A. External causes - I. Non-living environment: soil, atmosphere, temperature - II. Living environment: plants, animals - Complex interactions - Predisposing causes - No one factor works alone - Tangled problems of natural selection involved. B. So-called internal causes.
It is customary to classify the causes of disease in plants into two principal groups - (1) those due to the action of the non-living environment soil, atmosphere, physical conditions such as temperature, light, etc.; and (2) those brought about by the activities of living organisms - plants and animals of various species. Before passing to further subdivisions under these two heads, however, it is necessary to observe that no disease can be efficiently caused by an organism alone, since its powers for injury as a parasite, or otherwise, are affected by its non-living environment as well as by the host-plant. For instance, the spores of a parasitic fungus which would infect and rapidly destroy a potato plant in moist warm weather may be showered on to such a plant with impunity if the air remains dry and cool - or on to a cabbage under any circumstances as far as we know.
Again, probably no one factor of the nonliving environment ever suffices to induce a disease, possibly because no such thing as only one change at a time ever occurs. For instance, it is difficult to say, when a soil becomes sodden with water, whether the excess of water and dissolved matters, the want of air displaced by the water, the lowering of the temperature, or the accumulation of foul products, etc., is the principal factor in causing the damage which results, and we have to determine by the balance of experimental evidence which is the dominant factor in all such cases.
The study of aetiology of disease is in fact only a particular case of that of aetiology in general. Plants at high altitudes in the Alps acquire very different characteristics from the same species in the plains. Is this due to the low temperature, the rarer atmosphere, the more intense illumination, the changes in moisture, etc., etc.? The question is more difficult than it appears at first sight, and we must remember that, complex as are the factors working on the host, they are equally complex in their actions on a parasite attacking the host, whence the resulting disease becomes indeed a tangled problem of natural selection.
Finally it remains to say a few words about a numerous class of cases where no external cause of disease can be discovered. It was formerly the custom to group such cases of "Internal Causes" by themselves, but apart from the fact that many of these mysterious diseases have subsequently been shown to be due to the action of external agencies, the whole question of internal causes resolves itself into one of relations between the plant and its surroundings, and it becomes evident that no inherited or internal disease can be regarded as explained until we know the external causes which have so modified the structure and working of the living cells as to make them abnormal in their reactions to other parts of the plant. "Internal causes" of disease, therefore, is a phrase expressing our ignorance, but somewhat more emphatically than usual. If this is clearly understood there seems no reason against its employment for the time being in the artificial scheme of classification we require. With regard to external causes due to the non-living environment, excess or deficiency of materials in the soil, water, or atmosphere plays an important part, and - since we may neglect purely aquatic plants - it is customary to speak of diseases due to unsuitable soils or to injurious atmospheric influences. For instance, any deficiency in the supplies of the necessary mineral salts (compounds of calcium, magnesium, potassium with sulphuric, nitric and phosphoric acids, etc.) leads to pathological changes, as also does the lack of the necessary traces of iron. But it is equally true that the presence of such ingredients in excess or in combinations unsuited to the plants also leads to disaster, as also does the presence of minerals or other compounds which poison the root-hairs - e.g. products of decomposition, soluble salts of copper and other poisons. That these matters are bound up with the whole question of manuring and of proper soil-analyses will be evident.
Another essential factor is the nature and quantity of organic materials in the soil, whether leaf-mould and decomposing vegetable remains, stable manures, or other animal matters, all of which affect different species very differently, and produce very different results in different soils. It is necessary to apprehend in this connection what has been stated above: that soil is not a mere dead structureless medium, and that the root-hairs of ordinary plants cannot deal with large quantities of putrefying organic matter: that a good soil must abound in useful bacteria and fungi to render such substances available-and in very various ways-and that it must be open and aerated, of proper temperature and suitably supplied with water, and so forth, or disaster will result. Here, again, then we are brought into close contact with all that is known of fermentation, nitrification, and the various biological changes going on in soil, and the application of such knowledge to the practice of manuring and tillage in all its forms.
In view of the above remarks, the danger of "over-feeding," in this sense, has a real meaning for horticulturists, though it must not be forgotten that no substance is really a food until it is assimilable into the protoplasm: manures, etc., are food-materials, not food. The futility of mere chemical analyses to prove what a plant requires is now well known, and it is only on the basis of long and carefully conducted experiments that we can ever discover what a particular plant in a particular soil, situation, and climate requires for healthy development. Again, the quantity of water in soil may be too great or too small for given species, and this either on the average for the year, or during critical periods only; and it is obviously important whether the excess or deficiency is due to improper supplies of water, the depth or shallowness of the soil, its retentive powers, or the nature of the sub-soil and so on, again bringing the whole matter into connection with our understanding of the physical constitution and structure of soils, and the nature of soil-drainage.