Briefly explain the Classification of Plant Diseases

Explore the classification and types of plant diseases. Learn about common symptoms, causes, and preventive measures.

Classification of Plant Diseases introduction

A great many different ways of classifying plant diseases “have been suggested by various writers in the past. In the early history of the science dealing with plant diseases the field belonging exclusively to plant pathology was not clearly marked.

• The early writers sometimes included among plant diseases, troubles that now belong to other fields of science, for example, entomology. A chronological arrangement of a series of examples of systems of classification chosen from the different periods in the history of plant pathology would, in a way, illustrate the historical growth of the subject. Such a study would show that in the beginning the field was large and indefinite but that then: has been a gradual narrowing down of the field and a tendency to define care­fully and classify the subject-matter of plant pathology.

• There are several different, bases on which plant, diseases may be classified, namely, on basis of cause, on basis of symptoms, and on basis of Aost. The thing which determines which basis one would use in classifying plant diseases depends upon the phase of the subject in which one is chiefly interested. If the interest centers largely in the etiology of plant diseases, the logical basis on which to classify them would be that of the causal agent. If the gaining of familiarity with the effects of the disease on the host.

•  The either for the sake of the knowledge or as an aid in identifying plant diseases, were the chief object in view, then one would be inclined to use the symptom logic basis of classification. But. if one had already become familiar with the different kinds of causal agents and with the different types of symptoms encountered in plant disease studies, and if one were interested mainly in the diseases of certain crops, as potatoes, wheat, or apples, then it might prove con­venient to group diseases under their respective hosts.

HISTORY

• Historically, most systems of classification for plant diseases have been based on cause, although there are some notable exceptions to this rule. It will be of interest now to examine and analyze a few different systems of classification chosen from different historical periods as well as to repre­sent the different bases of classification. In doing this we shall famil­iarize ourselves with the development of the subject-matter of plant pathology sufficiently to enable us to work out a suitable classification of the plant diseases with which plant pathologists have to deal today.

• Classification of plant diseases on basis of cause. One of the earliest attempts to classify plant diseases was made by Tournefort (10). He made two main classes, namely, (a) diseases due to external causes, and (t>) diseases due to internal causes.

• knowledge of the true cause of plant diseases was non-existent at that date, at least in so far as parasitic diseases are concerned. His class due to external causes would probably correspond to the group we now know as non-parasitic diseases, that, is, diseases largely due to environmental factors such as weather conditions. His class due to internal causes was based largely upon the superstitious notions handed down from ancient and medieval times.
  

PLANT DISEASES CLASSIFIED ON BASIS OF SYMPTOMS 27

• Caused by bacteria.
• Caused by fungi.
• Caused by alga?.
• Caused by parasitic seed plants.

B. Caused by parasitic animals.

• Caused by nematodes.
• Virus diseases (infectious chloroses).
• Non-parasitic diseases (so-called  physiological ” diseases).
• Diseases of unknown origin.

This outline needs a word of explanation. Perhaps the most ques­tionable point centers around the place of the virus diseases in this scheme. Diseases of unknown origin. At present, however, we know a good many things about the nature of these troubles even though we do not know the exact cause.

We know so much about them, in fact, that there might be good reason for including them as a sub-group under parasitic diseases.

Rut since no parasite has been positively identified as the causal agent, it has seemed best to place the virus diseases tentatively in a class coordinate with parasitic and uon- parasitic diseases. If a causal parasite is finally demonstrated for this group of diseases, then of course they will have to be transferred to their proper position under parasitic diseases.

Plant diseases classified on basis of symptoms. — The fact that a great many of the early botanists were physicians who studied botany as an avocation accounts for most of the terminology formerly used in describing the symptoms of plant diseases. Zallinger  in “ Ik Morbis Plantarum, suggested the following classes of plant diseases based upon the symptoms:

• phlcgmasiac, or inflammatory diseases:
• paralysis, or debility; (c)
• discharges or draining; (d)
• cachexia, or bad constitution;
• chief defects of different organs.

• Thus list of symptoms certainly sounds more like animal pathology than plant pathology, but even today the plant pathologist still uses many- terms, in exact or modified form, that are used by the animal pathologist.

• Examples of present-day terms applied to plant diseases but borrowed from the medical profession arc: canker, oedema, necrosis, embolism, and anthracnose. In some cases these terms have been modified but nevertheless in the original they were borrowed from human pathology.
• While it is manifestly’ impossible to make exact comparisons between the symptoms of disease exhibited by members of the animal kingdom and those observed in diseased plants, yet there are many resemblances,
  

Classification of diseases on basis of hosts.

The classification of plant diseases on the basis of hosts is simply a matter of convenience. There is no fundamental principle involved in this system of classifica­tion. In many colleges and schools of agriculture it is customary to offer courses on the diseases of particular crops or groups of crops for the convenience of students interested in specialized fields of agriculture.

 For example,

 a course might be given for students specializing in vege­table gardening. Such a course might be entitled “ Vegetable  Diseases,” or “ Diseases of Vegetables.” Likewise another course designed espe­cially for students of pomology might be labeled “Fruit Diseases.” A still more specialized course might be entitled “ Diseases of Cane Fruits,” or “ Potato Diseases.”

Conclusion:

Such courses based on a host classification of plant diseases should as a rule follow a general course in plant pathology which should be prerequisite for the pursuit, of the specialized course. At least, some knowledge of the classification of diseases both on the basis of cause and on the basis of symptoms should he acquired, if not in a general beginning course, then as a part of the special course.

 The most fundamental principle in the science of plant pathology is certainly the fact that the cause of a disease must, be thoroughly understood before effective con­trol measures can be applied intelligently.

•  Any control measures not based on accurate knowledge of the etiology of the disease must neces­sarily be empirical and consequently many times more or less ineffective. Therefore, the causes of disease in plants should be accurately classified and each type of causal agent thoroughly understood. A knowledge of symptomatology is scarcely less desirable.

•  A study of all the effects of the disease upon the host should be made, including the pathological anatomy or histology of the diseased parts. The accurate classification of the disease on the basis of symptoms is necessary for accurate.

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