Classification of Diseases.

349

(b) Attacks of insects which suck their food, and which are, therefore, destroyed by caustic applications which injure the bodies of the pests. All the tribes of plant-lice and scale. insects belong here, and for these the kerosene emulsion, resin washes, and the like, are the specifics.

2. Parasitic fungous diseases, such as the applescab, black-rot and mildew of the grape, leaf-blight of the plum and pear, black- knot, and the like. These diseases are characterized by definite spots, discolorations or excrescences, which are more or less scattered over the surface of the leaf, fruit or branch. As a rule, the leaves and fruits which are attacked have a tendency to drop from the tree. The general treatment for these diseases is to spray with some fungicidal mixture, like the Bordeaux mixture or the ammoniacal carbonate of copper. The treatment is useful in proportion as it is applied early and thoroughly. After the fungus once gets into the tissues of the host-plant, it is difficult, if not impossible, to kill it. If, however, the fungicide is upon the plant before the fungus is, the parasite may not be able to obtain a foothold. Even after it does obtain a foothold, it is probable, however, that the spray will check its spread by preventing the development of its external parts.

3. The physiological and bacterial diseases, or those which are termed constitutional troubles. In these cases, there are rarely any definite spots, as in the attacks of parasitic fungi, but the entire

leaf, or even the entire plant, or a large part of it, shows a general weakening and disease, as if there were some cutting off of the accustomed source of nourishment. Such diseases are very likely to be seen in a general yellowing and death of the leaf, in the dying of the leaf along the main veins. and around the edges, showing that the difficulty is one which affects the entire leaf, and not any particular part of it. In general, there is a tendency for the foliage in plants so attacked to wither up and hang on the tree for a time. The peach yellows and pear blight are diseases of this kind. There are no specific treatments for troubles of this sort. They must be approached by what physicians call prophylaxis,—that is, by methods of sanitation and prevention. The diseased plants or parts are cut away and burned. All those conditions which seem to favor the development of the disease are removed. Varieties which are particularly susceptible are discarded. Careful management in matters of this sort is often much more important than any attempt at specific treatment.

Before taking up a discussion of spraying, it may be said that the best treatment for borers and similar pests is to watch the plants carefully, and to dig the insects out at least twice every year. In grounds which are kept in clean tillage, such insects are rarely as troublesome as they are in neglected areas. This is both because the insects find places of lodgment in neglected orchards, and because the fruit-grower is so seldom present that

he does not discover them in season. The various protective washes which are advised for keeping borers out of trees are of very doubtful efficiency.

The roots of fruit trees

and brambles are

very likely to be affected with large tuberlike swellings or galls, which have been the subject of a good deal of uneasiness in various parts of the country. One of these is shown in Fig. 64. So far as known, these galls are not contagious, and the amount of harm which they do has probably been overstated. The rootknot of the southern states and of greenhouses is a wholly different trouble, and is the work of a nematode worm. There is also a root swelling or gall on raspberries, due to the work of an insect. The nematode galls are commonly smaller and softer swellings, and occur on the younger or smaller roots, Fig. 64. Root gall on raspberry. and appear not to occur in Sta.)

(From Bull. 100, Cornell Exp.

the open in regions where the ground freezes deep. This subject of the root-galls has been fully gone over by various writers, and a summary of the subject is presented in Bulletin 117 of the Cornell Experiment Station (although this subject is by no means well understood), from which the following epitome is clipped: "The conclusion of the whole matter, then, as we now understand it, is that these rootgalls are not the work of a parasite, but are a mal-formation following some injury of the root, or some uncongenial

condition in soil or treatment. The galls may seriously interfere with the nutrition of the plant, in many cases causing it to become weak and sickly. It is probable that the trouble is not communicable, and that cutting off the gall averts further trouble from that source. As a precautionary measure, however, we much prefer to plant only trees with perfectly clean and normal roots."

The injuries caused by hail are very often confounded with those wrought by borers and other pests, and it is the delight of many persons to endeavor to puzzle the experimenters and teachers with specimens of such work. Plum limbs injured by hail-stones are shown natural size in Fig. 65.

Bagging Fruits.

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When it is desired to secure extra fine fruit, it is a good plan to tie up the fruits in paper bags. This keeps away the insects and fungi (although the whiteness of the bags is likely to attract thieves at night), and the fruit is apt to ripen earlier, and to be of higher quality because of the warmth which the bag gives. If it is desired to bring out the blossoms of a tree very early in the spring, it may be done by tying grocers' bags upon the spurs when the buds first begin to swell. The bagging of grapes is a frequent practice when exhibition or test specimens are desired. It is customary to pin the bags upon the clusters when the grapes are a third to a half grown. Bags made of mosquito netting are very useful later in the season, when it is desired to secure the full color of highly-colored fruit.

SPECIFIC REMARKS UPON SPRAYING.*

1. Spraying is only one of the requisites to success in fruit-raising.-Spraying has come into use so quickly, and so much of the attention of teachers and experiments has been given to it, that many people have come to look upon it as the means of salvation of our orchards. If spraying is to have the effect of obscuring or depreciating the importance of good cultivation and fertilizing, then it might better never have come into being. Trees must grow before they can bear, and this growth depends upon food and proper conditions of soil,

*Largely adapted from Bull. 101, Cornell Exp. Sta.