Does Spray Coverage Influence Fungicide Efficacy Against Dollar Spot?

Paul Vincelli and Ed Dixon, Department of Plant Pathology, University of Kentucky, Lexington 40546-0312

Corresponding author: Paul Vincelli. pvincell@uky.edu

Abstract

Chlorothalonil and myclobutanil were applied separately to creeping bentgrass using two sets of nozzles: two low-drift nozzles that provided incomplete coverage [Teejet Turbo Turfjet nozzle (1/4TTJ04-VS) and the Raindrop RA-4 nozzle (35654-2)], and two nozzles that provided nearly complete coverage [XR Teejet flat fan nozzle (XR11004-VS) and the Teejet Air Induction nozzle (AI1004-VS)], as assessed using water-sensitive paper. Spray volume and pressure were equivalent with all nozzles. For both fungicides tested, in 48% of our assessment dates over four trials, the complete-coverage nozzles controlled dollar spot better (P < 0.1) than one or both nozzles that provided incomplete coverage. We saw no instance when nozzles providing incomplete coverage controlled dollar spot better than nozzles providing complete coverage. These results support the hypothesis that achieving complete spray coverage with fungicide provides more consistent control of dollar spot than incomplete spray coverage.

Introduction

Spray coverage and penetration of the plant canopy can be influenced by nozzle factors (choice of nozzle, angle of orientation, spacing, and distance above the canopy), application factors (gallonage and pressure), and other factors (1,7). It seems intuitively sensible to assume that more complete spray coverage of turfgrass foliage would result in improved control of foliar diseases, based on the assumption that incomplete spray coverage would result in microsites free of fungicide which may provide an opportunity for infection. However, there are reasons to question this assumption:

• Redistribution of fungicide residues on leaf surfaces probably occurs readily in high-maintenance turfgrass ecosystems, considering the frequency of irrigation as well as mowing and other practices executed while foliage is wet from dew.

• Many of the most effective turf fungicides are xylem-mobile. It seems reasonable to expect that fungicide residues spattered onto turfgrass may be readily translocated throughout several tillers, especially given the high tiller density of high-maintenance turfs. Systemic translocation might therefore overcome deficiencies in canopy coverage.

Reducing off-target pesticide drift is an ongoing concern. Nozzle selection can help reduce drift because increasing droplet size is associated with less drift potential (6). Several low-drift nozzles are designed with a pre-orifice in front of the nozzle outlet, which acts to reduce liquid speed and pressure as it exits the nozzle, resulting in larger droplets. Certain low-drift nozzle designs (such as the air induction nozzle) allow air to be drawn into the nozzle body and incorporated inside spray droplets as they form. This increases the size of the liquid droplet, reducing drift (2). However, when this air-incorporated droplet strikes a plant surface, it breaks apart into many smaller droplets, potentially allowing for improved spray coverage when compared to other low-drift nozzles.

The objective of this study was to test the hypothesis that achieving complete spray coverage with fungicide provides more consistent control of dollar spot than incomplete spray coverage. We tested this hypothesis using a contact fungicide as well as a systemic fungicide. Annual reports of these tests were individually published previously (9,10,11); this paper presents a new statistical analysis combining data from all four tests.

Description of Field Trials

Four tests were conducted on ‘Penncross’ creeping bentgrass maintained under greens or fairway height of cut golf course conditions (Table 1). Plots measured 5 × 5 ft with 2-ft untreated borders, and were arranged in a randomized complete block design with three or four replications. Turf maintained as a putting green was mowed at 0.188 inch and fertilized twice with 1.0 lb nitrogen per 1000 ft² (delivered in a fertilizer with an analysis of 18-6-15): once in October of the preceding year and again in June. Turf maintained as a fairway was mowed at 0.75 inch; fertility information is not available.

Table 1. Summary of experimental conditions for field tests on dollar spot control in creeping bentgrass.

Test no.	Management*	Fungicide applications
Test no.	Management*	Start date	Interval (weeks)	No. of appl.
I	Putting green	6 May 2004	3	3
II	Putting green	19 Aug 2004	3	2
III	Fairway	21 Jul 2004	3	3
IV	Putting green	5 May 2005	4	3

* "Putting green" indicates USGA-type sand-based green maintained at 0.188-inch mowing height, "Fairway" indicates turf managed as a fairway (0.75-inch mowing height) on a Maury silt loam soil.

Fungicide Applications

We varied spray coverage by applying fungicides using each of four nozzles, each delivering a total of 1.5 gal/1000 ft² at 35 psi. Applications were made to plots by spraying half of the spray material in each of two perpendicular directions. Two were low-drift nozzles expected to provide incomplete spray coverage: the Teejet Turbo Turfjet nozzle (1/4TTJ04-VS, extremely coarse droplet size) and the Raindrop RA-4 nozzle (35654-2, coarse droplet size). Two nozzles were chosen to provide nearly complete spray coverage: the XR Teejet flat fan nozzle (XR11004-VS, medium droplet size) and the Teejet Air Induction nozzle (AI1004-VS, extremely coarse droplet size). The Raindrop nozzle is produced by Delavan Ag Spray (Mendota Heights, MN); the others are produced by Spraying Systems Co. (Wheaton, IL). Using the same sprayer and conditions used to apply fungicides, we applied water with each nozzle to water-sensitive paper (catalog no. 20301, R&D Sprayers, Opelousas, LA), and we verified that the nozzles selected for these tests were properly categorized with respect to spray coverage (Fig. 1).

Fig. 1. Spray pattern using a CO₂-pressurized hand-held sprayer fitted with one of 4 nozzles, each delivering 1.5 gal/1000 ft² at 35 psi. Water-sensitive paper turns from yellow to dark blue when wet; uniform blue represents the most uniform spray coverage.

Two fungicides were used: a contact, chlorothalonil (Daconil Ultrex 82.5WDG) and a systemic, myclobutanil (Eagle 20%EW). Daconil Ultrex and Eagle were applied at 3.2 oz/1000 ft² and 1.2 fl oz/1000 ft², respectively, in all applications. Applications were made using a CO₂-pressurized, hand-held sprayer fitted with a pair of the nozzles specified above spaced 24 inches apart and delivering 1.5 gal/1000 ft² at 35 psi at a boom height of approximately 20 inches.

Disease Assessments

Dollar spot infection centers (DSIC) were counted weekly from the central 2.5 × 2.5-ft area of each plot. Counts were log₁₀-transformed prior to analysis of variance, and statistical comparisons of treatment means for each assessment date were based on linear contrasts comparing the pooled results data for complete coverage nozzles vs incomplete-coverage nozzles. We used a P-value of 0.1 to infer a statistically significant difference because of substantial variability in the distribution of dollar spot in these naturally infested trials, as reflected in the fact that coefficients of variation exceeded 30% in over half of the assessments, even after log₁₀ transformation. Furthermore, given the relatively minor costs of choosing good-coverage nozzles as compared to the range of potential benefits of maximizing the efficacy of applied fungicides, we were interested in minimizing the risk of falsely rejecting our hypothesis. For the analyses presented here, assessment dates were included only if the mean disease level in water-treated control plots was at least 0.8 DSIC/ft², indicative of significant disease activity.

Spray Coverage and Dollar Spot Control

In 48% of our assessment dates in these four trials combined, nozzles providing complete spray coverage controlled dollar spot better (P ≤ 0.10) than nozzles that provided incomplete coverage (Table 2). This was true for both fungicides tested; there was no evidence that either fungicide was more sensitive to incomplete coverage. Often, when statistically significant differences were found, means in complete-coverage plots were in the range of 0 to 0.5 DSIC/ft² vs 0.2 to 2.0 DSIC/ft² in the incomplete-coverage plots. However, in other instances of statistically significant differences, means in complete-coverage plots were in the range of 0.6 to 3.8 DSIC/ft² vs 2.8 to 8.1 DSIC/ft² in the incomplete-coverage plots. Notably, we saw no instance where nozzles providing incomplete coverage resulted in better disease control than nozzles providing complete coverage. Thus, our study supports the hypothesis that achieving complete spray coverage with fungicides results in more consistent control of dollar spot than incomplete spray coverage.

Table 2. Number of assessment dates over four tests in which the nozzles indicated differed significantly in controlling dollar spot of ‘Penncross’ creeping bentgrass.^x

Fungicide test	Total no. assessment dates	No. assessments when complete-coverage nozzles provided better control than incomplete-coverage nozzles with specified P-value^y			No. assessments when incomplete-coverage nozzle provided better control than complete-coverage nozzles at P < 0.1
Fungicide test	Total no. assessment dates	0.1 > P > 0.05	0.05 > P > 0.01	0.01 > P
Chlorothalonil I II III IV Total	9 5 7 10 31	0 1 1 1 3	2 1 1 0 4	0 3 3 2 8	0 0 0 0 0
Myclobutanil I II III IV Total	9 5 7 10 31	2 1 0 3 6	2 1 2 0 5	4 0 0 0 4	0 0 0 0 0

^x Data were included in this table only from disease assessments where the mean number of dollar spot infection centers (DSIC) in water-treated plots exceeded 0.8 DSIC/ft², indicating moderate to severe disease pressure.

^y "Complete coverage" indicates pooled results for the Teejet Air Induction and XR Teejet flat fan nozzles; "incomplete coverage" indicates pooled results for the Raindrop and Teejet Turbo Turfjet nozzles. Linear contrasts were used for statistical comparisons.

Checking the degree of spray coverage provided by one’s sprayer is advisable, rather than assuming that adequate coverage is being achieved with a given spray configuration. For example, up to a point, one might expect better coverage with higher spray gallonage. However, in two tests, no difference was found in control of dollar spot on fairway-height creeping bentgrass between applications made in 1.0 to 1.1 gal/1000 ft² vs 2.0 to 2.5 gal/1000 ft² (4,7), and in another report, McDonald et al (5) found better control of dollar spot with chlorothalonil at 1.1 than at 2.5 gal/1000 ft². The flat-fan nozzles used in the study by Vincelli et al (8) provided approximately 80% coverage of water-sensitive paper at 1.0 gal/1000 ft² (Vincelli and Dixon, unpublished), which is considerably better than the incomplete-coverage nozzles used in the present study (Fig. 1). It seems possible that the flat-fan nozzles used in the studies by McDonald et al. (4,5) also provided acceptable coverage at both spray volumes tested, though evaluations of spray coverage were not reported in that study. Kaminski et al (3) reported that nozzles providing medium to fine droplet size and an air induction nozzle provided better dollar spot control than nozzles providing a coarse droplet size. Our present study, coupled with those just cited, suggest that it is probably important for golf course superintendents to consider degree of spray coverage being achieved as one selects sprayer configuration and other application delivery parameters.

Conclusions and Recommendations

Our results support the hypothesis that achieving complete spray coverage with fungicide provides more consistent control of dollar spot than incomplete spray coverage. Water-sensitive paper is easy to use and can quickly allow a superintendent to assess the degree of coverage expected from their equipment and spray conditions.

Literature Cited

1. Couch, H. B. 1995. Diseases of Turfgrasses, 3rd edn. Krieger Publishing Co., Malabar, FL.

2. Hofman, V., and Wilson, J. 2003. Choosing drift-reducing nozzles. Online. Coop. Ext. Serv. publ. no. FS919. Coll. of Agric. & Biol. Sci., SD State Univ., Brookings, SD.

3. Kaminski, J. E., Fidanza, M. A., Agnew, M., and Gregos, J. 2006. Impact of nozzle type on dollar spot control. Phytopathology (Abstr.) 96:S58.

4. McDonald, S. J., and Dernoeden, P. H. 2006. Preventive dollar spot control in creeping bentgrass as influenced by spray volume and a spring application of fungicides, 2005. Fung. Nemat. Tests 61:T017.

5. McDonald, S. J., Dernoeden, P. H., and Bigelow, C. A. 2006. Dollar spot control in creeping bentgrass as influenced by fungicide spray volume and application timing. Online. Applied Turfgrass Science doi:10.1094/ATS-2006-0531-01-RS.

6. Ozkan, H. E. 2001. Do "low-drift" nozzles work—an update. Turfgrass Trends 10:10-14.

7. Smith, J. D., Jackson, N., and Woolhouse, A. R. 1989. Fungal Diseases of Amenity Turf Grasses, 3rd edn. E. & F. N. Spon, London, UK.

8. Vincelli, P., Dixon, E., Williams, D., and Burrus, P. 2004. Efficacy of fungicides for control of dollar spot of creeping bentgrass managed as a fairway, 2003. Fung. Nemat. Tests 59:T007.

9. Vincelli, P., Dixon, E., Williams, D., and Burrus, P. 2005. Efficacy of fungicides and nozzle coverage for control of dollar spot on a creeping bentgrass fairway, 2004. Fung. Nemat. Tests 60:T006.

10. Vincelli, P., Dixon, E., Williams, D., and Burrus, P. 2005. Efficacy of fungicides and nozzle coverage for control of dollar spot on a creeping bentgrass sand-based green, 2004. Fung. Nemat. Tests 60:T005.

11. Vincelli, P., Dixon, E., Williams, D., and Burrus, P, 2006. Efficacy of nozzle coverageand fungicide control of dollar spot on a creeping bentgrass sand-based green, 2005. Fung. Nemat. Tests 61:T023.