AN ENCYCLOPEDIA OF HORTICULTURE. 



81 



Feronospora continued. 



a time, when on a suitable part of the plant, they push 

 out tubes, and these pass through the stomata into the 

 interior of the plant, and there reproduce the Fungus. 

 This is the usual process in summer. The conidia and 

 the zoospores, however, are unfit to resist intense cold ; 

 hence, another process of reproduction occurs in autumn, 

 resulting in the formation of the oospores in the tissues 

 of the host-plants. This latter process is as follows. 

 On two neighbouring branches of mycelium-threads, 

 inside the tissues of the host, two projections begin 

 to form. One of these (oosphere) is globular ; the 

 other (antheridium) is oval, or clubbed, and much 

 smaller, and a cell is cut off from the rest, at the end, 

 by a partition. The antheridium comes to lie against the 

 oosphere, and sends into it a kind of beak, which pierces 

 its wall, and fertilises the contents. These contract, 



X500 



x 300 



FIG. 91. PERONOSPORA NIVEA a, Part of Conidiophore, with 

 Conidia ; &, Conidium, inclosing five Zoospores ; c, Besting 

 Spore. 



become covered with a cell wall, and form the oospore (see 

 Fig. 91, c). The outer wall of the oospore often bears 

 ridges, or outgrowths, characteristic of the species. 

 The oospores remain unchanged during winter, and in 

 spring they reproduce the Fungus. 



Remedy. Owing to the fact that the Fungi live 

 inside the host-plants, and only produce their conidia 

 outside them, it is impossible to destroy the parasite 

 without destroying the host. Efforts should be directed 

 against the spread of the disease, since a cure of the 

 diseased plants is practically hopeless. All tissues 

 attacked by Peronosporce are liable to communicate the 

 disease to healthy plants; hence, the former should, if 

 possible, be burned, that being the only certain means 

 of preventing the spread of the disease. Damp places, 

 and confined air favour the Fungi; hence, drainage is 

 of value, and the plants liable to injury should be sepa- 

 rated, so far as can be done, to permit free circulation of 

 air. Any means to strengthen the plants will probably 

 render them better able to resist the evil influence of the 

 Fungi. Warm, close, showery weather is peculiarly 

 favourable to the spread of the latter, since it promotes 

 the production of conidia, and aids the zoospores in 

 diffusing themselves over neighbouring healthy plants 

 by means of the drops of rain or dew. 



In the case of the Potato-disease Fungus (P. infestans), 

 numerous remedies have been proposed, e.g., cutting off 

 stems and leaves as soon as disease appears, Jensen's 

 method of earthing-up the drills, &c. See Phyto- 

 phthora. 



The species of Peronospora are distinguished by 

 differences in the erect branches that bear the conidia, 

 and also in the markings of the outer coat of the 

 oospores ; but the latter bodies have, as yet, not been 

 detected in some of these Fungi. The genus has been 



VoL IIL 



Feronospora continued. 



divided into four sections by Professor de Bary, of 

 Strasbourg, who has employed, as the basis of his 

 arrangement, the modes of development of the zoo- 

 spores, or mycelium, from the conidia, in forming new 

 plants. This arrangement, however, is not of much 

 assistance in making out the species of such Fungi as 

 may be injuring any cultivated plants; and for ease of 

 reference, therefore, in the following descriptions, the 

 nature of the branching of the erect, fertile stems, and 

 the markings of the outer coat of the resting spores, 

 are chiefly employed. 



1. P. parasitica, abundant on, and destructive to, many 

 cruciferous plants, including Turnips and Cabbages. On 

 the larger kinds of Cruciferce, the Fungus is often con- 

 fined to spots on the leaves. It has large, branched 

 haustoria, or suckers, which often nearly fill the cells 

 of the host-plants, from which they draw nourishment. 

 The fertile stems are rather stout below, and five to 

 eight times bifurcated; the branches are rather crowded, 

 and end in slender, short branches, each of which bears 

 on its tip an elliptical, colourless conidium. The conidinm 

 shows no papilla, and, in germination, emits a mycelium 

 tube from any point of its surface. The oospore ia 

 globular, and has a thin, nearly smooth, outer coat. 



2. P. Vicice grows on Peas and Vetches, often doing 

 considerable damage. The fertile stems are rather slender, 

 six to eight times bifurcated, with the branches rather 

 stiff and spreading, and the terminal branchlets rather 

 longer than in most of the other species, and very slender. 

 The conidia are grey, with a pale violet tint, but otherwise 

 much as in P. parasitica. The oospore is globular, pale 

 brown, with a network of ridges on the outer coat. 



3. P. effusa attacks Spinach, and other Chenopodiacece, 

 causing the leaves to become thickened, and to speedily 

 decay. The fertile stems are rather short and thick, 

 and bifurcate from two to seven times, ending in short, 

 straight or hooked branches. They are grey, with a 

 tinge of violet, as are also the elliptical conidia. The 

 latter germinate as in P. parasitica. The oospore is 

 pale brown, with irregular, brown ridges which render 

 it, though globular, decidedly angular in outline. This 

 species forms very dense tufts on the lower surface of 

 the leaves. 



4. P. Schleideniana grows on Onions and their allies. 

 The fertile threads are rather stout, not, or scarcely, 

 jointed, giving off branches along the sides alternately; 

 branches usually three to five times bifurcated, and 

 ending in short, hooked or spreading branchlets; conidia 

 large, obovoid or nearly pear-shaped, all parts reddish- 

 grey or dull violet. The conidia germinate from any 

 part of their sides. The oospores are globular, brown, 

 smooth, except for an open network of ridges. 



5. P. arborescens lives on Poppies only, and hence 

 is of no very great practical importance in Britain. It 

 often causes distortions in the plants affected by it. The 

 fertile stems are rather longer and more slender than 

 usual, and are seven to ten times bifurcated; and the 

 branches end in spreading, or hooked and very slender 

 tips. The conidia are nearly round, small, colourless, and 

 germinate from the side. The oospores are globular, 

 brown, marked with a network of ridges. 



6. P. sparsa is one of the few Peronosporce on woody 

 plants. It grows, as a delicate grey mould, on the 

 under side of the leaves of Eoses, in conservatories. 

 The leaves become spotted with brown, and soon shrivel 

 up. The fertile stems of the Fungus bifurcate several 

 times, and end in curved tips, bearing round conidia, 

 which germinate from the side. The oospores of this 

 species have not been observed. 



7. P. nivea has been already described. The spreading 

 branches, bifurcated from one to three times; the ovoid, 

 rather small conidia, with a papilla at the tip, and pro- 

 ducing, not a germinal mycelium tube, but four or five 



M 



