A very important difference between the typical plant disease and the typical animal disease is as regards the definiteness of the symptoms shown. In animals and especially in man disease symptoms are usually very characteristic, so that it is often pos sible to identify (or diagnose) a particular disease from a study of the symptoms alone. With plants this is not generally possible.
Disease symptoms in the latter are much more generalized, i.e., the same or approximately the same symptoms can be produced by a great variety of causes. Wilting of the foliage, for example, is a symptom of disease in plants and its cause may be most varied— damage to the roots by fungal or insect attack, damage to the stem, presence of injurious salts in the soil, scarcity of water, and so on. Hence it follows that while a study of the symptoms may give valuable indications of the nature of a particular plant disease, it is in general necessary to discover the cause or aetiology of the disease before the latter can be definitely identified. De termination of the causal agent may be difficult and usually in volves considerable expense of labour and time.
While the main symptoms of plant disease are of a rather gen eralized nature, a number of different types may be distinguished. The more important of these are given in the following list : (a) Change of colour, such as paleness, silvering, reddening, blackening of leaves. The change of colour may be shown over the leaf or even over the whole plant, or it may be limited to spots, streaks or patterns, such as the well-known "mosaic" appearance.
(b) General wilting of the shoot, as when the plant suffers from drought, or from "damping off" disease, or from the effects of a parasite which destroys the roots or is present in the water-con ducting system.
(c) Premature shedding of leaves, blossoms, fruits, etc.
(d) Abnormal retention of leaves, which is especially well shown in certain diseases of deciduous trees. The autumnal shed ding of leaves is a vital or physiological process, due to the action of a definite absciss or cutting-off layer at the base of the leaf. If this layer is prevented from functioning, as for example by the killing action of a parasite which has invaded it, the leaves cannot be shed in the normal way.
(e) Dwarfing or stunting, which may be general to the whole plant or may only be shown in particular organs.
(f) Abnormal enlargement or multiplication of parts (hyper trophy) resulting in the formation of such structures as galls, tumours, knots, warts, etc. (fig. I). Hypertrophy is generally accompanied by distortion or malformation as in the rolling or puckering of leaves and the twisting of shoots. Frequently it takes the form of an abnormal development of buds, the majority of which would normally have remained dormant. The structures called "witches' brooms" (fig. i) which are common on certain trees (e.g., willow, birch) are produced in this way.
(g) Replacement of one organ by another, as for example the change of petals into foliage leaves. Some changes of this type, though properly speaking malformations, may be economically desirable and are therefore not considered as diseases. An example is the "doubleness" of some flowers.
(h) Change of habit of growth, e.g., from the prostrate to the erect, from the climbing to the non-climbing type of growth (fig. I).
(i) Change in the periodicity relationships of the plant, as for example the premature flowering ("bolting") of a biennial plant in its first year of growth, or the opening of foliage or floral buds at the wrong time of year.
(j) Replacement of parts of the plant by structures belonging to the parasite, such as the conversion of the rye grain into the resting organ (sclerotium) of the Ergot fungus (fig. 1). "Mum mification" of certain fruits is another example of this type.
(k) More or less rapid killing (necrosis) of the plant or parts of it. Killing is often accompanied by more or less pronounced rotting, as in the decay of fruits, buds, etc. (fig. I). Localized killing of leaves results in the formation of dead spots. The killed tissue may remain as such or may dry up and fall out, thereby producing a "shot-hole" effect.
(1) Formation of deep-seated wounds or cankers (fig. I). In a canker, the parasite slowly eats into the plant the part invaded being killed and to a large extent disappearing. Associated with this type of wound there is usually a certain amount of hypertrophy of the tissue which still remains alive round the margin of the invaded part.
(m) Various kinds of exudations, as in the gumming of plum trees or the abnormal flow of resin in certain conifers.