Abstract
Two percent Sunspray Ultra-fine SprayR oil and Safer Insecticidal ConcentrateR soap were applied 5 times in separate treatments to 30 species of trees and shrubs growing in containers under commercial nursery conditions in north Florida. A similar test was conducted on 17 species of container-grown ornamentals in south Georgia. Horticultural oil and soap were applied at 10 day intervals beginning July 16, 1990. Plants were visually rated for phytotoxicity prior to the second through fifth application and 10 days after the fifth application. A third test of only three applications of horticultural oil or soap on 9 species of plants was conducted at Monticello, Florida beginning August 28, 1990. All the treatments were applied from 9:30 - 11:00 A.M. and daily temperatures were above 35° each day. No phytotoxicity was observed on any of the plants tested.
Environmental and sociological concerns that accompany the application of conventional pesticides for pest control have shifted research attention to the evaluation of alternative methods. Traditionally, horticultural oils have been recommended for use in the dormant season for insect and mite control on a variety of plant species. Phytotoxicity caused by the older, less refined brands of horticultural oils precluded their use during the active growing season. However, many new, more refined horticultural oils and soaps reduce the risk of phytotoxicity. Thus they have much promise as effective alternatives to conventional pesticides, fitting well into integrated pest management programs that seek to reduce pesticide use (1, 2, 3).
Previous work in Maryland (2) and New York (1) discussed the results of both phytotoxicity and efficacy studies using Sunspray 6E horticultural oil under summer conditions. Discoloration was observed on 6 of 52 species tested (two species of maple, a juniper, a blue spruce, a red oak and an arborvitae) and the authors urged caution if 2% Sunspray oil was used repetitively on these plants (2). Excellent control of spider mites, scales, whiteflies, aphids, and mealybugs using 2-3% Sunspray 6E oil was documented in New York (1). Some phytotoxicity from 3% oil to several walnuf (Juglans sp.) cultivars and some apparently permanent foliage discoloration to several juniper cultivars were reported (1).
Repellency and toxicity of horticultural oil to greenhouse whitefly, Trialeurodes vaporariorum (Westwood) have been reported (3). Also a reduction in the spread of an aphid-vectored virus was observed as a result of mortality to the aphid vectors from insecticidal soap (3).
The present study was initiated to test the potential for phytotoxicity to ornamental plants under the environmental conditions of commercial nurseries in south Georgia and north Florida.
Materials and Methods
Plants for the test were selected at random for uniformity of size and appearance from a large bed of similar plants. Plants in experiment 1 were located at a commercial nursery in Monticello, Florida. Three plants (replicates) from each cultivar in 2-3 gal. containers were grouped together for each of the 3 treatments: untreated control, 2% Sunspray Ultra-fine Spray oil, and 2% Safer Insecticidal Concentrate soap. Plants in experiment 2 were located at a commercial nursery in Cairo, Georgia. Plants normally grown under shade conditions (i.e. aucuba, camellia and azalea) were held under shade during the test. The Georgia plants were in 1 gal. containers. Plants in experiment 3 were tested at the author’s laboratory in Monticello, Florida. Plants in this test were in 1–2 gal. containers. All plants were held under commercial nursery conditions of overhead irrigation and fertilizer, etc. No cultural factors were added or changed during the course of the tests.
The oil and soap treatments were applied to plants with a SoloR backpack sprayer to runoff in a 2% solution at a volume of 100 gal. per acre. All applications were made between 9:30 and 11:00 A.M. on clear, sunny days. Daytime temperatures were typical of the season in south Georgia and north Florida reaching above 35°C and above 65% relative humidity in the afternoon on all spray dates. Plants in experiments 1 and 2 were sprayed on 16 July, 27 July, 7 August, 17 August and 28 August. Plants in experiment 3 were sprayed on 28 August, 7 September and 17 September. Evaluations were made on the spray dates before the application of the treatments and 10 days after the last application date. Treated plants were observed and compared to untreated control plants for any changes in appearance of the foliage. No pests were observed on the plants and efficacy against pests was not evaluated.
Results and Discussion
Table 1 lists the cultivars tested in the 3 experiments. No phytotoxicity was observed on any of the plants from the repetitive oil or soap treatments. Aucuba sprayed with oil did yellow slightly after the third application. However, the change in appearance was not enough to affect the salable quality. In experiment 2, the three 1 gal. boxwood plants treated with oil died during the test. Foliage symptoms indicated that apparently this resulted from a root disease and not the oil. Boxwoods in 2 gal. containers were tested again in experiment 3 and phytotoxicity was not observed. No change was observed in the color of the juniper cultivars tested. However, changes in juniper color often do not show up until cool weather in the deep South and this problem needs further consideration. Previous work reported discoloration of some juniper cultivars (2).
List of plants sprayed with Sunspray Ultra-fine SprayR oil and Safer Insecticidal ConcentrateR soap in three separate experiments in south Georgia and north Florida during July-September, 1990.
It can be concluded from this test and the literature (1, 2) that the more refined horticultural oils can be safely used in the summer season in the eastern U.S. without general problems of phytotoxicity. Because of the large numbers of species and cultivars of plants grown in southern nurseries it will be impossible to rigorously test them all. No doubt some plants are sensitive to the oils and perhaps soaps and growers should spray a few plants of suspected sensitive species before applications to large numbers of plants. Phytotoxicity may be enhanced on plants that are under conditions of stress; although tests with horticultural oil on drought stressed plants indicated phytotoxicity to only a few cultivars (1,2). The application of adequate irrigation to plants before treatment with horticultural oil is highly recommended. The risk of phytotoxicity from horticultural oil may be higher to plants that have not yet hardened off during days in the fall when drastic changes in daily temperature often occur in the deep South.
Footnotes
↵1. We thank David Walker of Wright Nursery, Cairo, GA and Fred Beshears and Rex Purvis of Simpson Nursery, Monticello, FL for providing the plants and a site to conduct these tests. Florida Agricultural Experiment Station Journal Series publication.
- © 1991, International Society of Arboriculture. All rights reserved.
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