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TURFGRASS SCIENCE

Mowing Strategies and Dew Removal to Minimize Dollar Spot on Creeping Bentgrass

^a College of Agriculture and Technology, Dep. of Plant Science, State Univ. of New York, Cobleskill, NY 12043
^b Dep. of Horticultural Science, Room 305 Alderman Hall, Univ. of Minnesota, 1970 Folwell Ave., St. Paul, MN 55108
^c Dep. of Agronomy and Plant Genetics, Room 411 Borlaug Hall, Univ. of Minnesota, 1991 Upper Buford Circle, St. Paul, MN 55108

^* Corresponding author (ellramar{at}cobleskill.edu).

	ABSTRACT

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

 
Dollar spot (DS) disease (caused by Sclerotinia homoeocarpa F.T. Bennett) on bentgrass (Agrostis spp.) can be greatly reduced by implementing mowing and other cultural practices that reduce leaf wetness duration (LWD). Field studies were conducted in 2004 and 2005 to discern the effects of dew removal time (mowing at 0400, 1000, or 2200 h), method of dew removal (mowing with sharp or dull mower or squeegee), and frequency of dew removal (daily or alternate days) on the incidence of DS on fairway height (16 mm) creeping bentgrass (Agrostis stolonifera L.). The severity of DS was studied in laboratory experiments with 6, 12, and 18 h LWD. Dollar spot incidence on field test plots was lowest when mowing treatments were conducted at 0400 h. In addition plots mowed at 2200 h had significantly lower DS incidence than plots mowed at 1000 h. Mowing with either a sharp or dull reel mower proved more effective in reducing DS than alternating mowing with a squeegee for dew removal. Dull mower blades were as effective as sharp mower blades in reducing DS in our field studies. Dollar spot incidence was also lower when dew was removed daily than when dew was removed on alternate days. Mowing at 0400 h daily was the most effective treatment for reducing DS on creeping bentgrass plots in both 2004 and 2005. In laboratory experiments, DS lesion size increased as LWD increased. Specifically, disruption of leaf moisture after 6 h of uninterrupted LWD appeared to be most effective in reducing DS lesion diameter.

Abbreviations: DS, dollar spot • LWD, leaf wetness duration

Mowing Strategies and Dew Removal to Minimize Dollar Spot on Creeping Bentgrass

^a College of Agriculture and Technology, Dep. of Plant Science, State Univ. of New York, Cobleskill, NY 12043
^b Dep. of Horticultural Science, Room 305 Alderman Hall, Univ. of Minnesota, 1970 Folwell Ave., St. Paul, MN 55108
^c Dep. of Agronomy and Plant Genetics, Room 411 Borlaug Hall, Univ. of Minnesota, 1991 Upper Buford Circle, St. Paul, MN 55108

^* Corresponding author (ellramar{at}cobleskill.edu).

Dollar spot (DS) disease (caused by Sclerotinia homoeocarpa F.T. Bennett) on bentgrass (Agrostis spp.) can be greatly reduced by implementing mowing and other cultural practices that reduce leaf wetness duration (LWD). Field studies were conducted in 2004 and 2005 to discern the effects of dew removal time (mowing at 0400, 1000, or 2200 h), method of dew removal (mowing with sharp or dull mower or squeegee), and frequency of dew removal (daily or alternate days) on the incidence of DS on fairway height (16 mm) creeping bentgrass (Agrostis stolonifera L.). The severity of DS was studied in laboratory experiments with 6, 12, and 18 h LWD. Dollar spot incidence on field test plots was lowest when mowing treatments were conducted at 0400 h. In addition plots mowed at 2200 h had significantly lower DS incidence than plots mowed at 1000 h. Mowing with either a sharp or dull reel mower proved more effective in reducing DS than alternating mowing with a squeegee for dew removal. Dull mower blades were as effective as sharp mower blades in reducing DS in our field studies. Dollar spot incidence was also lower when dew was removed daily than when dew was removed on alternate days. Mowing at 0400 h daily was the most effective treatment for reducing DS on creeping bentgrass plots in both 2004 and 2005. In laboratory experiments, DS lesion size increased as LWD increased. Specifically, disruption of leaf moisture after 6 h of uninterrupted LWD appeared to be most effective in reducing DS lesion diameter.

Abbreviations: DS, dollar spot • LWD, leaf wetness duration

	INTRODUCTION

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

 
DOLLAR SPOT (DS) disease caused by the fungus Sclerotinia homoeocarpa F.T. Bennett is the most economically important disease on amenity turfgrasses in the USA (Vargas, 2005). Air temperatures from 15 to 30°C (Smiley et al., 2005) are most conducive to DS development allowing growth and spread of DS for a large portion of the growing season with several DS infection cycles on turfgrasses each year. On highly maintained turfgrass areas such as golf courses, tennis courts, and bowling greens, turf managers often make multiple applications of fungicides to preserve the playing quality demanded by clientele. Moreover, S. homoeocarpa attacks a wide range of turfgrasses including both warm and cool season species.

Sclerotinia homoeocarpa has developed resistance to several fungicides including heavy metal–based fungicides, benzimidazoles, anilazine, dicarboximides, and demethylation inhibitors (Vargas, 2005). Sclerotinia homoeocarpa resistance to demethylation inhibitors was confirmed only 11 yr after this family of fungicides was introduced to manage this disease (Golembiewski et al., 1995). Today, demethylation inhibitors are ineffective against DS on many golf courses where this fungicide family has been used extensively (Vargas, 2005).

The rapid development of fungicide resistance by S. homoeocarpa combined with the continuing movement to reduce pesticide use in turfgrass management has been the impetus for research on cultural practices to manage DS. One important cultural method of reducing plant diseases is to reduce leaf wetness duration (LWD). Prolonged LWD increases the severity of DS and other fungal diseases by providing a favorable environment for fungal penetration of leaf tissues (Huber and Gillespie, 1992, Williams et al., 1996; Gross et al., 1998; Walsh, 2000; Uddin et al., 2003). Walsh (2000) found that the minimum LWD for the development of DS for one isolate of S. homoeocarpa on creeping bentgrass (Agrostis stolonifera L.) was 12 h at 17.5°C. A longer LWD was required when the temperature was lower. Walsh also found that the size of the diseased area increased as LWD increased from 12 to 48 h.

Leaf wetness is frequently caused by the accumulation of dew on turfgrass. Dew is composed of fluids from within the plant (guttation) as well as water that condenses from the atmosphere and soil surrounding plants. The impact of dew accumulation and leaf wetness on DS occurrence was studied by Williams et al. (1996). This research indicated that removal of dew by mowing or dew whipping in the morning significantly decreased DS infection on ‘Penncross’ creeping bentgrass greens maintained at 0.6 cm and on creeping bentgrass–annual bluegrass fairways maintained at 1.1 cm. Williams (1996) also tested mowing and on alternate days dragging a hose. This method did not statistically reduce DS severity when compared to no moisture displacement because the research site utilized had inconsistent water holding capacity.

Another aspect of mowing generally believed to impact the incidence of DS and other turfgrass diseases is mower blade sharpness. It has been hypothesized that a dull mower blade shreds turfgrass tissue weakening the plant and leaving a greater amount of wounded tissue for pathogen invasion (Emmons, 1995). However, there has been no published research to test this hypothesis.

It is well documented that removing dew from turfgrass by mowing reduces DS (Williams et al., 1996), however many questions of practical significance to turfgrass management professionals remain unanswered. Therefore, the objectives of these studies were to

1. determine if mowing at different times (0400, 1000, or 2200 h) affects DS incidence on creeping bentgrass/annual bluegrass turf maintained at fairway height;
   
2. determine if mowing frequency (daily or on alternate days) affects DS incidence;
   
3. determine if removal of dew by mowing on alternate days and using a squeegee on days turf is not mowed affects DS incidence in the same manner as daily mowing treatments;
   
4. determine if the sharpness of the mower blade used to cut turf has an effect on the incidence of DS; and
   
5. determine the effect of LWD of 6, 12, and 18 h on development of DS on creeping bentgrass in a controlled environment.
   

	MATERIALS AND METHODS

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

 
Field Study
This study was conducted on an established turf sward, approximately 10 yr old, located on the Minnesota Agricultural Experiment Station, St. Paul, MN, in 2004 and 2005. Study dates in 2004 were 2 August through 9 September. Study dates in 2005 were 29 July through 7 September. The turf on this area was composed of approximately 90% Penncross creeping bentgrass and 10% annual bluegrass (Poa annua L.). All plots were mowed at 16 mm throughout the study period, and clippings were collected. Soil on site is categorized as a Waukegan silt loam (fine-silty over sandy or sandy-skeletal, mixed, superactive, mesic Typic Hapludoll).

Fertilizer applications on this site, which were made before the study in 2004, consisted of 24 kg ha^–1 of urea N on 5 May, 36.5 kg ha^–1 methylene urea N on 27 May and 2 July, and 12 kg ha^–1 ammonium nitrate on 28 July. In 2005, fertilizer was applied before the study period and consisted of 24 kg ha^–1 methylene urea N on 8 June, and 18 kg ha^–1 methylene urea N on 25 June and 13 July. Grass clippings from all individual plots were collected for total N analysis (Dumas method) at the beginning and the end of each study period in 2004 and 2005.

Plots were irrigated as needed to prevent drought stress. Irrigation was applied during midday hours only so as not to influence LWD.

Before beginning treatments, 60 plots (1.5 by 3 m) were artificially inoculated on 2 Aug. 2004 and 29 July 2005 with an isolate (no. 36) of S. homoeocarpa collected from annual bluegrass at the Les Bolstad University of Minnesota Golf Course. Four plots were left as uninoculated controls. The inoculation procedure consisted of first dragging a plastic leaf rake across the plots to lift grass to a standing position. A Gandy 24 drop spreader (Gandy Company, Owatonna, MN) was utilized to evenly distribute millet seed inoculum at rate of 25 g per plot. This resulted in approximately 3800 pieces of inoculum per plot. Plots were immediately rolled with a Toro Greensmaster 1000 walking greens mower (The Toro Company, Minneapolis, MN) to incorporate the inoculum and encourage disease development. Inoculum was watered in with 2 L m^–2 to keep inoculum moist until nightfall.

Inoculum Preparation
To begin preparation of S. homoeocarpa inoculum, 500-mL flasks were filled one-third full with dry millet seed. Deionized water was added to each flask until the water level was 3 cm above the level of the seed. Flasks were left overnight, allowing the seeds to imbibe the water. Excess water was drained after soaking overnight. Flask openings were subsequently covered with foil and flasks were autoclaved at 121°C for 30 min. Flasks were autoclaved a second time 24 h later for 30 min. Five 5-mm plugs of S. homoeocarpa (from the edge of actively growing cultures on potato dextrose agar) were transferred to each flask after cooling. Flasks were incubated in the dark at 25°C and shaken vigorously each day to minimize clumping and encourage fungal growth throughout the flask. After 7 d mycelium had infected the millet seeds. Seed inoculum was screened to break clumps into single seeds, transferred into an autoclaved paper bag, and allowed to dry for 3 to 5 d in a laminar flow hood.

Treatment Procedures
This study was set up as an incomplete factorial design examining the effect of mowing time, blade sharpness, and mowing treatment frequency with and without squeegee dew removal on DS incidence. Our design resulted in 15 different treatment combinations (Table 1 ). Each treatment combination was replicated four times. Three different methods were used for dew removal from turf: (i) mowing with a Toro 1000 walking greens mower with a dull blade adjusted so no contact was made between the reel and bedknife that left leaf tissue with visible tearing; (ii) mowing with a Toro 1000 walking greens mower with a sharp blade which cleanly cut standard printer paper placed between reel and bedknife; (iii) pulling a 45-cm floor squeegee (Ettore No. 61018, Alameda,CA) over plots on alternate days, and mowing with sharp blades on the days without the squeegee treatment.. Each of these treatments was conducted at three different times per day (0400 h, 1000 h, and 2200 h). The effect of mowing treatment frequency (daily or on alternate days) was also tested. The squeegee dew removal treatment was performed on plots that were mowed on alternate days with sharp blades, because this could be a realistic and practical method of dew removal, and was not performed together with daily mowing or mowing with dull blades. Four plots were not inoculated to serve as a control and were mowed on alternate days with the sharp Toro 1000 walking greens mower.

The squeegee method of dew removal involved dragging a 45-cm-wide floor squeegee four times across the plot being treated and wiping the dew from the squeegee with a cloth between passes. No downward pressure was applied except for the weight of the squeegee.

Data Collection
To collect onsite environmental data, a Campbell Scientific (CS) CR23X- Micrologger (Campbell Scientific, Inc. Logan, UT) was utilized. Air temperature, relative humidity and leaf wetness data were collected for later use in estimation of LWD. A CS 237 leaf wetness sensor grid was used for estimation of leaf wetness. In 2004, sensors were placed within the canopy of Kentucky bluegrass turf positioned 45° to the horizontal, facing north and immediately adjacent to the plot as described by Walsh (2000). This protocol was followed because Walsh et al. determined that the CS 237 leaf wetness sensor placed within canopy of Kentucky bluegrass mowed at 5 cm was the optimum sensor for determining LWD on creeping bentgrass mowed at 6.5 mm. In 2005, adjustments were made to the 2004 protocol in light of a new study (Sentelhas et al., 2004) indicating that sensors should be placed 30 cm above the turf positioned 45° to the horizontal and facing north to better approximate the moisture conditions in the turfgrass canopy. Relative humidity and temperature sensors (HMP45AC Vaisala Oyj, Finland) were also mounted 30 cm above the turf canopy adjacent to the plot area. Leaf wetness duration was estimated two ways. The first method utilized a model which predicts that leaves are wet when relative humidity is greater than or equal to 90% (Huber and Gillespie, 1992). The second method estimated leaves to be wet when resistance through the CS 237 sensor grid dropped below 500 K_ohms of resistance.

The amount of diseased area in each plot was assessed approximately every 7 d, after the plots were inoculated. Disease assessment dates were 13 August, 20 August, 27 August, 3 September, and 9 September in 2004. In 2005 data were collected 12 August, 19 August, 27 August, 2 September, and 7 September. Two digital images, approximately 1 m² in size, were taken (Nikon D100 camera, Nikon USA, Melville, NY) of each plot on each assessment date using a 2.5-m tripod. A 1 by 1 m frame was used in photographing plots and data collections were made in the same area of each plot. Images were saved for later analysis.

Captured images of plots were individually evaluated for percent infected tissue using Assess image analysis software for plant disease quantification (Lamari, 2003) utilizing a procedure similar to Horvath and Vargas (2005) modified after consultation with Dr. Lakhdar Lamari, the author of the software. Only the framed portion of each image was analyzed. Images were saved as .BMP files and analyzed using the hue function in Assess. Diseased tissue was differentiated from healthy tissue by adjusting lower and upper thresholds for images at 30 and 115, respectively. These thresholds provided clear visual definition between diseased and healthy tissue. After thresholding, percent diseased area was automatically calculated by Assess software. The mean of the disease rating for the two images taken on each plot on each sampling date were used for statistical analysis.

Data Analysis
The experiment was set up as a completely randomized design with four replications for a total of 64 plots. Plot size was 3 by 1.5 m with 3-m buffer areas between plots. All statistical analyses were performed with analysis of variance using PROC GLM of SAS v. 9.1 (SAS Institute, Cary, NC). Because the squeegee treatment was only used on alternating days with sharp blades, all levels of dew removal were not crossed with all levels of mowing frequency and blade sharpness. For this reason, effects for the squeegee treatment, blade sharpness, and mowing frequency were combined into the "treatment" effect. To determine the significance of these treatments and their interactions, the treatment effect and treatment x time interaction were subdivided into appropriate mutually orthogonal contrasts. Data were log transformed before the analysis to better conform to the underlying assumptions of the analysis of variance. All effects were considered fixed. F-tests were used to determine the significance of all main effects, interactions, and contrasts. Repeated measures effects were tested using the univariate repeated measures tests in PROC GLM. Error was partitioned into interactions with repeated measures effects and interactions not involving repeated measures for the purpose of these tests. Multiple comparison tests for treatment means were performed using Fisher's LSD ( = 0.05) on the transformed data.

Controlled Environment Study
The influence of LWD on the development of DS on Penncross creeping bentgrass was evaluated using three enclosed chambers (75 by 96 by 152 cm) with sliding doors constructed of Plexiglas and stainless steel that maintained relative humidities of 90 to 100%. Humidity and moisture was supplied to each chamber using an ultrasonic mister (HM480, Holmes Group, Inc., Milford, MA) with a mist cycle time of 20 s every 2 min (Phytotronics, Inc., Earth City, MO, Model 1626D). Plants within these chambers were subjected to 14-h days (35 µmol m^–2 s^–1 photosynthetically active radiation) and 10-h nights at 22 to 29°C. Watchdog Model 450 (Spectrum Technologies, Inc., Plainfield, IL) devices were placed inside the chambers to monitor temperature and humidity.

Penncross creeping bentgrass was established in the greenhouse from seed, in 9 by 9 cm pots, utilizing commercial growing medium (BM1, Berger Peat Moss, Quebec, Canada). Penncross creeping bentgrass seed was sown at a rate of 3 g m^–2. and mowed weekly at 16 mm once plants were established. All samples were maintained in a greenhouse with 12-h days (320 µmol m^–2 s^–1) and 12-h nights at 15–28°C for 6 wk before initiating the experiments. Plants were fertilized 14 d before the study at a rate of 1 g m^–1 N, using Plantex 17–5–19 Poinsettia Fertilizer (Plant Products USA, Canton, OH).

Inoculum was prepared from clippings of Penncross creeping bentgrass according to Walsh (2000). Briefly, 4 g of finely chopped, air-dried clippings were combined with 12 mL of deionized water in 100-mL beakers and covered with aluminum foil, then autoclaved for 25 min. After cooling, the clippings from each beaker were divided into two equal parts and transferred to sterile 9-cm-diameter petri plates. Clippings were inoculated with five 5-mm-diameter plugs of S. homoeocarpa (isolate no. 36) taken from the actively growing margin of colonies grown on potato dextrose agar. Inoculated grass clippings were then placed in a dark incubator at 25°C for 7 to 10 d until the clippings were completely covered with mycelium.

Inoculum was cut into 0.25-cm² pieces with a scalpel then placed inside the canopy of each pot of creeping bentgrass. Pots were immediately placed in the mist chamber and mist was run continuously until moisture was apparent on leaf tissue.

Experimental Design
The mist chamber study was set up as a completely randomized block design. Each block consisted of 10 individual pots. Each mist chamber contained one block; five pots that were inoculated and five pots that served as noninoculated controls. Experiments lasted 7 d and were repeated four times. Each of the three mist chambers were set to interrupt the leaf wetness period after either 6, 12, or 18 h of mist. Leaf wetness was interrupted every 6, 12, or 18 h by blotting pots with paper towels. A new towel was used for each pot so that no inoculum would be spread mechanically from pot to pot. After blotting, pots were kept in a dry chamber for the remainder of the 24-h period until the cycle was repeated. To minimize the influence of variation in chamber performance, each set of 10 pots was rotated between chambers during each treatment period. This resulted in each set of 10 pots being placed in each of the three chambers once per day.

Disease was scored 7 d after inoculation. Dollar spot lesion diameter (mm) was measured and digital images were taken of all treated and control pots. ANOVA was performed utilizing PROC GLM of SAS v. 9.1 (SAS Institute). Multiple comparison tests were performed using Fisher's LSD ( = 0.05).

	RESULTS AND DISCUSSION

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

 
Dew Removal Treatment Effects on Dollar Spot Incidence
Mean percent disease ratings for plots ranged from 0.6% to nearly 6.4% in 2004 and from 2.3 to 8.7% in 2005. ANOVA was performed on disease means for all disease assessment dates and is presented in Table 2 . Main effects of treatment, time, year, and date and the date x treatment interaction were determined to be significant (P < 0.0001). Interactions of date x time, and year x time were also significant (P < 0.01). The date x treatment interaction which occurred was a result of minor rank changes in treatment efficacy over the 10 disease assessment dates. Although statistically significant, these rank changes did not reveal a consistent trend in the data. Similarly, interactions of date x time and year x time resulted from minor rank changes in treatment efficacy over the 10 disease assessment dates. Although these interactions were statistically significant, they did not significantly affect the overall ranking of treatments. The significant treatment x time interaction was due mainly to the mowing frequency x time component of the interaction (Table 2). Daily dew removal at 0400 h resulted in the least diseased area. Alternate day dew removal at 0400 h and daily dew removal at 2200 h were intermediate in reducing disease, while daily treatments at 1000 h, alternate day treatments at 1000 h, and alternate day treatments at 2200 h resulted in the most diseased turf (Fig. 1 ).

View larger version (9K): [in this window] [in a new window]   Figure 1. Actual mean percent diseased area for combined effect of mowing time x mowing frequency for all disease assessment dates. Each mean is averaged across all dew removal methods. Means with different letters are significantly different (LSD 0.05) after means were log transformed to stabilize variance.

View larger version (6K): [in this window] [in a new window]   Figure 2. Actual mean percent diseased area by mowing time for all disease assessment dates. Each mean is averaged across all dew removal methods. Means with different letters are significantly different (LSD 0.05) after means were log transformed to stabilize variance.

View this table: [in this window] [in a new window]   Table 3. Dew development and drying estimates at St. Paul Minnesota Experiment Station 2004 and 2005.

View larger version (10K): [in this window] [in a new window]   Figure 3. Actual mean percent diseased area by treatment combination for all data collection dates. Each mean is averaged across all dew removal times and dew removal methods. Means with different letters are significantly different (LSD 0.05) after means were log transformed to stabilize variance.

Total tissue N may have also contributed to the lower initial disease observed in 2004. Total mean tissue N for all plots on 4 Aug. 2004 was 5.52% as compared to 4.46, 4.75, and 4.88% on 13 Sept. 2004, 1 Aug. 2005, and 9 Sept. 2005, respectively. The slightly elevated N level may have also helped to reduce initial DS infection. Dollar spot severity decreases noticeably when tissue N levels are greater than 5% (M. Boehm, personal communication, April 2007). Davis and Dernoeden (2002) also correlated lower DS severity with higher tissue N levels.

Controlled Environment Experiment
The mist chamber experiment provided additional data to support the hypothesis that reduced LWD results in less DS on creeping bentgrass. Lesion size data collected at the conclusion of each trial were combined and analyzed using the GLM procedure in SAS (Table 4 ). The main effects of LWD and run were highly significant. Overall, mean lesion diameter increased significantly as LWD increased from 6 to 18 h (Fig. 4 ). Variation in mean lesion diameter for the same LWD treatments occurred between experiments during this study. It is likely this variation was due to the temperature fluctuation in mist chambers between runs. Mean temperature ranged from 24.5 to 29.1°C for all runs in the study with a standard deviation range of 0.9 to 1.6°C.

View larger version (8K): [in this window] [in a new window]   Figure 4. Mean dollar spot lesion diameter by leaf wetness duration (LWD; number of hours of leaf wetness daily) in a controlled environment (mist chamber). All means with dissimilar LWD times were significantly different (LSD 0.05).

	CONCLUSIONS

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

 
Data indicate removing dew at a time which divides the length of continuous leaf wetness in half and minimizes LWD was most effective in reducing DS. Also, mowing disrupted dew more effectively than dragging a squeegee to remove dew. Frequency of dew removal was also important. Daily dew removal substantially reduced DS as compared to dew removal on alternate days. Knowledge gained from these studies should help turf grass management professionals design mowing and dew removal programs that enable significant reductions in DS incidence. In addition, this simple cultural practice might prove effective in managing other turf diseases such as brown patch (caused by Rhizoctonia solani Kuhn) and gray leaf spot (caused by Pyricularia grisea Cooke) that become more severe as a result of extended periods of leaf wetness (Gross et al., 1998; Uddin et al., 2003). Studies to discern the effect of mowing and other methods of dew removal (on fungal pathogens of turf aided by extended LWD) should be conducted to test this hypothesis. These data might serve as a starting point for a new model, used for the forecasting of DS disease based on LWD. Future studies comparing multiple isolates of S. homoeocarpa, varied LWD cycle length, varied temperature, and varying the number of LWD periods (from one to many), could help to develop an improved model for predicting this important turfgrass disease.

	ACKNOWLEDGMENTS

 
Thank you to Dr. Lakhdar Lamari for his assistance in digital image analysis and instruction in the use of Assess image analysis software. Thanks also to the Toro Company for their financial support of the LWD/mowing study.

	NOTES

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

 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication October 10, 2006.

	REFERENCES

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

 

• Davis, G.J., and P.H. Dernoeden. 2002. Dollar spot severity, tissue nitrogen, and soil microbial activity in bentgrass as influenced by nitrogen source. Crop Sci. 42:480–488.[Abstract/Free Full Text]
• Emmons, R. 1995. Turfgrass science and management. Delmar, Albany, NY.
• Golembiewski, R.C., J.M. Vargas, and A.R. Detweiler. 1995. Detection of demethylation inhibitor (DMI) resistance in Sclerotinia homoeocarpa populations. Plant Dis. 79:491–493.
• Gross, M.K., J.B. Santini, I. Tikhonova, and R. Latin. 1998. The influence of temperature and leaf wetness duration on the infection of perennial ryegrass by Rhizoctonia solani. Plant Dis. 82:1012–1016.[CrossRef]
• Horvath, B., and J. Vargas, Jr. 2005. Analysis of dollar spot disease using digital image analysis. Int. Turfgrass Soc. Res. J. 10:196–201.
• Huber, L., and T.J. Gillespie. 1992. Modeling leaf wetness in relation to plant disease epidemiology. Phytopathology 30:553–577.
• Lamari, L. 2003. Assess image analysis software. APS Press, St. Paul, MN.
• Sentelhas, C.S., T.J. Gillespie, M.L. Gleason, J.E. Monteiro, and S.T. Helland. 2004. Operational exposure of leaf wetness sensors. Agric. For. Meteorol. 126:59–72.[CrossRef]
• Smiley, R.W., P.H. Dernoeden, and B.B. Clark. 2005. Compendium of turfgrass diseases. 3rd ed. Am. Phytopathol. Soc., St. Paul, MN.
• Uddin, W., K. Serlemitsos, and G. Viji. 2003. A temperature and leaf wetness duration based model for the prediction of gray leaf spot of perennial ryegrass. Phytopathology 93:336–343.[Medline]
• Vargas, J.M. 2005. Management of turfgrass diseases. 3rd ed. John Wiley & Sons, Hoboken, NJ.
• Walsh, B.K. 2000. Epidemiology and disease forecasting system for dollar spot caused by Sclerotinia homoeocarpa F.T. Bennett. Ph.D. diss. Univ. of Guelph, Guelph, ON, Canada.
• Williams, D.W. 1996. The role(s) of dew in the epidemiology of dollar spot. Ph.D. diss. Univ. of Kentucky, Lexington.
• Williams, D.W., A.J. Powell, C.T. Dougherty, and P. Vincelli. 1998. Separation and quantification of the sources of dew on creeping bentgrass. Crop Sci. 38:1613–1617.[Abstract/Free Full Text]
• Williams, D.W., A.J. Powell, P. Vincelli, and C.T. Doughtery. 1996. Dollar spot on bentgrass influenced by displacement of leaf surface moisture, nitrogen, and clipping removal. Crop Sci. 36:1304–1309.[Abstract/Free Full Text]

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