These fruiting pustules which you will see on this bark are the structures which produce the reproductive bodies, these latter being known as the spores. There are two types of spores which are produced by this fungus. One is the type which is commonly spoken of as the summer spore, the other the type which is spoken of as the winter spore. The winter spore is known from the point of view of the mycologist as the perfect stage of the fungus, that is, it is the more characteristic of this particular fungus. If we should make a cross section of the bark, we should find that the vegetative stage is running through the middle bark, and commonly the inner bark, sometimes in one place only, sometimes in the other only, sometimes in both. This vegetative stage later sends up in various ways a mass of tissue which results in the formation of pustules. These appear on the surface, sometimes more or less regularly rounded, sometimes rather irregular. In the case of the summer spore stage, we have inside the pustules a mass of tissue which is formed into spores. The interior of the spore mass, or at least portions of it, is somewhat mucilaginous, so that when moisture is applied a swelling of the interior mass is produced at a certain stage and something has to break. As a result, we have a mucilaginous mass pressed out through the break in the shape of a twisted thread, much the same as if you take a collapsible tube of paste and pinch it.
Now, one of those summer spore threads may contain anywhere from one to five million spores. I have tried to estimate the number in a thread of this sort which was about an eighth of an inch long, and by taking a certain portion of that thread, mounting it in a drop of water, and then counting over a certain measured area under the microscope, I have estimated, by multiplying, that there were 2,400,000 spores in that one thread. So you can imagine how many of these spores may be produced by a single diseased area which has produced perhaps four or five hundred of those pustules, each pustule containing anywhere from one to twenty threads. Each one of those spores may develop a new diseased area, provided it is transported to a fresh break in the bark of a chestnut tree. Fortunately, only a very small fraction of one per cent ever reaches the proper place for growth.
This last is what I alluded to as the summer spore stage. There is a winter spore stage, or technically, the ascospore stage, which comes, as a rule, later in the development of the fungus. In this same pustule, later in the season, certain sacs are formed. These have long necks which extend to the top of the pustule. These sacs are sufficiently large to be seen with the naked eye. They are dark colored. Inside these, we have a lot of smaller transparent sacs or cases in each of which we get eight spores, sometimes in one row, sometimes in two rows. Each spore can propagate the fungus.
We have, then, two types of spores, either one of which can reproduce the fungus under suitable conditions. There is still another way by which the disease may be kept going. The vegetative stage can survive the winter and continue growing the following year.
I will say right here that I am planning to give you merely an outline of this disease, and have time afterwards for questions which I think in a meeting of this sort are one of the most productive sources of information.
In regard to the rapidity of spread of this disease, I will merely call your attention to two cases as illustrations, or to certain facts, rather. One is that the disease, so far as our attention has been directed to it, has developed over the area indicated on the map since the fall of 1904. Another case is one which has occurred in Rhode Island, where I have had a chance to watch its development a little more closely than in other places, that is, more constantly. In the fall of 1908, after I had made over thirty excursions around Rhode Island, I was unable to find a single trace of this disease, and no one else was able to find a single case of the disease in Rhode Island. In May, 1909, I happened to be about five miles west of the city of Providence, and I found two or three cases, all in one rather restricted spot. Later, it was discovered a little farther south, and soon, a little to the north, so that at the end of the season of 1909 we knew of about ten cases in Rhode Island. At the end of 1910, a season in which very few trips were made with the special object of surveying for the disease, we had more than doubled the number of infections found. That led to putting someone into the field in 1910 to make a survey of Rhode Island. A man was also put into the state of Massachusetts for the same purpose. Mr. Rankin, in cooperation with the United States Department of Agriculture, made a survey of New York State, which has resulted in this map. A man was put into Pennsylvania and one into Maryland for the same purpose. As a result of the survey in Rhode Island, where at the end of 1910 we knew of less than fifty cases at the outside, we now know of very nearly 4000 cases. It has been much the same story in Massachusetts. At the beginning of this year, there were four towns in which the disease was known; now there are seventy-one. At present in Connecticut, the disease is known in one hundred thirty-two towns of the one hundred sixty-eight in the state, and the southwestern part of Connecticut is very badly infected, just as badly as the adjoining portions of New York.[A]
So much for illustrations of the rapidity with which the disease develops. I am not going to say at this time anything special about the origin of the disease, simply because we haven't yet decided what was the probable origin. I will merely say there are some different theories in regard to the origin. One is that it was imported from the Orient, another, that it is a saprophyte, a fungus which has lived normally upon dead organic matter, but which has taken on the parasitic form, which develops on living organisms.
In connection with any disease of this sort, one naturally inquires, how are we going to recognize this disease? This past summer Pennsylvania has put into the field thirty or more men who have been trained to recognize this disease, with the idea of locating the infections in Pennsylvania. As perhaps all of you know, the legislature of Pennsylvania has passed a law relating to this particular disease, and has appropriated $275,000 to see if the disease can be controlled. Their idea is that they have perhaps fifty million dollars' worth of chestnuts, and if $275,000 can show whether or not this disease can be controlled, it is economy to try it.
So far as Pennsylvania is concerned, it means possibly the saving of the chestnuts in the middle and western parts of the state; but it also means that if they can check it there, it is likely to save the great area of chestnut growth along the southern Appalachians. I don't want to make any prophecy as to how that experiment is likely to come out, but, however it comes out, it will be a very great object lesson as to what can be done on a large scale with a disease of this sort.
One of the first things which had to be considered in Pennsylvania was to train a number of men to recognize the disease, so as to go over the country and locate the diseased spots. The method of recognizing the disease I will briefly outline. Of course, over a large country, many hundreds of square miles, it is a long, and laborious operation to look over every tree. It is perhaps impossible without a very much larger force than $275,000 could put into the field. But there are certain clues to the location of the disease which can be seen a long distance, a quarter of a mile, at any rate. The means of recognition is by what I commonly call danger signals. This fungus, when growing through the bark, starts from the common point of infection and grows in all directions, up the stem, down the stem, and around the stem. Wherever this vegetative stage, technically known as mycelium, penetrates, the bark is killed; and of course, you all know what that means. When this has succeeded in reaching around a twig, branch, or trunk, everything beyond that girdled area dies, not immediately, perhaps, but sooner or later it dies; and it dies in such a way that the leaves change color during the summer. The first obvious change which can be noted is a slight wilting of the leaf; then the leaf assumes a pale green color, and from the pale green it takes on a yellow stage; from this a reddish yellow stage, and then a brown, till the leaf is the ordinary dark dull brown of the dead leaves. This coloration which takes place is conspicuous. There is your guide, your danger signal. If the disease has worked very long, half a season, in one locality, you are almost sure of getting some of these danger signals. Where one is present, you can go and look up the cause of that danger signal. It may be a broken twig, but the point is to find out if it is this disease which has caused the danger signal. We start by looking at the danger signal, then at the base of the dead area. If we find here some of the reddish pustules which have been shown on this bark we are quite sure that the disease is present. Then by cutting into the bark a little, instead of the normal buff or yellowish tint of the fresh clean bark, we get, when the disease is present, a rather mottled effect, varying from a brownish to lighter or even darker. There is a peculiar fan-like effect to this mycelium which penetrates the bark, so that by shaving off the surface of the bark, you get this mottled appearance, which gives you another means of identifying the disease. So we look for the danger signals, and then look for the meaning of the danger signals. If we find those two things, the pustules and the mottled mycelium, we can very safely say that this disease is present.