Fruit Crops: A Summary of Research 1998

Research Circular 299-99

Influence of Pesticides and Water Stress on Photosynthesis and Transpiration of Apple

David C. Ferree, Franklin R. Hall, Charles R. Krause, Bruce R. Roberts, and Ross D. Brazee

Introduction

Chemical pesticides are frequently applied to crop plants that are already under environmental stress. In fact, the pesticide application process itself may be the cause of additional plant stress. Thus, crop plants are often subjected to two or more stress factors simultaneously, the combination of which may result in a different level of physiological response than that caused by individual stress factors alone. The effects of these combined stresses are often subtle in nature and may go unobserved until visual symptoms appear. Consequently, the direct measurement of some sensitive physiological processes such as photosynthesis (Pn) may be the best and earliest means of detecting potential plant injury.

Many of the early pesticides used in agriculture reduced growth and yield of the very crop plants they were designed to protect. These effects have been reviewed for many of the older sulfur, copper, and oil formulations as well as some of the newer organic materials (2, 8, 9). Other chemicals such as antitranspirants (29), growth regulators (11, 24), and spray adjuvants (15) can also influence Pn and transpiration (E). Insects and mites (16), foliar and vascular diseases (7, 20, 25, 28), air pollutants (3, 19, 22, 23), and foliar injury (16) as well as physical stress (13) have been shown to cause changes in Pn and E of numerous woody species.

Ferree and Hall (12) looked at the interactions of water stress and mites on potted apple trees and found that both mites and moisture stress reduced Pn and leaf water potential. Soil moisture stress reduced E, while mites increased it. There was no information between changed physiology of the leaf due to soil water stress and mite feeding as reflected in Pn and E measurements. Data from other studies (14), involving the interaction of mites with either scoring or shade, indicated that the influence of mites on Pn was independent of other stress factors, while the effect on E varied. In studies with containerized sweetgum seedlings, Roberts (21) reported no significant interaction between air pollution (sulfur dioxide) and drought on either Pn or stomatal conductance. But in similar experiments with potted red spruce, Roberts and Cannon (23) noted a greater impact of the combination of air pollution and drought on water relations (xylem water potential) than when either stress factor occurred singly. Cannon and Roberts (3) reported an important interaction involving the stomatal physiology of young yellow-poplar seedlings exposed to low levels of ozone and short periods of plant-moisture stress.

Very few studies have evaluated the impact of the new, more refined protective chemical formulations on host-plant physiology. The present series of nine studies was designed to evaluate some of the newer sulfur compounds and oil formulations that are proposed for use in the production of organic produce.

Materials and Methods

Common Procedures

MM.106 and MM.111 (Study IV) apple rootstocks were planted in a medium containing equal volumes of Wooster silt loam soil, peat, and perlite and grown in a greenhouse. Trees were cut off 10 cm above the soil line, trained to a single shoot, and fertilized every three weeks with a liquid fertilizer (20-20-20) in place of the normal watering. When the trees reached a height of 40-50 cm, the chemical treatments described with each experiment were applied using either a CO2-pressurized sprayer (Study I, II, III, V, VI) or a hand atomizer (Study IV, VII, VIII, IX) to thoroughly wet all the leaves on each plant. When systemic pesticides were used, the pots were covered with a plastic bag to avoid possible absorption by the roots.

Measurements of Pn and E were made using a portable gas analyzer (Analytical Development Corp., LCA-2) with an air supply unit and a Parkinson leaf chamber (6.25 cm²). The inlet to supply air to the analyzer originated outside the greenhouse to avoid uneven CO₂ levels in the compartment, and air flow was maintained at 300 ml/min. Trees were placed under a 400 W metal arc lamp suspended above the greenhouse bench to ensure saturating light levels above 800 mmol/m/s. When whole seedlings were treated, measurements of Pn and E were made on a recently fully expanded leaf, usually the sixth to eighth from the terminal end of the shoot at the time of treatment. Normally six single-tree replications were used in a randomized block design. Deviations from this protocol are mentioned in the description of each individual study.

Study I
On June 1, 1990, the materials and rates shown in Table 1 were applied. Pn and E were measured the day of application and 4, 11, 20, 27, and 45 days following application. Phytotoxicity appeared as translucent areas on the leaves and was rated visually on July 17, 1990, using a scale of 1 = no injury to 10 = severe injury. In addition to the degree of injury, the number of leaves showing symptoms and the number of leaves that had abscised were counted. The sixth leaf from the terminal end appeared fully expanded at the time of treatment and was used to measure Pn and E. At the conclusion of the study, the average leaf area of the six leaves above the measurement leaf was determined to evaluate the effect of treatment on developing leaf area expansion.

Study II
The pesticide treatments listed here were applied on June 21, 1990: (1) Control, sprayed with distilled water; (2) Microsulfur 80%, 6 g/l.; (3) Safer soap, 2 ml/l.; (4) Combination of 2 and 3; (5) Omite 6E, 0.8 ml/l.; (6) Combination of 3 and 5; (7) Vendex, 0.62 ml/l.; (8) Combination of 3 and 7. The carrier for all sprays was distilled water, and combination sprays were tank-mixed before application.

Study III
The miticides listed here were applied three times to the same trees on June 22, June 29, and July 6, 1990: Control, sprayed with water; Omite WP, 1.5 g/l.; Omite 6E, 0.8 ml/l.; Kelthane 35 WP, 1.3 g/l.; Kelthane 50WP, 0.9 g/l.; Vendex 4L, 0.62 ml/l.; Guthion 3E, 0.9 ml/l.; and Guthion WP, 0.85 g/l.

Study IV
'Red Prince Delicious' apple trees on MM.111 rootstocks were trained to single shoots as previously described. In each of three experiments, six leaves in the center of a 20-leaf plant were randomly assigned a pesticide treatment applied to wetness with a hand atomizer. The leaf receiving the spray was shielded from other leaves on the same plant. The materials used and the application dates are shown in Table 2. Each treatment was replicated six times.

Study V
Fruiting 'Starkrimson Delicious'/MM.106 apple trees in containers were sprayed to wetness with a CO₂- pressurized sprayer on May 15, 1991. Prime oil at 300 ml/l was combined with either 2.4, 4.8, or 7.2 g/l of microsulfur and treated plants were compared with untreated controls. Fruit size and shape were measured on each of 10 fruits per tree. Pn and E were also measured. Each treatment was replicated six times.

Study VI
Fruiting trees of 'Golden Delicious'/MM.106 in containers were sprayed weekly beginning May 15, 1991, with three applications of Safer soap, 20 ml/l; microsulfur, 6.0 g/l; or a combination of the two materials. Treated trees were compared to untreated controls. The diameter of 10 fruits on each tree was measured weekly. Pn and E were measured on a spur leaf of a fruiting spur beginning after the three applications had been made.

Study VII
In 1993, a number of fungicides (see Table 5 for materials and rates) and insecticides (see Table 6 for materials and rates) were evaluated for their effects on Pn and E with the intent of selecting a material to interact with water-stressed and unstressed (control) plants (see Table 7 for materials and rates). All plants were well watered until July 8, 1993, when pesticides were applied with a hand atomizer to thoroughly wet the leaves of whole trees. Trees were not watered following treatment, and Pn and E were measured two hours, one day, and six days after pesticide application. Xylem water potential was measured six days after treatment (26). Treatments were arranged in a split-plot design with water stress as the whole plot and pesticide treatment as the split-plot with eight single-tree replications.

Study VIII
Pesticide sprays (materials listed in Table 8) were applied April 11 and again on April 18, 1994. Following the April 18 application, water was withheld from half the seedlings, while the other half were watered as needed. As soon as some of the trees in the water-stressed treatment exhibited slight wilting, all trees in this treatment received 100 ml of water. This sequence was repeated until the study ended. Measurements of Pn and E began April 12, one day after the initial pesticide treatment, and were repeated 8, 10, 15, and 22 days after the initial treatment. The latter three treatments (10, 15, and 22 days) were made after initiation of water stress. Treatments were arranged as a 2 x 5 factorial in a randomized complete block design with six single-tree replications.

Study IX
In a second 1994 study, water was withheld from half the plants beginning April 11, with the remaining plants well watered. On April 16 several of the stressed seedlings exhibited wilted foliage, and all stressed plants received 100 ml water per pot. This procedure was repeated over the duration of the experiment as the stressed plants exhibited wilting. On April 18 most of the water-stressed plants exhibited signs of wilt, and the same pesticides used in Study VIII were applied again. Pesticide treatment was repeated a second time seven days later. Xylem water potential was measured as previously described. Treatments were arranged in a 2 x 5 factorial, randomized block design with six single-tree replications.

Results

Study I
Superior (70 sec) oil caused an immediate reduction in both Pn and E. The effect lasted for 11 days with E and for the entire 45 days with Pn (Figure 1). The 6E formulation of oil first reduced Pn four days after application and the effect persisted. Safer soap and the two formulations of sulfur first caused a reduction in Pn and E 11 days after application, and the effect persisted for the duration of the study. Leaf injury symptoms were apparent 11 days after application for both oil formulations, and these materials, along with microsulfur, resulted in a reduction in size of newly formed leaves (Table 1). The surfactants X100 and AG98 had no effect on Pn or E.

Figure 1. Influence of oil and sulfur formulations and adjuvants on net photosynthesis (A) and transpiration (B) of greenhouse-grown MM.106 apple trees. Each value is the average of six observations. Study I.		Figure 2. Influence of various pesticides combined with Safer soap on net photosynthesis (A) and transpiration (B) of greenhouse-grown MM.106 apple trees. Each value is the average of six observations. Study II.

Study II
Promoted as an acceptable "organic" insecticidal material, Safer soap had no effect alone on Pn and E, but it appeared to interact with other materials (Figure 2). A foliar spray of microsulfur reduced Pn and E, but a greater reduction occurred when it was combined with Safer soap. Omite 6E reduced Pn and E on two of the six measurement dates and combining with Safer soap made no difference in performance. Vendex had no effect on Pn or E when applied alone or when combined with Safer soap.

Study III
Pn and E were not affected by three weekly applications of miticides except for Omite 6E, which reduced both (Figure 3). The wettable powder formulation of Omite reduced Pn when measured immediately after the second application (seven days), but neither material had an influence at subsequent dates.

Figure 1. Influence of miticide formulations on net photosynthesis (A) and transpiration (B) of greenhouse-grown MM.106 apple trees. Each value is the average of six observations. Study III.

Study IV
The three experiments using three applications to single leaves of 'Delicious' trees confirmed that oil, this time as two formulations of Prime Oil, caused a reduction in Pn and E (Table 2). Omite 6E also generally caused reductions in Pn and E. Of the wide range of surfactants tested, Plex, Activate Plus, X-77, and Induce caused slight short-term reductions, while Nufilm 17 and Nufilm P caused short-term increases in Pn and E. The effect of Activate Plus on E persisted for the duration of the study.

Study V
Since it is often necessary to apply both a fungicide and an insecticide to produce quality fruit, the combination of microsulfur and Prime oil II was repeated three times, as might be required in an "organic" regime (Table 3). The low rate (2.4 g/l) of microsulfur combined with Prime oil II had no effect on Pn or E; however, the high rates of microsulfur (4.8 and 7.2 g/l) consistently reduced Pn and E. Very low rates of Pn were measured on June 13, and results appeared abnormal, while results of E were consistent with the other measurement dates. The highest rate of microsulfur and Prime oil II reduced early season fruit growth.

Study VI
Measurement of Pn on the spur leaf following three applications shows that the combination of Safer soap and microsulfur reduced Pn and E with no effect of either material alone (Table 4). Pn was still affected two weeks later with Safer soap alone also causing a reduction. Six weeks after the last spray no effect on Pn or E was noted, and no significant effect of fruit growth was observed.

Study VII
Two applications of various fungicides had little effect on Pn, but Rovral tended to decrease E. Banner and Benomyl had some effect on single dates (Table 5). Although a single spray of various oil formulations had no effect on Pn, two applications of superior oil, Safer soap, CS-7, or Prime oil reduced Pn for four days following the second application (Table 6). Well-watered trees were sprayed with superior oil or Alliette, and then soil moisture was withheld from half of the trees. Water stress was observed by a reduction in Pn and xylem water potential six days after withholding water (Table 7). Two hours after pesticide application, Alliette caused a reduction in Pn, and both pesticides reduced E. Pesticides had no effect one or six days following application. Application of superior oil caused a greater reduction in Pn than Alliette six days after application (Figure 4).

Figure 4. Interaction of soil-moisture stress and pesticide application six days after treatment on net photosynthesis of MM.106 apple trees. Study VII.

Study VIII
As a follow up to the pesticide-water stress interaction in Study VII, a similar study was conducted in 1994 using several rates of Alliette (Table 8). The plants were stressed from lack of soil moisture 10 days after withholding water, as indicated by reductions in both Pn and E. Pesticide treatment had little effect on Pn, E, or xylem water potential.

Study IX
In order to evaluate the influence of time of pesticide application to the development of water stress, a second study was conducted in 1994 where pesticides were applied to plants already showing signs of water stress (Table 9). Following two applications of the pesticide (15 days after initial treatment), all rates of Alliette and superior oil reduced both Pn and E and increased xylem water potential. The interaction of pesticide and water stress (Figure 5) show a similar increase in stress with both Alliette and superior oil treatment; however, the concentration of Alliette had no appreciable effect.

Figure 5. Interaction of soil-moisture stress and pesticides on water potential of MM.106 apple trees 15 days after pesticide application. Study IX.

Discussion

As was true in past studies (2, 9, 10, 27), applications of oil to apple foliage generally caused persistent reductions in Pn and E. The formulations of oil used in these studies made little difference (Figure 1, Tables 2, 3, and 6). Formulation did affect the response from some compounds (e.g., Omite). The emulsifiable formulation appeared to cause a greater decrease than the wettable powder formulation. Newer materials such as Safer soap, often advocated as "soft" materials for organic production tended to reduce Pn, particularly when combined with fungicides such as microsulfur. A number of earlier studies reported a marked reduction in Pn of apple leaves due to lime sulfur and other forms of sulfur (1, 5, 17, 18). Newer formulations of sulfur used in the authors' studies (Figures 1 and 2, Tables 3 and 4) also generally caused a reduction in Pn, particularly when combined with Safer soap or oil.

There was no strong interaction between pesticide application and water stress in the current series of studies. It appeared that the pesticide effect was greater when applied to plants already under water stress (Table 9) than when applied to well-watered plants that subsequently developed water stress (Tables 7 and 8). However, this cannot be definitely concluded since separate studies are reported, and a combined study was not conducted. The lack of a strong interaction may have been due to the smaller effect of pesticide on Pn compared to a greater effect due to water stress. Reports in the literature regarding the significance of interactions involving two or more stress factors on physiological activity tend to be somewhat contradictory. While interactions between mites and a series of other stress factors were not significant for apple leaf Pn or E (12, 14), other studies with air pollutants suggest that certain stress combinations may significantly interact to affect overall growth and physiological activity of other important woody species (3, 4, 6, 22). Without question, factors such as type/duration of stress, plant species, cultural conditions/practices, etc., all influence the degree to which interactions may impact growth and development.

In summary, some newer pesticide formulations of oils and sulfur or Safer soap may be deleterious to Pn of apple leaves. The majority of pesticides tested had little effect on Pn and E of apple trees. There did not appear to be strong interactions between the pesticide influence on Pn and the effect of water stress on Pn.

Acknowledgments

Appreciation is extended to J. C. Schmid, L. E. Horst, and K. A. Williams for technical assistance and to J. Sonowski and G. Cassidy for plant maintenance.

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