An Encyclopedia of Horticulture. 



207 



Potato — continued. 

 ^lnchang■e(l, but the following- summer there grow, from 

 those lying on or under the soil, from one to three 

 Pezizce, which have very long, slender, twisted stalks, each 

 of ^vhich ends in a cup, which very soon becomes flat on 

 top, and may reach iin. across (see Fig. 257). The upper 

 surface bears the very numerous asci standing side by 

 side ; each ascus incloses eight smooth, oval spores, which 

 are ejected from it if the air is at all dry. If the spores 

 fall on a suitable food-plant, they reproduce the Fungus. 



All Potato haulms, and other rubbish, should be burned, 

 instead of being left to infect the crop of next year. 

 Deep ploughing or digging would bury the sclerotia too 

 deep to allow the Peziza cups to reach the surface of the 

 soil. 



Potato Curl is a puzzling disease, since no evident 

 cause could be detected by some observers, while Fungi 

 have been discovered by others in the diseased plants. 

 It was first observed in England in 1704, and soon after- 

 wards was noticed in Rhenish-Germany and elsewhere. 

 It has become less hurtful since 1820, or thereabouts. 

 The young stems and leaves curve or curl up ; the whole 

 plant becomes sickly and stunted, and extremely brittle ; 

 the stems branch little, if at all ; the leaves are small, 

 and almost sessile ; and the flowers and fruit often fall 

 off prematurely, and all the green parts become mottled. 

 Tubers are either not produced, or they are very small, 

 and so watery as to be unfit for food. If used as seed 

 potatoes, the disease usually, if not always, appears in 

 the plants grown from them. Careful microscopic exami- 

 nation of the diseased plants has led to very different 

 conclusions as to the cause, some observers {e.ff., Kiihn) 

 failing to detect any trace of Fungi, while others {e.g., 

 (Hallier and Reinke) have found them in the interior of 

 the diseased tissues. Hallier asserts, and Reinke agrees 

 with him, that the disease is hereditary, or that diseased 

 tubers produce diseased plants, and that these plants are 

 not capable of forming tubers ; that mycelium of Fungi 

 is present in the inn?r tissues of the plants ; and that 

 infection with this mycelium will produce the same 

 disease in previously healthy plants. Reinke and Bert- 

 hold give the following account of the disease. The 

 mycelium is present, they say. in the woody bundles of 

 plants as soon as they begin to wither, and in badly- 

 affected plants it may be traced throughout from the 

 roots to tips of the leaf-stalks. It may also often be 

 found in plants that show no outward sign of disease. If 

 the plants are kept in a damp atmosphere, their whole 

 surface becomes covered with a white coat of conidio- 

 phores, or spore-bearers, of Fungi, pushed out from the 

 mycelium through the epidermis. These are, at fir.-t, 

 colourless ; and each bears two or three circles of short 

 branches at the ends of the cells, which, in a single 

 row, form the erect stem. There are from two to five 

 branches in each circle, and one or more of them may 

 bear one or two smaller branches. On the tip of each 

 branchlet there grows a small, oval spore, which falls off 

 on being wetted. They suggest for this Fungus the 

 name of Verticilliwm afro-album. Mycelium was found 

 in the tubers, even on plants that appeared moderately 

 healthy. 



In a second form of Potato Curl, the plants grow to 

 full size, but then the edges of the leaves begin to curl 

 backwards, and to become brown ; and this extends to 

 the whole of each leaf, and gradually back to the stems. 

 Microscopic examination shows no Fungi in the leaves, or 

 in the upper part of the stems ; but underground the 

 stems are marked with large, brown spots, and in the 

 cortex of these spots the cells are traversed by my- 

 celium, while the vessels show no trace of it. The 

 roots also are brown and diseased, and the seed-tubers 

 are often rotten. Cultivation of the Fungus showed it 

 to be V. atro-album. At times, both forms of disease 

 occur in the same plant. Tubers produced by plants 



Potato — continued. 

 affected in either of the above ways are almost always 

 diseased, and produce shoots that, from their first appear- 

 ance, are evidently diseased. These shoots develop slowly, 

 and remain small, stunted, and of an unhealthy colour. 

 Dark spots appear on the leaves and on the leaf-stalks, 

 and the leaves gradually wither from below upwards ; 

 and similar changes go on in the stems. The plants 

 perish without being able to form new tubers. No Fungi 

 have been detected in the leaves or stems of shoots 

 produced by diseased tubers ; but all the subterranean 

 parts have the bark permeated by mycelium, though 

 there is none visible in the woody bundles. The diseased 

 tubers show an abundant mycelium in the corky layers 

 of the skin. Cultivation of the mycelium, in each case, 

 has yielded V. of ro -album. Inoculation from diseased 

 plants rendered previously healthy plants diseased ; and 

 healthy tubers planted in soil impregnated with conidia 

 of T". atro-album produced diseased shoots. Reinke and 

 Berthold suggest that V. atro-album may be an imper- 

 fectly-developed condition of some Pyrenomycetous 

 Fungus of the genus Nectria. or closely allied to it, 

 and oppose the view advocated by Hallier, that the 

 cause of Potato Curl is Pleospora polyfricha ; nor do 

 they think the disease is caused by any species closely 

 allied to the genus Pleofipora. Schenck, in a series of 

 observations and cultivation of the diseased plants, ob- 

 tained from some of them Fungi which were much like 

 one form of conidia attributed to Pleospora herbaram, 

 and which he called Sporideswium exitiottum var. Solnni. 

 It is evident that there is need of further observations, 

 since there may be more than one cause of this disease, 

 and true parasites may be confounded with Fungi that 

 grow only on tissues already dead. No cure is known ; 

 hence, prevention is the aim to be kept in view. Diseased 

 plants should be pulled up and removed as soon as de- 

 tected ; and all the Potato-stalks should be collected into 

 heaps and burned. Care should also be taken to prevent 

 unsound tubers from being made use of as seed. In 

 short, the means employed to limit the spread of Potato 

 Rot, and of Peziza postuma, are equally applicable against 

 Potato Curl. 



In common with other herbaceous plants, the Potato 

 affords, in its dead stems and leaves, an abundant food 

 supply to many kinds of Micro-fungi ; but, as none of 

 these are known to be injurious to the plants during life, 

 they do not require even to be enumerated heje. 



Potato tubers are rendered unsightly, at times, by the 

 skin being more or less covei-ed with brown patches or 

 scabs. These may be due to various causes. In some 

 cases, a microscopic examination shows that the scab is 

 due to the growth of a Fungus named Tubercinia 

 srabiefi, the spores of which are formed of small cells, 

 grouped into a globe around an air space. Each spore 

 has a slender stalk at one side. There is often no trace 

 of this Fungus at harvest-time ; but, dur ng the winter, 

 it develops, and the spores form a layer beneath the 

 skin, often extending over a great part of the tuber. 

 After a time, the spores are set free by the bursting of 

 the skin. In other foi*ms of scale, the cells are filled 

 with mycelium of Fungi, and the formation of the scab 

 is probably due to the irritation caused by its presence 

 in the tissues. In others, there is no trace of the action 

 of Fungi ; and it has been conjectured that the cracks, 

 followed by scabbing, are due to contact with irritant 

 or corrosive substances in the soil, and that the scabs 

 are due to efforts at healing the injury ; but new cracks 

 form in them, and so the mischief goes on. The raw 

 surfaces of the cracks render the tubers more liable to 

 injury from Fungi, insects, frost, and other external 

 causes. Scabbed Potatoes are diminished in value 

 because of their unsightliness ; but they do not seem 

 unfitted for food when the skin is removed. They 

 should not, however, be used aa seed. When the cause 



