8 excessive winter desiccation and needle loss. However, com- mercial arborists often extend their oil treatments well into October–November in Westchester County and the Lower Hud- son Valley due to logistical reasons and seasonal temperature/ weather variation. Therefore, a second application was admin- istered in November to investigate possible injury that might appear by the following spring related to a late spray timing. A simple 0–5 rating system was used to evaluate possible injury to foliage, based upon expected symptoms from oil phytotoxicity. Branches chosen for treatment were examined, and the foliage condition was assigned a rating: 5 denoted a “perfect” appearance (i.e., abundant lush, green leaf cover free from chlorosis, necrosis), 4 indicated a very small degree of imperfection (i.e., one or two chlorotic or brown needles on a branch), 3 indicated significant aes- thetic compromise (i.e., many chlorotic/brown needles on a branch), 2 indicated widespread needle loss, 1 represented a nearly dead branch (minimal green foliage, growth), and 0 indicating complete necrosis. Pre-treatment appearance rat- ings were performed on August 18 and September 24, 2010, based on the trees’ overall foliage rating. Foliage quality ratings from the September and November treatments with horticultural oil were recorded on October 29 and Decem- ber 9, 2010, respectively, 30 days after each spray treatment. Informal examination of all treated hemlocks took place on three occasions in the spring/summer months of 2011 (March, May, and July) during regular plot maintenance. It was readily apparent that there were no notable changes/ differences in foliage quality (color, needle loss, necrosis) when compared to other portions of the trees that had been treated with horticultural oil the previous autumn. Armored scales and other arthropod populations (e.g., HWA, spruce spi- der mites) were not sampled due to limited availability of labor. Data Analysis Foliage quality ratings from both 30-day post-treatment dates were compared in one analysis of variance. Because there were potential differences in tree condition related to their planting site, the experiment was treated as a 3 × 2 × 3 factorial design experiment, where the oil treatment (3 levels) could interact with date of application (2 levels) and the site (3 levels), with five single tree replicates per site. Data were analyzed using Statis- tix 9 software (Analytical Software, Tallahassee, Florida, U.S.). RESULTS AND DISCUSSION Highly statistically significant differences in foliage quality rat- ings were found based on species, time of evaluation, site, and each of these factors’ two-way interactions (Table 1) (F > 4.96; df = 1 or 2, 168; P < 0.01); data did not require transformation to fulfill the assumptions for the analysis of variance. However, there was no evidence that horticultural oil sprays were associ- ated with any symptomatic injury to foliage, as the oil treatment’s main effect and its interactions were all statistically insignificant (F < 1.51, df = 2 or 4, 144; P > 0.2). Indeed, the overall foliage quality ratings were very high for T. chinensis at all ratings and sites (averaging between 3.9 and 5, grand mean of 4.4) (Figure 2). Ratings were more variable for T. canadensis (grand mean of 4.1), but appearances were influenced by site conditions. In follow-up informal observations of the treated specimens made ©2013 International Society of Arboriculture Harper and Cowles: Susceptibility of Chinese Hemlock Figure 2. Foliage quality ratings (mean ± SE) of eastern and Chi- nese hemlocks from three replicated common garden plantings at Lasdon Arboretum. Quality ratings were on a subjective 0–5 scale (dead–dark green), and were recorded on October 29 and December 9, 2010 (date effect). Foliage ratings from eastern and Chinese hemlocks were averaged over the oil application dos- ages, for which there were no significant differences. Each main effect mean for date, site, and species significantly differed (Tukey HSD test, P < 0.05). Each bar represents five trees. in the spring and summer of 2011, there were no delayed phy- totoxic effects observed on either species in any oil treatment. Paraffinic horticultural oils are believed to have the potential to cause injury to plants through two mechanisms (Hodgkinson et al. 2002). Acute toxicity is characterized by necrotic spot- ting on leaves, sudden abscission of green leaves, and injury to meristematic tissue, which results from penetration of oil and/or formulation emulsifiers into the leaves where they dissolve and disrupt cell membranes. The oil used in this study, Lesco Horti- cultural Oil, has only 1.2% formulation by weight of emulsifiers, 0%–20% percent heavy paraffinic petroleum, and a relatively low 50% distillation point of 212°C, all of which minimize the likelihood of acute toxicity to plants while maintaining pesticidal activity. Chronic toxicity results from persistence of oils on leaf surfaces and temporary interference with basic biological pro- cesses in plants, such as transpiration and respiration. Oil-caused stress added to other existing stressors can cause failure of func- tion in the plant tissues, characterized by leaf yellowing or poor growth. In spite of this trial being carried out in mildly drought- stressed conditions, neither species of hemlock was adversely affected by the application of oil at 1% or 2% concentrations. In general, color ratings were higher for T. chinensis than for T. canadensis, with the exception of the T. canadensis in the Hem- lock Hedge Plot, which featured excellent color where the ratings of 4.5 were equivalent for the two species. A number of underly- ing factors may have been responsible: this planting site is forest- ed and offers a generally cooler, well-shaded environment simi- lar to where hemlocks grow naturally. In comparison, the Tsuga spp. in the other two plots (i.e., Front Gate and Magnolia Garden, average ratings of 4.2 and 3.6, respectively) are shaded only from early to mid-afternoon through early to mid-morning (depending on the angle of the sun) and experience significantly more sun- light with associated hotter and drier conditions. Informal obser-
January 2013
| Title Name |
Pages |
Delete |
Url |
| Empty |
Ai generated response may be inaccurate.
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success.
Downloading PDF
Generating your PDF, please wait...
This process might take longer please wait
