The constitutional and bacterial diseases usually affect the whole plant, or at least large portions of it; and the seat of attack is commonly not so much in the individual leaves as in the stems, the sources of food supply being thereby cut off from the foliage. The symptoms of this class of diseases are general weakening of plant when the disease affects the plant as a whole or when it attacks large branches; or sometimes the leaves shrivel and die about the edges or in large irregular discolored spots, but without the distinct pustular marks of the parasitic fungi. There is a general tendency for the foliage on plants affected with such diseases to shrivel and to hang on the stem for a time. One of the best illustrations of this type of disease is the pear-blight. Sometimes the plant gives rise to abnormal growths, as in the “willow shoots” of peaches affected with yellows (Fig. 215).
Another class of diseases are the root-galls. They are of various kinds. The root-gall of raspberries, crown-gall of peaches, apples, and other trees, is the most popularly recognized of this class of troubles (Fig. 216). It has long been known as a disease of nursery stock. Many states have laws against the sale of trees showing this disease. Its cause was unknown, until in 1907 Smith and Townsend, of the Bureau of Plant Industry, United States Department of Agriculture, undertook an investigation. They proved that it is a bacterial disease (caused by Bacterium tumefaciens); but just how the bacteria gain entrance to the root is not known. The same bacterium may cause galls on the stems of other plants, as, for example, on certain of the daisies. The “hairy-root” of apples, and certain galls that often appear on the limbs of large apple-trees, are also known to be caused by this same bacterium. The disease seems to be most serious and destructive on the raspberry, particularly the Cuthbert variety. The best thing to be done when the raspberry patch becomes infested is to root out the plants and destroy them, planting a new patch with clean stock on land that has not grown berries for some time. Notwithstanding the laws that have been made against the distribution of root-gall from nurseries, the evidence seems to show that it is not a serious disease of apples or peaches, at least not in the northeastern United States. It is not determined how far it may injure such trees.
Of obvious insect injuries, there are two general types,—those wrought by insects that bite or chew their food, as the ordinary beetles and worms, and those wrought by insects that puncture the surface of the plant and derive their food by sucking the juices, as scale-insects and plant-lice. The canker-worm (Fig. 217) is a notable example of the former class; and many of these insects may be dispatched by the application of poison to the parts that they eat. It is apparent, however, that insects which suck the juice of the plant are not poisoned by any liquid that may be applied to the surface. They may be killed by various materials that act upon them externally, as the soap washes, miscible oils, kerosene emulsions, lime-and-sulfur sprays, and the like.
There has been much activity in recent years in the identification and study of insects, fungi, and microorganisms that injure plants; and great numbers of bulletins and monographs have been published; and yet the gardener who has tried assiduously to follow these investigations is likely to go to his garden any morning and find troubles that he cannot identify and which perhaps even an investigator himself might not understand. It is important, therefore, that the gardener inform himself not only on particular kinds of insects and diseases, but that he develop a resourcefulness of his own. He should be able to do something, even if he does not know a complete remedy or specific. Some of the procedure, preventive and remedial, that needs always to be considered, is as follows:—
Keep the place clean, and free from infection. Next to keeping the plants vigorous and strong, this is the first and best means of averting trouble from insects and fungi. Rubbish and all places in which the insects can hibernate and the fungi can propagate should be done away with. All fallen leaves from plants that have been attacked by fungi should be raked up and burned, and in the fall all diseased wood should be cut out and destroyed. It is important that diseased plants are not thrown on the manure heap, to be distributed through the garden the following season.
Practice a rotation or alternation of crops (p. 114). Some of the diseases remain in the soil and attack the plant year after year. Whenever any crop shows signs of root disease, or soil disease, it is particularly important that another crop be grown on the place.