Passing now to those causes of disease which are connected with the living environment, we may obviously divide them into two groups of agents, animals and plants.
Among animals, the various vertebrata, including man, are especially responsible for the larger kinds of wounds and wholesale destructive processes due to breakage, stripping of leaves and bark, cutting and biting, and so forth. Cattle, rabbits, rats and mice, squirrels and birds of various kinds stand out prominently as enemies to trees and other plants, to which they do immense injury in various ways by their horns, teeth, claws, and beaks; and the damage which an ignorant gardener or forester can do with his ill-guided footsteps, axe, spade, and knife can only be appreciated by one who knows the habits of plants.
It is among the invertebrata, however, especially insects and worms, that the most striking agents of disease in plants are to be found, for, with the exception of certain rodents - and we may logically include also human invasions - vertebrate animals do not often appear in such numbers as to bring about the epidemics and scourges only too commonly caused by insect pests.
Insects injure plants in very various ways. Some, such as locusts, simply devour all before them; others, e.g. caterpillars, destroy the leaves and bring about all the phenomena of defoliation. Others, again, eat the buds - e.g. Grapholitha; or the roots - e.g. wire-worms, and so maim the plant that its foliage and assimilation suffer, or its roots become too scanty to supply the transpiration current. Many aphides, etc., puncture the leaves, suck out the sap, and produce deformations and arrest of leaf-surface, as well as actual loss of substance, and when numerous such insects induce all the evils of defoliation. Others, such as the leaf-miners, tunnel into the leaves, with similar results on a smaller scale.
It must be remembered that a single complete defoliation of a herbaceous annual, or even of a tuberous plant like the potato, so incapacitates the assimilatory machinery of the plant, that no stores can be put aside for the seeds, tubers, etc., of another year, or at most so little that only feeble plants come up.
In the case of a tree the case is different, and since most large trees in full foliage have far more assimilatory surface than is actually necessary for immediate needs, a considerable tax can be paid to parasites or predatory insects before the stores suffer perceptibly. Still, it should be recognised that the injury tells in time, especially in seed years.
Many larvae of beetles, moths, etc., bore into the bark and as far as the cambium or even into the wood or pith of trees, the local damage inducing general injuries in proportion to the number of insects at work: moreover, the wounds afford points of entrance for fungi and other pests.
Galls and similar excrescences result from the hypertrophy of young living tissues pierced by the ovipositors of various insects, and irritated by the injected fluid and the presence of the eggs and larvae left behind. They may occur on the buds, leaves, stems, or roots, as shown by various species of Cynips on oak, Phylloxera on vines, etc., in all cases the local damage being relatively small, but the general injury to assimilatory, absorptive, and other functions is great in proportion to the number of points attacked.
Many grubs - larvae of flies, beetles, etc. - bore into the sheaths or internodes of grasses, or the pith of twigs, or into buds, fruits, and other organs of plants, and do harm corresponding to the kind and amount of tissues injured.
Various species of so-called eelvvorms - Nematodes - also cause gall-like swellings on young roots, or they invade the grains of cereals.
Finally, various slugs and snails cause much injury by devouring young leaves and buds and diminishing the assimilatory area.
Plants as agents of disease or injury fall naturally into the two main categories of flowering plants (Phanerogams) and Cryptogams, among which the fungi are the especially important pests.
Beginning with weeds, we find a large class of injurious agents. Weeds damage the plants we value by crowding them out in the struggle for existence, as already stated, and when the weed-action is simply due to superfluous plants of the same species, we speak of overcrowding. But it must not be overlooked that the competition between crowded plants of the same species - where every individual is acting as a weed to the others - may be more dangerous than between plants and weeds belonging to other species and genera, because in the former case they are struggling for the same minerals and other necessary food-materials: a matter of importance in connection with the rotation of crops.
The question of allowing grass to grow at the foot of fruit trees, as in orchards, is a good case in point. Such grass may increase the damp and shade, thus favouring fungi at one season, and dry up the moisture of the soil to the injury of the fine superficial roots at another, as well as exhaust the soil, owing to the competition of the roots for salts and other materials. On the other hand, the checking of surface roots by competition with the grass has been claimed as advantageous. In this connection probably the whole question of the composition of the turf arises, as well as that of possible cropping for hay, and manuring.
As regards any particular weed, the cultivator should learn all he can respecting its duration, seeding capacity, method of dissemination, the depth and spread of its root-system, and any other particulars which enable him to judge when and how to attack it. It is only necessary to see the victory of such drought-resisting weeds as Hieracium pilosella, Plantains, Hypochaeris, on lawns to realise how weeds may win in the struggle for existence with the finer grasses.