Impact of Potential Organic Pesticides and Potential Fruit Crop Load Regulators on Photosynthesis and Growth of Apple

Final Report for GS04-033

Project Type: Graduate Student
Funds awarded in 2004: $10,000.00
Projected End Date: 12/31/2005
Grant Recipient: University of Arkansas
Region: Southern
State: Arkansas
Graduate Student:
Major Professor:
Dr. Curt Rom
University of Arkansas
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Project Information


Three experiments were conducted between 2004-2006 to test the effectiveness of a greenhouse model for screening potential organic pesticides and crop growth regulators. In experiment I, 21 compounds were tested at 2 % concentrations for their effect on plant gas exchange and growth. Out of these 21 compounds, 11 had significant effects on plant gas exchange. In experiment II, three of the most potential compounds from experiment I were chosen to screen at various concentrations. Potassium sulfate (K2SO4) and copper sulfate (CuSO4) reduced plant gas exchange at various concentrations. However, reduced clove oil concentrations had no effect. In experiment III, a field study was used to test the potential of these compounds in an apple orchard. Although treatments were not significantly different than control, tree limbs treated with 2% Crocker’s fish oil (CF), 2% K2SO4, and 2% CuSO4 showed the highest reduction of fruit retention compared to all other treatments. The model system was useful but had limitations due to plant-to-plant variability.

Tables, figures or graphs mentioned in this report are on file in the Southern SARE office.
Contact Sue Blum at 770-229-3350 or for a hard copy.


Fruit production is an important component of the agricultural market-basket of states in the southern region. Fruit production in the region is characterized by high intensity and management inputs, the high capital investment costs, the high production risk, but also the potential for high value return on the investment. Therefore many fruit production systems in the southern region are small acreages, are family farms, have multiple crop production systems, and the crop is typically direct marketed through roadside markets, local farmers’ markets, and local/regional retail outlets. Fruit growers in the southern region are looking for means of reducing production costs while looking to enhance crop value. Fruit growers also realize that their production system is a long-term commitment to both the enterprise and to their land due to the perennial nature of the crops. Therefore, issues of sustainability are paramount to fruit growers. For the fruit crop production system in the southern region to be sustained and to continue to develop, while minimizing negative impacts on the environment or resources, new technologies for sustainable production must be developed and evaluated. A primary focus of our fruit crops research group at the University of Arkansas has been to study crop physiology and the impact of management pursuant to developing technologies that harness that information and improve the productive potential of the crops. More recently, we have launched extensive efforts to develop and test methods for use in organic fruit production.

There has been a recent increase in the production of certified organic and sustainable tree fruit production based upon philosophies of environmental stewardship, enhancing soil and productivity of soil and the soil-plant production relationship, maintaining and improving water quality, and capitalizing on crop value and thus the economic sustainability of the system. Although organic fruit production is increasing, it is limited by a lack of science-based technology. Farmers producing certified organic fruit currently have limited use of IPM technologies due to restrictions on pesticides, which may be used in conventional management. Typically fruit produced in organic systems require the farmer to spray multiple applications of numerous chemicals to achieve a similar response to the few chemicals used in conventional IPM. The farmer is faced with additional costs and labor.

Alternative crop load regulators, also known as fruit thinners, and pesticides are needed for both certified organic and conventional fruit production. Prior to creating new management technologies, it is necessary to have an understanding of the physiological basis or impacts of the management treatments. Gas exchange and growth by plants are sensitive indicators of plant response to management or environment. For example, a transient reduction in photosynthesis (Pn) has proven to be an effective technique used for fruit thinning. Conversely, pesticides, which reduce Pn, may be detrimental to plant growth.

Previous studies have shown pesticide applications cause a decrease in photosynthesis (Pn) and growth due to stomatal clogging, osmotic stress, or disturbance in the electron transport chain (Aucther and Roberts, 1933; Ayers and Barden, 1975; Barden 1971; Byers, et al. 1990; Ferree and Hall, 1975, 1978; Heinecke and Foot, 1966). Preliminary studies in our lab have shown that surface-active compounds such as salts, osmotics and oils may reduce Pn (unpublished data). Before recommendations for chemical use in organic production can be made, it is necessary to understand the impact on and physiological response of the tree to those chemicals. Compounds which may have a place in organic production, such as oils (biological and mineral), salts, minerals, or bio-regulants may have an impact on plant Pn and resulting growth and therefore should be studied.

Fruit thinning is essential to fruit production in order to ensure fruit quality and size, and sustains the tree to annually bear marketable fruit. A number of chemical thinning treatments are available to conventional growers but none are currently registered or recommended to certified organic growers. Organic growers typically rely on mechanical removal of excessive fruitlets by hand labor, which is costly to the farmer. In order to test potential organic fruit thinning treatments and develop reliable, economic thinning technologies, it is necessary to create a model system to test organically certifiable compounds and understand their mechanism of action.

Over the past four decades, research has demonstrated the value and appropriate timing of fruit thinning techniques. Early studies showed a relationship between factors such as shade, the Pn reduction, and affects on fruit growth (Heinecke, 1966). Currently, fruit thinning is accomplished through synthetic plant growth hormones, herbicides and caustic chemicals, or by mechanical means (hand removal of flowers or fruitlets). Due to the expense of hand removal of flowers and fruitlets, most fruit thinning research has focused on chemical methods of application. There have been many studies on conventional means of fruit thinning. Past studies have shown that herbicides such as terbacil, insecticides such as carbaryl, growth regulators such as ethephon are useful thinning techniques (Byers et. al,1990). Other chemicals such as naphthaleneacetic acid (NAA) and 1-napthyl methylcarbomate (carbaryl) have been studied intensively over the years (Ferree and Hall, 1978).

Previous research on organic fruit thinning agents is limited. Recent studies, however, in our lab and elsewhere have shown there is potential to use organic pesticides as chemical thinning agents in apple and peach. Applications of lime sulfur (LS), which is registered for the control of Erwinia amylovora (fireblight), has the potential to act as a caustic agent for pesticide and fruit-thinning purposes (unpublished data). This compound was studied in the early 1900’s and was proposed to be an effective thinning agent (Auchter and Roberts, 1933; McDaniels, et. al, 1935). Our lab analysis shows this compound at recommended concentrations has a pH in the range of 10-11 and strong electrical conductivity was acting as a strong base and dielectric salt in solution. When applied as a thinning agent, it is theoretically capable of stressing the leaves of the tree due to osmotic tension on stomatal and epidermal cells therefore lowering Pn. LS has been studied as a bloom thinner (Auchter and Roberts, 1933) acting by means of stressing flowers or “burning” floral parts preventing pollination, but it may also be effective by other means.

Oils are used as an insecticide for aphids and may have potential for fruit thinning. Organic apple growers in the Northwest are using combinations of 2% LS and 2% fish oil as a thinning agent based upon our work and that of other labs. Many oils used for pesticides have been observed to promote flower or fruit drop. Soybean oil has been studied as a bloom thinning agent and showed significant thinning potential on dormant trees. It reduced the labor of hand thinning and quickened fruit maturity in peach trees (Moran, et. al, 2000). A study of oils in combination with other chemicals caused reduced Pn in apple (Ferree and Hall, 1975). Weller and Ferree (1975) showed a pinolene base antititranspirant could reduce transpiration and therefore impacted net Pn, shoot growth, and fruit growth. We have observed that various concentrations of oils (CF, soybean oil, corn oil, etc.) can be mixed with non-ionic surfactants and cause reduced Pn. Different oils may also show varying effects on phytotoxicity and other potential effects on the leaf’s morphology. These characteristics are ideal for a potential contact herbicide. As demonstrated by Byers, a slight herbicidal effect may be a useful strategy for fruit thinning.

In our preliminary studies (McAfee and Rom, 2003), we have found that foliar applications of various salts such as sodium chloride (NaCl), potassium bisulfate (KHSO4), potassium bicarbonate (KHCO3) and calcium chloride (CaCl2) can stress leaves and cause a reduction in Pn, stomatal conductance, and transpiration. This may result in damage to guard cells surrounding the stomates of the leaf. Although high concentrations can defoliate a tree, at low concentrations, these salts may have the potential to be an effective thinning agent.

Information on the effect of pesticides on physiology and growth of crop plants may have two outcomes. If it is found that a pesticide, organic or conventional, reduces Pn and growth of the plant, that information is important to determining how the chemical fits into a management program. As a result of that knowledge informed decisions may be effectively made about spray concentration, timing, repeat applications, crop load management, etc. Further, it has been demonstrated that apple trees may have some ability over the long-term to “compensate” photosynthetically for damaged foliage (DenHerder and Rom, 1992). Although a chemical reducing Pn may be perceived as “negative information”, there are significant positive uses. Chemicals that cause a transient reduction in Pn may be useful as fruit thinning chemicals or as growth regulating agents. The proposal is an attempt to search for those compounds for that specific use.

It will be important to consider other effects of these treatments on the crop. Many thinners have shown to cause significant damage on fruit appearance (russet). During the 2nd Annual National Organic Tree Fruit Research Symposium, Dr. Jim Walker said that more focus should be put on finding new methods or alternatives to lime sulfur need to be found due to sensitivity amongst different apple cultivars. Various cultivars have shown sensitivity to sulfur treatments such as phytotoxicity and fruit russet. Due to its relative effectiveness, this treatment will be a good treatment to compare with new potential treatments. Other treatments in the study may show similar side effects as lime sulfur. It will be important to evaluate this in the field studies.

One of the problems in tree fruit research of this nature is the field production cycle. Apple trees only bloom during a short period in the spring each year. If potential thinning agents were tested directly in the field, it would take immense effort, acreages, and resources to screen and test, and make appropriate measurements in the field. Further, those field studies would be prey to environmental catastrophe such as frosts, hail, heat, etc., over which there may be little control but be ruinous to the study. In this proposal, we intend to use small, vegetatively growing apple trees as a model plant system to screen and test potential organic pesticides in order to create a physiological-based information set from which more appropriate field tests may later be based.

Therefore, a three-year study is proposed to measure plant response to foliar applications of various acids as potential organic pesticides or growth regulating treatments. Treatments of alternative crop load regulators were applied to vegetative apple trees under controlled environmental conditions to study effects on gas exchange and growth. Gas exchange measurements included photosynthetic assimilation (A), evapotranspiration (Et), and stomatal conductance (gs). Growth measurements included stem diameter, stem length, stem weight, average leaf area, average leaf weight, and specific leaf weight. This model system for screening new compounds will establish a base for studying additional compounds that may have the potential to be effective organically certifiable pesticides or fruit thinning agents.

Project Objectives:
  1. To evaluate potential alternative crop growth regulators for effects on A, Et, gs, and plant growth of vegetative model apple trees grown in the greenhouse.

    To evaluate potential alternative crop growth regulators at various concentrations.

    To evaluate most effective concentrations in the orchard for crop growth regulating potential.


Click linked name(s) to expand
  • Jason D. McAfee


Materials and methods:

Experiment I evaluated potential crop growth regulators at 2% concentrations. Five classes of potential crop load regulators and pesticides were evaluated in five studies with a total of 21 treatments. Treatments for the five studies were classified as:
Study 1. Fatty oils

Study 2. Sulfur compounds
Study 3. Organic acids
Study 4. Miscellaneous compounds
Study 5. Essential oils
All compounds were screened at treatment concentrations of 2%, applied to small, vegetatively growing apple trees grown in a greenhouse and evaluated for effects on gas exchange, growth, and leaf morphology as described below.

M.106 apple rootstock liners were planted in 1.9 L pots with a sufficient soil medium and grown in the greenhouse. At planting, trees were cut back to three nodes and subsequently new growth was trained to a single shoot and all lateral buds were removed. Plants were grown in a greenhouse with temperatures of 25-30/18-20 degrees C (day/night). Trees were watered as needed. Pests were controlled only if detected by scouting. Trees were divided into a number of replications based on the number of treatments (n+1) following previous experience and model power analysis in order to show separation of treatments. The trees were sorted by size at the beginning of the study. A completely randomized design was used for the experimental design. Mean separation was analyzed using LSD from the SAS Proc GLM procedure. For all studies planned, a “water-sprayed” or untreated control was used. Treatment data was analyzed as untransformed data but expressed graphically as a percent of the control. When shoots were approximately 10-15 cm in height, treatments were applied. Treatments will be applied once with 1 L spray bottles until leaves were thoroughly drenched.

After treatment, gas exchange was measured over a 21 day response period using a CIRAS-1 differential CO2/H2O infra-red gas analyzer with integral cuvette air supply unit and Parkinson leaf cuvette with an automatic light control (LED unit). Leaf chamber conditions were set for 50% RH, 350 ppm [CO2]. PAR light saturation for all measurements was set at 1000 µmol/m2/s and temperature of 25-32 degrees C at the leaf surface maintained. The CIRAS-1 was used for the measurements of photosynthetic assimilation (A), evapotranspiration (Et), and stomatal conductance (gs). Two sample leaves, 5-7 nodes from the most recently unfolded leaf, per treated tree was labeled and measured on various dates. Each leaf was labeled for continued measurement of the same leaf.

Growth measurements and leaf morphology included shoot height (cm), diameter (mm), shoot dry-weight (g), average leaf area (cm2), average leaf dry weight (g) and specific leaf weight (g/cm2). Following the 21-day treatment period, all treated trees were destructively sampled for the same measurements. Leaves were removed and divided between treated and those emerged subsequent to treatment. Changes in growth and leaf morphology during the treatment period as effected by treatment were calculated.

Experiment II evaluated potential crop growth regulators at various concentrations. Three compounds were chosen from the five classes of chemicals from experiment 1. Three studies were conducted to evaluate three potential compounds at various concentrations. Citric acid and potassium sulfate (K2SO4) were evaluated at 0, 0.25, 0.5, 1, and 2 % concentrations. Clove oil was evaluated at 0, 0.025, 0.05, 0.125, and 0.25 % concentrations. Measurements for Pn and growth are the same as experiment 1.

Experiment III evaluated potential crop growth regulators in the orchard. Trials were established in the field to test post-bloom fruit thinning potential using spray applications of the most promising treatments. Prior to full bloom, trees for test were selected and tagged in a completely randomized design with approximately 10 replications. Ten flower clusters were counted per limb to represent one replication. Each treatment was replicated one time per tree. Approximately 5-10 days after bloom when fruits are 10-15mm diameter, treatments were applied. Treatments included:
1. 2% CF + lime sulfur (LS)
2. 2% LS
3. 2% CF
4. 0.5% Cinnamon oil
5. 0.25% Clove oil
6. 2% Ammonium sulfate ((NH4)2SO4)
7. 2% Copper sulfate (CuSO4)
8. 0.5% Cedarwood oil
9. 2% Potassium sulfate (K2SO4)
10. 2% Citric acid
11. 2% Acetic acid
At 45 days after bloom and harvest, fruits on sample limbs will be counted to determine fruit set as fruit/10 flower clusters.

Research results and discussion:

Experiment I included five studies that included treatments of 2% fatty oils, 2% sulfur compounds, 2% organic acids, 2% miscellaneous compounds, and 2% essential oils. In Study 1, fatty oils did not affect A and Et. Trees treated with CF significantly reduced gs eight days after treatment (Table 1). No significant tree growth effect was observed among treatments (Table 2). In Study 2, sulfur compounds reduced A and Et. Trees treated with ferric sulfate (Fe2(SO4)3) had increased A seven days after treatment. Treatment with CuSO4, (NH4)2SO4, and K2SO4 significantly expressed reduced Et seven days after treatment. No significance was observed for gs (Table 3). Specific leaf weight was significantly higher for control plants (Table 4). In Study 3, trees treated with organic acids showed no significant difference for A, Et, and gs (Table 5). Treatment with citric, acetic, and oxalic acids significantly reduced average leaf area (Table 6). In Study 4, tested miscellaneous compounds reduced A, Et, and gs. Trees treated with coumarin, chitosan, catechin, and thymol had significantly reduced A one day after treatment. Treatment with catechin significantly reduced A five days after treatment. Treatment with coumarin significantly increased gs one and 12 days after treatment (Table 7). Treatment with chitosan, thymol, and catechin reduced average leaf weight (Table 8). In Study 5, trees treated with selected essential oil treatments had no significant effect on A for treatments. However, clove oil was very phytotoxic and defoliated all trees in this study. Trees treated with cedarwood oil significantly decreased Et and gs one day after treatment (Table 9). Differences in plant growth were not significantly different among essential oil treatments excluding clove oil (Table 10).

Experiment II included three studies that included various concentration treatments of citric acid, clove oil, and K2SO4. In Study 1, trees treated with 0.5% citric acid concentration significantly reduced A 1 and 21 days after treatment. No differences were observed for Et and gs (Table 11). Differences were not observed for plant growth (Table 12). In Study 2, trees treated with 0.125% concentration of clove oil significantly increased A, Et, and gs 20 days after treatment (Table 13). No differences in plant growth were observed (Table 14). In Study 3, trees treated with 2% concentration of K2SO4 significantly reduced A 22 days after treatment (Table 15). No differences in growth were observed (Table 16).

In Experiment III, potential fruit thinning compounds at various concentrations were evaluated in the orchard during the post-bloom period. Tree limbs treated with 2% CF, 2% K2SO4, and 2% CuSO4 expressed the highest reduction of fruit retention compared to all other treatments (Table 17).

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:
  • McAfee, Jason (2006) Effects of Potential Organic Apple Fruit Thinners on Gas Exchange and Growth. Unpublished. University of Arkansas.

    McAfee, J. and C. R. Rom. 2005 Effects of oils, sulfur compounds, and various acids on CO2 assimilation, evapotranspiration, stomatal conductance, and internal co2 of apple trees. HortScience 40: 88 [abstract].

    McAfee, Jason and C. R Rom. 2003 The effects of potential organic apple fruit thinners on gas exchange and growth of model apple trees: A model plant study of transient photosynthetic inhibitors and their effect on physiology and growth. HortScience 38:1265 [abstract].

    McAfee, Jason, and C. R. Rom. 2005. Impact of potential organic pesticides and potential fruit crop regulators on photosynthesis and growth of apple. In, Granstein, D. and A. Azarenko (eds), Proceedings 3rd National Organic Tree Fruit Research Symposium. p. 78-79.

    Rom, C.R. and J. McAfee. 2006. Tree Fruit Crop Load Management; An update of current practices and new research. Proceedings of the 25th Arkansas-Oklahoma Horticulture Industries Show, p. 51-53.

    Rom, C. R. and J. McAfee. 2005. Strategies for Fruit Thinning. Proceedings of the 24th Arkansas-Oklahoma Horticulture Industries Show, p. 109-111.

Project Outcomes

Project outcomes:

This project provided a new model for screening potential organic pesticides and crop growth regulators. Clove oil, K2SO4, citric acid, and other compounds may have potential as fruit thinners but will need additional field studies to determine their effectiveness.


Areas needing additional study

Additional field studies will be needed to find the most effective concentrations of each compound in relation to crop growth management. A standard should be developed to compare plant gas exchange with a known standard in order to identify appropriate concentrations of compounds. In addition, costs and economic analysis should be determined for each compound to determine if the treatment is a feasible option to a fruit grower.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.