CHAPTER XV.
SPREADING OF DISEASE AND EPIDEMICS.
Dissemination of fungi by the aid of snails, rabbits, bees, and insects—Man—Distribution in soil, on clothes, through the post, etc.—Worms, wind—Puffing of spores—Creeping of mycelia—Lurking parasites—Spread of insects and other animals—Losses due to epidemics.
The dissemination of plant diseases is a subject which has been far too much neglected, but our knowledge of it is slowly increasing. The spores of fungi such as Rusts and Erysipheae are often carried from plant to plant by snails; those of root-destroying and tree-killing Polyporei by rabbits, rats, and other mammals which rub their fur against the hymenophores. Bees have been shown to carry the spores of Sclerotinia and infect the stigmas of Bilberries, etc., with them; and flies convey the conidia of Ergot from grain to grain. Insects, indeed, of all kinds are great disseminators of disease—as witness also the part played by mosquitoes in transferring the malaria parasite to man—and beetles, bees, flies, etc., of all sorts probably play more active parts in this work than has yet been proved, since they not only carry spores attached like pollen to their hairy bodies, but in many cases in their alimentary canal, to be spread later in the dung.
The part played by man in conveying fungi from plant to plant counts for much. Not only do gardeners and farm labourers carry spores on their boots and clothes as they pass from infected to non-infected areas, but carted soil and manure are frequently infested with spores of Smuts, Fusarium, Polyporus, and the sclerotia or rhizomorphs of Sclerotinia, Agaricus melleus, Dematophora, etc. Man also sends diseases through the post, and by rail and ship, by spores or mycelia attached to seedlings, bulbs, fruits, flowers, etc., as shown in several cases of potato, vine, hollyhock, lily, and hyacinth diseases. Every time a carpenter saws a piece of fresh timber with the saw which has been used previously for cutting wood attacked with dry rot, he risks infecting it with the fungus. Similarly in pruning: every cut with a knife which the gardener has used on infected branches may infect the tree.
Cuttings made with a soil-contaminated knife and stuck into ordinary soil in dirty boxes covered with equally dirty glass, present every chance for infection by soil organisms; bacteria and fungi obtain access to the vessels, and derive plenty of food from the juices, and the wonder is not that so many cuttings "damp off," but that any are raised at all under ordinary conditions.
That worms bring buried spores to the surface can hardly be doubted after Pasteur's experiments with Anthrax, and the principle of Darwin's discoveries of the important bearing of the habits of earthworms on this subject, and that the soil attached to the feet of ducks and other birds teems with small seeds, applies to fungi also. Wind is also responsible for distributing fungus-spores over wide areas, as may be easily proved by fixing a glass slide smeared with glycerine in the course of a breeze passing over an infected area.
But although the fungi are, generally speaking, passive in regard to their distribution, such is by no means always the case. Apart from the fact that some forms attract insects by means of honey dew (Ergot), or by sweet odours (Spermogonia, Sclerotinia), the zoospores of Pythium, Phytophthora, etc., are motile, and although they cannot move far in the films of water in which they travel, nevertheless in a wet potato field, with the wind flapping the leaves one against the other, some dissemination of importance must be actively brought about, and similarly with the amoebae of Plasmodiophora in the soil.