The purpose of the list of plants given below is not to discourage any prospective planter, even though the list is a formidable one, nor to catalogue all the ornamental plants which may be affected by insects or disease. It should be remembered that ornamental plants, like animals, are much less subject to disease, and less likely to be harmed also when in good condition. Thus the average person who grows ornamental plants will, if he keeps them from being damaged by mechanical factors such as wounds, lack of food and water, poor soil aeration, and so on, not need to worry very much as to whether they will succumb easily to disease or insect attacks.
The true diseases of plants are either physiological, such as tip burn, due to over-transpiration of water during a hot wind, or parasitic. The parasitic diseases either kill by the secretion of toxins and enzymes which destroy plant tissues, or these toxins and enzymes cause excessive tissue growth or diversion of food substances of the plant to the use of the parasite. When the plant tissue is killed a rot, blight, or leaf spot appears and when the second effect takes place the result is a gall, leaf curl, rust, or smut. The first effect, which results in the immediate death of the plant tissue, is of course the most harmful. Anthracnose is a disease caused by one definite sort of fungus or parasite.
While the diseases of plants are not something new, since the historical writings of the ancients mentioned rusts, cankers, and smuts, the study of how to combat them is not only a new but an everchanging subject. Accordingly, the reader is urged to consult the latest bulletins of his State Experiment Station or of the U. S. Department of Agriculture before embarking very far upon a programme of spraying or of otherwise combating these diseases.
Not all insects which live on or about plants are harmful. Thus the honey bees, the "lady-bugs," and many others are beneficial since they either pollenate the flowers or eat other harmful insects. The types of injurious insects are as follows: plant lice, scales, borers, fruit-eating insects, root-feeding insects, and leaf or bud-feeding insects. The leaf or bud-feeding insects are the most harmful to the appearance of ornamental plants, though borers and scales do a vast amount of damage. Plant lice also, during some seasons of the year, spoil the appearance of some ornamental plants as well as do damage.
Much valuable study and thought have been given to the subject of eradication of insect pests and the information available on this subject is very complete. The spray calendars published by the State Experiment Stations should be consulted for details of how to keep ornamental plants clean and thrifty.