Air Pollution Effects on Azaleas and Rhododendrons
Laurence D. Moore
Virginia Polytechnic Institute and State University
Natural sources of air pollution have been with us for a long time, but air pollution and acid rain did not become major problems until recent times. The use of coal has polluted the air with sulfur dioxide; while various industries have contributed fluorides, heavy metals, hydrochloric acid, chlorine, ammonia, some nitrogen dioxide, and other such compounds. Acid rain is a combination of sulfuric and nitric acids as well as a number of other compounds often emitted from industrial sources. Ozone, peroxyacetyl nitrate (PAN), and nitrogen dioxide have become major pollutants because automobile exhaust releases compounds into the atmosphere which interact with sunlight to form these phytotoxic compounds.
The effects of ozone on rhododendrons and azaleas have been studied more extensively than any other pollutant world-wide. Ozone injury on rhododendrons and azaleas appears as an upper surface stippling. The stipples are reddish-brown spots up to 2mm in diameter. They occur on mature fully-expanded foliage, not on expanding or old foliage. Of 21 cultivars we tested, only the rhododendron cultivar 'Nova Zembla' and the azalea cultivars 'Delaware Valley White', 'Roadrunner', and 'White Water' exhibited injury when fumigated with relatively high levels of ozone. Most of the cultivars were highly tolerant of ozone toxicity. Moreover, fumigation alone caused no significant change in the growth indexes of six azalea cultivars, but fumigation with ozone coupled with inoculation with Phytophthora cinnamomi resulted in significant reductions in the growth indexes of five of the cultivars.
The effects of sulfur and nitrogen dioxides on rhododendrons have not been reported, but eight azalea cultivars have been tested and reported to be very tolerant to fumigation with a combination of sulfur and nitrogen dioxides. When ozone was included with both the sulfur and nitrogen dioxides, 'Hershey's Red' and 'Glacier' were found to be relatively sensitive but the other six cultivars were very tolerant.
There are few reports concerning the effects of the other types of air pollutants on either azaleas or rhododendrons. Chlorine has been found to cause a dark upper surface stipple on rhododendrons but on azaleas the most common symptom was leaf necrosis followed by abscission. Azaleas gassed with ethylene were observed to be very tolerant. Three days of gassing were required to cause any damage to the azaleas. The treatment of azaleas with acidified water at pH levels as low as 2.6 reportedly had no effects on the plants.
On the air pollutants important in the United States possibly only ozone will be of major concern to most azalea and rhododendron growers. Ozone not only can cause foliar damage and reduced growth but may also increase the susceptibility of the plant to fungal disease. Most likely the other air pollutants would be of concern only if they were present in relatively high levels and/or present with ozone. Since most natural rain has a pH above 3.8, the possibility that azaleas and rhododendrons would be sensitive to naturally acidic rain appears to be minimal.
Jones, R.K., and R.C. Lambe. 1982. Diseases of woody ornamental plants and their control in nurseries. North Carolina Agr. Ext. Pub. AG-286.
Moore, L.D., R.C. Lambe and W.H. Wills. 1984. Influence of ozone on the severity of Phytophthora root rot of azalea and rhododendron cultivars. J. Environ. Hort. 2:12-16.
Sanders, J.S., and R.A. Reinert. 1982. Screening azalea cultivars for sensitivity to nitrogen dioxide, sulfur dioxide, and ozone alone and in mixtures. J. Amer. Hort. Soc. 107:87-90.
This article is a summary of Dr. Moore's research, funded in part by the American Rhododendron Society, and a chapter on air pollution published in a Compendium of Rhododendron and Azalea Diseases by the American Phytopathological Society Press, St. Paul, MN. L.D. Moore is Professor and Head of the Department of Plant Pathology, Physiology and Weed Science at Virginia Polytechnic Institute and State University.