120 



THE DICTIONARY OF GARDENING, 



Phytophthora continued. 



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 



Fio. 142. PHYTOPHTIIORA INPESTANS o, Entire Conidiophore, and 

 base of a second, coming through a stonia of the leaf ; on the 

 branches are several Conidia (the thicker parts show the 

 successive tips on which Conidia are formed in the way 

 characteristic of the genus Phytophthora) ; b, Conidium, with 

 five Zoospores inside it ; c. Free Zoospore, with two Cilia ; 

 d, Zoospore, after settling down ; , Zoospore, emitting Myce- 

 lium. o is magnified about 80 times ; the remainder about 400 

 time*. 



the threads of the Fnngns ; and numerous oonidiophores, 

 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 " swarm- 

 spores " (zoospores) in their interior, and these readily 

 emit mycelium threads, which push in through the sur- 

 face 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 I'erono- 

 spora, 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 ger- 

 minating 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 



Phytophthora continued. 



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 pre- 

 vious 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 

 destructire in some years than in others. The Fungus 

 attacks several plants belonging to the same order as 

 the Potato (Solanacece), such as the Bitter-sweet (Sola- 

 num Dulcamara), Schizanthus, and the Tomato (Ly co- 

 per sicum esculentum). The damage done by it to the 

 latter plant is sometimes considerable. Occasionally, 

 it grows on plants of the order Scrophularinece. The 

 appearance of the diseased Potato-plants is too well 

 known to require a long description. On the leaves 



Fio. 143. POTATO-LEAF ATTACKED BY PHYTOPHTHORA INFESTANS, 

 in the dark spots, k, k, k. 



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 dis- 

 tinctly on a watery-looking, discoloured border round 

 the spot. This whitish coat consists of numerous conidio- 

 phores, 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 



