White oak is a valuable forest and landscape species native to the eastern United States (Elias, 1987). Since the early 1980s, loss of interveinal tissues giving leaves a lacy appearance has been reported on white oak (Haugen et al., 2000). In the spring of 2003, Wisconsin nursery inspectors found leaf tatters on oak trees at garden centers [Wisconsin Department of Agriculture, Trade and Consumer Protection (WDATCP), 2003]. Leaf tatters has also been reported in states from Minnesota, south to Missouri, and east to Pennsylvania (Haugen et al., 2000). The problem can affect a substantial portion of a tree's canopy reducing the aesthetic appeal, overall tree vigor, and health. This makes affected trees more susceptible to other stressors such as adverse environments, air pollution, or pests (Haugen et al., 2000).
Haugen et al. (2000) and WDATCP (2003) proposed that insect feeding, environmental factors, or herbicide drift could cause leaf tatters. Our preliminary research eliminated insect feeding as a cause of leaf tatters and found trees near agricultural fields were most likely to be injured. In northern Illinois, leaf tatters occur in early to mid-May when corn (Zea mays L.) is planted and before widespread soybean [Glycine max (L.) Merr.] planting or postemergence applications of glyphosate. Based on these observations, we theorized that leaf tatters was caused by drift from herbicide applications before or at corn planting.
Atrazine, glyphosate, s-metolachlor, and acetochlor are the most common herbicides applied to corn in the midwest (U.S. Department of Agriculture, 2006). In Illinois, misapplications of dicamba or 2,4-D (growth regulators) are the most common cause of drift injury complaints (Mohr, 2004). There are no published reports of these specific herbicides causing injury to white oaks but all have caused injury to other plant species. Preemergent applications of acetochlor have injured sorghum [Sorghum bicolor (L.) Moench] (Roeth et al., 1983). Chrysanthemum (Chrysanthemum leucantheum L.) and kale and collards (Brassica oleracea L. var. acephala DC.) were stunted by applications of s-metolachlor (Derr, 1993; Harrison et al., 1998). S-metolachlor caused transient leaf crinkling and malformed growth to some collard and kale cultivars (Harrison et al., 1998).
Simulated glyphosate drift on pepper (Capsicum annuum L.) caused chlorotic terminal buds and later chlorosis of the foliage developing from the buds (Gilreath et al., 2000). Al-Khatib et al. (1992) reported that glyphosate applications to rose (Rosa dilecta) caused leaf distortion, crinkling, cupping in developing leaves, and chlorosis of young leaves. Symptoms were aggravated with increasing rates of glyphosate. Rose treated with reduced rates of 2,4-D or 2,4-D + glyphosate had epinasty of stems and petioles along with leaf crinkling, curling, and cupping. After 10 d, roses treated with the 2,4-D + glyphosate combination had only symptoms consistent with 2,4-D injury (Al-Khatib et al., 1992). On boxelder (Acer negundo L.), leaf blight injury symptoms were caused from exposure to 2,4-D concentrations as low as 0.01 μg (Phipps, 1963). Simulated 2,4-D or dicamba drift to soybeans or field bean (Phaseolus vulgaris L.) caused leaf and stem elongation and leaf malformation, including cupping and crinkling (Behrens and Lueschen, 1979; Lyon and Wilson, 1986).
The impact of herbicide drift on native species is difficult to evaluate and quantify compared with annual crop species. Native species are often perennials and present in communities and not in monocultures (Obrigawitch et al., 1998). Also, the effects of herbicide drift on these species can be subtle such as increased mortality, reduced biomass, lower fecundity, and modifications in morphology and development (Freemark and Boutin, 1995; Gove et al., 2007). Herbicide drift can cause alterations of species composition and diversity; devaluation of wildlife habitat, recreational uses, or aesthetic vistas; reduction in timber or wood pulp production; and lower livestock-carrying capacity (Obrigawitch et al., 1998). Although herbicides at recommended rates can selectively affect growth and species composition of seminatural vegetation, knowledge on the impact of sublethal herbicide concentrations is scarce (Marrs et al., 1991). Our objective was to determine the effects of field corn herbicides on white oak at different stages of development.
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