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they are growing. They remain dormant during the winter, and when spring comes germinate, and make their way into the nearest grape-vines. It would not be unwise in the autumn to collect and burn all grape-leaves — that is, as far as practicable — in districts which have during the summer suffered from the Uncinula. In this place we should bear in mind what was said of the black knot extending from wild species to cultivated. The Uncinula can extend in a similar way, as it is found on wild vines; but, as far as we yet know, the fungus does not grow on any wild plants except grape-vines. If we turn now to the Peronospora which grows on grapevines, we see that the preventive measures, which in the case of the Uncinula would be of advantage, would here be of little avail; because the fungus is not confined to the surface, but pervades the whole plant, and, in fact, does not grow through the breathing-pores into the air until it has already traversed a considerable part of the interior of the leaves and stems. The first warning of the presence of the fungus, viz., the white spots on the under surface of the leaves, is not to be interpreted as showing that the disease is beginning, but that it is already far advanced. To sprinkle sulphur on the leaves is quite useless in the case of the Peronospora ; for it will not affect the mass of the fungus which is in the interior. The only thing which will check the disease is to diminish the moisture in the air; but that, unfortunately, is beyond human control. If the season happens to be dry, all very well; if very wet, then the Peronospora, once started, will grow in spite of every thing. The disease spreads from plant to plant during the summer by means of the conidial spores. They may be carried about by the wind and rain, just as the conidial spores of the Uncinula ; but, when they fall on a place sufficiently moist, they germinate, but in a different way from the conidial spores of Uncinula. The contents of the spores separate into a number of distinct bodies, which break through one end of the spore, and escape, leaving the empty spore-wall behind. The bodies which escape, to which the name of “zoöspores” is applied, swim about by means of two hair-like threads called “cilia,” which are in constant motion. Being very small, they are able to move about in the moisture which is found on the . ground and on plants when it is not dry weather. They
swim about for only a short time, and then the cilia drop off, and the zoöspores come to rest. They then give off threads like the conidial spores of Uncinula, and the threads penetrate into the interior of the grape-plants on which they may be. Once inside, the threads constitute a mycelium, which extends through the plant at a rate corresponding with the external moisture; and finally the threads make their way through the breathing-pores into the air, and produce new spores. It will be seen that the conidial spores of the Peronospora have an advantage over those of the Uncinula, because they produce a number of zoöspores, generally from five to fifteen, each of which is capable, under favorable hygrometric conditions, of producing a new mycelium and spores. Like the corresponding bodies in Uncinula, the conidial spores are destroyed by cold. The winter spores of the Peronospora are thick and tough, and are produced in the interior of the grape-leaves by a rather complicated process, which, although interesting from a botanical point of view, need not be described here. They fall to the ground with the leaves in the autumn, and are set free by the rotting away, during the winter and spring, of the leaves in which they are contained; and, as the season advances they germinate, and enter the nearest vines, but the details of the germination have not yet been made out. The remarks already made about the burning of grape-leaves apply also in this connection. The mycelium of the Peronospora in the interior of the vines affected during the summer may remain dormant during the winter, and start up again when the warm weather returns. Just how much harm the Peronospora does to the grape-vines is not easy to decide. I have, on a previous occasion, expressed the view that it is not unlikely that the harm done has been exaggerated, because the fungus never attacks the berries, and it does not cause the leaves to shrivel and dry up until comparatively late in the summer, when, as some say, their room is better than their company; for in Massachusetts the thing to be desired is, that there shall be plenty of sunlight to ripen the grapes, which is not the case when the foliage is luxuriant, and covers up the branches. Whether the shrivelling of the leaves in the latter part of August permanently injures the vines and injures the crop is a point to be settled, not by the botanist, but empirically by the grapegrower; and, as far as can be learned, on this point opinions differ. My object in describing, in what you may perhaps consider too great detail, the two principal blights on American grapes, has been to show that an accurate and scientific knowledge of the causes of disease in plants requires a careful microscopic study, and that such study is not without definite and even practical results. The time has passed when the labors of botanists should be considered of interest only to special students of science. From them the farmer may learn certain facts of which he cannot afford to be ignorant. The high science of one decade, it must be remembered, becomes, in the course of three or four decades, the popular belief, and is then honored with the name of common sense, just as though, not more than half a century previously, people had not been considered fools for believing just such things. Only within a few years have fungi been recognized as the cause of disease in plants; and there is a growing tendency to account for almost all obscure plant-diseases by saying that they are caused by fungi. If a disease suddenly makes its appearance, and inquiry is made as to its cause, up jumps Dr. A., and says, “It is a fungus: I have found some mycelium.” Or Professor B. startles the community with the announcement that he has found “spores.” Neither Dr. A. nor Professor B. tells the public to what form the mycelium and spores belong; nor do they apparently know that it is almost impossible to find a leaf or stem in which, or on which, there are not some traces of mycelium or spores. The spores and mycelia of the common moulds are everywhere; and, if one is determined to see in fungi the cause of all diseases, he has not to look long before finding them in abundance, — such as they are. It savors decidedly of quackery to make a little bit of mycelium, or a few spores of some ordinary mould, explain the appearance of wide-spread and devastating diseases. A few years ago every thing was laid to insects by the agricultural quacks; but, as a knowledge of entomology spread, that became dangerous ground, and they then took up fungi, about which the public were not so well informed. Before long, it is to be hoped, there will be such a general knowledge of the habits of fungi, that the-war cry, “Mycelium ! Spores ' " will have lost its terrors. Where, then, will the quacks take refuge 2 At the lowest limit of the vegetable kingdom, some would say below the lowest limits, is a large group of very minute beings, called “Bacteria.” They are very small; they are found everywhere; their study taxes the highest powers of the first scientific men. It will be a long time before the scientific world knows much about them, and longer'still before the public do. What a paradise for quacks Without being a prophet, it will be safe to predict, that, within the next ten years, the agriculturist will have to listen to an immense amount of nonsense about the harm these small bodies do, and the diseases they cause. In the mean while, let us not underrate the harm done by fungi, while deprecating all attempts to make them responsible for every disease which may make its appearance; and here, as in other things, a little knowledge is a dangerous thing, for it is only by cautious and careful research that we reach results which are really valuable, either scientifically or for practical application.
Mr. FLINT. I find this question in the question-box: Is mildew-seed in the air, or in the land 2 I would like to have Professor Farlow make some statement, if he has any thing to say upon that question.
Professor FARLow. It is in both. This fungus, you know, has two kinds of seeds, or spores. The seeds that are found on the under surface of the leaf drop off on the ground, or at least into the air, and can be carried about by the wind. The other seeds are found in the tissue of the leaves; and as the leaves fall off, and fall upon each other, of course there is quite a respectably thick layer of leaves upon the ground. These are covered by snow; the leaves rot; and these spores will be left upon or in the ground, because they will be more or less buried.
Mr. HILLs of Arlington. I proposed that question. The fact is, I have raised considerable lettuce, and I am troubled with mildew. I have noticed, that, when I went on to new lands, I was not troubled with it. I have been to considerable expense in moving my fences, so as to enclose land where lettuce had not been grown; and some men in the same business think it was foolish to do so. They say the spores are in the air. * * * . . . .
Professor FARLOW. Some of these seeds will be killed in
the fall, and others will be left over: so, if you find lettuceleaves affected, do not let them be, but pull them off and burn them. If you simply pull them off, these seeds will not be injured. They will come out the next spring with full force. Mr. STRONG. The lecturer states that these spores are perishable, unless they have a sufficient amount of moisture. Now, it is known that this fruit-mildew does not prevail until about August, when we have warm weather, and very rapid generation. I would like to know how these spores live from spring until fall. Professor FARLow. The spores that are preserved through the winter will be those that germinate, and enter into the plants when they appear above the ground. The threads are found in the leaves and stems, from top to bottom. After a while, when they have gained sufficient force, they burst through ; but they do not grow much for a time: they remain quiescent; but the next moist weather they will develop rapidly. Mr. STRONG. You are not aware of the existence of the disease until you see these spots on the leaf. The fungus may nearly fill up a grape-vine, and not do much harm: but, the moment it has acquired force enough to break through into the air, it seems to have attained much greater vigor: and, simultaneously with breaking through on the surface, you will see a wilting and destruction of the leaves. Professor STOCKBRIDGE. We have found an application of sulphur destructive to the fruit-mildew, when it has broken through, and becomes visible to the naked eye. Mr. PAUL. Is there any remedy for the fungus on the grape-leaf 2 I am not a judge of these matters; but I will state a fact, which, by analogy, may apply to the question now under consideration. A neighbor of mine picked the leaves off of his strawberry-plants, and none of the berries ripened. I have seen on my own vines, where from some blight the leaves shrivelled and dropped off, that the grapes never matured afterwards. Mr. FISHER. Some grape-growers have pulled the leaves from their vines to let the sun in ; and the result has been exactly what any man of sense might have expected. Grapes never ripen except in the presence of the upper leaves.