CHAPTER FIVE .... IcRooRGANrsMs and environ- ditions causing disease are often not mental stresses can cause dis- visible to the naked we and mav not ease symptoms on any plant part. Diseased roots may be enlarged, stunted, or rotted. Sap may drip from infected branches or trunks. Leaves or stems may become spotted, stunted, swollen, discolored, distorted, orwilt- ed, or they may die. The severity of symptoms expressed by affected plants depends on the interaction among the plant, its environment, and the pest or causal agents (Figure 5;l). Biotic or living causal agents include nematodes or tiny roundworms (discussed in Chapter 81, as well as the bacteria, fungi, and viruses dis- cused in this chapter. Nonliving (non- infectious or abiotic) causes such as overwatering or underwatering, tox- HdLlmi- suess&t-- canlead to disease are discussed in Chapter 6. Disease diagnosis is often difficult. Symptoms of different diseases may be similar to each other or they may be difficult to distinguish from other causes, such as damage by certain insects. The organisms or abiotic con- be readily apparent at the time disease symptoms develop. Dise symp toms are often vaiEable, there are commonly several contributing fac- tors, and many of the organisms involved are microscopic. Profession- al help and laboratory tests may be needed to positively identify the cause I! 11 of a disease. '1 Types of Pathogens - Microorganisms that cause diseases are called pathogens. Funs, bactena, vimes, and nematodes (see Chapter 8) are the most common pathogens ~swgd~t..t.. Abhongtr pathogenic microorgamsms feed on living organisms, many fungi, bacte- na, and nematodes are beneficial, they attack and kill pests or feed on dead organic matter and help to decompose dead plants and animals so that nutrients become available for plant growth.b, birch, fuchsia, pine, and sycamore are among the plants most Wly damaged by air pollution. Omne naturally occurring in the atmosphere shields plants and dimah from harmful soh radiation; ene near the ground damages phrs. Ozone is produced when the -%en oxides and hydrocarbons aritted during combustion are wed to sunlight and react with rggM. &one causes pale fl& or small Irk patches to appear on needles &I leaves. Discolored areas can &ge and foliage may drop prema- luy. Pine needle tips can turn ,b and die or banding may appear qmrEfccted needles. Ozone retards and increases phnts' suseepti- to diseases and insects. -FUR OXIDES xides or oil are pr I, refin ,educed by ling petrok d some minerals, and%y motor Wes. Sulfur oxides cause tip wck and yellow or brown bands rmnifer needles. Broadleaf foliage Mes and dies beginning between &weins. Affected foliage may be e, stunted, and grow in tufts. die or banding es aBected by Air pollution like this causes plant damage that is difficult to diagnose because many symptams with other causes are similar to, and are aggravated by, air pallution. Ozone pollution damage can be subtle, like the yellowish patches in these Japanese maple leaves. MANAGEMENT OF AIR POLLUTION INJURY Provide proper cultural care and con- trol other causes of stress to keep plants vigorous and increase their tol- erance to pollution. Grow more plants because plants help in several ways to reduce air pollution. Particu- lates become trapped by foliage and are washed by precipitazion from foliage to the soil. Gaseous air pollu- tants are absorbed by bark and taken into plants through leaf stomata. Plant tolerant species in areas where air quality is especially poor. Exten- sive lists of species especially suscep- tible or tolerant to pollution have been developed (see references in Diseases of Trees and Shrubs). Air pollution is best controlled at its source. Reduce pollution by using alternative means of transportation and energy, properly maintaining vehicles and engines, conserving resources and materials, and support- ing appropriate regulations. ABIOTIC DISORDERS