Disease in plants

by H. Marshall Ward

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 eelworms—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.

Many so-called weeds are, however, partially parasitic, with their roots on the roots of others—e.g. Rhinanthus, Thesium, etc., and much damage is done to meadow grasses and herbage by the exhaustive tax which these semi-parasites impose.