Such trees, known as chlorotic dwarfs, have thin chlorotic crowns and the foliage has a tufted appearance. Black cherry appears to be another good indicator of ozone. Other forest species that may serve as bioindicators of ozone pollution are blackberry, milkweed and sorrel. Blackberry and sorrel exhibit a dark purple-red stippling of the upper leaf surface that often coalesces over most of the leaf surface. The symptom on milkweed is similar to that on blackberry, except that the colouration is purple-black.
Plants often respond to different stresses in a similar fashion.
Several incitants other than air pollution are known to cause symptoms similar to those induced by O 3 . On broadleaf plants, leafhoppers and spider mites often cause a chlorotic stippling that may become pigmented with time. Sunburn, sunscald or severe late-season droughts may cause bronzing or general purpling of the upper leaf surface. Viruses can also induce chlorotic mottling. Marginal necrosis produced by O 3 on an expanding leaf may cause leaf crinkling and be misdiagnosed as a virus.