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a Dep. of Natural Resource Sciences and Landscape Architecture, Univ. of Maryland, College Park, MD 20742-3721
b Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907-2054
* Corresponding author (pd{at}umd.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: AUDPC, area under disease progress curve • DS, dollar spot • GLS, gray leaf spot • IC, Sclerotinia homoeocarpa infection centers plot–1 • PB, paclobutrazol • % PAB, percentage of plot area blighted • WA, wetting agent
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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During summer months, water from irrigation is applied frequently to fairways to maintain turf health and promote vigor. The two most common approaches to irrigating fairways during summer in the mid-Atlantic region are light and frequent and deep and infrequent irrigation. Frequent irrigation (i.e., >5 times wk–1 during rain-free periods) in summer is common for numerous golf course managers, primarily for aesthetic reasons. Frequent irrigation promotes wet soils and longer leaf wetness durations, which can be important in the development of several turf diseases. Deep and infrequent irrigation is performed for playability and agronomic reasons and involves maintaining soil as dry as possible until symptoms of wilt are observed. Deep and infrequent irrigation typically is recommended as a cultural disease management strategy as this practice promotes a drier soil condition, which is generally less conducive to infection by most foliar pathogens (Couch, 1995; Vargas, 2004; Smiley et al., 2005). A greenhouse study, however, demonstrated that low soil moisture levels enhanced DS in mature Kentucky bluegrass (Poa pratensis L.) (Couch and Bloom, 1960). In that study, soil moisture levels 
75% of field capacity resulted in a 45 to 55% increase in DS severity, when compared with plants maintained at field capacity. Jiang et al. (1998), however, found that DS in fairway height perennial ryegrass was enhanced by 50% when it received daily irrigation in 1 yr of a 2-yr field study in Kansas. Watkins et al. (2001) studied the influence of two irrigation regimes on DS in creeping bentgrass in Nebraska. In that field study, turf was irrigated daily at 100 or 60% (between April to mid-June) to 80% (between mid-June and 9 September) of potential evapotranspiration. Dollar spot severity was not affected by either irrigation regime. There have been no field studies that have investigated the influence of soil moisture on GLS severity. Previous research, however, has demonstrated that DS and GLS are more prevalent during periods that favor increased leaf wetness durations (Couch, 1995, Hall, 1984, Uddin et al., 2003a, Williams et al., 1996).
Many golf course managers use a variety of chemicals to maintain fairways at the desired quality level. Some of these chemicals include plant growth regulators (PGRs), wetting agents and fungicides. Paclobutrazol is a PGR that is commonly applied to manage excess clippings, improve turfgrass color and density, and to suppress annual bluegrass (Poa annua L.) populations. Paclobutrazol has fungistatic effects on S. homoeocarpa in creeping bentgrass (Burpee et al., 1996; Dernoeden et al., 2002), but its effect on GLS is unknown. Wetting agents are used on fairways to improve water infiltration and to alleviate hydrophobic soil conditions. Polymeric polyoxyalkylene 95% oxoalkonyl hydroxyl polyoxlalkane diyl 5% (Primer Select, Aquatrols Corporation of America, Paulsboro, NJ) is a nonionic surfactant, which has been shown to suppress DS (Dernoeden et al., 2002).
Chlorothalonil is perhaps the most widely used fungicide on turfgrasses and it effectively controls DS and GLS (Vargas, 2004). There have been no reported cases of pathogen resistance to chlorothalonil, but there have been resistance problems with other fungicides used to control DS and GLS (Burpee, 1997; Detweiler and Vargas, 1982; Vargas, 2004; Vincelli and Dixon, 2001). The U.S. Environmental Protection Agency placed restrictions on the use of chlorothalonil in 1999 for turfgrass use (Vincelli and Dixon, 2003). These restrictions have created the need to elucidate approaches to improving chlorothalonil performance. Since chlorothalonil, paclobutrazol and wetting agents, and various tank-mixes are widely used as part of fairway management programs, research regarding their effect on disease under relatively wet or dry soil conditions is warranted.
The purpose of this 3-yr field study was to evaluate the influence of two irrigation regimes (light and frequent nighttime versus deep and infrequent morning irrigation) and three commonly used chemicals (chlorothalonil, paclobutrazol, wetting agent) and various tank-mixes of these materials on fairway height creeping bentgrass and perennial ryegrass for their impact on DS and GLS incidence and severity.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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2002
In April 2002, soil in eight 3.0- by 10.5-m blocks was tilled and leveled by hand raking. These blocks will be described further in the irrigation protocol to follow. Blocks were split (1.5 m x 10.5 m) and ‘Crenshaw’ creeping bentgrass and ‘Figaro’ perennial ryegrass were separately seeded into each half block at 49 and 292 kg seed ha–1, respectively. Following the mid-April seeding, the soil was raked and firmed by rolling. Crenshaw was chosen on the basis of its known high susceptibility of DS and Figaro because of its susceptibility to GLS (T. R. Turner, personal communication). The study area received a total of 120 kg N ha–1 during the 2002 study period from either a 16–4–8 or 19–0–19 fertilizer. All plots were mowed three times per week to a height of 19 mm with a reel mower and clippings were removed. Treatments were initiated on 17 June, and the final applications were made on 26 Aug. 2002. Dollar spot or GLS were not visually active at the time the study was initiated. Occasionally during the 3 yr, flutalonil {N-[3-(1-methylethoxy) phenyl]-2-(trifluoromethyl) benzamid} and metalaxyl-M [methyl N-(methoxyacetyl)-N-(2,6-xylyl)-D-alaninate] were applied to control brown patch (Rhizoctonia solani Kühn) and Pythium blight (Pythium spp.), respectively. The aforementioned fungicides would not be expected to control DS or GLS (Dernoeden, 2001).
2003
The 2002 site was renovated using glyphosate [N-(phosphonomethyl)glycine] in October 2002. The site was vertical cut and seeded with Crenshaw creeping bentgrass (49 kg seed ha–1) on 12 Oct. 2002. ‘Brightstar’ perennial ryegrass was substituted in 2003 in anticipation that it was not as susceptible as Figaro. The Brightstar perennial ryegrass, however, was not seeded (440 kg seed ha–1) until 5 May 2003, since seedlings would be expected to be more susceptible to GLS. Plots were mowed two to three times per week as previously described. Creeping bentgrass received a total of 60 kg N ha–1, while the perennial ryegrass received a total of 146 kg N ha–1 from either the aforementioned fertilizers or urea during the study period.
Treatments were initiated in the creeping bentgrass on 6 June and repeated on 19 June 2003. Dollar spot became a problem in the creeping bentgrass before the time when the perennial ryegrass plants were mature enough to impose treatments. Hence, all bentgrass plots were treated with chlorothalonil (10.2 kg a.i. ha–1) on 27 June to control DS and to provide time for infection centers to heal so that irrigation treatments could be imposed simultaneously on both species. Treatments finally were re-initiated in both species on 10 July and continued every 14 d until 21 August.
2004
The 2003 site was renovated with glyphosate on 17 Sept. 2003. Irrigation blocks were reversed to avoid potential long-term irrigation bias, such as variation in inoculum levels. Hence, the frequently irrigated blocks became infrequently irrigated blocks and vice versa. Similarly, the perennial ryegrass and creeping bentgrass were reversed in each block. Crenshaw creeping bentgrass (78 kg seed ha–1) and Figaro perennial ryegrass (370 kg seed ha–1) were seeded as previously described on 15 October. The study area received 86 kg N ha–1 from 20–20–20 and 31–0–0 in the autumn of 2003 and 86 kg N ha–1 from 20–20–20 or urea during the 2004 study period. Plots were mowed as previously described for 2002.
Chemical and irrigation treatments were initiated 1 June 2004 and chemical treatments were reapplied to the bentgrass on 15 and 29 June, 15 and 30 July, and 16 August. The rate of chlorothalonil was reduced in 2004 in the creeping bentgrass to 4.5 kg a.i. ha–1. Initially, chlorothalonil was applied to the perennial ryegrass at 4.5 kg a.i. ha–1. The 2002 data suggested that chlorothalonil applied on a 14-d interval would not effectively control GLS. Once GLS became active in 2004, the rates and application intervals were adjusted as noted below. Perennial ryegrass plots received chlorothalonil (4.5 kg a.i. ha–1) treatments on 15 and 29 June and 15 July 2004. Gray leaf spot became active on 20 July 2004 and thereafter, chlorothalonil (8.0 kg a.i. ha–1) was applied to the perennial ryegrass plots that were to receive chlorothalonil either alone or in a tank-mix on 20 and 28 July and 6 and 16 August. Paclobutrazol and WA-alone treatments were applied on a 14-d interval beginning 1 June and ending 16 August.
Irrigation Treatments
Eight (3.0 x 10.5 m) independently irrigated blocks were outfitted with pop-up, matched precipitation spray irrigation heads (Weathermatic Model 5520; Weathermatic Irrigation Company; Dallas, TX.) that delivered 22.3 L min–1 to each block. This is equivalent to a precipitation rate of 0.15 mm water min–1. The two irrigation regimes were (i) infrequent irrigation to a soil depth of 6 to 8 cm and (ii) light (on average 5.0 mm water) and frequent (daily during rain-free periods) irrigation. Each irrigation treatment was applied to four, randomly assigned blocks.
The deep and infrequent morning and light and infrequent nighttime blocks were irrigated 0600 and 2100 h, respectively. These times were chosen on the basis of the practices of many golf course managers. Infrequently irrigated blocks only received water when visible drought stress symptoms were observed or when soil moisture measurements indicated that the soil had fallen well below field capacity (i.e., <0.17 cm3 cm–3 soil moisture). Both irrigation regimes were adjusted on the basis of weather patterns and irrigation was withheld if rainfall were forecast or had recently occurred (>6.0 mm). Rainfall was recorded by a rain gauge (Rain Gauge, Spectrum Technologies, Inc. Plainfield, IL.).
Approximately 25 to 38 mm of water from irrigation or rainfall per week were applied to both frequently and infrequently irrigated plots in 2002, which was a drought year. In 2002, there was an attempt to apply about the same amount of water per week in each regime. This was not possible in 2003 or 2004 because of frequent rainfall. In 2003, there were 47.6 cm precipitation during study period, and therefore there were only a few dates when supplemental irrigation was applied to the frequently irrigated blocks. The 2004 season was another wet year (51.4 cm precipitation during the study period), and tarps were used to cover infrequently irrigated blocks before the onset of rain. Tarps (3.3 x 11 m) were constructed from 12 mil black/white reinforced polyethylene sheeting (Model 12 BW; Integra Plastics, Madison, SD). Tarps only were used on the infrequently irrigated plots to promote soil drying. The white side of the tarps faced up, and they usually were removed within 15 min after weather had cleared and were used on 14 occasions between 4 June and 21 August. The frequently irrigated blocks received water on rain-free days at the aforementioned amounts and times of day. The frequently irrigated blocks received an average of 64 mm water wk–1, while the infrequently irrigated plots received 21 mm wk–1 from either irrigation or rainfall during 2004.
Chemical Treatments
Six chemical treatments were applied as follows: chlorothalonil (Daconil Ultrex 82.5WDG, Syngenta Crop Protection, Greensboro, NC) at 8.0 kg a.i. ha–1; paclobutrazol (Trimmit 2SC, Syngenta Crop Protection, Greensboro, NC, PB) at 0.13 kg a.i. ha–1; wetting agent (Primer Select, WA) at 6.3 L product ha–1; chlorothalonil + PB; chlorothalonil + WA; and chlorothalonil + PB + WA; and there was an untreated control. Rates for tank-mix treatments were the same as for each chemical applied alone. The rate of chlorothalonil was reduced to 4.5 kg a.i. ha–1 in the creeping bentgrass in 2004 because no DS differences were observed among these treatments in 2002 and 2003. Chemical treatments were applied on the dates described above and footnoted in the data tables. Chemical treatments were applied with a CO2 pressurized sprayer (262 kPa) equipped with an 8004E flat fan nozzle and calibrated to deliver 468 L ha–1. On treatment days, the chemicals were allowed to dry on foliage and irrigation (
6.4 mm water) was applied to the entire area 8 to 10 h after application to promote PB uptake by roots.
Soil Moisture Measurements
Volumetric soil moisture was recorded two to three times per week by time domain reflectometry (TDR) (Soilmoisture Equipment Corp., Santa Barbara, CA). On measurement days, soil moisture was recorded during early morning (700–800 h). The two probes of the TDR were 15 cm in length and pushed into the soil so that the top of the probes were flush with the thatch layer. The TDR takes the average of the dielectric constant within those 15 cm and records the values as cm3 cm–3. Seven measurements were taken randomly in each perennial ryegrass and creeping bentgrass block. Soil moisture measurements then were averaged for each irrigation regime and species. A soil moisture release curve for this field soil was determined from 0.001 to –1.5 MPa using the pressure plate method (Klute, 1986). Soil moisture data were subjected to an ANOVA in SAS MIXED (SAS Institute, Inc., 2003), and significantly different means were separated at P 0.05 according to Fischer's least significant difference (LSD test). Soil moisture measurements were charted over the course of each study year with 1 ± SE bars shown in Fig. 1
and 2
. Correlation analyses were performed between volumetric soil water content and dollar spot severity on dates when significant disease x soil moisture interactions were observed using PROC CORR (SAS Institute, Cary, NC).
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(yi + yi + 1)/2[ti + 1 – ti] where i = 1, 2, 3....n – 1, yi is the amount of disease (either ICs or percentage of plot area blighted, % PAB), and ti is the time of the ith rating (Campbell and Madden, 1990). For DS, AUDPC values were calculated separately for early season (i.e., IC data) and late season (i.e., % PAB data) data collection periods. The 2002, 2003, and 2004 % PAB AUDPC values include ratings taken 32, 24, and 42 d after the last fungicide application, respectively.
Treatment structure was a 2 (irrigation treatments) x 7 (6 chemical treatments and 1 untreated control) factorial with four replications. Disease data were square-root transformed, but actual means are shown in data tables. The data were subjected to a one-way analysis of variance (ANOVA) in SAS MIXED (SAS Institute, 2003). When interactions were observed, data were subjected to a two-way ANOVA in SAS GLM and separated at P 0.05 according to the protected Tukey's least significant difference test (Steel et al., 1997).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Dollar spot was slow to develop, and a significant interaction between chemical treatment and irrigation regime was not observed until 22 August (Table 1). On that date, the WA applied to frequently irrigated plots had provided a level of DS suppression equal to all other chemical treatments. Conversely, the WA applied to infrequently irrigated plots had DS levels equal to the untreated control. Similar results were observed on 26 August. Dollar spot severity was greater in infrequently irrigated creeping bentgrass versus frequently irrigated creeping bentgrass on 22 and 26 August.
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On 23 September, frequently irrigated blocks had an average of 5.4% PAB, while blocks that received infrequent irrigation had more than twice as much disease (13.1% PAB) (Table 2). A significant irrigation x chemical interaction also was observed on 23 September. In frequently irrigated blocks, PB and WA reduced DS when compared with untreated plots. In the infrequently irrigated regime, the WA was not as effective as chlorothalonil treatments or PB; however, DS levels in the WA-treated plots were less than those observed in the untreated control. In frequently irrigated blocks on 1 October, WA and PB provided the same level of DS suppression. However, in the infrequently irrigated blocks, PB (18.8% PAB) provided better DS suppression than the WA (40.3% PAB).
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2003
The year was marked by unseasonably cool temperatures and frequent rainfall. Dollar spot pressure peaked between 6 and 13 August, and soil moisture levels at this time ranged from 0.19 to 0.23 cm3 cm–3 and 0.27 to 0.29 cm3 cm–3 in infrequently and frequently irrigated blocks, respectively (no 2003 data shown). On 5 September, a significant chemical by irrigation treatment interaction was observed. Within the frequent irrigation regime, all treated plots had equal levels (0.0–2.3% PAB) of DS, which was equal to the untreated control (2.7% PAB). In infrequently irrigated blocks, plots treated with PB alone (3.7% PAB) had less DS than WA-treated plots (5.7% PAB) and the untreated control (6.9% PAB). The PAB AUDPC values, which were calculated for the period of 24 August to 15 September, showed that chlorothalonil-alone and tank-mix treatments provided a higher level of DS control (2.3 to 17.8 disease x time), when compared with plots treated with PB, WA and the untreated control. Although plots treated with PB-alone were severely damaged by DS, they had less disease (153.5 disease x time), when compared with WA-treated plots (334.4 disease x time) and the untreated control (413.5 disease x time). While soil moisture levels were lower in infrequently irrigated blocks (0.21 versus 0.26 cm3 cm–3) when DS peaked on 13 August, there were no significant DS differences between irrigation regimes in 2003. There was, however, a trend for less DS in frequently (127 disease x time) versus infrequently (141 disease x time) irrigated blocks.
2004
Because of frequent rainfall in 2003, tarps were used on 14 occasions in 2004. Soil moisture in frequently irrigated blocks remained above 0.30 cm3 cm–3 between 10 June and 12 September (Fig. 2). Except between 29 July and 18 August, infrequently irrigated blocks had soil moisture levels below 0.25 cm3 cm–3 between 10 June and 12 September.
There was a significant chemical x irrigation interaction on 21 July (data not shown). Plots treated with WA-alone (10.8 ICs) and PB-alone (6.0 ICs) in frequently irrigated blocks had DS levels equivalent to the untreated control (10.3 ICs). In infrequently irrigated blocks, PB-alone (0.0 ICs), but not WA-alone (8.5 ICs), provided a level of DS control equivalent to all chlorothalonil-treatments (0.0–2.0 IC's).
The IC AUDPC data, which were calculated for the period of 7 June to 10 August, showed that plots treated with WA-alone (1292 disease x time) had DS levels equivalent to the untreated control (1308 disease x time) (data not shown). Plots treated with PB-alone (441.6 disease x time) had less DS than plots treated with WA-alone (1292 disease x time). Chlorothalonil + PB (19.2 disease x time) and chlorothalonil + PB + WA (25.6 disease x time) provided a higher level of DS control, when compared with chlorothalonil-alone (115.2 disease x time) or chlorothalonil + WA (134.4 disease x time). There were no DS differences between irrigation regimes during this period.
Between 30 August and 17 September, there was less DS in frequently irrigated (4.1–11.5%) versus infrequently irrigated blocks (9.2–24.0%, Table 3). There were, however, few DS differences among chemical treatments, except WA-alone on 24 and 30 August and 3 September (Table 3, all data not shown). During the aforementioned period, WA-alone-treated plots had DS levels equivalent to the untreated control in both irrigation regimes. Within frequently irrigated blocks on 17 September, plots treated with PB-alone, chlorothalonil + PB, and chlorothalonil + PB + WA had lower DS levels than the untreated control (Table 3). All chemical treatments in the infrequently irrigated blocks had less DS than the untreated control at this time.
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Gray Leaf Spot
2002 and 2003
The 2002 summer was marked by prolonged periods of drought and heat stress. The AUDPC values for both irrigation regimes showed that PB-alone and WA-alone had no effect on GLS (data not shown). There were no differences in GLS levels among plots treated with chlorothalonil-alone (939% PAB x time) and the tank-mix treatments (762–1183% PAB x time). The AUDPC values showed that there was no difference in GLS levels between irrigation regimes in 2002. The 2003 study year was marked by cool temperatures and frequent rainfall, and GLS did not develop.
2004
In 2004, the rate and application interval for chlorothalonil treatments were adjusted as previously described. Gray leaf spot had dramatically intensified and there was a significant interaction by 24 August. There were greater disease levels in frequently (11.4% PAB) versus infrequently irrigated blocks (3.3% PAB) on 24 August (Table 4). All chlorothalonil-treated plots were disease-free, but plots treated with PB-alone and WA-alone were more severely blighted in the frequently irrigated blocks. Similar results were observed on 30 August. On 22 September, frequently irrigated blocks (52.0% PAB) were more severely blighted than infrequently irrigated blocks (40.1% PAB). By 22 September, GLS severity had increased substantially in all chlorothalonil-treated plots (15.0–18.8% PAB) in the frequently irrigated blocks. In infrequently irrigated blocks, however, blight levels only ranged from 0.5 to 2.5% PAB in chlorothalonil-treated plots. No GLS differences, however, were observed among plots treated with chlorothalonil-alone and the tank-mixes on any rating date in 2004 (Table 4).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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As previously noted, the wilting point of this soil was determined to be 0.22 cm3 cm–3. From the time that soil moistures levels consistently fell below 0.23 cm3 cm–3 in late summer of 2002 and 2004, DS became more severe in the infrequently irrigated blocks. In 2002, soil moisture averaged 0.15 cm3 cm–3 in the infrequently irrigated blocks on 10 July and DS developed in those blocks on 19 July (Fig. 1). Dollar spot pressure increased during late August and early September in 2002, when 87 ICs were observed in the infrequently irrigated untreated plots and 30 ICs were observed in frequently irrigated untreated plots (data not shown). Soil moisture levels during this period ranged from 0.22 to 0.24 cm3 cm–3 and 0.29 to 0.32 cm3 cm–3 within the infrequently and frequently irrigated regimes, respectively. Dollar spot severity was negatively correlated with soil moisture on 22 Aug. (r = –0.80) and 26 Aug. 2002 (r = –0.78). There was, however, only a nonsignificant trend for less DS in frequently versus infrequently irrigated blocks in 2003. Soil moisture levels fell to close to 0.20 cm3 cm–3 on three occasions in 2004 (8 June, 19 July, and 5 September) in the infrequently irrigated blocks. A significant irrigation effect, however, was not observed until 24 Aug. 2004, when soil moisture averaged 0.23 cm3 cm–3 in infrequently irrigated blocks. On 24 August (r = –0.77) and 30 August (r = 0.80), dollar spot severity was negatively correlated with soil moisture. On 3 Sept. 2004, soil moisture in the infrequently irrigated blocks averaged 0.19 cm3 cm–3, and a significant irrigation effect was observed. Dollar spot pressure in 2004 peaked in the infrequently irrigated untreated control (63.3% PAB) plots on 17 September, at which time soil moisture averaged 0.23 cm3 cm–3. On the same day in the frequently irrigated blocks, soil moisture averaged 0.36 cm3 cm–3 and only 18.5% PAB was observed in untreated control plots. On 9 (r = –0.88) and 17 Sept. 2004 (r = –0.96), disease severity was negatively and strongly correlated with soil moisture. Hence, when moisture levels for this soil approached a range of 0.20 to 0.23 cm3 cm–3 in late summer, DS became more severe in the creeping bentgrass. Low soil moisture levels in a similar range (0.16 to 0.23 cm3 cm–3) occurring earlier in the season, however, were not associated with increases in DS severity in any year. It is conceivable that when soil moisture levels were low earlier in the season, S. homoeocarpa inoculum levels were not sufficient to incite severe blighting. Indeed, it was only early in the season when ICs could be counted since disease pressure was low, and IC AUDPC values from 2002 and 2004 between irrigation regimes were not significantly different. Another important factor to consider is that Crenshaw creeping bentgrass was the host. Dollar spot can be more severe in other bentgrass cultivars in Maryland in late spring or early summer. Typically, however, peak DS activity in Crenshaw often occurs in late summer in Maryland (Bigelow et al., 2002; Dernoeden and Kaminski, 2000). Hence, these results only may apply to situations in which severe outbreaks of DS occur in late summer.
The mechanism by which low soil moisture conditions in late summer predispose creeping bentgrass to more severe damage from S. homoeocarpa is unknown. It is possible that S. homoeocarpa is more competitive under conditions of low soil moisture or that drought and other summer stresses weaken plants, rendering them more susceptible to the pathogen. Furthermore, turf managed under low soil moisture levels or drought stress grows more slowly and is less likely to recover rapidly from damage incited by S. homoeocarpa. It also is possible in soils with high moisture levels that there is an increase in microbial populations, which antagonize, compete with, or in some way reduce the capacity of S. homoeocarpa to infect plants. Finally, more N may have been available to plants grown in the frequently irrigated blocks because of an increase in mineralization in warm and wet soils. The availability of more N can stimulate recovery from blighting caused by S. homoeocarpa (Couch, 1995). It is likely, however, that other environmental (e.g., temperature, soil type, and pH, etc.) and host–pathogen interaction factors were involved and that a soil moisture level near or below the wilting point in late summer is not the only factor involved.
In all years, the chlorothalonil-alone treatment provided acceptable DS control (8–10 ICs or 0.5% PAB) on 15 of 16 rating dates early in the season, when IC data were collected (all data not shown). In both irrigation regimes over the 3 yr, however, late season PAB data showed that chlorothalonil-alone only provided an acceptable level of control on 12 of 23 rating dates. It is important to note that on nine PAB rating dates, data were obtained 14 or more days after chlorothalonil was last applied. When a chemical by irrigation interaction is observed, chemical treatments in one irrigation regime cannot be compared with the other. However, some agronomically important differences were observed between regimes when interactions occurred. For example, chlorothalonil-alone provided slightly better DS control on 9 of 13 ratings in frequently versus infrequently irrigated blocks (all data not shown). Hence, chlorothalonil, even at the reduced rate evaluated in 2004, generally provided acceptable DS control in frequently irrigated turf in late summer, when disease pressure was greatest.
The AUDPC values showed that PB-alone reduced DS levels, when compared with the untreated control in all years. On six occasions over the 3 yr when the threshold had been exceeded in untreated plots, PB-alone provided acceptable DS control (all data not shown). Another important observation was that PB-alone provided better DS control in the frequently irrigated blocks on 9 of 13 rating dates, when compared with the infrequently irrigated blocks in 2002 and 2004 (all data not shown). Over all years, PB-alone reduced DS blighting by 40 to 60% on eleven rating dates and by greater than 60% on 20 rating dates, when compared with the untreated control (all data not shown). It should be noted that the PB rate evaluated (0.12 kg a.i. ha–1) was a low label rate. Results from this study support those of Burpee et al. (1996), corroborating that PB-applied alone (0.16 kg a.i. ha–1) does provide an agronomically significant level of DS suppression.
The WA-alone was able to provide acceptable DS suppression on only one occasion when the disease exceeded the IC threshold in the untreated control. The ability of the WA to reduce DS likely is due to its ability to displace dew and thus reduce leaf wetness duration. The WA may have more effectively displaced dew in the frequently irrigated blocks, where disease pressure was generally less. On seven dates over the 3 yr, the WA reduced DS by as much as 30 to 50%, when compared with the untreated control (all data not shown). When examining all AUDPC values, however, the WA-alone only reduced DS levels twice, when compared with the untreated control.
When chlorothalonil-alone treated plots had exceeded the threshold, the mixtures of chlorothalonil + PB and chlorothalonil + PB + WA provided acceptable DS control on only three rating dates (1 Oct. 2002, 5 Sept. 2003, and 24 Aug. 2004; all data not shown). In 2004, the rate of chlorothalonil was reduced and IC AUDPC values (calculated for the period of 29 June to 10 August) showed that chlorothalonil + PB provided a higher level of DS control, when compared with all other chlorothalonil-treatments (Table 3). On 2 and 10 Aug. 2004, chlorothalonil + PB and chlorothalonil + PB + WA provided a higher level of DS control, when compared with chlorothalonil-alone and chlorothalonil + WA (data not shown). The 2004 PAB AUDPC data (i.e., late season data), however, showed that there were no DS differences among plots treated with chlorothalonil-alone or a tank-mix. Hence, data indicated that PB, which was applied at a low label rate, was more effective in suppressing DS when disease pressure was low early in the 2004 season. The AUDPC values for each rating type (i.e., IC and PAB) showed that plots treated with chlorothalonil + WA generally had DS levels equivalent to plots treated with chlorothalonil-alone. Hence, there sometimes was a benefit from applying a combination of chlorothalonil + PB, but there was no benefit provided by tank-mixing chlorothalonil + WA.
Data from this study have shown that maintaining moisture levels above 0.25 cm3 cm–3 for this soil (i.e., above the wilting point) in late summer through frequent irrigation can reduce DS severity and improve the ability of chlorothalonil, PB, and WA to suppress the disease in fairway height creeping bentgrass. Hence, in creeping bentgrass fairways where DS is a chronic problem the use of PB and WA may help to reduce DS levels as well as provide other important agronomic benefits. Because the rate and application intervals for chlorothalonil are restricted, these data will help golf course superintendents use chlorothalonil to manage DS more efficiently. Since environmental conditions are widely variable among regions, these finding and conclusion may only apply to creeping bentgrass grown in a transition zone climate in the mid-Atlantic region.
Gray Leaf Spot
Gray leaf spot was severe in 2002 and 2004, but the disease did not develop in 2003. There was no irrigation regime effect on GLS in 2002. Chlorothalonil treatments provided an acceptable level of GLS control on only 2 of 8 rating dates in 2002, and there were no disease differences among chlorothalonil treatments (data not shown). The WA and PB had no apparent impact on GLS. The 2002 data indicated that the 14-d chlorothalonil spray interval was too long to provide effective GLS control.
The application interval for all chlorothalonil treatments was reduced to 8 d and the rate was increased from 4.5 to 8.0 kg a.i. ha–1 after GLS was observed in 2004. The AUDPC values showed that GLS was more severe in frequently irrigated blocks, when compared with infrequently irrigated blocks. The chemical treatments provided an acceptable level of GLS control (0.5–1.0% PAB) on 8 of 13 rating dates in 2004 (all data not shown). On three of 13 rating dates in 2004, when interactions were observed, plots treated with chlorothalonil in frequently irrigated blocks (13.8–28.3% PAB) exhibited substantially more blighting, when compared with the same treatments applied in infrequently irrigated blocks (0.5–8.8% PAB). The difference between 2002 and 2004 was due to less disease pressure in infrequently irrigated plots in 2004, which resulted in a greatly improved level of GLS control provided by chlorothalonil in those blocks. While no benefit was observed from tank-mixing PB or WA with chlorothalonil, there also did not appear to be any negative effects when using these chemicals during a GLS epidemic.
These studies showed that under conditions of high disease pressure, and insufficient use of fungicides, GLS was very destructive, regardless of soil moisture level. Using a closer spray interval (8 d), however, chlorothalonil (8.0 kg a.i. ha–1) did effectively control GLS, but only under conditions of less frequent irrigation, which resulted in lower soil moisture (0.26 cm3 cm–3). Gray leaf spot occurs during periods of moderate to warm weather that is accompanied by periods of prolonged leaf wetness (Couch, 1995; Uddin et al., 2003a). Large numbers by P. grisea conidia undoubtedly were alighting on leaves of infrequently irrigated perennial ryegrass but may have had a reduced capacity to germinate and infect leaves because of a shorter leaf wetness duration. Less frequent irrigation also would have reduced the number of wetting and drying cycles and contributed to generally drier soil conditions, which may have reduced the capacity of the pathogen to sporulate, infect leaves, and blight. Other environmental and biological factors probably contributed to the inability of P. grisea to more severely blight perennial ryegrass in the infrequently irrigated blocks in 2004. Additional research is needed to better understand the relationship between irrigation practices and the incidence and severity of GLS.
Received for publication December 22, 2005.
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