Effect of application method on the control of powdery mildew (Bulmeria graminis) on spring barley

ARTICLE IN PRESS

Crop Protection 22 (2003) 949–957

Effect of application method on the control of powdery mildew (Bulmeria graminis) on spring barley J.A.S. Barbera,1, C.S. Parkinb,*, A.B.M.N.U. Chowdhuryc b

a Cranfield University, Silsoe, Bedford MK45 4QT, UK Silsoe Research Institute, Silsoe, Bedford MK45 4HS, UK c Portsmouth University, Portsmouth PO1 2DY, UK

Received 10 October 2002; received in revised form 13 January 2003; accepted 14 April 2003

Abstract Two field experiments were conducted to establish the effect spray application parameters have on the response of barley powdery mildew to fungicides. Measurements of the distribution of spray deposit within and below the plant canopy were carried out during both experiments using techniques based on the fluorescent tracer Tinopal. The volume deposited was determined by fluorescence spectrometry and the surface coverage was determined by image analysis. To remove the influence of application volume and formulation concentration, all treatments were made at 200 l/ha. In the first experiment three chemicals with differing modes of action, sulfur, prochloraz, and epoxiconazole, were used. Their intrinsic fungal pathogenicity was determined using a laboratory bioassay technique. The control of mildew by the chemicals was found to be significantly altered by nozzle type. Sprays classified under the BCPC scheme as fine and medium provided more disease control at full and half dose compared to a coarse spray and an air-included very coarse spray. In the second experiment, the influence of nozzle type on the field dose response of epoxiconazole was determined. The coefficient of variation of spray coverage was the only factor that provided a significant correlation between biological response and spray deposit. This demonstrated the significance of application method for controlling fungal pathogens, particularly at reduced dose, and highlighted the importance of uniform coverage for achieving control. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Sprays; Nozzles; Application method; Epoxiconazole; Sulfur; Propiconazole; Dose response; Mildew; Barley; Fluorescent tracer

1. Introduction Minimising chemical inputs can be achieved not only by the correct choice of chemical but also by improved application technology. There is much evidence (Cooke et al., 1985; Hislop, 1987; Holman and Miller, 1995; Chapple et al., 1997; Matthews, 1999) that this can be achieved by optimising conventional hydraulic nozzle technology. In common with all optimisation problems this requires knowledge of the response of the system to inputs; in this case the response of the pathogen to the product with varying methods of application. Whilst optimisation could be carried out purely from biological response data, previous field-based research with herbicides has indicated that more informed decisions could *Corresponding author. E-mail address: [email protected] (C.S. Parkin). 1 Present address: PHEREC, 4000 Frankford Ave., Panama City, FL 32405, USA.

be developed from knowledge of spray deposition on the target and its link to the corresponding biological response (Enfalt et al., 1997). During the development of pesticides, dose response curves are constructed from laboratory data to indicate the level and form of response over a range of doses. Although laboratory dose response curves can indicate the intrinsic activity of the pesticide, its behaviour in the field is modified by environmental factors and by the variability of spray distribution. Thus, the recommended field dose is usually established through experiments that progress from the glasshouse to the field. The end user is usually presented with either a fixed recommended dose rate or perhaps a limited range of dose rates, but within the recommendations allowance has to be made for many biotic and abiotic factors and for sub-optimal application methods. In this paper we seek to demonstrate how the field response cereal fungicides can be influenced by changes by spray application method. We test the hypothesis

0261-2194/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0261-2194(03)00110-8

ARTICLE IN PRESS 950

J.A.S. Barber et al. / Crop Protection 22 (2003) 949–957

that coverage influences control of mildew under field conditions with natural infection and differences in spray coverage resulting from nozzle selection. Furthermore, we aim to show how detailed measurements of distribution of spray in the field and intrinsic fungal pathogenicity could provide a rationale for decisions concerning application method and dose. The model system chosen to illustrate the approach was spring barley infected with powdery mildew (Bulmeria graminis f sp. hordei). This system was chosen for a number of reasons. Barley is an important crop, powdery mildew is one of the primary fungal diseases in temperate agriculture and application methods are known to influence the performance of chemicals used to control powdery mildew (Last and Partin, 1988). Two field experiments were carried out. To provide a link between changes in biological performance and the use of different nozzles spray deposits on the crop were measured using a tracer technique that measured both volume deposited and coverage. The first experiment, using three chemicals and four nozzle systems, investigated how variations in spray deposit effected the performance of chemicals with differing modes of action. Since field performance is dependent upon chemical mode of action, formulation and fungi toxicity, to enable qualified conclusions to be made on the effects of systemicity, the mildew toxicity of chemicals used in the experiment was measured using a laboratory bioassay. The second experiment focused on the influence of application method on the field dose response. Eight different doses of one of the chemicals were applied using three nozzle systems. For all experiments application volume was fixed at 200 l/ha. This enabled effects due to variations in spray coverage to be isolated from changes in spray volume. It also ensured that changes in biological activity were not influenced by changes in the concentration of active ingredient or formulating agents. However, as a consequence, the vehicle speeds and pressures used in the field experiments are not typical of those used by farmers.

2. Method 2.1. Chemicals For the study three chemicals were selected for their varying modes of action (Table 1). Sulfur (BASF Kumulus DF 80% w/w SG) is a contact compound with primarily germination inhibition protectant mode of action; prochloraz, (Agrevo Sportac 450 g/l EC) is a semi-systemic compound in terms of surface movement; epoxiconazole (BASF Opus 125 g/l SC) is a low dose rate fungicide with acropetal systemic activity.

Table 1 Biochemical properties of chemicals used in field experiments Chemical

Biochemistry

Epoxiconazole Inhibition of C-14-demethylase in streol biosynthesis Prochloraz Ergostreol biosynthesis inhibitor Sulfur Unknown (Hassall, 1982)

2.2. Laboratory dose response Bioassays for powdery mildew are not well reported, since it is a biotrophic pathogen, with a complex infection process where the mycelium can grow only on a dry surface. Investigations into inhibition of germination are at present only possible on agarose blocks, glass, and leaf surfaces. Culturing methods (Blaich et al., 1989) have been tried with varying degrees of success. Agarose block inhibition of germination has been shown to be more successful than leaf dip assays. On the agarose block the fungicide can contact directly with the conidia (Koga et al., 1979), whereas during the leaf assay conidia size (20–30 mm) and the hispid nature of the phyloplane reduces spore fungicide contact. One issue with mildew bioassays is the application of known quantities of spore to the fungicide-impregnated substrate. Mildew spores are lysed by water and so dilutions of spores for in vitro studies are not possible. Most researchers inoculate by brushing conidia onto samples, but recently a practical technique has been developed for the controlled distribution of spores onto a variety of surfaces (Chowdhury et al., 2003). In the work reported here, spore viability was measured on an agar medium containing fungicide using the technique developed by Chowdhury et al. (2003). Powdery mildew spores were inoculated onto impregnated agar plates via a Potter Tower modified for the controlled uniform distribution of spores. The membrane was vibrated for 5 min using an audio speaker, displacing the conidia through the mesh. The conidia were then left to settle for 15 min. The equipment was set to apply 5.5 mg of spores at 65–70 spores/mm2. The full dose rates were 0.625, 1.8, and 3.4 g a.i./l for epoxiconazole, prochloraz, and sulfur, respectively. Impregnated agar was prepared at eight-, two-, and 10-fold dilutions from the full dose. After spore application plates were sealed, with water applied to the lid to create high humidity, and stored in a glasshouse. Germination counts were taken at 24, 48, and 72 h. 2.3. Field experiments 2.3.1. Field treatments The plots were subjected to normal farming practice. However, the only fungicides applied to the crops were

ARTICLE IN PRESS J.A.S. Barber et al. / Crop Protection 22 (2003) 949–957

those reported here. The first experiment, examining the influence of application on chemical performance, was carried out on the 14 June 1999 with the crop at growth stage 45 (Tottman, 1987). The second experiment, investigating the influence of application method on dose response was carried out on 22 May 2000 at growth stage 50. On both occasions mildew infection was well established on the crop. Small randomised plot trials can tend to underestimate the efficacy of fungicides since unsprayed control plots provide a continuing heavy inoculum of mildew spores to nearby treatments. To counteract problems between treatments crop-free guard areas were cultivated between each plot and unsprayed control plots were further separated down wind (prevailing wind direction) from the treatments to minimise any negative infection pressure (Van der Plank, 1960; Jenkyn and Bainbridge, 1974; Bainbridge and Jenkyn, 1976). For the chemical performance experiment in 1999 treatments were arranged as randomised blocks to avoid a possible soil compaction gradient. Each plot was 3
3 m2 with a downwind guard distance of 3 m and an across wind guard distance of 2.6 m. The variety was Prisma (CPB Twyford). The field chosen for the dose response experiment in 2000 was relatively homogeneous without any noticeable change in soil type or other interference. A less susceptible variety Regina (Banks Agriculture) was selected. Each plot was 3
2 m2 with a 2.6 m wide guard area on all sides. In both experiments control crops were separated by a further 5 m down wind from the treatments in both trials. Free water is known to inhibit mildew development (Parry, 1990) requiring two sets of controls. One set of controls was untreated whilst the other was sprayed with a solution of the tracer and surfactant, using each nozzle type, but not including pesticide. The spray system consisted of four spray nozzles on a side-mounted boom fed from a 25 l tank by an electrically powered pump. The system was fitted to an All-Terrain Vehicle in 1999 and a tractor in 2000. The nozzles were positioned 50 cm above the crop and at 50 cm spacing as recommended by the nozzle manufacturer. The proper function of the nozzles was checked by patternator measurements (Barber, 2001). In the chemical performance experiment three types of flat-fan hydraulic nozzles, classified by the BCPC scheme (Southcombe et al., 1997) as producing fine, medium, and coarse sprays, and an air-induction nozzle classified as producing a very coarse spray, were used. Details of the nozzles and their operating parameters are given in Table 2. Sulfur was applied as a contact at 160, 80, and 40 g a.i./ha. Sulfur is normally applied at (B10 kg/ha) as a foliar feed and has no UK label recommendation for mildew control on barley. The rate was selected by comparison with application to other crops where it is used only as a fungicide. Prochloraz

951

Table 2 Application parameters Nozzle descriptiona

Speed (km/h)

Application volume (l/ha)

Pressure (bar)

BCPC spray class

XR 110015 XR 11004 XR 11006 AI 110025

4 8 8 6

204 194 206 198

4 2 1 3

Fine Medium Coarse Very coarse

a

Spraying Systems Cos, Wheaton, Illinois, USA.

was applied at 200, 100, and 50 g a.i./ha and epoxiconazole was applied at 64, 32, and 16 g/ha. For the dose response experiment, to limit the number of plots required, the air-included nozzle was not used and a single chemical was applied. The application rates of epoxiconazole were 125, 62.5, 31.25, 15.625, 7.813, 3.91, 1.953, 0.976 g a.i./ha. 2.3.2. Disease assessment The level of disease was assessed as the percentage area of flag leaves infected. To minimise variability constant reference was made to a standardised key. As in similar studies, the basal leaves were not used because they were senescing. The readings were taken on three separate occasions and grouped as blocks in order to reduce experimental variability. 2.3.3. Spray volume deposit measurement All treatments were applied with 2% w/v of the fluorescent tracer Tinopal CBS-X added. HPLC analysis and spectroscopic analysis (Barber, 2001) showed that there was no interaction between the fungicide and tracer. It can therefore be assumed that by using the tracer and fungicide together neither fungicidal activity nor fluorescence was compromised. Calibration standards were taken from each treatment tank for volumetric calculations of deposit using fluorescence spectroscopy. Because photo-degradation can be an issue with fluorescent tracers (Hall et al., 1993), standards were prepared using topical applications of 50 ml of a stock solution to leaves cut from the canopy. These were exposed for 40 min, a time determined as being the maximum interval between spraying and sampling. Once the spray deposits had dried, 10 tillers were taken at random from the centre of each plot. In the field, samples were temporarily placed in marked bags and stored under black-out sheeting. Once in the laboratory, they were stored in controlled temperature dark room at 0 C. Each tiller was sub-sampled into three zones (upper, middle, and lower canopy); leaves from each zone were then removed and transferred into watertight bags. Thus, approximately 10 leaves were taken from each zone in each plot, with only separate

ARTICLE IN PRESS 952

J.A.S. Barber et al. / Crop Protection 22 (2003) 949–957

plots acting as replicates. If there was no significant difference in deposit between plots where only dose was varied, then the results would be grouped by application type, thus increasing the number of true replicates. In addition to leaf sampling, six plots for each nozzle type were sampled to include stems and awns. The subsampling procedure was essentially the same; plants were divided into three zones, the stems cut at the intercept, and all material bagged. 2.3.4. Spray coverage deposit measurement Measurements of the proportion of leaf covered by spray (spray coverage) and its uniformity were made using an Optomax V image analyser fitted to a Leitz fluorescence microscope. Each sample was visualised using
40 magnification (field of view 6.11 mm2). Spray coverage was measured on three areas of leaf (apex, centre, and base) on both the adaxial and abaxial surfaces of each sample. Previous work (Last and Parkin, 1987) had shown that upward facing surfaces receive more spray than downward, but because of natural changes in leaf orientation in the field canopy this cannot be related to any distinction based on adaxial and abaxial surfaces. Therefore, surface coverage on both surfaces was combined for statistical analysis. For analysis of variance, spray coverage data for each section of the leaf and canopy was recorded as deposit area (mm2). A coefficient of variation (CV) was determined for each data set to provide an indication of the variability of coverage across the leaf, through the canopy or within treatments. The visualised spray deposits on leaves were subsequently taken for volumetric analysis using an MSE Spectro-Plus D spectro-fluorimeter. A calibration standard was created from each treatment tank sample. Each leaf sample was washed in its bag with 50 ml of distilled water and left for 3 min before volumetric measurement. To enable the volume deposited on leaves to be expressed as volume per unit area, leaf area measurements were made using the Optomax V linked up to a video camera and macro-lens. Degradation standards were collected throughout the spraying operation, analysed and the effects of photo-degradation quantified.

3. Results and discussion During the period of the experiments meteorological conditions were recorded at the nearby Silsoe Campus weather station2 (Table 3). Precipitation occurred 2 h following the dose response experiment (Table 3). This was after deposit sampling and was unlikely to 2 Weather station coordinates are Latitude 52 00,06 N Longitude 00 25,25 W 60 m above sea level.

Table 3 Meteorological data for the field experiments

Mean temperature ( C) Mean solar radiation (MJ/m2) Mean wind speed (km/h) Rain (mm/day) Relative humidity (%)

1999 chemical performance

2000 dose response

14.5 18.3 2.2 0.0 64

12.3 9.4 5.1 1.4 84

significantly effect the biological performance of epoxiconazole (Personal Communication, BASF). The results obtained from the photo-degradation samples indicated that tracer deposits were degraded by 14% in 1999 and 28% in 2000. Allowance was made for this in the calculations of deposit. 3.1. Laboratory dose response The laboratory dose response (Fig. 1) showed significant differences between the three chemicals tested with epoxiconazole showing the lowest inhibition. Because the bioassay method adopted provided direct contact between the spores and fungicides, the results demonstrate the direct effect of the fungicides and ignore any differences caused by movement within the leaf. It is therefore likely that the effectiveness of epoxiconazole, which shows considerable acropetal movement (Akers et al., 1990), was underestimated. Although sulfur appeared to be more fungi toxic the bioassay only considered germination inhibition and no other toxicological effect. Nevertheless, the results provided support for the choice of application rate for sulfur. 3.2. Field experiment—chemical performance (1999) 3.2.1. Disease control The experiment showed that epoxiconazole provided superior control in comparison to both sulfur and prochloraz (Table 4). It was presumed that this was because and the disease was already well established. The latter compounds act primarily as protectants but epoxiconazole, having systemic properties, can have curative properties. The fine and medium nozzles provided significantly better control than the coarse and very coarse (air induction) nozzles (Table 5). 3.2.2. Spray volume deposits Despite the better disease control exhibited by the fine and medium nozzles (Table 5), the fine nozzle deposited significantly (P ¼ 0:044) less chemical to the plant canopy (Table 6) than the coarse medium and very coarse nozzles. No significant interactions were observed between nozzle type and canopy height.

ARTICLE IN PRESS J.A.S. Barber et al. / Crop Protection 22 (2003) 949–957

953

100

% mortality

80

60

40

20

0 10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

1

Fractional concentration Sulfur

Prochloraz

Epoxiconazole

% Þ; where R Fig. 1. Laboratory dose response curves for the different chemical types fitted to the logistic curve (Fitted equation R ¼ a þ c=ð1 þ ebðxxÞ is the response (% mortality) to x the fractional concentration. a; b; c are fitted constants.) (72 h germination assessment).

Table 4 Average arcsine transformed % mean infection from 1999 field experiment by chemical (SED=0.567, Po0:001; LSD=1.113)

Infection

Epoxiconazole

Sulfur

Prochloraz

7.38

10.84

10.74

Table 7 Mean coverage for a 6.11 mm2 field of view from 1999 field experiment by spray class (SED=0.0471, Po0:001; LSD=0.0924) Spray class

2

Area covered (mm ) Table 5 Arcsine transformed % mean infection data from 1999 field experiment by spray class (SED=0.655, Po0:001; LSD=1.285) Spray class

Infection

Fine

Medium

Coarse

Very coarse

8.74

8.54

10.22

11.12

Table 6 Mean volume ml/cm2 deposited on barley leaves from 1999 field experiment by spray class (SED=0.0293, P ¼ 0:044; LSD=0.0581) Spray class

2

Volume (ml/cm )

Fine

Medium

Coarse

Very coarse

0.383

0.460

0.449

0.417

3.2.3. Spray coverage Significant differences were observed between canopy level, nozzle type and leaf section. The fine nozzle produced significantly lower coverage through the canopy than the coarse and very coarse (air inclusion) nozzles (Table 7). There was no significant difference between the medium and very coarse nozzle. The variation in the uniformity of spray cover with application technique and level, expressed as CV of the % cover measurements, is shown in Table 8. The very

Fine

Medium

Coarse

Very coarse

0.284

0.340

0.439

0.480

Table 8 Mean % coefficient of variation from 1999 field experiment by spray class and canopy level (SED=11.2, P ¼ 0:002; LSD=22.11) Spray class Canopy level

Fine

Medium

Coarse

Very coarse

Lower Middle Upper

69 53 58

61 91 54

90 54 74

103 108 100

coarse air-included spray produced significantly higher CVs compared to the conventional flat-fan nozzles; additionally, deposits from the fine spray were more uniform than the coarse spray. 3.3. Field experiment—dose response (2000) 3.3.1. Disease control To provide a broad comparison between laboratory and field activity of epoxiconazole, the laboratory and field dose responses (for all treatments) are plotted in Fig. 2. The dose response curves show that 104 times the dose effective in the laboratory was necessary to get a similar effect in the field. However, it should be emphasised that the purpose of the field experiments

ARTICLE IN PRESS J.A.S. Barber et al. / Crop Protection 22 (2003) 949–957

954

Fig. 2. Laboratory (solid line) and mean field (dashed line) dose response curves for powdery mildew controlled by epoxiconazole using logistic curves fitted to assessments of spore germination 72 h following treatment (Full dose is based on 125 g a.i./ha or 0.625 g a.i./l.) (Po0:001 laboratory curve; Po0:002 field curve).

Table 9 Arcsine transformed response of powdery mildew to epoxiconazole treatment from 2000 field experiment by spray class (SED=0.344, P ¼ o0:238; LSD=0.674) Spray class

Response

Fine

Medium

Coarse

21.93

21.83

22.38

was to determine differences caused by application not to provide full disease control. Thus, mildew infection in the field was allowed to be established at the time of treatment and inoculum pressures were such that complete control was not possible. To remove the difficulties experienced in analysing fractional data, the field infection data were arcsine transformed for statistical analysis and are presented in Table 9. There was a 77% probability that the fine and medium sprays improved control in comparison to the coarse spray. Although the process of arcsine transformation allowed infection data to be statistically analysed, it could have suppressed trends. Therefore, to show more clearly the differences between nozzle types the mildew response to epoxiconazole treatment was calculated from primary infection data. The data showed that epoxiconazole treatments reduced mildew infection levels compared to the control plots at full and half dose rates. Using regression analysis the response to all three sprays was plotted (Fig. 3) using a logistic curve (Po0:001). Conversion of the original epoxiconazole data to a percentage response more clearly defined the improvement in control found with the fine and medium

sprays when compared to arcsine transformation. The fine spray improved control by 18% and 38% at half and full dose, respectively, compared to the coarse spray, whilst the medium spray improved control when compared to the coarse spray by 26% and 37% at half and full dose, respectively. 3.3.2. Spray volume deposits Analysis of variance of the data relating to the volume per unit area deposited on leaves (Table 10) showed that the only significant differences were between levels in the canopy (Po0:001), with the upper canopy retaining the greatest volume. Whilst the middle and lower levels retained 40% and 24%, respectively, of the volume deposited to the upper level, no difference was observed between nozzle types. However, analysis of the upper canopy alone (Table 11) showed that the coarse spray deposited more (P ¼ 0:199) compared to the fine and medium sprays. The lower and middle levels retained similar quantities for each nozzle. Analysis of the spray volume deposited per sample weight (Table 12) showed similar differences (Po0:001) between the canopy zones, the upper canopy retained the greatest volume, whilst the middle and lower zones retained 35% and 16% of the upper zone volume, respectively. For whole plants there was no significant difference between nozzles. It is worth noting that the fine and medium nozzles deposited larger volumes in all zones compared to the coarse spray although this was not statistically significant. The leaf samples in Table 13 obtained from the upper canopy showed that significantly more spray was deposited by the coarse nozzle. In Table 12 where the leaves stems and awns were

ARTICLE IN PRESS J.A.S. Barber et al. / Crop Protection 22 (2003) 949–957

955

40 35 30

% Control

25 20 15 10 5 0 -5 0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

Fractional Dose

Fig. 3. Effect of spray quality on the field dose response of powdery mildew to epoxiconazole. Fine spray (dashed line) E; medium spray (dotted line) ’; coarse spray (solid line) m. Data fitted to logistic curves (Po0:001 fine and medium sprays, Po0:002 coarse spray).

Table 10 Mean volume (ml/cm2) deposited on whole plant samples from 2000 experiments, by canopy position (SED=0.1371, P ¼ 0:001; LSD=0.2771)

Table 12 Mean volume per sample weight (ml/g) for whole plant samples for 2000 field experiment by spray class (no significant difference) and canopy level (SED=0.1371, P ¼ o0:001; LSD=0.482) Spray class

Canopy level

2

Volume (ml/cm )

Lower

Middle

Upper

Canopy level

Fine

Medium

Coarse

Mean

0.605

1.320

3.536

Upper Middle Lower Mean

3.69 1.41 0.64 1.91

3.54 1.33 0.63 1.84

3.37 1.22 0.55 1.71

3.54 1.32 0.61 —

Table 11 Mean volume per unit area (ml/cm2) deposited on individual leaves in the 2000 field experiment by spray class (no significant difference) and canopy level (upper—SED=0.01626, Po0:001; LSD=0.03207, other levels—SED=0.0423, P ¼ 0:199; LSD=0.0851) Spray class Canopy level

Fine

Medium

Coarse

Mean

Upper Middle Lower Mean

0.333 0.141 0.082 0.186

0.327 0.140 0.086 0.181

0.397 0.127 0.080 0.192

0.340 0.136 0.083 —

measured there was no significant difference. This difference in results could have occurred because the fine and medium sprays deposited more to the vertical stems and awns than the coarse spray. 3.4. Spray coverage As shown in Table 13, significant (Po0:001) differences were observed between levels in the canopy, nozzle type, and leaf section. Leaves at the top of the canopy had the highest coverage, whilst those at the bottom of the canopy had the lowest. In the upper canopy the

Table 13 Mean cover (mm2) per 6.11 mm2 field of view for the 2000 field experiment by canopy level and spray class (SED=0.01991, Po0:001; LSD=0.03905), and % coefficient of variation (CV) by spray class and canopy level Spray class Canopy level

Fine

Medium

Coarse

All classes CV%

Upper Middle Lower All levels CV %

0.301 0.142 0.061 70.9

0.335 0.169 0.103 74.6

0.511 0.177 0.086 90.0

55.9 78.9 96.7 —

coarse spray had the greatest coverage, whilst the fine and medium sprays produced similar coverage. In the middle level of the canopy all sprays produced similar coverage. However, in the lower canopy the medium spray had the greatest coverage, whilst the results from the fine and coarse sprays were similar. The CV of spray coverage increased in the order fine, medium, and coarse and with depth through the canopy. Analysis between

ARTICLE IN PRESS 956

J.A.S. Barber et al. / Crop Protection 22 (2003) 949–957

leaf sections showed that there was more coverage on the apex of leaves than the centre or base.

4. Conclusions In terms of application method, in both experiments fine and medium sprays achieved better levels of control of powdery mildew than more coarse sprays. Although the volumes deposited onto the crop by each nozzle class were often similar, significant differences were only apparent when the leaves were assessed alone. Moreover, where differences in deposit were apparent they were often contrary to the differences in control. Therefore, it is concluded that the differences in the total volume of spray deposited were unlikely to have caused the differences in control observed. In this instance it appears that for all the chemicals used the uniformity of the distribution of spray on the crop was the causal factor. This is supported by the fact that CV of spray coverage was the only deposit characteristic that significantly segregated the efficacy attained by the fine and medium sprays. In the dose response experiment there was evidence of increased deposition to the stems and awns with the fine spray and control of infection on stems. This could have reduced overall infestation levels by removing disease harbourage and hence reinfection. However, improvements in control occurred with both the fine and medium sprays, indicating that the causal factor is likely to be the one factor that they shared; uniformity of deposit as indicated by lower values of the coefficient of variation of spray coverage. Thus, the data supports the hypothesis that fine uniform coverage is more likely to produce superior fungal control because of the greater probability of spores contacting fungicide. Furthermore, although epoxiconazole is systemic, its movement is acropetal; therefore, good uniform coverage may still be necessary especially at the leaf base and stem. The field dose response curve showed that spray class can have a marked effect on control of mildew in cereals. The lack of control at higher doses using the coarse spray indicates that it may be more difficult with coarse sprays to maintain control when infection pressures are high or when dose is reduced. This also suggests that the use of coarse and very coarse sprays may have a detrimental effect on the control of important cereal pathogens. Further work on the model system used here could create a more robust field dose response curve that would indicate the dose rate below which failure in control is likely to occur and provide evidence of the associated risks. The approach could be extended to the further use of air-induction nozzles particularly early in the growing season where deposit patterns can be more

variable (Webb et al., 2002) and target size effects more significant (Powell et al., 2002). The experiments demonstrate an approach that could be more widely adopted. The spray distribution measurements and disease assessment techniques used in this study provided useful data that can support a more dynamic field dose selection. Although the data obtained is not extensive and the conclusions that can be drawn are limited, it has been demonstrated that more informed decisions on field dose could be developed from by linking biological response to knowledge of spray deposition on the target.

Acknowledgements The authors would like to thank the Ministry of Agriculture Fisheries and Food for funding J.A.S. Barber. We would also like to thank Professors Daryl Joyce of Cranfield University, Paul Miller of SRI and Martyn Ford of Portsmouth University for their helpful discussions. Dr. Philip Russell of AgrEvo provided technical information and advice on epoxiconazole. We are also grateful to Charles Marshall for assistance with the statistics. Special thanks go to Bob Walker, Peter Grunden, Dr. Se! amus Murphy and Matthew Fisher for their assistance with the fieldwork.

References Akers, A., Kohle, H.H., Gold, R.E., 1990. Uptake, transport and mode of action of BAS 480F, a new triazole fungicide. Proceed. Brighton Crop Prot. Conf.—Pests Dis. 2, 837–845. Bainbridge, A., Jenkyn, J.F., 1976. Mildew reinfection in adjacent and separated plots of sprayed barley. Ann. Appl. Biol. 82, 477–484. Barber, J.A.S., 2001. Development of assessment techniques to facilitate reduced dose pesticide applications. Ph.D. Thesis, Cranfield University, Silsoe, p. 300. Blaich, R., Heintz, C., Wind, R., 1989. Studies on conidial germination and initial growth of the grapevine powdery mildew Uncinula necator on artificial substances. Appl. Microbiol. Biotechnol. 30, 415–421. Chapple, A.C., Wolf, T.M., Downer, R.A., Taylor, R.A.J., Hall, F.R., 1997. Use of nozzle induced air entrainment to reduce active ingredient requirements for pest control. Crop Prot. 16 (4), 323–330. Chowdhury, A.B.M.N.U., Bremer, G.B., Salt, D.W., Miller, P.C.H., Ford, M.G., 2003. A novel method of delivering Blumeria graminis f. sp. hordei spores for laboratory experiments. Crop Prot., accepted. Cooke, B.K., Herrington, P.J., Jones, K.G., Western, N.M., Woodly, S.E., Chapple, A.C., Hislop, E.C., 1985. A comparison of alternative spray techniques in cereals. Symposium on Application and Biology. No. 28, BCPC Monogram, University of Reading pp. 299–309. Enfalt, P., Enqvist, A., Bengtsson, P., 1997. The influence of spray distribution and drop size on the dose response of herbicides. Brighton Crop Prot. Conf.—Weeds 2, 381–389. Hall, F.R., Kirchner, L.M., Downer, R.A., 1993. Some practical limitations of fluorescent tracers used to measure off-target

ARTICLE IN PRESS J.A.S. Barber et al. / Crop Protection 22 (2003) 949–957 pesticide deposition. In: Divesetty, B.N., Chasin, D.G., Berger, P.D. (Eds.), Pesticide Formulations and Application Systems, 12, ASTM 1146. American Society for Testing and Materials, Philadelphia, USA. Hassall, K., 1982. The Biochemistry and Uses of Pesticides. Macmillan Press, London. Hislop, E.C., 1987. Can we define and achieve optimum pesticide deposits? Aspects Appl. Biol. 14, 153–172. Holman, K., Miller, PC.H., 1995. Precision techniques right place right dose. Br. Sugar 63 (3), 36–37. Jenkyn, J.F., Bainbridge, A., 1974. Disease gradients and small plot experiments on barley mildew. Ann. Appl. Biol. 76, 269–279. Koga, H., Toyoda, H., Mayama, S., Shishiyama, J., 1979. A new bioassay for detection of the substances to inhibit infection establishment of powdery mildew of barley. Ann. Phytopathol. Soc. Jpn. 45, 258–260. Last, A.J., Parkin, C.S., 1987. The measurement of spray deposits on natural surfaces by image analysis. Aspects Appl. Biol. 14, 85–95. Last, A.J., Parkin, C.S., 1988. Effect of spray deposit characteristics on the efficacy of propiconazole. Proceedings of the Symposium

957

International sur les Techniques D’Application des Produits Phytosanitaires, Paris, 18–19 January, pp. 241–248. Matthews, G.A., 1999. Application of Pesticides to Crops. Imperial College Press, London, p. 325. Parry, D., 1990. Plant Pathology in Agriculture. Cambridge University Press, Cambridge, UK. Van der Plank, J.E., 1960. Errors due to spore dispersion in field experiments with epidemic disease. Report of the Sixth Commonwealth Mycological Conference, UK. Powell, E.S., Orson, J.H., Miller, P.C.H., Kudsk, P., Mathiassen, S., 2002. Definining the size of target for air induction nozzles. Aspects Appl. Biol. 66, 65–72. Southcombe, E.S.E., Miller, P.C.H., Ganzelmeier, H., Van de Zande, J.C., Miralles, A., Hewitt, A.J., 1997. The international (BCPC) spray classification system including a drift potential factor. The Brighton Crop Protection Conference—Weeds, Brighton, BCPC, 5A-1, pp. 371–380. Tottman, D.R., 1987. The decimal code for the growth stages of cereals, with illustrations. Ann. Appl. Biol. 110, 441–454. Webb, D.A., Parkin, C.S., Andersen, P.G., 2002. Uniformity of the spray flux under arable boom sprayers. Aspects Appl. Biol. 66, 87–94.