Nonparasitic plant diseases
Physiological plant diseases
Physiogenic plant diseases
Plants in an adverse or unfavourable environment or subject to injury by physical or chemical agents may vary so far from their normal growth habits that they are considered diseased. Accidents, poisons or defects in the environment often result in diminished growth or diseased tissues. Subsequently, the plant is more susceptible to attack by microorganisms which may cause further damage.
Crops may be seriously damaged by fire, water, frost, insects and poor agricultural techniques. Pollution of the air by noxious gases, dust and smoke may provoke injury and disease. A growing problem in developed countries is plant-pathogenic air pollutants. The most important are sulphur dioxide, ozone and peroxyacetyl nitrate. The last two are photochemical pollutants formed from the interaction of other pollutants. Agricultural chemicals, when improperly used, can cause serious physiological diseases. Damage is fairly common from the misuse of sprays, seed treatments, fumigation, fertilizers, herbicides, soil treatment and hormones. Too high a temperature may result in sun scald and death in the tips and margins of leaves. With insufficient light, plants may become chlorotic. Low temperatures may cause damage, such as net necrosis in potatoes, and frost injury is common. Too little water causes stunting, wilting and burning; but too much water may induce flooding of tissues, resulting in such diseases as water core of apple, celery heart rot, and tomato blossom end rot. For healthy growth, plants require various essential elements. If these elements are absent, or present in insufficient quantities, plants will display characteristic symptoms of deficiency diseases. Besides nitrogen, potash and phosphorus, which plants need in relatively large amounts, smaller quantities of sulphur, magnesium and calcium are required. Trace elements, which are necessary in minute amounts for healthy plant growth, include boron, iron, copper, manganese, molybdenum and zinc.