When we come to enquire into what circumstances bring about those severe and apparently sudden attacks on our crops, orchards, gardens, and forests by hosts of some particular parasite, bringing about all the dreaded features of an epidemic disease, we soon discover the existence of a series of complex problems of intertwined relationships between one organism and another, and between both and the non-living environment, which fully justify the caution already given against concluding that any cause of disease can be a single agent working alone.
The statement of prophecy that a particular insect or fungus need not be feared, because it is found to do so little harm in particular cases or districts examined, will thus be seen to be a dangerous one: any pest may become epidemic if the conditions favour it!
In 1844 and 1845 the potato disease assumed an epidemic character so appalling in its effects that it is no exaggeration to say that it constituted a national disaster in several countries. It was stated at the time that this disease had been known for some time in Belgium, in Canada and the United States, in Ireland, in the Isle of Thanet, and in other parts of the world. Similar, but less devastating epidemics have occurred in various years since. It was generally noticed during such epidemics that the plants themselves were full of foliage, surcharged with moisture, and of a luxuriant green colour promising abundant crops. The now well-known spots, at first pale and then brown and fringed with a whitish mouldlike growth - the conidiophores of the Phytophthora - were observed during the dull cloudy and wet weather, cooler than usual, when the atmosphere was saturated for days together, in July and August. The actual amount of rain does not appear to have been excessive, but most observers seem to agree that dull weather with moist air had succeeded a warm forcing period of growth. So rapidly did the disease run its course that in a few days nearly all the plants were a rotting blackened mass in the fields, and the potatoes dug up afterwards were either already rotten or soon became so in the stores. Further experience has confirmed this, and we now know that the epidemic is very apt to appear in any region where potatoes are grown on a large scale, in dull moist weather, especially in fields exposed to mists, heavy dews, etc., about July and August, when the foliage is full and turgid. Similarly on heavy wet soils, unless the season is remarkably open and dry; but also on dry light soils in rainy seasons. So evident was this that many believed that the mists and dew brought the disease - harking back to the superstitions of earlier days. We must remember that prior to 1860 the life-history of Phytophthora was not known. Since De Bary's proof of the germination of the zoospores and of the infection of the leaves, the course of the hyphae in them and in the haulms, the origin of the conidia, etc., and the confirmation by numerous competent observers of the true fungus nature of this disease, we are now in a position to understand the principal factors of the various epidemics of potato disease.
It is not merely that the potato-fields afford plenty of food for the fungus, and that the dull weather causes the tissues to be surcharged with moisture, owing to diminished transpiration, but the mists and dew - to say nothing of actual rain and the flapping of wet leaves - favour the germination and spread of the zoospores throughout the field. Whether the dull light also favours the accumulation of sugars in the tissues, and the partial etiolation of the latter implies less resistance to the entering hyphae, may be passed over here, but in any case it is clear that we have several factors of the non-living environment here favouring the parasite and not improving the chances of the host, even if they do not directly disfavour it.
As another instance I will take the Larch-disease, which is due to the ravages of a Peziza (Dasyscypha Willkommii) the hyphae of which obtain access by wounds to the sieve-tubes and cambium of the stem, and gradually kill them over a larger and larger area and so ring the tree, with the symptoms of canker described below.
Now the Larch fungus is also to be found on trees in their Alpine home, but there it does very little damage and never becomes epidemic except in certain sheltered regions near lakes and in other damp situations. How then are we to explain the extensive ravages of the Larch disease over the whole of Europe during the latter half of this century? The extensive planting, providing large supplies for the fungus, does not suffice to explain it, because there are large areas of pure Larch in the Alps which do not suffer.
In its mountain home the Larch loses its leaves in September and remains quiescent through the intensely cold winter, until May. Then come the short spring and rapid passage to summer, and the Larch buds open with remarkable celerity when they do begin - i.e. when the roots are thoroughly awakened to activity. Hence the tender period of young foliage is reduced to a minimum, and any agencies which can only injure the young leaves and shoots in the tender stage must do their work in a few days, or the opportunity is gone, and the tree passes forthwith into its summer state.
In the plains, on the contrary, the Larch begins to open at varying dates from March to May, and during the tardy spring encounters all kinds of vicissitudes in the way of frosts and cold winds following on warm days which have started the root-action - for we must bear in mind that the roots are more easily awakened after our warmer winters than is safe for the tree.
It amounts to this, therefore, that in the plains the long continued period of foliation allows insects, frost, winds, etc., some six weeks or two months in which to injure the slowly sprouting tender shoots, whereas in the mountain heights they have only a fortnight or so in which to do such damage. That the lower altitude and longer summer are not in themselves inimical to Larch is proved by the splendid growths made by the trees first planted a century ago. Then came the epidemic of Larch-disease: the fungus, which is merely endemic - i.e. obtains a livelihood here and there on odd trees, or groups of trees in warmer or damper nooks - in the Alps, was favoured by the more numerous points of attack afforded to its spores by injuries due to insects - Coleophora, Chermes, etc. - and frost wounds, as well as by the longer periods of moist dull weather, and the longer season of foliation. Moreover, as time went on almost every consignment of young Larch-trees sent abroad was already infected. Here again, then, we find the factors of an epidemic consisting in events which favour the reproduction and spread of a fungus more than they do the well-being of the host.