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.
Notes to Chapter XV.
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.
