Fruit Crops: A Summary of Research 1998

Research Circular 299-99

Pesticide Deposition in Orchards:
Effects of Pesticide Type, Tree Canopy, Timing, Cultivar, and Leaf Type

Franklin R. Hall, Jane A. Cooper, and David C. Ferree

Introduction

Pesticides are expected to continue to play a major role for the foreseeable future in protecting most crop systems from insect and disease damage. Increased concern about pesticide pollution (drift and groundwater contamination) and development of pesticide resistance combined with recent advances in low-volume spraying and integrated pest management (IPM) make it even more important that the correct amount of pesticide is applied to the target.

Orchard spraying is generally regarded as an inefficient, although usually effective, process. In the orchard, the target is complex; it may be an insect, a pathogen, or a mite, or it could be the leaves, or the fruit, or more. In addition, the target location within the canopy varies, e.g., outside edges of canopy or inside top center. Tree size and shape change during an individual growing season as well as during the span of the tree's age. Most sprays applied by traditional techniques produce a satisfactory biological result. However, a large proportion of the pesticide may never reach its intended target due to factors such as tree density, seasonal growth patterns, pruning/management characteristics, the differential retention properties of leaves from different cultivars, and sprayer-to-tree size mismatch (8,9).

This report summarizes the findings from a large project designed to determine the major parameters – for example, apple-orchardgeometry, cultivar, leaf type, spray timing, and pesticide type – governing within-canopy and off-target pesticide movement.

Materials and Methods

General

The nine-year-old apple orchard (Malus domestica Borkh) used in this study consisted of four blocks of trees containing an array of cultivars and management systems. 'Smoothee Golden Delicious' and 'Lawspur Rome Beauty' were established either in a four-wire trellis hedgerow trained as oblique palmettes on M.9 rootstock, or freestanding trained as a central leader on M.7 rootstock. A three-tree plot of each combination was established at the recommended spacing for that cultivar and rootstock ('Smoothee' – M.7, 4.5 m x 6 m; M.9, 2.5 x 3.5 m.; 'Lawspur' – M.7, 3.5 x 5 m; M9, 2 x 3.5 m), and two plots of three trees were established in each row at half the recommended row spacing. Trees in the plots at half spacing (i.e., close spacing) were either rootpruned (annually at full bloom) or mechanically hedged (annually in mid-August) to achieve additional tree size control. The cultivars 'Smoothee' and 'Lawspur,' the systems' trellis and central leader, and the treatments of wide and close spacings were compared.

An application of 1% calcium nitrate (used to mimic the application of a pesticide) at 748 liters per hectare was made using a Swanson airblast sprayer. Application was made from the west side of the canopy only, with the sprayer passing as close to each tree as possible without actually touching the canopy. Sprays were applied in April, June, and November of a single growing season.

All three trees of each plot were sprayed, but only the center tree was used for these assessments. Each plot was replicated once in each of the four blocks.

Tree canopy volumes (hence, spray collection efficiencies) can change substantially during an individual season as well as during the life span of the trees because of rootstock/cultivar interactions. One estimate of target volume and, therefore, spray volumes and delivery characteristics needed to treat a canopy is the leaf area index (LAI) which is the ratio of total area of foliage divided by the ground area (7). LAIs were developed for each planting in these studies.

Targets

Plastic tape (0.05 m wide and 2.4 m long) was used to detect calcium nitrate passing through the canopy. The tape was placed in an aluminum holder on the east (nonspray) side of the canopy, behind the center of the tree and the same distance as the radius of the widest canopy in the study. The center of the tape was positioned at the same height as the center of the tree, i.e., approximately 1.5 m for trellis and 2 m for freestanding canopies. The conductivity of each tape was plotted as a trace using a "tape-washing machine" (developed by the USDA-Application Technology Research Unit [ATRU], Wooster). This was converted into amount of calcium nitrate per cm2 tape using a calibration curve.

Leaves and pipe cleaners were used as targets to measure within-canopy deposition. Three four-leaf samples from each of four trees were taken at random from each of the west, the middle, and the east third of each canopy, at approximately the same height as the center of the tree. All targets were removed after each spray pass, taken back to the laboratory after spray completion, and stored in a refrigerator until analysis. Each sample was shaken together with 10 ml of tap water for 20 seconds, and the conductivity of the resultant solution measured using a hand-held conductivity meter. The amount of calcium nitrate per cm2 leaf surface was calculated using the regression equation from a previously obtained calibration curve, and from measuring the area of leaves using a LI-3000 (LI-COR, Lincoln, Nebr.) portable area meter.

Pipe cleaners were used as "pseudo-leaves" to measure within-canopy deposition when the leaves were very small or absent (April and November, respectively). Pipe cleaners were analyzed in the same way as leaves.

Pesticide Retention

Retention trials were carried out at three times during a growing season, at post-petal-fall (May), full foliage (July), and pre-harvest (October). Shoots and spurs were selected at random from individual trees, but the fifth leaf from the base of a shoot was always sampled, as was the fifth leaf in the rosette pattern of any spur. Leaves were stored flat in plastic bags in the laboratory until sprayed.

Six pesticides (three emulsifiable concentrates and three wettable/dry flowable powders) were chosen as representative of pesticides presently used by orchard growers (Table 1). All were applied at concentrations within the rate range recommended on the label. Tap water was sprayed as a seventh treatment.

Immediately before spraying, the weight of each individual leaf was determined using a Mettler PM460 Balance. Leaves were positioned (in a support, as per Cooper and Hall [5]) under an 8001-E nozzle and sprayed to runoff. Eight replicate leaves were sprayed from each of the cultivars, 'Smoothee,' 'Oregon Spur,' 'Empire,' and 'Lawspur.' Leaves were reweighed after 30 seconds of simulated air current, and leaf areas were measured with the LI-3000.

Equilibrium surface tension of each pesticide was measured using the DuNouy ring method. Data was analyzed using ANOVA with mean separation using LSD (14). Estimation of leaf hair density and statistical analysis were performed as per Hall et al., (10).

Results and Discussion

Canopy Deposition

Table 2 shows a decrease in within-canopy spray deposit as the distance from the sprayer increased, with approximately 50% less deposit on the east side of trellis canopies compared to more than 60% in the larger central leader canopies for the July timing only.

More spray was deposited on leaves from trellis trees than central leader trees and on 'Smoothee' leaves than 'Lawspur.' This can be explained by the leaf-area-index (LAI) data in Table 3 which shows a higher LAI value (and thus a greater density of leaves to "capture" more spray) for both 'Smoothee' and the trellis system. Significantly more spray was deposited on wide-spaced trees than close-spaced ones in this July study.

The quantity of pesticide retained on a leaf depends upon many complex factors, including the nature of the foliar surface, the physico-chemical properties of the spray solution, and the application method used. As Table 4 indicates, retention was influenced by pesticide type and formulation. Those pesticides formulated as emulsifiable concentrates (ECs) were retained less than those formulated as dry flowables (DF) or wettables (W). This study indicates that when apple leaves are sprayed to run-off, the quantity of spray retained is closely related to equilibrium surface tension, with a decrease in surface tension corresponding to a decrease in retention. Under other spraying conditions – for example, when not spraying to run-off – the use of a surfactant could result in an increased deposit.

In an overview presentation of within-canopy deposition data (Table 5), pipe cleaners clearly showed the superior capture efficiency vs. leaf or tape (Table 6) methodologies. There were also differences in wide vs. close plantings, system, and target site (near sprayer vs. opposite side of tree). The pipe cleaner data also demonstrated the lack of foliar interference with increased spray capture in November when leaves were absent.

Off-Target Deposition

Off-target data (Table 6) generally showed increased spray drift from wide- compared to close-planted systems, 'Smoothee' trellis vs. 'Lawspur' trellis, and November timing vs. in-season sprays. Drift generally decreased as distance from target trees increased, which follows accepted spray drift deposition rules.

Spray Retention

The amount of spray retained on the leaves during the study decreased at each subsequent application, with 30% more being retained at the start of the season than at the end (Table 7). This corresponded to a 60% decrease in the number of leaf hairs during the study, with the density decreasing dramatically at the October application. In the five months between the first and last spray applications, the apple leaves would have undergone physiological changes as the leaf changed from newly emerged to mature to pre-senescent. In addition, the leaves would have experienced environmental changes (e.g., abrasion) or damage by pests or disease.

Other authors have reported changes in pesticide retention with the age of the leaf, and that the effect was species specific. Bukovac et al. (4) found the chemical composition and pesticide retention of a peach-leaf surface to vary with the age of the leaf. Baker (2) noted that environmental conditions can modify the size and distribution of the surface wax structure. In many species, the epicuticular wax system of young leaves is less well-developed than that of older leaves, and the quantity of wax increases as the leaf ages (3, 4, 12, 15).

In addition, aphids, for example, generally feed preferentially on younger leaves; leaf miners occupy slightly older leaves; and various apple diseases possess similar preferential infection sites. While it is currently accepted theorem that cultivars differ in susceptibility to pests and diseases, there remains a lack of organized crop protection guidelines illustrating the complex interactions between pest density, cultivar tolerance to pests (damage sensitivity), and pesticide retention advantages offered by specific actives, adjuvant, and spray-delivery combinations.

From a chemical perspective, Hartley and Graham-Bryce (11) provide fundamental information about pesticide characteristics and pesticide/leaf-surface phenomena. Forshey (7) briefly summarizes some of the major studies of growth regulators where retention, penetration, and growth responses are measured in/on apple and other tree fruit leaves. The parameter that showed the largest difference in spray retention in these studies was leaf surface (Table 7). Significantly more spray was retained on the abaxial than the adaxial surface. This corresponded to the abaxial surface having a much denser covering of leaf hairs.

Ennis et al. (6) found marked differences in the amount of spray retained by pubescent and glabrous soybean plants, with the pubescent plants retaining more spray than the glabrous ones. Bukovac et al. (4) showed the two surfaces of peach leaves to differ markedly in the composition of the epicuticular wax, and Anderson et al. (1) noted that the abaxial surface of wheat leaves was devoid of crystalline wax, whereas the adaxial surface was covered with platelets.

There was a small, but significant, difference in the spray retention between shoot and spur leaves (Table 7). Spur leaves consistently retained slightly less spray throughout the season than shoot leaves, which corresponded with spur leaves having fewer leaf hairs than shoot leaves. In this study [and consistent with Schechter et al. (13)], shoot leaves were significantly heavier and larger than spur leaves at each of the three timings (Table 8).

Finally, the cultivars retained significantly different amounts of spray (Table 7). When the data were averaged over all three timings, 'Empire' retained twice as much spray as the cultivar 'Smoothee.' Again, the retention followed the trend of leaf-hair density, with an increase in pubescence corresponding to an increase in spray retention. An analysis of covariance (Table 9) showed there to be a significant correlation between number of leaf hairs and spray retention. However, there was only one variable, experiment timing, which gave a significant correlation with spray retention.

The obvious implication for crop protection is that rates may be able to be reduced by 50% on Empire. However, these studies were not designed for that objective, nor do data or retention consistently yield equivalent biological results. In addition, commercial apple plantings frequently contain a two- to three-cultivar mixture, thus complicating still further any crop protection decisions designed to reduce rates by cultivars. However, where solid blocks occur, rate reductions to take advantage of cultivar susceptibility, pesticide retention data, and other factors, as well as modification of spray-timing intervals, could be utilized to optimize crop-protection strategies.

Conclusions

Growers can do much with information, technology, and equipment they already have on the farm. Although airblast sprayers are not easy to adjust, operations can be accomplished to improve delivery to the wide array of tree canopies and geometries. The problem is making growers aware of the need to do so (economic, environmental, and IPM advantages) and the potential that exists for their operation by:

• Matching sprayer delivery/canopy geometry for each block.
• Developing a block-by-block crop-protection strategy (the variation in cultivar susceptibility/tolerance to various pests is under-used by most growers).
• Recognizing that time of year, system/cultivar interactions, and pesticide type/cultivar/leaf interactions all play key roles in effective spray-capture efficiencies.

An intimate knowledge of these relationships combined with historical information on cultivar/pest infestation histories and cultivar susceptibilities will aid in making effective crop protection decisions and developing successful IPM strategies.

Keeping pesticides on target – i.e., defining that target and making appropriate adjustments in spray-delivery protocols throughout a growing season – is going to be a very important issue for the tree fruit grower as the next century approaches. Faced with increasing spray costs, increasing regulations, and public pressure to use less (pesticides), management strategies that address these issues will clearly pay dividends for the grower who is willing to invest the management expertise to solve these problems.

Literature Cited

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