Phytophthora
(from the Greek phyton, plant, and phthora, destruction). A small group of parasitic Fungi, very closely allied to Peronospora in all important points of structure (see Peronospora), except that the stems bearing conidia (conidiophores) do not, as in that genus, produce only a single conidium on the tip of each branchlet. Instead of this, after a conidium is formed at the tip, the branch grows on from just below it, and produces a new conidium (see Fig. 142); and this process may be repeated several times. The reproduction by zoospores formed in the conidia, and also by oospores, or "resting spores," is much like that of Peronospora nivea. Few species are known, but of these one is the dreaded Potato-disease Fungus (P. infestans), too often seen wherever Potatoes are cultivated. Another is P. Fagi (also called P. omnivora), the cause of very widespread and serious disease in Beech seedlings, in many parts of Europe. It also grows well in seedlings of most of the commonly-cultivated coniferous trees, in the commoner Maples, and in many low-growing plants, e.g., Sempervivum, Clarkia, &c. This latter Fungus has not yet caused damage in Britain; but, from its wide distribution, and its hurtfulness abroad, there is reason to dread its ravages should it appear in these islands. A short notice of it will, therefore, not be out of place here. It is injurious to trees only in the seedling stage, but, when it breaks out in a bed of seedlings of the kinds named above, the disease makes very rapid progress around the centres of infection. This progress is more rapid in warm, damp weather, and in shady situations, and, most of all, in beds crowded with young plants.
In the Beech, in which the disease has been most fully studied, the seedlings become black, and perish almost before germination; or they form the seed leaves and the first true leaves before the disease forms spots on the stems and on the various leaves. In damp weather, the diseased parts soon rot; in dry weather, they become dry and brown. All the diseased parts are penetrated by the threads of the Fungus; and numerous conidiophores, each producing two, or less often three, egg-shaped conidia, are pushed out through the skin of the stem or leaf. The conidia, if they fall into dewdrops, emit one or more mycelium threads, or they produce a number of "swarmspores" (zoospores) in their interior, and these readily emit mycelium threads, which push in through the surface cells of the host-plants, and give rise to the disease anew. The damage thus spreads very rapidly. The Fungus is propagated in the following spring by means of the resting spores. These are produced as in Peronospora, and are formed in the diseased tissues. When the latter decay, the spores are scattered in the soil, there to remain till the warmth of the following spring promotes their germination. If a suitable host is at hand, the Fungus bores into it, and runs the same course as before. It has been found, experimentally, that resting spores can retain their capacity for germinating even for four years. Seed-beds of forest-trees of the species liable to injury should be established in places free from shade, and airy, to allow of free evaporation of superfluous moisture. All dead or dying plants should be at once picked out and burned. The seedlings must not be crowded. Ground where a diseased seed-bed has stood must not be used for making a new bed for some time, lest disease should break out afresh from germinating resting spores left in the soil, thus perpetuating the evil.
The Potato-disease Fungus (P. infestans) is one of the most hurtful of all Fungi, because of the damage it inflicts on the Potato crop; in some years, a half, or more, of the yield is destroyed. In the year 1845, the disease produced by this Fungus first came prominently into notice, since, in that year, it spread over the West of Europe, including Britain and Ireland, and over the Northern United States of America. In the latter half of August, the crop in our own country was blighted, causing heavy losses to farmers; while severe and fatal famine and disease followed in Ireland. In years previous to 1845, the disease of Potatoes had been observed occasionally in various places; but, since 1845, it has never been wholly absent from any country where it has once appeared, though it is much more destructive in some years than in others. The Fungus attacks several plants belonging to the same order as the Potato (Solanaceae), such as the Bitter-sweet (Solanum Dulcamara), Schizanthus, and the Tomato (Lycopersicum esculentum). The damage done by it to the latter plant is sometimes considerable. Occasionally, it grows on plants of the order Scrophularineae. The appearance of the diseased Potato-plants is too well known to require a long description. On the leaves brown spots appear (see Fig. 143), rapidly extending, and becoming rotten towards the centre. The lower surface of the spot bears a thin, whitish coating, seen most distinctly on a watery-looking, discoloured border round the spot. This whitish coat consists of numerous conidiophores, emerging from the stomata in small groups. Each has a few branches, with conidia near the tips (see Fig. 142). The tissues of the leaf are full of the mycelium of the Fungus, pushed between, and lying in contact with, the cells of the leaf, and causing them to rot. Zoospores are formed in the conidia in water (e.g., a dewdrop), as described under Peronospora (which see). They are set free by the conidium bursting, and each moves, by means of two fine hairs (see Fig. 142, c) in the water on the leaf. Each settles down after a little while, and emits (see Fig. 142, e) mycelium, which, in a short time, penetrates the leaf, and gives rise in it to a new diseased spot. Any movement of the diseased plants, by wind or otherwise, however slight, is sufficient to spread the conidia all round; and thus, from a single plant, a whole field may be infected in a very short time. But the whole plant becomes filled with the mycelium, alike in leaves, stems, and branches; and the tubers also become infected, both in this way, and by the entrance of mycelium tubes emitted by zoospores, washed down upon them if they lie near the surface of the soil. In whatever way the mycelium reaches the tuber, the latter becomes rotten, in patches or entirely; and, on careful examination, the starch grains are found to be dissolved and fissured. Where the tuber has not passed beyond the first stage of disease, it shows little sign of injury externally; and such tubers are often used for planting for next year's crop, with the frequent result of diseased plants growing from them. So far it may be said that all mycologists are agreed in regard to the development of this Fungus; but what remains to be stated here is still hotly discussed. Globular cells, three or four times the diameter of the mycelium, possessing a thick, warty outer coat, and a thin inner one, were observed in diseased potatoes, even as early as 1845. Mr. W. G. Smith has found such bodies very abundantly in old potatoes, and in diseased leaves kept in a vessel over water (i.e., in moist air); and has observed, in regard to them, a process of development like that in Peronospora nivea, &c.; and he asserts that these brown, globular cells are the resting-spores of the