Pathology of Plants

Definition and Scope.

While it is true that the distinction between a healthy and a diseased plant is often perfectly obvious, it is not easy to frame a definition of the diseased condition which will meet all cases. The border line between health and disease is not always sharply marked. It will be shown below that the major ity of plant diseases result from the invasion of the plant tissues by a definite organism, the parasite. Nevertheless, the presence of a parasite does not always necessarily mean disease in the com mon acceptation of the term. For example, the root-nodules of leguminous plants (pea, bean, clover, etc.) are produced by the attack of an organism—a bacterium—and far from producing damage, the presence of the bacterium is essential to the proper development of the plant. The important part played by certain fungi in the germination and subsequent growth of many plants, such as orchids and heaths, is a further illustration. The living to gether of two organisms does not necessarily produce injury to either partner, in which case one speaks of a symbiotic relation ship. Every gradation may be traced from the purely symbiotic type, in which the two partners are benefited or not obviously in jured, to the distinctly parasitic type in which one partner thrives at the expense and to the detriment of the other. In fact both types of relationship may be shown between the same two part ners—that is, a symbiotic or harmonious relationship is at first set up, but by and by one of the partners suppresses the other and the relationship becomes one of parasitism. There is thus a gradual transition from a condition of health to one of disease.

So far reference has only been made to the diseases of para sitic nature. Over and above these there is a class of diseases in which no parasite is present at all, viz., the so-called physiological diseases. The existence of the latter increases the difficulty of de fining disease in terms of the causal agent.

Perhaps the most adequate definition is obtainable by consider ing the normal development of a plant. Over a certain range of conditions, which one reckons as normal, a particular species of plant develops in a certain way—e.g., has a certain appearance, reaches a certain size, and so on—all of which are the reflection of a normal physiology or functioning of the plant. If now a par ticular individual diverges from the normal and if the abnormali ty impairs the functioning and more especially imperils the life of the plant, that plant may be considered as being diseased.

From the purely practical or economic point of view, a satisfac tory definition of disease presents no great difficulties. Any given crop is grown in the expectation of a certain yield, both as regards quality and quantity. Disease in its economic aspects may there fore be defined as a condition of the crop in consequence of which it fails to give a satisfactory yield.

General Classification.

Diseases, whether of plants or ani mals, may be classified on the basis of the symptoms produced or of the causal agent concerned. For reasons given more fully below,

the latter system of classification is more generally adopted in connection with plant diseases, and it is convenient to give at this point a general classification along such lines in order to limit the scope of the present article. Classified on the basis of the causal agency, plant diseases are as follows :— ( ) Diseases due to the non-living environment.

(2) Diseases due to so-called internal causes.

(3) Diseases due to living agencies, which may be plants or animals.

The first two groups constitute the physiological diseases. In group (I) the disease symptoms can be related to certain physical or chemical features of the environment. In group (2) no such association has yet been demonstrated, nor has it been possible to explain the symptoms as being due to the presence of a parasite. The second group is thus defined on purely negative characters, and is in fact a kind of limbo into which are put all diseases the cause of which is still obscure. Some of these may prove, with advancing knowledge, to be referable to either of the other groups.

The members of the third group constitute the parasitic diseases. The plant, technically termed the host, is attacked by another organism called the parasite. The animal parasite may be various —mammal, bird, snail,: insect, eelworm, etc. The attack of a crop by such an animal as a rabbit is not .usually. described as a disease, though there is no difference in kind between such attack and that produced by mites or greenflies. Economically the most important diseases of animal origin are those produced by in sects, and these constitute in the main the province of the ento mologist. (See ENTOMOLOGY : Economic.) Plant parasites may be bacteria, algae, fungi (including slime fungi) or higher plants. Plant diseases of bacterial origin were at one time believed not to exist, on account of the supposed fact that the acidity of plant juices is inimical to bacterial growth. Many bacterial diseases of plants are now known, some of great economic importance. The recorded cases of parasitism due to algae are few and unimportant. Diseases caused by higher plants, some of which will be cited later, are on the whole of greater scientific interest than of economic importance. The fungi (q.v.) are by far the most important group of disease-producing organisms, so far as plants are concerned.

Broadly speaking, therefore, the plant pathologist is by train ing either an entomologist or a mycologist. As far as diseases of parasitic type are concerned, only those caused by plants, and therefore chiefly of fungal origin, will be dealt with in the present article. At the same time it is important to note that the two aspects of the study of plant disease, the entomological and the mycological, cannot in practice always be separated.