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Published online 1 January 2005
Published in Crop Sci 45:399-406 (2005)
© 2005 Crop Science Society of America
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Genotypic Variation for Snow Mold Reaction among Creeping Bentgrass Clones

Z. Wanga, M. D. Caslerb,*, J. C. Stierc, J. S. Gregosc and S. M. Millettc

a Formerly Dep. of Agronomy, Univ. of Wisconsin-Madison, Madison, WI 53706
b USDA-ARS, U.S. Dairy Forage Res. Ctr., Madison, WI 53706-1108
c Formerly Dep. of Plant Pathology, Univ. of Wisconsin-Madison, Madison, WI 53706

* Corresponding author (mdcasler{at}wisc.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Creeping bentgrass (Agrostis stolonifera L.) is currently the most desirable grass for golf courses in temperate climates. In temperate climates, creeping bentgrass is highly susceptible to snow mold fungi which can cause significant injury and mortality. The objectives of this study were to survey a collection of creeping bentgrass clones for reaction to snow mold fungi (Typhula spp.), to identify clones with potential resistance to snow mold fungi, and to identify ecological factors related to necrotic reactions of creeping bentgrass clones to snow mold fungi. Three hundred sixty creeping bentgrass clones, selected from old golf courses in Wisconsin, were evaluated for necrosis reaction during incubation and recovery periods following inoculation with Typhula ishikariensis Imai or as noninoculated controls. Genotypic variation was observed for tolerance to snow mold and cold, dark (noninoculated) conditions but the two tolerances were uncorrelated with each other. Clones from fairways were more tolerant of snow mold, most likely due to long-term natural selection in the absence of fungicide applications. In a second experiment involving 72 selected clones, selection was successful in identifying divergent groups of clones, although differences between experiments indicated that future selection and breeding should make use of multiple inoculation runs. Resistance to snow mold in creeping bentgrass appears to be nonspecific with respect to race and species of snow mold isolates.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
CREEPING BENTGRASS is the most widely used temperate grass species on golf course putting greens. It is often used on fairways at relatively low mowing heights, but the adaptation of many other grasses to fairway management results in a wider array of species and a reduced frequency of creeping bentgrass on fairways compared with greens. Creeping bentgrass is highly self-incompatible and seeded populations are highly heterozygous and heterogeneous (Warnke et al., 1997). The genetic heterogeneity of creeping bentgrass populations results in development of distinct clonal populations following a few decades of selection, mortality, and adaptation to local stresses.

Snow molds, caused by at least two Typhula spp., are one of the most important factors limiting the use of creeping bentgrass on fairways and putting greens in northern North America (Hsiang et al., 1999). Snow molds can cause mortality in creeping bentgrass, resulting in stand decline (Hsiang et al., 1999) and invasion of less desirable species, such as Poa annua L. In wheat (Triticum aestivum L.), resistance or tolerance to snow mold fungi appears to be related to accumulation of soluble carbohydrates, particularly highly polymerized fructans (Gaudet et al., 2001; Mohammad et al., 1997). The most resistant cultivars accumulate relatively high levels of soluble carbohydrates and metabolize them relatively slowly during the winter (Kiyomoto and Bruehl, 1977; Yoshida et al., 1998). Depletion of carbohydrate reserves appears to predispose wheat plants to infection and colonization by snow mold fungi (Nakajima and Abe, 1994).

Fungicide applications can be utilized to control Typhula spp., but they are expensive, time-consuming to apply, have limited efficacy, and may have potential negative environmental impacts (Burpee et al., 1990; Cox, 1997; Hsiang et al., 1999). Host-plant resistance would be an economical and effective method of controlling snow mold on creeping bentgrass. Cultivar and clonal tests have revealed some genetic differences within creeping bentgrass for reaction to snow mold (Beard, 1966; Dahl, 1934; Landschoot and Bachman, 1997).

Golf courses that were established late in the 19th century or early in the 20th century were often planted with seed of South German Bentgrass (Duich, 1985). Creeping bentgrass typically made up the minority of this mixture, reported to be approximately 1% of the seed composition (Piper, 1918). These seed populations were obtained from old pastures in Austria, Hungary, and other parts of Europe (Duich, 1985). Because these populations represented natural variation with little or no selection history by humans, they likely contained large amounts of genetic variability, both within and among populations.

Frequent mowing prevents sexual reproduction of creeping bentgrass on a golf-course fairway or putting green. The prostrate and stoloniferous growth habit of creeping bentgrass allows individual plants to survive and persist in stressful environments without the need for sexual reproduction. Such an extreme environment is likely responsible for a large degree of selection pressure on heterogeneous creeping bentgrass populations. Only the plants with the greatest vegetative fitness and local biotic and abiotic stress tolerances will survive long term. Old putting greens dominated by creeping bentgrass typically have a patchy or mottled appearance, demonstrating phenotypic variability for patch size, color, leaf texture, and tiller density. This phenotypic variation often provides the basis for collection of creeping bentgrass germplasm from old golf courses. Natural variation present on old golf courses has been extremely useful as the foundation of several creeping bentgrass breeding programs (Engelke et al., 1995; Hurley et al., 1994; Robinson et al., 1991). Creeping bentgrass clones collected from old golf courses in Wisconsin show considerable spatial variability across the Wisconsin landscape (Casler et al., 2003). This variability is related to turf management (fairway vs. putting green) and to historical presettlement vegetation of the region (deciduous vs. mixed coniferous–deciduous forest).

The objectives of this study were to survey a collection of creeping bentgrass clones for reaction to snow mold fungi (Typhula spp.), to identify clones with potential resistance to snow mold fungi, and to identify ecological factors related to necrotic reactions of creeping bentgrass clones to snow mold fungi.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Host Germplasm
Over 700 clones of creeping bentgrass were collected from golf courses throughout Wisconsin in summer 1996, spring 1997, and summer 1997 (Table 1). Putting-green clones were collected during the summer, based on three principal criteria: clone size >30-cm diameter, absence of P. annua contamination, and fine texture. Fairway clones were collected in early spring, just after snowmelt, on the basis of absence of reaction to snow mold fungi [Typhula spp. and Microdochium nivale (Fries) Samuels & Hallett]. Fairway collections were made only on courses with reliable snow cover (generally persisting throughout the winter, which only occurs in the northern half of Wisconsin), without a history of fungicide applications for snow mold prevention, and at a time when snow-mold symptoms were severely evident. Golf courses were chosen largely on the basis of their age, with a minimum of 75 yr since establishment.


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  		Table 1. Names, locations, and abbreviations used for 23 Wisconsin golf courses that served as the sources of creeping bentgrass germplasm.

 
A subset of 360 clones were utilized for this study, consisting of those clones that propagated well in the greenhouse, providing vigorous and well-established transplants for the study. All clones were potted in the greenhouse from one tiller. Clones were maintained in the greenhouse by clipping, fertilization, and occasional repotting to maintain vigor and purity of each clone. Clones were replicated by splitting crowns.

Experiment 1
Inoculum
Isolate Z3.116 of Typhula ishikariensis Imai var. ishikariensis, originating from Trout Lake Golf Course near Woodruff, WI, was used for the experiment. This isolate belongs to Biological Species I of the T. ishikariensis complex (Millett et al., 2001). Five sclerotia were placed in a 100-mm Petri dish containing half-strength potato dextrose agar (PDA) amended with gentamycin sulfate (50 µg g–1) to suppress bacterial growth and incubated at 10°C for 1 to 2 wk. A single colony was selected, subcultured on full strength potato dextrose agar (PDA), and incubated at 10°C without light. After about 30 d, several 5-mm diameter agar discs with sclerotia or mycelia were cut and grown in potato dextrose broth (PDB) in glass flasks at 4°C to stimulate mycelium growth. After 35 d, the mycelium was harvested from four to eight 500-mL flasks, air-dried, pooled, vacuum-filtered through cheesecloth, and macerated in a blender for about 35 s. Dry matter of the mycelium was determined after drying a sample in a microwave oven. The mycelium was diluted in sterile, distilled water at 0.05 g DM mL–1. The inoculum suspension was used immediately after its preparation.

Hardening and Inoculation
Before hardening, all of the plants to be inoculated were raised in the greenhouse and clipped on a weekly basis to a 1-cm height. Each plant was grown in a 2.5- x 2.5- x 5-cm plastic pot containing a 1:1 mix of silt loam soil and peat moss. Plants were arranged in flats of 72 plants each. To begin the experiment, plants were transferred to a walk-in, controlled-environment chamber and hardened as follows: 15°C and 12-h daylength for 10 d; 10°C and 11-h daylength for 10 d; and 5°C and 10-h daylength for 20 d. Humidity was maintained at approximately 50% during the hardening period. The experimental design was a randomized complete block with six replicates. Replicates 2 and 5 were maintained as noninoculated controls. Replicates 1, 3, 4, and 6 were inoculated with T. ishikariensis isolate 3.116. All replicates were conducted during the same time period.

Sterile plastic pipettes with 0.5 cm of the tip removed were used to deliver 5 mL of inoculum solution to the leaves of each plant on 1 Aug. 1999. After inoculation, the plants were kept at 5°C and 95% humidity without light for 56 d. Necrotic symptoms, whitish-gray mycelium, and/or leaf necrosis, were first observed 15 d after inoculation. Necrosis was evaluated by a scale from 0 to 10, where 0 indicated no evidence of chlorosis or necrosis; 1 indicated approximately 10% of leaves were chlorotic or necrotic, and 10 indicated a plant that had completely necrotic or chlorotic leaves. Necrosis of each plant was scored 21 d after inoculation then at 7-d intervals for 42 d. Each flat was removed before rating and returned to the growth chamber after rating, minimizing light exposure inside the growth chamber. After the sixth rating, made on 26 September, all plants were transferred back to the greenhouse and allowed to recover at approximately 20 to 22°C and 16-h daylength, supplemented with high-pressure sodium-halide lights. Recovery ratings were made 7 and 14 d after transfer to the greenhouse, by the same necrosis scale previously described.

Statistical Analyses
Necrosis ratings at each rating time were analyzed by univariate analysis of variance using general linear models. Before statistical analysis, two linear regressions were computed for each experimental unit: the linear regression of necrosis rating on time for the six incubation times (Days 21 to 56) and the linear regression of necrosis rating on time for the three recovery times (Days 56 to 70, Day 56 counting as both the end of the incubation and the beginning of the recovery period). For each of the eight necrosis ratings, a new variable was created to provide a direct measure of disease reaction. Within each replicate, inoculated and noninoculated values were merged for each clone. The noninoculated value, measuring cold/dark tolerance, was subtracted from the mean of the two inoculated values (measuring tolerance to cold/dark conditions and to the pathogen), providing a direct measure of reaction to the pathogen.

Analyses of variance were computed for the 10 ratings using replicates, blocks, inoculation, turf type, golf courses, and clones as factors (Table 2). Inoculation and turf type were considered to have fixed effects and all other factors were considered to have random effects. A second set of analyses of variance were computed separately for each of the fixed effects (two inoculation classes in combination with two turf types). Variance components for golf courses and clones within golf courses were computed by equating mean squares to their expectations (Gaylor et al., 1970) and their significance levels were determined by the P value of the mean square associated with the variance component.


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  		Table 2. Analyses of variance for 10 variables measured on 322 creeping bentgrass clones incubated for 8 wk at 5°C and 24-h darkness, followed by 2 wk of recovery in the greenhouse.

 
A third set of analyses of variance were computed on the data from noninoculated plants, from inoculated plants, and from the differences. These analyses ignored the structure among the clones and treated both blocks and clones as random effects. Broad-sense heritability was computed from these analyses and the 95% confidence interval for heritability was computed according to Milliken and Johnson (1984). Phenotypic correlation coefficients were computed from the means of the 322 clones evaluated under inoculated and noninoculated conditions. Phenotypic correlation coefficients were tested using the Bonferroni-Holm sequential adjustment at a tablewise Type 1 Error rate of P = 0.01 (Rice, 1989). Autocorrelation structures of phenotypic correlation matrices were modeled by using mixed models analysis (Littel et al., 1996).

On the basis of the results of Exp. 1, 72 clones were selected for Exp. 2. Selection was based on the mean over Days 42, 49, and 56 of necrosis ratings measured on inoculated plants. Twenty-four clones per group were chosen from within three groups: high, medium, and low (mean necrosis reactions of 8.2 ± 0.5, 5.8 ± 0.1, and 3.5 ± 0.3, respectively).

Experiment 2
Inoculum and Environment
Experiment 2 was initiated with new clonal material, held in reserve during Exp. 1. Two isolates of T. ishikariensis, Z3.116 and Z1.83, and one isolate of T. incarnata Fr., Z1.41, were used in this study. Isolate Z1.83 was collected at Christmas Mountain Golf Course near Lake Delton, WI, is classified as T. ishikariensis Imai var. canadensis Smith & Arsvoll, and belongs to Biological Species II of the T. ishikariensis complex (Millett et al., 2001). Isolate Z1.41 was collected from Meadow Springs Golf Club near Jefferson, WI.

The experimental design was a randomized complete block with three replicates and a split-plot randomization restriction. Isolates (two strains of T. ishikariensis, one strain of T. incarnata Fr., and the noninoculated control) were whole plots and the 72 clones were the subplots. Each whole-plot comprised one flat of 72 plants. Plants were inoculated on 17 May 2000. Hardening, inoculation, incubation, recovery, and data collection were conducted as described for Exp. 1, with one exception. Plants were transferred to the greenhouse on 12 July and allowed to recover under ambient light conditions (approximately 16-h daylength). Experiment 2 was repeated in a second run, inoculated on 21 Aug. 2000, transferred to the greenhouse for recovery on 16 October (with supplemental light for a 16-h daylength), and all conditions and procedures as described for Exp. 1.

Statistical Analyses
Differences between necrosis ratings on inoculated and noninoculated plants within each block of each run were computed as described for Exp. 1. Analyses of variance were computed for the 10 ratings using runs, blocks, isolates, reaction groups (defined in Exp. 1), and clones as factors. Isolates and reaction groups were considered to have fixed effects and all other factors were considered to have random effects. A second set of analyses of variance were computed for the raw data of inoculated plants, the raw data of noninoculated plants, and the differences between inoculated and noninoculated plants. Means of the three clone groups defined in Exp. 1 were compared by contrasts: high necrosis rating vs. low necrosis rating and medium necrosis rating vs. the mean of high and low.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Experiment 1
Genotypic variation for necrosis rating was observed among creeping bentgrass clones at all 10 rating times in Exp. 1 (Table 2). In addition, there was genotypic variation among clones for the linear regressions of necrosis rating on incubation time and recovery time (Slope-I and Slope-R). The two turf types, fairway and green, differed in mean necrosis rating for eight of the 10 rating times and for the linear regression of necrosis rating on incubation time. The inoculation x turf type interaction was significant for nine of the 10 rating times and for both slope variables, indicating that differences between fairway and putting-green clones were not consistent for inoculated and noninoculated experimental units. The inoculation x turf type interaction increased in magnitude with incubation time and decreased in magnitude with recovery time indicating that, as snow mold symptoms became more severe this interaction was more severe, lessening again as plants recovered from the disease.

Under noninoculated conditions, fairway clones had slightly more necrosis than putting-green clones, but the changes in mean necrosis ratings over incubation times and recovery times were not different between the two turf types, averaged over all clones (Fig. 1 ; Table 3). Fairway clones averaged 9.3 to 20.7% more necrosis during incubation and 2.7 to 7.3% more necrosis during recovery than putting-green clones. Noninoculated plants appeared to be under stress from persistent cold and dark conditions during the incubation period, as indicated by the significant decrease in necrosis ratings during the recovery period.


Figure 1
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  		Fig. 1. Plot and regressions of mean necrosis ratings as a function of time after inoculation for 322 creeping bentgrass clones. Open symbols represent inoculated plants and closed symbols represent noninoculated plants. Squares and solid lines represent the mean of all fairway clones; circles and dashed lines represent the mean of all putting green clones. Times up to and including Week 8 represent incubation conditions (5°C and 24-h darkness); Weeks 9 and 10 represent the recovery period in the greenhouse. Linear regression coefficients and significance levels are shown in Table 3.

 

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  		Table 3. Mean linear regression coefficients for the regressions of necrosis rating on incubation or recovery time for several groups of creeping bentgrass clones inoculated or not inoculated with Typhula ishikariensis.

 
Under inoculated conditions, mean necrosis ratings of putting-green clones increased dramatically relative to that of fairway clones under incubation (Fig. 1, Table 3). Rates of recovery for fairway and putting-green clones were similar. Because the changes in mean necrosis rating were similar for fairway and putting-green clones under noninoculated conditions, the differences between fairway and putting-green clones, for both means and slopes, suggest greater tolerance to snow mold infection for the fairway clones. Furthermore, differences in slopes between inoculated and noninoculated plants were greater for putting-green clones compared to fairway clones, also indicating that putting-green clones were more sensitive to the effects of inoculation compared to fairway clones. This was reflected by a significant inoculation x turf type interaction for the rate of necrosis response to incubation and recovery times (Table 2).

On an individual basis, 22 of the 24 most snow-mold tolerant clones (defined as the lowest differences between inoculated and noninoculated necrosis ratings for Days 42–56) were fairway clones. These 22 clones originated from six golf courses located across the northern half of Wisconsin (BSCC, LFGC, MCC, MeCC, RMGC, and SCVGC). Although there were differences among golf courses in mean necrosis reactions, snow-mold tolerant clones can be found at many sites that possess the proper conditions for natural selection. Two clones among these 24 clones originated on putting greens at two of the northern golf courses, despite regular fungicide applications to protect these greens from snow mold damage.

Both fairway and putting-green clones were collected from environments containing visually heterogeneous populations of creeping bentgrass that had a minimum 75-yr historical presence. Apart from the obvious difference in mowing height between the two turf types, putting greens had a long history of protection from snow mold infection by routine application of snow-mold inhibiting fungicides, largely eliminating natural selection for snow mold tolerance as a possible factor causing changes in putting-green populations of creeping bentgrass. Conversely, fairways had a long history of snow mold infection, creating the potential for natural selection among creeping bentgrass clones for tolerance to snow mold infection. Natural selection would be largely manifested by mortality or loss of vigor for snow-mold-tolerant clones. These results suggest that natural selection was a likely cause for the differences in snow mold reaction between fairway and putting-green clones. However, other forces, such as differential gene migration into fairways and putting greens through overseeding or contamination, differential mutation rates between turf types, and the differential mowing height as a potential selective factor cannot be ruled out.

Broad-sense heritability of necrosis reaction under cold and dark (noninoculated) conditions was moderate and fairly constant across incubation and recovery times (Table 4). These values indicted that tolerance to cold and dark conditions is under some degree of genetic control in creeping bentgrass. Broad-sense heritability for necrosis of inoculated plants averaged 26% higher than for noninoculated plants. This increase in heritability was likely due to an expansion of the phenotypic scale due to the combination of multiple stresses imposed on inoculated plants. The difference between inoculated and noninoculated plants, a measure of necrosis reaction to snow mold inoculation per se, increased in heritability from near zero at 3 wk post-inoculation to a maximum value at the end of the recovery period. The repeatability of difference values for these clones confirms the presence of genotypic variation for snow mold reaction among these clones. Genotypic variation for snow mold reaction per se could not be detected during the early stages of incubation, because snow mold symptoms were insufficient to cause differential reactions between inoculated and noninoculated plants. Most necrosis symptoms at this stage of incubation were a result of cold and dark conditions. As snow mold symptoms became more severe, necrosis ratings of inoculated vs. noninoculated plants diverged—more so for intolerant clones, less so for tolerant clones—resulting in a heritable trait for the latter states of inoculation and for the recovery period. Broad-sense heritability estimates were similar to those observed for freezing and snow mold tolerance in orchardgrass, Dactylis glomerata L. (Tronsmo, 1993).


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  		Table 4. P values and broad-sense heritability estimates, including 95% confidence intervals, for 322 creeping bentgrass clones evaluated for necrosis reaction with or without inoculation by T. ishikariensis Z3.116.

 
These results are supported by phenotypic correlation analysis (Table 5). The phenotypic correlation between inoculated and noninoculated plant means declined throughout the incubation and recovery periods (slope = –0.092 ± 0.007 wk–1), reflecting the increase in snow mold symptoms and an increasing phenotypic divergence in necrosis ratings caused by the biotic vs. abiotic stresses. This result was reflected in the lack of correlation between necrosis rating of noninoculated plants (tolerance to cold and dark conditions) and the difference between inoculated and noninoculated plants (snow mold tolerance). Field results on perennial grasses show mixed results regarding a correlation between snow mold tolerance and freezing tolerance, with some studies showing a positive relationship (Abe and Matsumoto, 1981; Tronsmo, 1984; 1993) and other results showing little or no relationship (Tronsmo, 1985). In the latter paper, the author suggests that snow mold tolerance and freezing tolerance are conditioned by different mechanisms.


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  		Table 5. Phenotypic correlation coefficients among 10 variables measured on noninoculated plants (above the diagonal), inoculated plants (below the diagonal), or for inoculated vs. noninoculated plants (underlined values on the diagonal) for 322 creeping bentgrass clones.

 
The correlation structure for both noninoculated and inoculated plants followed a typical first-order autoregressive structure, with a linear decline in the phenotypic correlation coefficient of –0.071 ± 0.016 wk–1 for noninoculated plants and –0.158 ± 0.011 wk–1 for inoculated plants (Table 5). The greater decline for inoculated plants was an additional indication of the increased manifestation of necrosis symptoms resulting from a combination of both abiotic and biotic stresses, causing differential expression of phenotypes among clones, because of the combined effects of tolerances to cold-dark conditions and the snow mold fungus. These results support separate mechanisms for tolerance to snow mold fungi vs. cold and dark conditions in creeping bentgrass.

Experiment 2
Analysis of variance reveled no significant interactions of bentgrass clones with runs or Typhula isolates. Ranking and differences among clones were consistent across both runs and all three fungal isolates. Therefore, all results are presented as means over both runs and three isolates of Typhula. Good repeatability between runs was also observed by Jönsson and Nilsson (1985). The lack of clone x isolate interaction is supported by results from numerous other studies on both perennial grasses (Abe and Matsumoto, 1981; Tronsmo, 1992; Wu and Hsiang, 1998) and wheat (Bruehl, 1982). These results suggest that snow mold tolerance seems to be non-race-specific and non-species-specific in grasses, including creeping bentgrass. This nonspecificity appears to also transcend boundaries of fungal genera, with a strong positive correlation between tolerance to T. ishikariensis and M. nivale observed in two perennial grasses (Tronsmo, 1992).

Divergent selection for necrosis rating of inoculated plants had little effect on necrosis of noninoculated plants (Fig. 2A) . Significant differences between high and low group means occurred only on the two recovery dates. Conversely, divergent selection resulted in significant differences between high and low group means for all but the first rating time for necrosis rating of inoculated plants (Fig. 2B). Divergent selection resulted in significant differences for most rating times for differences in necrosis rating between inoculated and noninoculated plants (Fig. 2C). The medium reaction group was intermediate to the high and low groups for necrosis rating of inoculated plants and for differences observed at most rating times (Fig. 2B and C). Despite the significance levels, differences among group means were very small compared to the results of Exp. 1. For necrosis ratings taken on Days 42 to 56 of inoculated plants, the selection criterion, the range among group means was only 11% as large as the range among group means for Exp. 1.


Figure 2
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  		Fig. 2. Means of three groups of creeping bentgrass clones selected for high, medium, or low necrosis reaction following inoculation with Typhula in Exp. 1: A. Necrosis reaction without inoculation in Exp. 2; B. Necrosis reaction following inoculation in Exp. 2, expressed as means over three Typhula isolates; and C. Difference in necrosis reaction between inoculated plants and noninoculated plants in Exp. 2. Times up to and including Week 8 represent incubation conditions (5°C and 24-h darkness); Weeks 9 and 10 represent the recovery period in the greenhouse. Numerical values inside the figure are P values for the contrast of high vs. low group means for each time. Means were computed over two runs, three replicates, and 24 clones per group.

 
While the dramatic reduction in mean differences among groups suggests a repeatability problem, the relatively consistent P values and ranking of high, medium, and low group means indicates that clonal selection based on a single experiment can successfully identify differences in snow mold reaction among creeping bentgrass clones. While there would be considerable expense associated with multiple runs, a mean over multiple runs would be a better selection criterion, increasing the repeatability of this selection criterion. As an example, by the same selection criterion as applied to the clones evaluated in Exp. 1, but on the basis of weighted means over Exp. 1 and 2, the six clones with the highest or lowest mean necrosis ratings were selected (Table 6). The greatest difference was for the mean of inoculated plants, the selection criterion, for which the high group had a mean 38% higher than the low group. For differences between inoculated and noninoculated plants, the high group had a mean more than double that of the low group. There was no observed difference between groups for necrosis of noninoculated plants, mimicking the original selection results for Exp. 2 on the basis of selection from Exp. 1. These results are supported by studies on other perennial grasses in which selection for snow mold tolerance was successful (Jönsson and Nilsson, 1985, 1992; Posselt and Altpeter, 1994).


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  		Table 6. Mean necrosis ratings for the six creeping bentgrass clones with the highest or lowest mean necrosis measured on inoculated plants and averaged across all isolates and both experiments for Days 42–56 after inoculation.

 
As expected, the largest and most consistent differences between selection groups were for the means of inoculated plants, which was the selection criterion. The fairly consistent selection effects for differences between inoculated and noninoculated plants, regardless whether selection was based on Exp. 1 or the mean of both experiments, suggested that genotypic differences in resistance to Typhula were the major reason for the overall selection response. Selection appeared to have little effect on tolerance to cold and dark conditions, but appeared to have a positive effect on tolerance to snow mold fungi. The lack of isolate x group and isolate x clone interactions indicated that these responses were consistent across all isolates, suggesting that snow mold resistance in these clones is a generalized form of resistance, most likely non-race-specific. Lack of race or species specificity will simplify the process of selection and breeding Typhula-resistant cultivars of creeping bentgrass, requiring screening with only a single isolate of one species of Typhula. However, if selection is to be conducted in growth chamber conditions, it should be based on means over multiple runs. It does not appear necessary to include noninoculated controls, as resistance to Typhula does not appear to be correlated with tolerance to cold and dark conditions.


   ACKNOWLEDGMENTS
 
We thank the superintendents, managers, and/or owners of the golf courses listed in Table 1 for their support and kind permission to collect grasses on their golf course.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
This research was supported by the College of Agric. and Life Sci. and Hatch formula funds.

Received for publication April 1, 2004.


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


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