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. Herbicide injury may also be confused with pollution injury. Individual plants within the same species may differ in pollution sensitivity. Genetic and microclimatic differences may produce apparently sensitive and tolerant individuals side by side, but multiple species are often found to exhibit injury at the same time. Symptoms generally appear over a whole tree rather than a single stem or branch and may be more severe in plants growing on open sites.