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.
The shooting of ascospores into the air by certain species of Peziza, from the discs of which the spores may be seen to puff out in clouds, affords further evidence that fungi cannot be regarded as entirely passive in respect to distribution of their spores. But when we come to certain of the soil fungi - e.g. Agaricus melleus, Dematophora, etc. - the active creeping forward by growth in the soil of their rhizomorphs and mycelial strands afford examples of active spreading of considerable importance in the vineyard and forest, since they pass from root to root and from tree to tree and may infect the entire area in course of time.
Not the least significant mode of dissemination is that by which what I have termed "lurking parasites" are spread: such are fungi which attach themselves to the seeds, fruits, tubers, etc., of other plants and so obtain all the advantages of being carried and sown with the latter - e.g. Ustilagineae and Uredineae which adhere to grain, Verticillium, Nectria, etc., in potatoes and other plants.
The spread of diseases due to animals, especially insects, is of course more active, in consequence of the motility of the distributing agents. This is most marked in the winged species, of which locusts, beetles, moths and butterflies, flies and wasps furnish well-known examples; and is not inconsiderable in the case of wingless and merely creeping species. It is noteworthy that many forms wingless in the parasitic stage are winged at certain periods, e.g. the females of Phylloxera.
That man also spreads insect pests is well known and acted upon, as witness the phylloxera laws - which, however, it is to be feared too often only illustrate once more the adage concerning the shutting of the stable door after the horse has gone.
It would be tedious to attempt anything like a complete account of the estimates of loss in different countries, due to the ravages of insects and fungi, but the following examples should surely serve to convince anyone of the magnitude of these losses and of the economic importance of the whole question, and the reader may be referred to the special literature for further details.
The coffee leaf-disease of Ceylon, due to the fungus Hemileia, is estimated to have cost that Colony considerably over £1,000,000 per annum for several years. One estimate puts the loss in ten years at from £12,000,000 to £15,000,000. The hop-aphis is estimated to have cost Kent £2,700,000 in the year 1882. In 1874 the Agricultural Commissioner of the United States estimated the annual loss, due to the ravages of insects on cotton alone, to amount to £5,000,000; and in 1882 the annual loss to the United States due to insects, calculated for all kinds of agricultural produce, was put at the appalling figure of from £40,000,000 to £60,000,000 sterling. In India, the annual loss due to wheat-rust alone has recently been estimated at 4,000,000 to 20,000,000 rupees, and one insect alone is said to have cost the cotton planters a quarter of the crop - valued at seven crores of rupees - in bad years. Similarly, in Australia the annual loss from wheat-rust has been put at from £2,000,000 to £3,000,000. In 1891 the loss in Prussia alone from grain-rusts was officially estimated at over £20,000,000 sterling. Need more be said? Even allowing for considerable exaggerations in such estimates it is clear that the damage to crops in any country soon amounts to sums which even at low rates of interest would easily yield incomes capable of supporting the best equipped laboratories and staffs for investigations directed to the explanation of the phenomena in detail, the sole basis on which intelligent preventive and therapeutic measures can be based. But it is far from likely that the estimates are exaggerated. The planting and agricultural communities are as a rule opposed to the publication of statistics - or at least have been so in various countries and at different times - and if we knew the damage done to all crops even in our own Empire, the results would probably astonish us far more than the above figures have done.
On the dissemination of fungi, the reader will find Fulton, "Dispersal of the Spores of Fungi by the Agency of Insects," Ann. Bot., Vol. III., 1889, p. 207, and Sturgis, "On Some Aspects of Vegetable Pathology and the Conditions which Influence the Dissemination of Plant Diseases," Botanical Gazette, Vol. XXV., 1898, p. 187, both useful papers. Further information will be found in Zopf, Die Pilze, Breslau, 1890, pp. 79 - 95 and 228, and Wagner, "Ueber die Verbreitung der Pilze durch Schnecken," in Zeitschr. f. Pflanzen Krankh., 1896, p. 144. The estimates as to losses due to epidemics are taken from Watt, Agricultural Ledger, Calcutta, 1895, p. 71; Balfour, The Agricultural Pests of India, London, 1887, pp. 13 - 15;
Eriksson and Henning, Die Getreideroste; the publications of the U.S. Department of Agriculture, The Kew Bulletin, and elsewhere. The reader will find further examples in Massee, Text-Book of Plant Diseases, 1899, pp. 47 - 51. Both these subjects are well worth further attention, and I know of no complete account of them.