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© 2012 Plant Management Network.
Accepted for publication 14 May 2012. Published 17 June 2012.


Multiyear Evaluation of an Orange Peel Oil-based Spray Additive for Managing Insect Pests and Brown Rot of Nectarine


G. Schnabel and D. Fernández-Ortuño, School of Agricultural, Forestry & Life Sciences, Clemson University, Clemson SC 29634; W. C. Bridges, Department of Mathematical Sciences, Clemson University, Clemson, SC 29634; and S. B. Hudson, Musser Fruit Research Farm, Clemson University, Seneca, SC 29678


Corresponding author: G. Schnabel. schnabe@clemson.edu


Schnabel, G., Fernández-Ortuño, D., Bridges, W. C., and Hudson, S. B. 2012. Multiyear evaluation of an orange peel oil-based spray additive for managing insect pests and brown rot of nectarine. Online. Plant Health Progress doi:10.1094/PHP-2012-0617-01-RS.


Abstract

Adjuvants are often added to pesticide formulations to enhance their effectiveness. In this multiyear field study, a proprietary formulation of orange peel oil-based spray additive containing alcohol ethoxylate (OPO-AE) was used alone or in combination with the pyrethroid insecticide esfenvalerate or the demethylation inhibitor fungicide fenbuconazole for pest and disease control of nectarine. Field performance of fenbuconazole for preharvest and postharvest brown rot control was not improved when OPO-AE was added to the spray tank. However, OPO-AE alone reduced preharvest but not postharvest fruit rot compared to the nontreated control. Esfenvalerate was consistently the best treatment to control chewing insects but the insecticidal effect of the pyrethroid was negated when the adjuvant was added. Leaf damage likely due to insect feeding was significantly higher in the esfenvalerate + OPO-AE treatment compared to esfenvalerate alone. The OPO-AE treatment by itself had significantly greater leaf damage compared to the untreated control indicating that the orange peel oil-based formulation is an insect attractant likely due to the presence of d-limonene. In conclusion, our studies do not support claims that OPO-AE formulated as Vintre increases insecticidal or fungicidal efficacy of commercial products.


Introduction

The application of pesticides, such as insecticides, herbicides, or fungicides, is a key component of modern agriculture but biological barriers must be overcome to maximize effectiveness. The leaf cuticle constitutes the main barrier controlling the rate of transcuticular diffusion of active ingredients (5,10,18,24). It is an extracellular lipoid membrane consisting of the biopolymer cutin and associated cuticular waxes, which are semicrystalline solids responsible for low solubility and mobility of organic solutes (21,27,28). To overcome this barrier, adjuvants are added to the pesticide formulation of an active ingredient or added as a separate product to spray tanks. They enhance the effectiveness (bioavailability) of the pesticide formulation by raising the solubility or the compatibility of the active ingredients with other spray constituents; improving adsorption, penetration, and translocation into the target; increasing rain fastness, and altering selectivity of the active ingredient toward different plants (13,14,15). Most adjuvants are either solvents or surfactants, with non-ionic forms of the latter often being utilized (15,32,33).

Of the non-ionic surfactants, alcohol ethoxylates (AEs) are used as adjuvants in a wide variety of agrochemical formulations to enhance active ingredient effectiveness (20). Numerous studies have demonstrated that, in addition to improving spray retention and leaf wetting, AE adjuvants may also increase cuticular permeability (11). Research with adjuvants has predominantly focused on herbicide performance (14). Only few publications refer to other types of pesticides. For example, performance enhancements were shown for AE adjuvants in combination with dimethomorph and 1-(4-chlorobenzyl)-4-phenylpiperidine fungicides in controlling grapevine downy mildew and barley powdery mildew, respectively (16,17). With respect to insecticides, synergistic effects were seen with different insecticide classes when combined with some linear AEs for German cockroaches control and management of other insects (29).

VINTRE (Oro Agri, Trophy Club, TX) is a proprietary formulation of a cold-pressed orange peel oil-based spray additive containing alcohol ethoxylate (OPO-AE). It is currently registered for use on vines and trees throughout the United States with the exception of Arkansas, Kentucky, Mississippi, Tennessee, Utah, West Virginia, and Wyoming. According to the company’s prospectus, VINTRE improves the distribution and efficacy of miticides, insecticides, fungicides, herbicides, nutrients, and plant growth regulators in vineyards and orchards, but to our knowledge very few scientific publications are available that support such claims. The objective of this study was to investigate the potential beneficial effects of the adjuvant VINTRE for insect and disease management in nectarine. Specifically, VINTRE was applied in three consecutive years with the demethylation inhibitor fungicide fenbuconazole and the pyrethroid esfenvalerate for brown rot and insect control, respectively.


Establishment of Experiments

The experimental nectarine orchard was located at the Clemson University Musser Fruit Research Farm, Seneca, SC. The orchard consisted of cultivar ‘Juneprincess’ trees established in 2003 on ‘Guardian’ rootstock. Spacing between trees and rows was 6 m and a standard herbicide strip was formed. Insecticides, fungicides, and herbicides were applied according to recommendations of the Southeastern Peach Spray Guide to all trees until the phenological stage of “pit hardening” after which no more cover sprays (insecticides and fungicides) were applied. Thinning was conducted by hand twice within 2 weeks after the phenological stage of “shuck split.” Treatments were arranged in a randomized complete block design with 4 single-tree replicate trees per treatment. Trees were re-randomized each year. OPO-AE was applied at 0.2% vol/vol in form of VINTRE (OroAgri) (containing 8.92% AE); insecticide esfenvalerate was applied at 992 g/ha in form of Asana XL14 (DuPont, Wilmington, DL); and the fungicide fenbuconazole at 142 g/ha in form of Indar 75 WSP (Bayer Crop Science, Raleigh, NC). The application timing is indicated in Tables 1, 2, and 3.

Trees were sprayed to runoff (17 liter/tree) at 1724 kPa pressure with a handgun sprayer 28, 14, or 7 days preharvest, on 15 and 29 May and 5 June 2009; 18 May and 2 and 12 June 2010; and 19 May, 2 and 9 June 2011, respectively. When fruit were at commercial shipping maturity on 12 June 2009, 20 June 2010, and 16 June 2011, 100 fruit from each experimental tree were rated visually while still hanging on the tree for the presence of oozing, catfacing, russeting, chewing, and brown rot, and percentage damage/disease incidence was calculated. Afterwards but on the same day, 50 asymptomatic fruit of similar maturity stage were arbitrarily picked from each tree and stored for 7 days. Fruit were placed in two 25-pocket plastic trays in cardboard boxes, and stored in an air-conditioned room at 21 to 22°C. Decay was calculated as a percentage 3 and 7 days postharvest, which was 15 and 19 June 2009, respectively, 23 and 27 of June 2010, respectively, and 19 and 23 June 2011, respectively. Leaf quality was rated at the day of harvest each year in form of missing leaf tissue (leaf damage). Ten randomly selected one-year-old branches were selected around each experimental tree and the youngest 10 fully developed leaves on those branches (100 leaves per tree) were rated visually and individually on a scale of 0, 1, 2, or 3 corresponding to 0, >0 to 5, >5 to 25, and >25% leaf damage severity. Leaf damage incidence was the sum of all means in leaf severity categories within a treatment.


Data Analysis

A statistical model was developed that related the responses (chewing, total fruit damage, oozing, etc.) to the experimental factors of treatment, year, and treatment by year interaction. The method of least squares was used to estimate the model terms associated with the factors, and analysis of variance was used to test for a significant effect of the factors on the response means. If a factor was found to be significant, mean separation (using student’s t test) was used to further determine the nature of the effect of the factor on the responses.

Due the nature of the data collected for leaf damage and the resulting distribution, a non-parametric analysis based on ranks was used to estimate the model terms associated with the factors and test for significance. The results of the non-parametric analysis were largely consistent with a traditional analysis of variance results, and therefore we present the results for traditional analysis of variance for ease of interpretation. Only changes in significance when conducting the non-parametric analyses were in the year effects. For the > 5 to 25 category, the year effect changed from significant (P = 0.0004) in a regular analysis to not significant (P = 0.2957) in a non-parametric analysis. For the > 25 to 100 category, the year effect changed from not significant (P = 0.3006) in a regular analysis to significant (P = 0.0001) in a non-parametric analysis. All calculations were performed using the statistical package JMP version 9.0.0 (2010, SAS Institute Inc., Cary, NC) and all tests were performed with a = 0.05.


Fenbuconazole Did Not Perform Better in Combination with VINTRE

Adjuvants are additives commonly applied with pesticides to improve spray performance (30). However, the potential benefits of adjuvants on improving fungicide efficacy have been demonstrated with relatively few chemistries and crop species under field conditions. Numerous studies have shown a benefit from adjuvants in laboratory and greenhouse environments (1,31), but few have translated into improved disease suppression in the field (30). It was concluded that the interaction of adjuvant and the pathogen, pesticide, and plant in the field is complex and variable. In this study, the adjuvant VINTRE did not improve the efficacy of the fungicide fenbuconazole for preharvest or postharvest brown rot control. There was a significant treatment and year interaction for preharvest- (P ≤ 0.0001), but not for postharvest brown rot control (P = 0.32 for 3 dph and P = 0.91 for 7 dph treatments). The interaction occurred due to conducive environmental conditions leading to heavy brown rot pressure in experimental year 2009, which caused disease incidence values to be higher compared to years 2010 and 2011. The comparisons among the treatments within years were very similar for all three years, however, and therefore we combined the means across years to simplify the display and interpretation of our results (Table 1). All treatments, except esfenvalerate + OPO-AE significantly reduced brown rot incidence at harvest, the most efficacious treatments being the ones containing fenbuconazole. After 3 and 7 days of storage, fenbuconazole-containing treatments were still superior compared to all other treatments, but there was no difference between the fenbuconazole and the fenbuconazole + OPO-AE treatments. Similarly, OPO-AE did not improve the performance of esfenvalerate at harvest (Table 1).


Table 1. Average of 3 years' results from 2009 to 2011 on the effect of fenbuconazole and esfenvalerate alone or in combination with OPO-AE on brown rot of nectarine at harvest and 3- and 7 days post harvest (dph).

Treatment Application
timing
(days
before harvest)
Fruit rot (%)
Harvest 3 dph 7 dph
Untreated  22.63 a* 45.82 a 70.45 a
OPO-AE 14,7 14.89 b  39.73 ab 59.47 a
Fenbuconazole 14,7  4.39 c  6.36 c 15.4 b  
Fenbuconazole + OPO-AE 14,7  5.40 c  7.06 c 14.86 b
Esfenvalerate 28,14 15.38 b  37.45 ab 64.29 a
Esfenvalerate + OPO-AE 28,14  17.57 ab 33.07 b 58.23 a

 * Numbers within columns followed by the same letter are not significantly different according to ANOVA followed by student's t test (a = 0.05). Values are means of four single-tree replicates per year; each replicate consisted of 100 (preharvest) or 50 (postharvest) peach fruit.


Similar failure of adjuvants to provide beneficial effects in tank mixtures for the control of tree fruit and grape vine diseases were reported recently (6,7,26), but some field trials reported slight improvement of plant disease control (23,34). We are only aware of two other peer-reviewed studies using VINTRE as additive. In the first study VINTRE applied at 0.25% (v/v) in mixture with quinoxyfen (Quintec, Dow AgroScience) provided significantly better powdery mildew control of grape compared to quinoxyfen alone but the same study also showed no significant improvement of the activity of trifloxystrobin (Flint, Bayer CropScience) when tank mixed with VINTRE (9). In the second study VINTRE at 0.25% provided no benefit for powdery mildew control when added to myclobutanil (Rally, Dow AgroScience) and quinoxyfen used in a rotational program (8).


Esfenvalerate Did Not Perform Better in Combination with VINTRE

Esfenvalerate is used to control numerous biting and sucking insects affecting peach and nectarine fruits, leaves, twigs, limbs, and trunk. Some of the most common ones are plant bugs and stink bugs causing fruit distortion (also called catfacing); thrips causing fruit russeting especially on nectarines; Japanese beetles and grasshoppers causing chewing damage on fruit and leaves; and plum curculio and others causing oozing of fruit (19). Most catfacing and russeting of nectarine fruit was most likely initiated early in the season prior to the first application of experimental treatments, which explains the lack of difference in mean values among treatments. The chewing and oozing activity, however, occurs throughout the season. The treatment by year interaction was found to be not significant for oozing (P = 0.43), catfacing (P = 0.21), russeting (P = 0.34), chewing (P = 0.64), and total fruit damage (P = 0.6). Therefore, the data are shown combined for all experimental years (Table 2). While very little oozing of fruit was observed in this study, chewing activity was comparably high. Esfenvalerate was consistently the best treatment but interestingly, the insecticidal effect of the pyrethroid was negated when OPO-AE was added.


Table 2. Average of 3 years' results from 2009 to 2011 on the effect of fenbuconazole and esfenvalerate alone or in combination with OPO-AE on fruit damage of nectarine.

Treatment Application
timing

(days
before
harvest)
Fruit damage (%)
Oozing Cat-
facing
Russet-
ing
Chewing Total
Untreated 0.97 1.03 2.20   6.27 bc* 10.48 ab
OPO-AE 14,7 1.16 0.74 3.15  8.90 ab 13.95 a
Fenbuconazole 14,7 0.65 1.30 3.43  9.59 ab 14.98 a
Fenbuconazole
+ OPO-AE
14,7 0.60 0.62 3 11.23 a   15.46 a
Esfenvalerate 28,14 0.67 0.69 1.55   4.57 c    7.50 b
Esfenvalerate
+ OPO-AE
28,14 0.61 1.01 2.28   9.14 ab 13.03 a

 * Numbers within columns followed by the same letter are not significantly different (P < 0.05) according to ANOVA followed by student's t test. Values are means of four single-tree replicates per year; each replicate consisted of 100 fruit.


VINTRE May Contain an Insect Attractant

The treatment by year interaction was found to be significant for the “>0 to 5%” and “>5 to 25%” leaf damage severity and the “leaf damage incidence” categories (P values of 0.0011, < 0.0001, and <0.0001, respectively). The interaction was not significant for the “>25%” category (P = 0.09). However, careful analysis of the interaction between treatments and years revealed that although the absolute numbers between years varied due to variable disease pressure and associated disease incidence and severity, the comparisons among treatments were very similar. We therefore combined the means of years to simplify the display and interpretation of our results (Table 3). The esfenvalerate + OPO-AE treatment contained almost twice as many leaves with missing tissue compared to the esfenvalerate treatment alone. In fact, even OPO-AE alone showed significantly more leaf damage than the control, suggesting the presence of an insect attractant in the VINTRE formulation. Feeding insects, however, were not observed during data collection. Phytotoxicity cannot explain the leaf damage because leaf damage did not increase when VINTRE was applied at 1.5 and 2× the label rate to single nectarine trees (data not shown) and because phytotoxicity would have been expected to cause greater leaf damage in the fenbuconazole + OPO-AE treatment.


Table 3. Average of 3 years' results from 2009 to 2011 on the effect of fenbuconazole and esfenvalerate alone or in combination with OPO-AE on leaf damage severity and incidence of nectarine

Treatment Application
timing

(days
before
harvest)
Percent leaves with Damage
incidence

(%)
>0 to 5% damage >5 to 25% damage >25% damage
Untreated 30.64 de  8.65 c 1.24 ab 40.53 bc
OPO-AE 14,7 56.93 a   19.26 b 0.83 ab 77.02 a  
Fenbuconazole 14,7 27.61 e    6.56 c 1.11 ab 35.28 c  
Fenbuconazole
+ OPO-AE
14,7  41.36 bc  6.73 c 0.69 ab 48.78 b  
Esfenvalerate 28,14  37.75 cd  2.94 c 0.49 b    41.18 bc
Esfenvalerate
+ OPO-AE
28,14  49.23 ab 27.29 a 2.31 a 78.83 a  

 * Values within columns followed by the same letter are not significantly different (P > 0.05) according ANOVA followed by student's t test. Values are means of four single-tree replicates per year; each replicate consisted of 100 peach leaves.


According to the label, VINTRE contains 8.92% alcohol ethoxylate and 91.08% constituents ineffective as spray adjuvants likely derived from the orange peel. The citrus peel contains a large volume of essential oils, and most components of the essential oils are monoterpene compounds. (+)-Limonene is the most abundant of these compounds (97% of total terpene in orange fruits) (12). The cold-pressed orange oil in VINTRE consists of approximately 90% d-limonene in bound form according to a company’s prospectus. However, the strong citrus fragrance left in the orchard after aerial application does indicate that not all d-limonene is bound or inactive. There is strong evidence that monoterpenes like limonene are involved in signaling and inducing plant defenses (3). In fact, limonene appears to act as an attractant for insects. For example, males of C. capitata were less attracted to fruit expressing low levels of limonene in the peel compared to the control fruit (25). In the same study, fruit with reduced limonene contents were more resistant to the fungus Penicillium digitatum or the bacterium Xanthomonas citry subsp. citri, indicating that limonene accumulation in the peel of citrus fruit is involved in host-pathogen interactions. Another study showed stimulation of egg-laying behavior of Lobesia botrana when exposed to plant volatiles from grape bunches with high amounts of limonene (2). In other studies, limonene has been found to act as a repellent, however, limonene and other terpene compounds of citrus peel conferred partial resistance in fruit to the Mediterranean fruit fly, medfly (Ceratitis capitata) a major pest of citrus species worldwide (22) and to other tephritid pests (4).


Conclusions and Management Recommendations

VINTRE did not enhance the activity of esfenvalerate or fenbuconazole for pest control of nectarine, adding new evidence to the small but increasing body of literature that the addition of OPO-AE to spray tanks do not provide consistent benefits under field conditions. The evidence presented in this study also suggests VINTRE is an insect attractant with potentially associated negative impacts on leaf quality and plant health.


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