This section is from the book "The Potato: A Compilation of Information from Every Available Source", by Eugene H. Grubb, W. S. Guilford. Also available from Amazon: The Potato: A Compilation Of Information From Every Available Source.
Potato Leaf Blight (Alternaria Solani). The following description is from "Bulletin No. 71" of the Wyoming Agricultural Experiment Station:
This disease has probably been long in existence. Our knowledge of it, however, is exceedingly recent. So long as the real nature of potato diseases were not understood, the different kinds of such diseases were not discriminated. They were all classed under one name, if named at all. Our first definite knowledge of the early blight was worked out in this country in the early '90's, though references to it occur somewhat earlier in the nineteenth century in European literature. It had been overlooked or confounded with the late blight, but it is now fully understood that the parasitic organism causing it is wholly different from the one causing the late blight in structure, in method and time of development, as well as the conditions under which it occurs.
The fungus, like most other plant parasites, lives within the tissues of the host, spreading its mycelium through the intercellular spaces of the leaf. It consists of slender threads (hyphse), more or less branched, which tend to become aggregated (in certain areas, the tissues of which die, producing the characteristic brown spots. While in full growth and while the tissues of the host are supplying an abundance of food, there are few, if any, reproductive bodies (spores) produced. When the leaves become partially exhausted and dry, spore reproduction takes place freely and the characteristic several-celled spores, formed in chains, occur abundantly.
Since the early blight has only recently attracted attention in this state, it is not generally known to our growers. It may, however, be readily recognized and easily distinguished from the late blight and the other potato diseases. Early blight begins to show itself about the time that the blossoms appear, which, with us, is usually in July. More rarely it attacks plants scarcely six inches high. The first indications are relatively small grayish brown spots, which, as they become larger, are marked with faint concentric circles, giving a target-like appearance to them. The spots may increase in size until several of them run together and form large patches of dead tissue. In the course of a few days these spots become brown and withered, while the rest of the leaf takes on a yellowish, sickly color, though the stems may remain green. Sometimes the disease progresses quite slowly and the vitality of the plant is only gradually reduced. In any case, however, the tubers either stop growing entirely or remain so small as to make them of little value. The death of the vines in this way is often mistaken for early ripening and it then occasions a surprise to find that no tubers of value are present. •
Any injury to the foliage, such as insect bites or bruises from hail, seems to furnish the condition for the entrance of the fungus into the leaf. Likewise any decline in the vigor of the plant seems to invite attack. Drought, poor soil, delayed development due to cold weather, excessive heat tending toward wilting or sun-scald, all make the plants less able to withstand the attacks of this blight. In other words, the more nearly perfect the plant and the more vigorous its growth the less likely it is to suffer from this parasite.
But little is known concerning the source of the disease. The tubers seem to be wholly free from attack, and there is therefore no reason for suspecting that the seed potatoes carry the disease over from one year to the next. Certain it is that somewhere the several-celled black spores winter over and start the disease again the following season. That this might happen where the dead tops are not destroyed, but are scattered about over the field and farm, is easily understood.
Satisfactory treatment for this disease has not yet been found. Many experiments, however, have shown that the effects of the disease may be greatly reduced by two or three thorough sprayings with Bordeaux mixture. The spraying must be thoroughly done and the first application must be made previous to the appearance of the blight. After the leaves have become filled with the mycelium and the spots are beginning to show, it is too late. Prevention must be the aim, and this is accomplished by putting the leaves in such a condition by the application of the Bordeaux that the spores cannot germinate upon the leaf surface."