It is due to a species of plant, and although that species is small, it is as easily separated from allied plants as species of flowering plants can be separated from each other.  This plant was known in South America before it made its appearance in this country.  It has been traced from South America to North America, and to Australia, and it made its first appearance in Europe in Belgium, in 1844, and within a very few days after it appeared in Belgium, it was noticed in the Isle of Wight, and then within almost a few hours after that it spread over the whole of the south of England and over Scotland....  When the disease begins to make its appearance, the fungus produces these large oblong bodies (conidia), and the question is how these bodies are spread, and the disease scattered....  I believe that these bodies, which are produced in immense quantities, and very speedily, within a few hours after the disease attacks the potato, are floating in the atmosphere, and are easily transplanted by the wind all over the country.  I believe this is the explanation of the spread of the disease in 1844, when it made its appearance in Belgium.  The spores produced in myriads were brought over in the wind, and first attacked the potato crops in the Isle of Wight, and then spread over the south of England.  The course of the disease is clearly traced from the south of England toward the midland counties, and all over the island, and into Scotland and Ireland.  It was a progress northward....  This plant, the Peronospora infestans, will only grow on the Solanum tuberosum, that is, the cultivated potato....  Just as plants of higher organization choose their soils, some growing in the water and some on land, so the Peronospora infestans chooses its host plant; and its soil is this species, the Solatium tuberosum.  It will not grow if it falls on the leaves of the oak or the beech, or on grass, because that is not its soil, so to speak.  Now, the process of growth is simply this:  When the conidia fall on the leaf, they remain there perfectly innocent and harmless unless they get a supply of water to enable them to germinate....  The disease makes its appearance in the end of July or the beginning of August, when we have, generally, very hot weather.  The temperature of the atmosphere is very high, and we have heavy showers of rain.”

The warmth and moisture are, in fact, the conditions necessary for the germination of the conidia.  Their contents (zoospores) are liberated, and quickly grow in the leaf, and soon permeate every tissue of the plant.