicated a significant genetic effect especially when based on an 11-plant mean, the next logical step in understanding the gene action and inheritance of resistance is to study the progeny of controlled crosses. The mean dollar spot average of the progeny from each of the controlled crosses is reported in Table 1 and Fig. 2 through 6 . Individual selfed controlled crosses of the parents yielded no progeny, indicating that seed production by accidental self-pollination was infrequent and also that a high level of self-incompatibility among these creeping bentgrass clones existed. Most grasses, including creeping bentgrass, have a high level of self-incompatibility (Aston and Bradshaw, 1966; Stuckey and Banfield, 1946).

View larger version (42K): [in this window] [in a new window]   Fig. 2. Population distribution of F1 progeny of a cross between two dollar spot susceptible creeping bentgrass clones. A) 7418-3 as the female parent pollinated by Crenshaw-5. B) The reciprocal cross, Chrenshaw-5 as the female parent pollinated by 7418-3.

View larger version (54K): [in this window] [in a new window]   Fig. 6. Population distribution of F1 progeny of a cross between one dollar spot susceptible and one resistant creeping bentgrass clone. A) Crenshaw-5 (susceptible) as the female parent pollinated by Penncross-2 (resistant). B) The reciprocal cross, Penncross-2 as the female parent pollinated by Crenshaw-5.

View larger version (44K): [in this window] [in a new window]   Fig. 3. Population distribution of F1 progeny of a cross between one dollar spot susceptible and one dollar spot resistant creeping bentgrass clone. A) 7418-3 (susceptible) as the female parent pollinated by L93-10 (resistant). B) The reciprocal cross, L93-10 as the female parent pollinated by 7418-3.

View larger version (49K): [in this window] [in a new window]   Fig. 4. Population distribution of F1 progeny of a cross between one dollar spot susceptible and one resistant creeping bentgrass clone. A) Crenshaw-5 (susceptible) as the female parent pollinated by L93-10 (resistant). B) The reciprocal cross, L93-10 as the female parent pollinated by Crenshaw-5.

View larger version (48K): [in this window] [in a new window]   Fig. 5. Population distribution of F1 progeny of a cross between one dollar spot susceptible and one resistant creeping bentgrass clone. A) 7418-3 (susceptible) as the female parent pollinated by Penncross-2 (resistant). B) The reciprocal cross, Penncross-2 as the female parent pollinated by 7418-3.

Detection of Major Genes. The observation of continuous distribution of phenotypes is taken as support for a large number of genes of roughly equal effect. This is an assumption, however, because the presence of major genes can be obscured if the environmental variation is large or if the major genes are at a low frequency (Lynch and Walsh, 1998). The detection of major genes by means of the equation proposed by Fain (1978) and described by Lynch and Walsh (1998) resulted in a P value = 0.66 for the quadratic b₂ term, indicating that a major gene was not segregating for the families evaluated in this study and that environmental factors and other polygenes could contribute to the differences between families. This further supports the idea that dollar spot resistance may be inherited quantitatively.

Heterosis and Maternal Effects. Progeny means of all the controlled crosses were not significantly different from the midparent mean (data not shown) on the basis of the two-sample t-test statistic (Kitchens, 1998). This indicates that heterosis was not present for these crosses and suggests that dominance or epistasis may not be the main gene effects involved in disease resistance. However, it could be that 7418-3 or Crenshaw-5 had a group of loci conferring susceptibility and L-93-10 and Penncross-2 may contain loci with opposite effects or recessive alleles that when crossed negated the gene effects of either parents resulting in progeny with dollar spot responses in between that of the two parents, or loci segregating in L93-10 and Penncross-2 may not be linked to loci segregating in 7418-3 or Crenshaw-5. Falconer and Mackay (1996) conclude that if some loci are dominant in one direction and some in the other, their effects will tend to cancel out, and no heterosis may be observed, in spite of dominance at the individual loci. Maternal effects, determined by comparing progeny means from reciprocal crosses, were not significant for the resistant x susceptible crosses, but was significant for the susceptible x susceptible cross (Table 1). These data indicate that 7418-3, when used as the female parent, had a positive effect on disease resistance compared with Crenshaw-5. Additional crosses including F₂ and backcrosses to both original parents are needed to determine a clear and educated explanation of the gene action involved in the inheritance of dollar spot resistance.

Chi-Square Analysis. Progeny of all the controlled crosses classified as either resistant or susceptible resulted in more susceptible than resistant progeny, so the segregation ratios of the progeny were fit to both a single dominant locus or two dominant locus model for susceptibility (Table 2). The progeny segregation ratios for dollar spot resistance did not fit the single dominant locus model in simplex (data not shown). Two crosses and their reciprocals (Penncross-2 x Crenshaw-5 and Crenshaw-5 x 7418-3) fit the single-dominant-locus-in-duplex model and the two-dominant-loci model with disomic inheritance and selective pairing with susceptibility to dollar spot being dominant over resistance.

Minimum Number of Effective Factors (Loci). In most cases, this formula gives a conservative estimate of the number of effective factors involved because it does not take into account the presence of linkage, dominance, or unequal effects at different loci. The presence of epistasis can cause either an over or an underestimation of the actual number of segregating genes (Bjarko and Line 1988; Das and Griffey, 1994). Effective factor calculations varied between 1.8 and 4.2 for the resistant x susceptible crosses (Table 5) . Therefore, it is assumed that there may be more than two to five genes involved in dollar spot resistance in creeping bentgrass depending on the particular cross. The Penncross-2 x Crenshaw-5 crosses had minimum values of 1.69 and 1.87, which is in agreement with the Chi-square analysis of the two dominant loci model (Tables 2 and 5). The other crosses may have more than two genes segregating between them, which would explain why the Chi-square analysis for one or two genes did not fit the segregation ratios observed. Better correlations between chi-square analysis and effective factors may occur with a larger number of crosses and/or larger number of progeny to score within each cross. It is also possible that different races of the fungus may be interacting with different resistance loci. These crosses were evaluated with a mixture of three different isolates to simulate natural infection and identify durable resistance. It is unknown whether these results are due to major resistance genes specific to each isolate or due to a larger number of genes indicating quantitative disease resistance. Preliminary results of a subsequent study comparing isolates separately, however, indicate that there may be interactions between host and pathogen genotypes for quantitative disease resistance (Bonos and Meyer, 2002). Results from this study suggest that dollar spot resistance in creeping bentgrass may be quantitatively inherited. Future research studies including effects of specific isolates and the determination of quantitative trait loci associated with dollar spot resistance in creeping bentgrass are currently underway to determine a more conclusive estimate of the number of loci and the gene action involved in dollar spot resistance in creeping bentgrass.

	ACKNOWLEDGMENTS

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Research was supported by the Rutgers Center for Turfgrass Science, New Jersey Agric. Exp. Stn., and New Jersey Turfgrass Association Journal No. D-12264-12-01

Received for publication March 29, 2002.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 

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