Relative Ozone Susceptibility of Selected Woody Ornamentals1

Abstract

Abstract More than 1000 plants representing 15 species and/or cultivars of woody ornamentals were exposed to 0.25 ppm ozone for 8 hours at bi-weekly intervals throughout the 1973 growing season. A different set of plants were utilized in each bi-weekly exposure. Plants injured at this rate, in descending order of susceptibility, were Rhododendron obtusum Planch. ‘Hinodegiri’ (Hinodegiri Hiryu azalea), Rhododendron poukhanensis Leveille (Korean azalea), Ailanthus altissima Swingle (tree-of-heaven), Ulmus parvifolia Jacq. (Chinese elm), Philadelphus coronarius L. (sweet mock-orange), Viburnum setigerum Hance (tea viburnum), and Viburnum dilatatum Thunb. (linden viburnum). Plants resistant at this rate were Ilex crenata Thunb. ‘Hetzii’ (Hetz Japanese holly), Ilex opaca Ait. (staminate and pistillate American holly), Kalmia latifolia L. (mountain-aurel kalmia), Lingustrum amurense Carr. (amur privet), Nyssa sylvatica Marsh, (black gum), Taxus × media Rehd. ‘Densiformis’ (dense Anglojap yew), Taxus × media Rehd. ‘Hatfieldii’ (Hatfield Anglojap yew), and Tilia americana L. (American linden). The most common ozone symptom on the broadleaved plants was a tan or dark red to black stipple on the upper leaf surface. Premature defoliation occurred on susceptible plants. Plants were more susceptible to ozone in mid- to late summer than in early spring.