All parts of plants, both above and below ground, are subject to infestation by pests and diseases. Under garden conditions, cultivation is intensive, and plants are grown year after year on the same ground in surroundings much more sheltered and crowded than in the open field. Sound garden management is therefore an important control factor, and the following features are fundamental:—
Condition of Soil.—The vigour of plants is dependent on the soil, which therefore must be kept in the right state; it must be well tilled, and must contain the requisite nourishment and moisture available for plant use, and as far as possible be free of an abnormal population of root feeding pests, such as eelworms and the larvæ of many insects. Proper cultivation is therefore the important factor in bringing the soil into the state most favourable to plant life, as all inimical factors, including pests, are reduced. Wherever practicable, as in glass-houses, soil-inhabiting pests and diseases can be completely controlled by steam sterilisation.
Importation of Pests.—One of the readiest methods of infesting a garden is the importation of pests on plants, and every care should be taken to secure only pest-free stock. In this respect, also, must be mentioned the use of stable and barnyard manure, in which pests such as insect larvæ, woodlice and eelworms are introduced; artificial fertilisers are therefore safer.
Overcrowding.—The tendency to overcrowd, especially in household gardens, is to be avoided; a favourite habit is to plant something in every available space. Under such conditions pests and diseases will abound, and before attempting to spread over a large area, and so lessen the effect of their depredations, they concentrate in mass formation within the confines of the garden as long as the food supply lasts; further, plants tend to be less vigorous and more susceptible to infestation under crowded than under more open conditions.
Injury to Plants.—Care should be taken not to injure plants with garden tools during cultivation, and a clean cut should always be the object in pruning. Mechanical injury opens the way for infestation by diseases and some insects.
Garden Sanitation.—Clean gardening is an extremely important control factor. In most gardens there are rank growths of grass and weeds in out-of-the-way places, along boundaries, and bordering cultivated plots. Such growths, especially when the weeds are related to the garden plants, are always favourite breeding places for many pests that move on to cultivated plants immediately they appear above ground. If these growths are cut and burned in the winter, and the ground thoroughly dug, many a spring infestation will be suppressed by the control of hibernating pests; it is the control of spring infestations that will save a great deal of trouble throughout the summer and autumn.
The compost heap, where garden refuse is dumped until sufficiently rotted, may be a source of infestation; not only does it attract and breed many destructive underground pests, but it may be infested with the spores of diseases harboured by the plant refuse of which it is composed; it is thus a ready means of reinfesting the soil. Diseased and pest-infested refuse should be burned without delay, and only healthy refuse used for the compost heap if not dug into the ground, where it will rot.
Crop Rotation.—Growing the one type of crop on the same piece of ground for several seasons encourages the development of pests and diseases; but by a rotation of different kinds of plants the continuity of the conditions favourable for the pests and diseases is broken, and the latter do not have the chance of becoming thoroughly established.
Diseases Spread by Pests.—It should be borne in mind that the fewer the animal pests, the less chance there is for diseases to spread. It is now well known that many pests, though not necessarily epidemic themselves, are carriers from plant to plant of certain destructive fungous, bacterial and virus diseases.