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TURFGRASS SCIENCE

Dollar Spot Severity, Tissue Nitrogen, and Soil Microbial Activity in Bentgrass as Influenced by Nitrogen Source

Dep. of Natural Resource Sciences and Landscape Architecture, Univ. of Maryland, College Park, MD 20742

* Corresponding author (pd9{at}umail.umd.edu)

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
There is increasing interest in the use of bio-organic nitrogen (N) sources to suppress turfgrass diseases. The objectives of this field study were to evaluate nine N-sources for their effects on dollar spot (caused by Sclerotinia homoeocarpa F. T. Bennett) severity; to elucidate the relationship among N-sources, tissue N, soil microbial activity, and the severity of dollar spot; and to evaluate N-source effects on thatch and soil organic matter levels, and plant parasitic nematodes. Nitrogen sources included urea, sulfur-coated urea (SCU), Milorganite (Milwaukee Metropolitan Sewerage District, Milwaukee, WI), Sustane Medium (Sustane Corporation, Chaska, MN), Earthgro 1881 Select (Earthgro Inc., Lebanon, CT), Earthgro Dehydrated Manure, Ringer Lawn Restore (Ringer LR, Ringer Corporation, Minneapolis, MN), Com-Pro (Blue Plains Sanitation Commission, Silver Spring, MD), and Scotts All Natural Turf Builder (Scotts ANTB, Scotts Company, Marysville, OH). The N was applied annually at a rate of 200 kg N ha^-1 yr^-1 between 1994 and 2000 to a stand of fairway height ‘Southshore’ creeping bentgrass (Agrostis stolonifera L.). Data were collected between 1998 and 2000. Ringer Lawn Restore (all 3 yr) and urea (1999 and 2000) delayed dollar spot to within an acceptable threshold from May to early or mid-June, when disease pressure was low to moderately severe. Com-Pro (all 3 yr), however, enhanced dollar spot. No N-source reduced dollar spot when data were averaged over-the-season. No N-source was consistently associated with higher levels of soil microbial activity. There was a weak negative correlation between soil microbial activity and dollar spot on one date in 2000, when disease pressure was low. When disease pressure was moderately severe, there was a strong negative correlation between the amount of foliar N and dollar spot severity in June 1999, but no correlation was found in 2000. Only Sustane Medium-treated plots had higher organic matter levels in the upper 2.5 cm of soil on all rating dates, but there was no increase in soil organic matter in the 2.6 to 5.0 cm soil zone with any N-source.

Abbreviations: Earthgro 1881 Select (Earthgro S) • Earthgro Dehydrated Manure (Earthgro DM) • Ringer Lawn Restore (Ringer LR) • Scotts All Natural Turf Builder (Scotts ANTB) • Sustane Medium (Sustane) • dollar spot (DS) • nitrogen (N)

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
THERE IS INCREASING INTEREST in the use of natural or bio-organic nitrogen (N) sources and soil amendments for use on turfgrasses. Several natural fertilizers are good N sources and their use has been linked to suppressing turfgrass diseases. The mechanism for disease suppression by natural organic fertilizers may be due in part to their effect on increasing soil microbial activity. Increased microbial activity in soil presumably diminishes the activity of plant pathogens by antagonizing, parasitizing, or competing with pathogens.

The severity of dollar spot, Pythium damping-off and root rot (caused by Pythium graminicola Subramanian), and necrotic ring spot (caused by Ophiosphaerella korrae J.C. Walker & A.M. Sm.) have been reduced following applications of various organic fertilizers and composts (Craft and Nelson, 1996; Harman, 1991; Landschoot and McNitt, 1997; Liu et al., 1995; Markland et al., 1969; Melvin and Vargas, 1994; Nelson and Craft, 1991a,b). Several researchers reported significant reductions in dollar spot severity following applications of Milorganite, an activated sewage sludge (Cook et al., 1964; Landschoot and McNitt, 1997; Markland et al., 1969). Sand topdressing amended with Ringer Compost Plus, Ringer Green Restore, and Sustane as well as selected composts prepared from turkey litter and sewage sludge and non-composted blends of plant and animal meals also were reported to suppress dollar spot (Nelson and Craft, 1991b; Nelson and Craft, 1992).

Liu et al. (1995) evaluated alginate (Norwegian kelp meal), ammonium nitrate, Milorganite, Ringer Lawn Restore, Ringer Greens Super, Ringer Turf Restore, Sandaid (granular sea plant meal, Emerald Isle Ltd., Ann Arbor, MI), sewage sludge, and sulfur coated urea for management of dollar spot in creeping bentgrass. They reported that applications of Ringer fertilizers, ammonium nitrate, and sulfur-coated urea gave rise to significantly higher populations of microorganisms on turfgrass leaves and in the thatch and soil. They also reported that higher microbial populations associated with the use of certain organic fertilizer treatments may have been related to dollar spot suppression. The researchers, however, did not apply the amendments and fertilizers at a uniform rate (50–260 kg N ha^-1). Consequently, the level of dollar spot suppression observed might have been the result of increased rates of N applied to the turf, which could have allowed the turf to outgrow or recover more rapidly from the disease.

Landschoot and McNitt (1997) compared five natural organic fertilizers, urea, and ureaform for their effects on dollar spot suppression in creeping bentgrass. Their results showed that urea provided equal or better dollar spot suppression than the natural organic fertilizers. They also reported that on the majority of rating dates, dollar spot severity decreased as turf color improved, suggesting that as N-availability increased, disease severity decreased.

General suppression of a pathogen is directly related to the total amount of microbial activity in the soil or on the plant at a critical time in the life cycle of the pathogen (Cook and Baker, 1983). Essentially, no one microorganism or specific group of microorganisms is responsible for general suppression to occur. The inability of microbiologists to isolate and culture all of the species present in soil is the greatest obstacle for analyzing microbial communities. In fact, only 1 to 10% of all microbes in soil can be cultured on artificial media (Alexander, 1998). To avoid this difficulty, some assays have been developed that do not require isolation of specific microorganisms. For example, general levels of microbial activity can be determined in soil by measuring the rate of fluorescein diacetate (FDA) hydrolysis (Boehm and Hoitink, 1992; Craft and Nelson, 1996; Kim et al., 1997; and Schnrer and Rosswall, 1982). Fluorescein diacetate is hydrolyzed by numerous enzymes, such as proteases, lipases, and esterases. The product of this enzymatic conversion is fluorescein, which can be quantified by spectrophotometry (Schnrer and Rosswall, 1982).

Craft and Nelson (1996) used the FDA technique to determine general levels of microbial activity in composts. They tested different batches of brewery sludge; a biosolids compost; and chicken, horse, and turkey manure. They then determined the relationship between levels of microbial activity in these composts and the suppression of Pythium damping-off of creeping bentgrass. Their results indicated that as the rate of FDA hydrolysis increased, the levels of Pythium damping-off decreased.

Few studies have been designed to take into account the suppressive effects of N and microorganisms both together and independently (Landschoot and McNitt, 1997). For example, Nelson and Craft (1992) reported that dollar spot suppression in turf receiving certain composts was due to their effect on increasing microbial activity. Nelson and Craft (1992), however, did not take into account the relationship between dollar spot and the turf response to N. Landschoot and McNitt (1997) suggested that improved turf growth in response to N, as measured by turf color, could have reduced dollar spot severity. They did not, however, assess the possible effects of the fertilizers on soil microbial activity. Results from Liu et al. (1995) suggested that higher microbial populations associated with certain organic and inorganic fertilizer treatments may have been related to dollar spot suppression. Liu et al. (1995), however, did not apply the amendments and fertilizers at a uniform N rate. Consequently, the dollar spot suppression observed also may have been the result of increased rates of N. The lack of information taking into account the suppressive effects of N and microorganisms both together and independently on dollar spot suppression indicates a need for further research.

The primary objectives of this study were to evaluate nine N-sources and composts for their effects on dollar spot severity and turf quality. Other study objectives were as follows: (i) to elucidate the relationship among the N-sources and the amount of N in foliar tissue, soil microbial activity, and the severity of dollar spot, and (ii) to evaluate the N-sources for their impact on turf quality, thatch production, soil organic matter levels, and plant parasitic nematode population densities.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
An established stand of Southshore creeping bentgrass at the University of Maryland Cherry Hill Turfgrass Research Facility in Silver Spring was used as the study site. Soil was a Sassafras sandy loam (fine-loamy, siliceous mesic Typic Hapludult) with a pH of 6.5 and 28 mg of organic matter g^-1 soil. Soil phosphorus (P) and potassium (K) levels were 145 and 112 kg ha^-1, respectively. The N sources evaluated were urea (46-0-0), sulfur-coated urea (37-0-0), Milorganite (6-2-0), Sustane Medium (5-2-4), Earthgro 1881 Select (8-2-4), Earthgro Dehydrated Manure (2-2-2), Ringer Lawn Restore (10-2-6), Com-Pro (1-2-0), and Scotts All Natural Turf Builder (11-2-4). Urea and sulfur-coated urea (SCU) are synthetic organic N fertilizers. Milorganite is an activated sewage sludge and Com-Pro is composted sewage sludge. Sustane Medium, Earthgro 1881 Select, Earthgro Dehydrated Manure, Ringer Lawn Restore, and Scotts All Natural Turf Builder are composed primarily of poultry waste materials. The N-sources were applied annually from 1994 to 2000 at a rate of 50 kg N ha^-1 in October, November, December, and May with a shaker jar. Plots were irrigated immediately after each application. Plots measured 1.5 by 2.1 m, and were arranged in a completely randomized design with three replications. Plots were not inoculated and dollar spot (DS) developed naturally and uniformly across the study site. The Southshore was mowed to a height of 15 mm at least twice weekly with a reel mower and the clippings were removed. The turf was irrigated only to prevent wilt and was aerified in September 1995. In September 1999, the study site again was core-aerated to a depth of about 6.0 cm and the cores were removed prior to fertilizer applications. Core aerification was performed because the turf had become fluffy and was scalped repeatedly in 1999. No fungicides were applied during the data collection periods so that the N-sources could be evaluated for disease suppression. Chlorothalonil (tetrachloroisophthalonitrile; 10 kg ai ha^-1) or propiconazole (1-[[2-(2,4-dichlorophenyl)-4-propyl-1,3-dioxolan-2-yl)methyl]-14-1,2,4-triazole; 0.38 kg ai ha^-1) were applied to stop DS epidemics and assist in the recovery of the turf after data were collected in each year.

Dollar Spot Severity, Turf Quality, Thatch Depth, and Organic Matter
The plots were evaluated weekly for disease by counting the number of S. homoeocarpa infection centers plot^-1 from 1998 to 2000. A typical golf course fairway threshold for DS severity was judged to be 10 infection centers plot^-1 (3.3 m²), at which time a golf course superintendent would likely apply a fungicide.

Dollar spot data were used to calculate the relative area under the disease progress curve (AUDPC) as described by Lawton and Burpee (1990). The AUDPC values, expressed as percent disease x day, were calculated with the following formula: å[(y_i + y_i+1)/2][t_i+1], where i = 1,2,3Yn - 1, y_i is the number of infection centers; and t_i is the time of the ith rating (Campbell and Madden, 1990). The AUDPC data then were standardized by dividing the AUDPC value by the total time duration (t_n - t₁) of the epidemic (Fry, 1977).

The plots were evaluated weekly for overall turf quality (i.e., color and density) by a visual rating scale where 0 = entire plot area brown or dead, 8 = minimum acceptable quality for a golf course fairway, and 10 = optimum greenness and density. Thatch depth was evaluated by taking five (2.5-cm diam and 8 cm deep) soil cores from each plot, and the uncompressed thatch depth of each core was measured with a ruler. No significant thatch differences were measured in 1998 or 1999. In 2000, the sample size was increased to nine soil cores. Organic matter was determined by removing 15 soil cores (2.5-cm diam and 8 cm deep) from each plot on 3 Nov. 1998, and on 1 March and 28 June 2000. The foliage and thatch were removed, and the amount of organic matter in the 0- to 2.5-cm and 2.6- to 5.0-cm zones was determined by the organic matter loss on ignition process (Storer, 1984). Soil samples were dried for 1 h at 125°C and then weighed. Samples then were combusted for 2 h at 360°C and reweighed. Percent organic matter was determined by dividing the soil weight after the first drying period by the soil weight after combustion and multiplying by 100. The average thatch depth and amount of organic matter (expressed as mg g^-1 soil) for all cores from each plot were used in the statistical analysis. All data were subjected to analysis of variance performed on Statistical Analysis Software (SAS Institute, Cary, NC, 1995). Data were analyzed by the PROC MIXED procedure in SAS and significantly different means were separated using the least significant difference (LSD) t-test at P = 0.05.

Leaf Tissue N
Leaf tissue was sampled every 2 wk from 21 May to 3 September of 1999 and from 11 May to 23 June 2000 for N content. Fifteen grams of disease-free clippings were harvested from each plot by means of scissors. Clippings were dried, ground, and then analyzed to determine the amount of N per gram of dry weight of tissue (expressed as mg g^-1 dry weight). Approximately 0.2 g of leaf tissue were placed into a tin capsule and analyzed with a Carbon-Hydrogen-Nitrogen Determinator, Model CHN-600 (Leco Corp., St. Joseph, MI). The sample was combusted at 360C and the percent N was measured by thermal conductivity (Campbell, 1992). Tissue N data were statistically analyzed as previously described. Tissue N data also were subjected to correlation analysis performed on Statistical Analysis Software (SAS Institute, Cary, NC, 1995) using the PROC CORR procedure. The later analysis was used to determine if there were correlations between percent foliar N and DS severity.

General Soil Microbial Activity
Plots were sampled every 2 wk between 11 June and 8 Sep. 1999 and between 11 May and 24 June 2000 to determine the general level of soil microbial activity. Samples were collected randomly by removing five soil cores (2.5-cm diam. by 8 cm deep) from each plot. The samples were immediately taken to the lab and processed. The foliage and thatch layer were removed with a knife. Five g of soil were taken from the first 2.5 cm of soil below the thatch from each core and placed into a sterile milk bottle (160 mL). By the procedure described by Schnrer and Rosswall (1982), 20 mL of sodium phosphate buffer and 400 µg of fluorescein diacetate (FDA) were added to each bottle. The bottles then were incubated on a rotary shaker at 90 rpm for 1 h. After 1 h, 20 mL of acetone were added to each bottle to stop FDA hydrolysis. Soil residues were removed from the mixture by filtering the suspension through filter paper (Whatman No. 1). The filtrate was collected in a test tube and the concentration of fluorescein (µg of FDA hydrolyzed min^-1 g^-1 dry weight of soil) was determined spectrophotometrically (Milton Roy Spectronic 21 Spectrophotometer, Milton Roy Company, Rochester, NY). To compensate for background absorbance from soluble components in each sample, absorbance blanks consisting of 5 g of soil from each treatment plus buffer and no fluorescein were used.

Absorbencies (500 nm) were compared against a standard curve. Standard curves were prepared for each treatment on each harvest date by the method described by Schnrer and Rosswall (1982). Aliquots of 0, 100, 200, 300, and 400 µg of FDA were added to screw cap tubes with 5 mL of phosphate buffer. Tubes were capped tightly and incubated for 60 min in boiling water to hydrolyze the FDA. Upon cooling, the fluorescein and buffer in each test tube were added to a sterile milk bottle (160 mL) containing 5 g of soil from each treatment and an additional 15 mL of phosphate buffer. Bottles were incubated on a rotary shaker at 90 rpm for 45 min, after which time 20 mL of acetone were added. The samples then were filtered and FDA hydrolysis was determined by the procedure described above. Standard curves for each treatment were generated by regression analysis. Finally, the equation for each curve was used to determine the amount of fluorescein hydrolyzed in each sample. Fluorescein diacetate data were analyzed as previously described. Correlation analysis, as described previously, was used to determine if there were a relationship between general microbial activity and DS severity.

Plant Parasitic Nematode Population Densities
Plots were sampled in June (1999 and 2000) and September (1999) to quantify plant parasitic nematode population densities. Samples were collected randomly by removing 15 soil cores (2.5-cm diam by 5.0 cm deep) from each plot. A 250 cm³ soil sample from each plot was used to determine the population densities of the following nematodes: Pratylenchus penetrans Cobb (lesion), Meloidogyne sp. (root-knot), Tylenchorhynchus claytoni Steiner and T. maximus Allen (stunt), Helicotylenchus pseudorobustus Cobb (spiral), Hoplolaimus galeatus Cobb (lance), Xphinema americanum Cobb (dagger). Nematodes were extracted from soil by the modified Baermann funnel technique (Christie and Perry, 1951). Nematode adults and juveniles were counted and their species identified at x40 magnification by S. Sardanelli (Nematode Testing Laboratory, University of Maryland, College Park, MD). Data were analyzed as described previously.

	RESULTS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
Dollar Spot Severity
1998.
Unfertilized plots received no N between 1994 and 2000. As a result, turf density was poor and DS levels were generally low. Hence, DS data from unfertilized plots were somewhat irrelevant, particularly since most golf course fairways are fertilized. Dollar spot was active throughout May and June 1998. Except for Com-Pro, which intensified disease, there were no significant differences in the number of S. homoeocarpa infection centers among treatments between 5 May and 30 June (Table 1, all data not shown). There were higher DS levels in Com-Pro- treated plots, when compared with nearly all other treatments on all rating dates. Considering a golf course threshold level of ten infection centers 3.3 m^-2, there were certain fertilizers on some dates that reduced dollar spot to within acceptable limits. For example, plots treated with Ringer LR did not reach the threshold level until after 9 June, whereas the Com-Pro-treated plots already had reached the threshold level by 5 May. All other fertilizer-treated plots reached the threshold by 26 May. Com-Pro-treated plots had a higher AUDPC value, when compared with all other fertilizer treatments in 1998 (Table 1). Lowest AUDPC values were associated with Ringer LR, but values among all treatments except Com-Pro were similar statistically.

2000.
Plots treated with Com-Pro had the highest number of S. homoeocarpa infection centers between 25 May and 23 June (Table 2, all data not shown) Plots treated with SCU and Ringer LR had the fewest number of infection centers on most dates, but data did not vary significantly from most other treatments. Plots treated with urea, SCU, Sustane, and Ringer LR did not exceed the threshold level until after 8 June, whereas, the Com-Pro-treated plots had reached the threshold level by 25 May. All other fertilizer-treated plots exceeded the threshold by 8 June. The AUDPC value was highest for Com-Pro-treated plots. Lowest AUDPC values were observed in plots fertilized with SCU, Sustane and Ringer LR. Except for plots treated with Earthgro DM and Com-Pro, however, AUDPC data for SCU, Sustane and Ringer LR did not vary significantly from the other treatments including the unfertilized control.

2000.
A spring drought injured the turf and quality ratings were low between 5 and 18 May. The turf recovered by late May and plots receiving urea, SCU, Sustane, Earthgro S, Ringer LR, and Scotts ANTB exhibited acceptable turf quality (i.e., >8.0) between 25 May and 8 June (Table 4). Following an N application on 29 May, turf quality improved by 2 June in all plots except those treated with Com-Pro. Poorest turf quality generally was observed in unfertilized, Com-Pro, and Earthgro DM-treated plots on most dates. By 23 June, all plots had unacceptable quality; however, relatively good turf quality (i.e., >7.0) was observed in plots fertilized with SCU, Sustane and Ringer LR. All N-sources generally provided improved turf quality, when compared with unfertilized, Com-Pro, and Earthgro DM-treated plots.

All treatments increased thatch when compared with unfertilized plots (Table 5). Highest thatch levels were detected in Com-Pro-treated plots and lowest levels were observed in plots fertilized with Scotts ANTB and Earthgro S. All other treatments had similar thatch levels.

Tissue N
1999.
Tissue N was quantified between 21 May to 3 September (Table 6, all data not shown). There were significant tissue N differences among treatments in samples taken on 21 May, 4 and 18 June, and 8 July, but not after 8 July. Foliage in plots treated with urea, SCU, Earthgro S, and Ringer LR generally contained higher N between 21 May and 8 July, but tissue N levels were not always greater than that from plots fertilized with Scotts ANTB or Milorganite. Plots fertilized with Sustane generally had tissue N levels higher than plots fertilized with Com-Pro, Earthgro DM, and the unfertilized control. On 21 May and 4 June, higher tissue N levels were observed in all plots receiving a fertilizer, when compared with Earthgro DM and unfertilized plots. Higher tissue N levels generally were observed among all fertilizer treatments, when compared with Earthgro DM, Com-Pro, and the unfertilized plots on 18 June and 8 July.

2000.
Tissue N was quantified between 11 May (prior to DS appearance) to 23 June (Table 6). There were significant tissue N differences among treatments on all rating dates. Foliage from plots treated with urea, SCU, Earthgro S, and Milorganite contained higher N than unfertilized turf on three of four rating dates, but tissue N levels were not always greater than that from plots fertilized with Sustane, Ringer LR, or Scotts ANTB. Tissue N levels generally were lower in Earthgro DM, Com-Pro and unfertilized plots, but these levels often were equivalent to most other treatments. No significant correlations between the number of S. homoeocarpa infection centers and tissue N were observed on any date.

General Soil Microbial Activity
1999.
No N-source was consistently associated with higher levels of soil microbial activity, when compared with unfertilized turf (Table 7). On 11 June, all treatments except Ringer LR and Scotts ANTB-treated plots had significantly higher levels of soil microbial activity, when compared with unfertilized plots. The highest level of microbial activity on 28 June occurred in the unfertilized plots, although levels were not significantly higher than plots receiving Ringer LR or Com-Pro. On 15 July, only Earthgro DM and Scotts ANTB-treated plots had higher levels of microbial activity, when compared with unfertilized turf. Plots receiving urea, Sustane, Earthgro S, and Scotts ANTB had higher levels of microbial activity, when compared with unfertilized turf on 28 July. By 8 September, SCU, Sustane, and Earthgro S-treated plots had higher levels of soil microbial activity, when compared with unfertilized turf. There was no significant correlation between the number of S. homoeocarpa infection centers and soil microbial activity.

Plant Parasitic Nematode Population Densities
1999.
Plant parasitic nematode populations were quantified on 18 June and 7 Sep. 1999. There was no significant interaction between nematode species and fertilizer treatment on either rating date and the data were combined for the analysis. On 7 September, plots treated with Com-Pro had plant parasitic nematode population densities higher than those recovered from the unfertilized plots (Table 5). Plant parasitic nematode population densities were lower in plots that received Sustane and Scotts ANTB, when compared with plots fertilized with Com-Pro and Earthgro S, but not the other treatments.

2000.
Plant parasitic nematode populations were quantified on 25 June 2000, and there was a significant interaction between only lance nematode population densities and fertilizer treatments (Table 5). Plots fertilized with Milorganite and Earthgro S had significantly higher lance population densities when compared with plots fertilized with SCU, Sustane, Ringer LR, Com-Pro, Scotts ANTB, and unfertilized plots. None of the N-sources reduced the population densities of any species assayed, when compared with unfertilized turf.

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
Southshore creeping bentgrass was fertilized with nine N-sources for seven continuous years, and data were collected in the final three years. What distinguishes this study from others (Landschoot and McNitt, 1997; Liu et al., 1995; Nelson and Craft, 1992) is that the N-sources were applied at the recommended rates and timings for moderate to high management golf course fairways in the transition zone of the Mid-Atlantic region. For example, in this study 150 kg N ha^-1 were applied in the autumn months and only 50 kg N ha^-1 were applied in the spring. Conversely, Landschoot and McNitt (1997) applied two N rates (24 and 50 kg N ha^-1) monthly between May and September. Liu et al. (1995) also applied N every 4 wk from early June to early September and once in late November.

Ringer LR and SCU generally provided the most consistant level of higher quality when DS was active. Urea, Milorganite, Scotts ANTB, Sustane, and Earthgro S generally provided an intermediate level of turf quality. Lowest turf quality in all years was observed in plots treated with Earthgro DM, Com-Pro, and unfertilized turf. Tissue N levels in bentgrass treated with Com-Pro and especially Earthgro DM generally were low when compared with other N-sources in May and early June. The immobilization of N or lack of appreciable mineralization of N from Com-Pro and Earthgro DM was likely a major factor contributing to their inability to improve turf quality, when compared with the other N-sources.

The AUDPC values showed than none of the N-sources reduced DS over-the-season. Ringer LR (all 3 yr), urea (1999 and 2000), Milorganite (1999), Sustane (2000), and SCU (2000) delayed DS to within the acceptable threshold in May and early to mid-June, when disease pressure was in the low to moderate range. Conversely, Com-Pro (all 3 yr) and Earthgro DM (1999) had intensified DS. The mechanism for the enhanced DS associated with Com-Pro and Earthgro DM is unknown. Com-Pro contains numerous wood chips, which may have provided a favorable energy source for S. homoeocarpa. The wood chip also may have immobilized N.

None of the treatments reduced DS significantly when disease pressure became moderately severe in 1998 or 1999. In 2000, SCU, Sustane, and Ringer LR reduced DS during moderately high disease pressure in late June, when compared with Com-Pro-treated and unfertilized plots. As was observed by Landschoot and McNitt (1997), none of the natural organic products evaluated in this study consistently reduced DS when compared with a synthetic organic N-source (i.e., urea or SCU) in any year. Except Com-Pro and Earthgro DM, data also showed that mostly autumn-applied N had little impact on DS after the disease began to intensity in mid-to-late June. Continuous applications of N during the summer, however, do reduce DS (Landschoot and McNitt, 1997; Liu et al., 1995).

It is widely stated that turf maintained with very low inputs of N is more likely to be severely damaged by S. homoeocarpa (Couch, 1995; Monteith and Dahl, 1932; Smiley et al., 1992). Data from this study, however, revealed that unfertilized bentgrass turf with poor density is not necessarily more likely to be severely damaged by DS, when compared with turf receiving N. It has been well documented, however, that DS can be reduced as N level is increased in fertilized turf (Couch, 1995; Landschoot and McNitt, 1997; Liu et al., 1995).

General soil microbial activity and tissue N levels were quantified in 1999 and 2000. No N-source consistently was associated with higher levels of soil microbial activity, when compared with unfertilized plots. There was no correlation between general soil microbial activity in 1999. In 2000, however, DS was correlated negatively with soil microbial activity on the first rating date, when disease pressure was low. Hence, these results do not strongly support the premise that natural organic fertilizer suppress DS by enhancing soil microbial activity as suggested by Liu et al., 1995 and Nelson and Craft, 1992. In the aforementioned studies, however, N was applied at different times and rates than used in this study. It is conceivable that sampling the foliage and thatch for FDA activity may have provided more definitive insights on the influence of the N-sources on microbial activity in turf.

When disease pressure was moderately severe in 1999 (i.e., May and June), there was a strong negative correlation (P = <0.01) between the amount of foliar N and DS severity. By the second week in July when disease pressure was severe, however, there no longer was a significant correlation between DS severity and tissue N. In 2000, there was no correlation between tissue N and DS. Although DS severity was not correlated with tissue N in 2000, plots with highest quality (i.e., SCU, Sustane and Ringer LR) had the least DS on 23 June when disease pressure was high. Except for Milorganite and SCU on 23 June, none of the N-sources was associated with elevated tissue N-levels when disease was intensifying after 30 May 2000. Hence, it appeared that N availability was the major factor in reducing DS severity earlier in the season. Although not quantified, the mineralization rates of the N-sources likely was key factor in the improved DS suppression associated with selected N-sources. These observations support those of Landschoot and McNitt (1997), who reported improved color responses with decreasing DS. According to Couch (1995), applications of N reduce damage from S. homoeocarpa by stimulating growth, resulting in the need for more frequent removal of necrotic tissue during periods less favorable for growth of the pathogen.

Soil organic matter was higher in the upper 2.5 cm of soil in plots fertilized with Sustane, Earthgro S, Earthgro DM, Com-Pro, and Scotts ANTB in 1998, whereas all fertilizers and composts were associated with an increase in organic matter in March 2000. By June 2000, however, only Sustane and Milorganite-treated plots had organic matter levels higher than unfertilized plots in the upper 2.5-cm zone. Hence, only those plots treated with Sustane had higher organic matter levels in the upper 2.5-cm zone, when compared with urea-treated and unfertilized plots on all three rating dates. No fertilizer treatment was associated with an increase in soil organic matter in the 2.6- to 5.0-cm soil zone in either year.

The large increase in organic matter by early 2000 likely was due to the core aeration performed prior to the autumn 1999 applications of the N-sources. Aerification allowed the N-sources to more effectively penetrate the upper 2.5 cm of soil. Improved aeration provided by coring combined with a facilitated entry of nutrients into the aeration channels probably promoted an increase in root biomass. The highest organic matter levels were found in the upper 2.5-cm zone of Com-Pro-treated plots in March 2000. This high organic matter level in Com-Pro-treated plots likely was due to the collection of samples from some aerification holes containing large amounts of Com-Pro. This would appear to explain why organic matter levels in Com-Pro-treated plots were similar to unfertilized plots by June 2000. Regardless, data suggest that fertilizers may be more effective in enhancing organic matter levels by applying them following core aeration.

None of the N-sources reduced thatch, when compared with unfertilized plots. Com-Pro, however, increased thatch. The low thatch levels in unfertilized plots were anticipated, but the minimal increase in thatch (3–15 mm) in fertilized plots was unexpected. This may have been due in part to the annual destruction of tissue by S. homoeocarpa over the 7-yr period. However, highest thatch levels were in Com-Pro-treated plots, which had intensified DS. Because of a low N analysis, large amounts of Com-Pro had to be applied and there simply may have been a build up of material on the surface. There was, however, less thatch in plots treated with Earthgro S (22 mm) and Scotts ANTB (21 mm), when compared with other fertilizer treatments. Except for Com-Pro (33 mm), however, the magnitude of the thatch depth differences among fertilized plots was small (1–6 mm).

Natural organic N-sources and organic soil amendments are believed to suppress plant parasitic nematodes (Rodriguez-Kabana, 1986). The population densities of the parasitic nematodes recovered from fertilized plots generally were similar to populations recovered from the unfertilized plots. Com-Pro-treated plots on one rating date in 1999, however, had higher parasitic nematode population densities than were found in the unfertilized plots. In 2000, higher lance nematode population densities were found in plots treated with Milorganite and Earthgro S. None of the N-sources reduced the population densities of any of the nematode species assayed.

	CONCLUSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
Ringer LR was consistently associated with reduced DS when disease pressure was low to moderately severe and provided good turf quality. None of the natural organic products evaluated, however, consistently reduced DS when compared with the synthetic organic N-sources. The suppression of DS early in the season was associated more with N-availability rather than enhanced microbial activity. Results also showed that the natural organic fertilizers did not reduce thatch or the population densities of plant parasitic nematodes when compared with synthetic N sources. Conversely, Com-Pro increased DS, thatch depth, and on one rating date was associated with elevated levels of plant parasitic nematodes. Sustane was the most consistent N-source shown to increase organic matter levels in the upper 2.5-cm soil zone, but no N source increased organic matter at the 2.6- to 5.0-cm sampling depth.

	ACKNOWLEDGMENTS

 
We thank Dr. Thomas R. Turner for initiating the study and applying the N-sources between 1994 and 1997. We are grateful to Ms. Sandra Sardinelli for extracting, identifying and quantifying parasitic nematodes.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
This manuscript is a contribution from the Maryland Agric. Exp. Stn.

Received for publication May 4, 2001.

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