The Late Blight (Phytophthora infestans) is a very serious disease. The following is from "Bulletin 71" of the Wyoming Experiment Station:

Though this disease had not been fully worked out until in comparatively recent times, yet there are references in literature to potato epidemics which devastated the fields of Europe at intervals during the nineteenth century, which were undoubtedly due to it. The first recognizable description occurs first in 1845. Its life history, however, has now been known for some time, though as late as the '80's and '90's this trouble was still confused with the early blight. For a considerable time it was not known that the rot which usually follows an attack is also due to the same parasite. While probably of rare occurrence in the Rocky Mountain states, late blight is feared more than any other disease in the potato districts of the Eastern States. It is estimated that the loss in New York alone sometimes amounts to $10,000,000 a year.

Though this fungus resembles the early blight in many respects, yet it is easily distinguished from it by its mode of growth, the effect it produces on the leaf tissues, and especially by the spores and the way in which they are produced. It finds entrance into the potato leaf through the stomata, and the mycelium once having found entrance spreads by numerous branching hyphse through the leaf among its cells, from which the fungus draws its nourishment. After the leaf has become filled, as it were, with the mycelium, the fruiting period of the fungus is reached. Some of the hyphae then grow out through the stomata, branch, and produce small pear-shaped bodies on the tips of the branches. These latter structures, known as sporangia, serve to spread the disease to other parts of the field. They are very readily detached from the filament upon which they are grown and then fall upon the soil, or are carried far by the wind. If they happen to fall upon a potato leaf they will begin to grow just as soon as a little moisture either from rain or dew is present. This growth consists either in the formation and discharge upon the surface of the leaf of several free swimming spores, capable of infecting the plant, or in the direct formation of a filament which enters the leaf through a stoma and develops again a mycelium. From this mycelium other similar reproductive bodies are formed, in turn, to further infect the field.

During the time that the fungus is spreading its mycelium through the tissues of the leaf there is little to indicate its presence. When the fruiting stage is reached it soon becomes evident enough by the formation of brown spots, which grow gradually larger and larger, finally turning black and a little later decomposing and emitting a disagreeable but characteristic odor. If one of these infested areas be examined closely it will be found to be bordered by a grayish white mildew. This latter, under examination with a lens, is seen to be the branched fruiting hyphse bearing the sporangia described in the preceding paragraph.

For the development of the mycelium - that is, for the growth of the fungus within the potato plant - moderately cool weather seems the most favorable. For this reason this disease rarely proves troublesome where high temperatures prevail for considerable periods of time. Spore production, however, seems to be hastened and enormously increased when a few days of warm, cloudy and muggy weather alternate with the longer, cooler periods. Under such conditions a field showing but slight infection may in a few days look as if it had been swept by fire or frost. It rarely attacks early potatoes, mostly appearing upon the late varieties during the tuber-forming period.

Late Blight Diseased leaf and tuber of potato. From the Journal of the British Board of Agriculture and Fisheries.

Late Blight. The last stages of the disease. When this is reached the field looks as if fire had swept over it. (From the New York Experiment Station, Geneva, Bulletin 241).

Various experiment station workers have tried different remedies for holding this disease in check. At some stations these experiments have been carried on for many years. While several have given results which were of value, no treatment has been as uniformly successful as the application of Bordeaux mixture. The universal experience is that spraying with this fluid will so nearly control the late blight as to make it possible to secure a crop even in those years when this disease is most prevalent. It requires, however, that the spraying should be begun in time and continued at intervals throughout the growing season. As already stated, it must be a precautionary measure. If not begun until after the blight is evident in the field only partial control can be expected. If the spray is applied thoroughly from the beginning, not only will the blight be controlled, but the rot of the crop which usually follows a severe attack is altogether prevented.

It has been almost conclusively proved that the rot of the tuber which follows an attack of late blight is really due to the infection of the tuber by the spores which have fallen upon the soil and which, in the course of the season, are carried by rains or irrigation waters into contact with the tuber itself. Here it may begin growth at once or it may develop after the potato has been dug and stored. Sometimes a large portion of the crop is thus lost even after it has been harvested. Thorough spraying of the vines will, at the same time, impregnate the surface of the soil with the copper-sulphate solution. Thus not only is the formation of any considerable number of spores prevented, but the spores that do happen to reach the soil are destroyed.

It is believed that the spores of the fungus do not live through the winter. If that be true the mycelium of the fungus must either live over in the dead tops that are left strewn about the field, or else the tubers carry the disease over from one season to the next. The latter is thought the more probable, as it has been seen that the blighting of the tops (if not checked by spraying) is very likely to be followed by rot of the tubers, either before.or after digging. Of course, no one would think of planting badly rotted potatoes, but those that are but slightly affected may escape notice. These, if planted, will be sufficient to start the infection the next year, and once started it soon goes over the field."

This disease is very serious in Europe. In "Leaflet 23" of the Board of Agriculture and Fisheries of Great Britain is the following:

This disease, well termed by agriculturists the ' potato disease, has in the past been the cause of immense loss, and is even at the present day the chief trouble with which potato growers have to contend.

The first sign of this disease is the presence of yellowish spots on the leaves. These spots gradually increase in size and become brown, this condition being followed by the curling of the leaves. If the under surface of a diseased leaf is examined with a magnifying glass, the fruiting branches of the fungus will be seen forming a delicate white mold.

The spores of the fungus are exceedingly numerous and minute, and are scattered by wind, or by ground game and other animals running amongst the plants. When it is stated that every spore brought into contact with a damp potato leaf is capable of starting a new centre of infection, the rapid spread of the disease under favorable conditions will be readily understood. The disease develops and spreads with the greatest rapidity during damp, warm weather, such as often occurs in July.

Spores that fall to the ground are washed through the soil by rain and may infect young potatoes, especially those growing near the surface. It is probable, too, that the mycelium of the fungus passes down diseased stems into the young potatoes. If the season be wet and warm the mycelium present in the potato continues to grow, soon causing brown spots to appear, and ending in the rotting of the tuber. On the other hand, if potatoes that are infected be kept dry, the mycelium in their substance may remain stationary until the following spring, when it may commence growth and infect the new crop, afterward appearing in the fruiting condition on the leaves.