1911 Encyclopædia Britannica/Potato
POTATO (Solanum tuberosum), a well-known plant which owes its value to the peculiar habit of developing underground slender leafless shoots or branches which differ in character and office from the true roots, and gradually swelling at the free end produce the tubers (potatoes) which are the common vegetable food. The nature of these tubers is further rendered evident by the presence of “eyes ” or leaf-buds, which in due time lengthen into shoots and form the haulm or stems of the plant. Such buds are not, under ordinary circumstances, formed on roots. The determining cause of the formation of the tubers is not certainly known, but Professor Bernard has suggested that it is the presence of a fungus, Fusarium solani, which, growing in the underground shoots, irritates them and causes the swelling; the result is that an efficient method of propagation is secured independently of the seed. Starch and other matters are stored up in the tubers, as in a seed, and are rendered available for the nutrition of, the young shoots. When grown under natural circumstances the tubers are relatively small and close to the surface of the soil, or even lie upon it. In the latter case they become green and have an acrid taste, which renders them unpalatable to human beings, and as poisonous qualities are produced similar to those of many Solanaceae they are unwholesome. Hence the recommendation to keep the tubers in cellars or pits, not exposed to the light. Among the nine hundred species of Solanum less than a dozen have this property of forming tubers, but similar growths are formed at the ends of the shoots of the common bramble, of Convolvulus sepium, of Helianthus tuberous, the so-called Jerusalem artichoke, of Sagittaria, and other plants. Tubers are also sometimes formed on aerial branches, as in some Aroids, Begonias, &c. The production of small green tubers on the haulm, in the axils of the leaves of the potato, is not very infrequent, and affords an interesting proof of the true morphological nature of the underground shoots and tubers. This phenomenon follows injury to the phloem in the lower parts of the stem, preventing the downward flow of elaborated sap. The injury may be due to gnawing insects, and particularly to the fungus Corticium vagum, var. Solani (Rhizoctonia).
The so-called fir-cone potatoes, which are elongated and provided with scales at more or less regular intervals, show also very clearly that the tuber is only a thickened branch with “eyes” set in regular order, as in an ordinary shoot. The potato tuber consists mainly of a mass of cells filled with starch and encircled by a thin corky rind. A few vessels and woody fibres traverse the tubers. The chief value of the potato as an article of diet consists in the starch it contains, and to a less extent in the potash and other salts. The quantity of nitrogen in its composition is small, and hence it should not be relied on to constitute the staple article of diet. Letheby gives the following as the average composition of the potato—
| Nitrogenous matters | 2.1 | Saline matter | 0.7 |
| Starch, &c. | 18.8 | Water | 75.0 |
| Sugar | 3.2 | ||
| Fat | 0.2 | 100.0 |
—a result which approximates closely to the average of nineteen analyses cited in How Crops Grow from Grouven. In some analyses, however, the starch is put as low as 13-30, and the nitrogenous matter as o-92 (Dehérain, Cours de chimie agricola, p. 159). Boussingault gives 25.2% of starch and 3% of nitrogenous matter. W arington states that the proportion of nitrogenous to non-nitrogenous matter in the digestible part of potatoes is as 1 to 10.6. The composition of the tubers evidently varies according to season, soils, manuring, the variety grown, &c., but the figures cited will give a sufficiently accurate idea of it. The “ash” contains on the average of thirty-one analyses as much as 59.8% of potash, and 19.1% of phosphoric acid, the other ingredients being in very minute proportion. Where, as in some parts of northern Germany, the potato is grown for the purpose of manufacturing spirit great attention is necessarily paid to the quantitative analysis of the starchy and saccharine matters, which are found to vary much in particular varieties, irrespective of the conditions under which they are grown.
It is to the Spaniards that we owe this valuable esculent. The Spaniards met with it in the neighbourhood of Quito, where it was cultivated by the natives. In the Cronica de Pern of Pedro Cieca (Seville, 1553), as well as in other Spanish books of about the same date, the potato is mentioned under the name “battata” or “papa.” Hieronymus Cardan, a monk, is supposed to have been the first to introduce it from Peru into Spain, from which country it passed into Italy and thence into Belgium. Carl Sprengel, cited by Professor Edward Morren in his biographical sketch entitled Charles de l'Escluse, sa vie et ses œuvres, states that the potato was introduced from Santa Fé into England by John Hawkins in 1563 (Garten Zeitung, 1805, p. 346). If this be so, it is a question whether the English and not the Spaniards are not entitled to the credit of the first introduction; but, according to Sir Joseph Banks, the plant brought by Drake and Hawkins was not the common English potato but the sweet potato.
In 1587 or 1588 De l'Escluse (Clusius) received the plant from Philippe de Sivry, lord of Waldheim and governor of Mons, who in his turn received it from some member of the suite of the papal legate. At the discovery of America, we are told by Humboldt, the plant was cultivated in all the temperate parts of the continent from Chile to Colombia, but not in Mexico. In 1585 or 1586, potato tubers were brought from what is now North Carolina to Ireland on the return of the colonists sent out by Sir Walter Raleigh, and were first cultivated on Sir Walter's estate near Cork. The tubers introduced under the auspices of Raleigh were thus imported a few ye'axs'later than those mentioned by Clusius in 1588, which must have been in cultivation in Italy and Spainfor some years prior to that time. The earliest representation of the plant is to be found in Gera1d's Herbal, published in 1597. The plant is mentioned under the name Papus orbiculalus in the first edition of the Catalogns of the same author, published in 1596, and again in the second edition, which was dedicated to Sir Walter Raleigh (1599). It is, however, in the Herbal that we find the first description of the potato, accompanied by a woodcut sufficiently correct to leave no doubt whatever as to the identity of the plant. In this work (p. 781) it is called “ Battata virginiana sive Virginianorum, et Pappus, Potatoes of Virginia.” 1 The “common potatoes” of which Gerard speaks are the tubers of Ipomoea Balatas, the sweet potato, which nowadays would not in Great Britain be spoken of as common. A second edition of the Herbal was published in 1636 by Thomas Johnson, with a different illustration from that given in the first edition, and one which in some respects, as in showing the true nature of the tuber, is superior to the first. The phenomenon of growing out or “ super-tube ration ” is shown in this cut. Previous to this (in 1629) Parkinson, the friend and associate of Johnson, had published his Paradisus, in which (p. 517) he gives an indifferent figure of the potato under the name of Papas sen Ballalas Virginianormn, and adds details as to the method of cooking the tubers which seem to indicate that they were still luxuries. Chabraeus, who wrote in 1666, tells us that the Peruvians made bread from the tubers, which they called “ chunno." He further tells us that by the natives Virginieae insulae the plant was called “ openauk, ” and that it is now known in European gardens, but he makes no mention of its use as an esculent vegetable, and, indeed, includes it among “ plantae malignae et venenatae.” Heriot (De Bry's Collection of Voyages), in his report on Virginia, describes a plant under the same name “ with roots as large as a walnut and others much larger; they grow in damp soil, many hanging together as if fixed on ropes; they are good food either boiled or roasted.” The plant (which is not a native of Virginia) was probably introduced there in consequence of the intercourse of the early settlers with the Spaniards. The cultivation of the potato in England made but little progress, even though it was strongly urged by the Royal Society in 166 3; and not much more than a century has elapsed since its cultivation on a large scale became general.
Botanists are agreed that the only species in general cultivation in Great Britain is the one which Bauhin, in his Phytopinax, p. 89 (1596), called Solanum tuberosum esculentum, a name adopted by Linnaeus (omitting the last epithet), and employed by all botanical writers. This species is probably native in Chile, but it is very doubtful if it is truly wild farther north. Baker (Journ. Linn. Soc., 1884, xx. 489), has reviewed the tuber-bearing species of Solanum from a systematic point of View as well as from that of geographical distribution. Out of twenty so-called species he considers six to be really distinct, while the others are merely synonymous or trifling variations. The six admitted tuber-bearing species are S. tuberosum, S. Maglia, S. Commersoni, S. cardiophyllum, S. Jamesii and S. oxycarpum.
S. tuberosum is, according to Mr Baker, a native not only of the Andes of Chile but also of those of Peru, Bolivia, Ecuador and Colombia, also of the mountains of Costa Rica, Mexico and the south-western United States. It seems most probable, however, that some at least of the plants mentioned in the northern part of America are the descendants of cultivated forms. S. Maglia is a native of the Chilean coast as far sou th as the Chonos Archipelago, and was cultivated in the garden of the Horticultural Society at Chiswick in 1822, being considered by Sabine, in his paper on the native country of the wild potato, to be the true S. tuberosum and the origin of the cultivated forms. This species was also found by Darwin in Chile, and was considered by him, as by Sabine before him, to be the wild potato. Baker refers to the plants figured by Sabine (Trans. Hart. Soc. Lond. v. 249) (fig. 1) as being without doubt S. Maglia, but A. de Candolle (Origins des Plantes cultivées, p. 40) is equally emphatic in the opinion that it is S. tuberosum. S. Commersoni occurs in Uruguay, Buenos Aires and the Argentine Republic, in rocky situations at a low level. Under the name of S. Ohrondii it has been introduced into western France, where it is not only hardy but produces abundance of tubers, which are palatable, but have a slightly acid taste. S. cardiophyllum, described by Lindley in the Journ. Hort. Soc. is a native of the mountains of central Mexico at elevations of 8000 to 9000 ft. S. Jamesii is a well-defined species occurring in the mountains of Colorado, New Mexico and Arizona, and also in Mexico. In a wild state the tubers are not larger than marbles. S. oxycarpurn is a
- (From Sabine's figure in the Trans. Hort. Soc. Lond., 1824, vol. v. pl. ii. See text.)
Fig. 1.—Wild Potato-plant in bloom. (14 nat. size.)
little known but very distinct tuberous species from central Mexico.[1]
A review of the localities in which the presence of S. tuberosum and its tuber-bearing allies has been ascertained shows that, broadly, these varieties may be divided into mountainous and littoral. In either case they would not be subjected, at least in their growing season, to the same extremes of heat, cold and drought as plants growing on inland plains. Again, those forms growing at a high elevation would probably start into growth later in the season than those near the coast. The significance of these facts from a cultural point of view is twofold: for, While a late variety is desirable for culture in Great Britain, as ensuring more or less immunity from spring frost, it is, on the other hand, undesirable, because late varieties are more liable to be attacked by the potato disease (Phytophthora infestans) which as a rule appears about the time when the earliest varieties are ready for lifting, but before the late varieties are matured.
In cultivation the potato varies very greatly not only as to the season of its growth but also as to productiveness, the vigour and luxuriance of its foliage, the presence or relative absence of hairs, the form of the leaves, the size and colour of the flowers, &c. The tubers vary greatly in size, form and colour; gardeners divide them into rounded forms and long forms or “kidneys,” and there are of course varieties intermediate in form. The colour of the rind, yellowish, brown or purple, furnishes distinctions, as does the yellow or white colour of the flesh. The colour of the eyes and their prominence or depression are relatively very constant characteristics. These variations have arisen chiefly through cross-breeding, though not entirely so, there being a few cases upon record of the production of “sports” from tubers that have become the parents of new varieties, but authentic cases of the sporting of tubers are few and far between. If, on the other hand, the true seeds of any of our cultivated varieties are sown, the seedlings show very wide variations from one another and from the parents. In this Connexion it is very interesting to observe that Messrs Sutton of Reading find that the seedlings of many of the varieties of potato that occur spontaneously in different parts of America come quite true to type from seed.
The potato thrives best in a rather light friable loam; and in thin sandy soils the produce, if not heavy, is generally of very good quality. Soils which are naturally wet and heavy, as well as those which are heavily manured, are not suitable. Indeed it is best, except when there is ample space, to grow only the earlier kinds in gardens. If the soil is of fair quality the less manure used upon it the better, unless it be soot or lime. Gypsum, bone-dust, super phosphate of lime and nitrate of soda may also be used, and wood ashes are advantageous if the soil contains much vegetable matter; but the best results are usually obtained when farmyard manure is supplemented by artificial, not by using artificial alone. Potatoes are commonly propagated by planting whole tubers or by dividing the tubers, leaving to each segment or “set” one or two eyes or buds. The “sets” are then planted in rows at a distance varying from 15 in. to 3 ft., the distance being regulated by the height of the stems, and that between the sets varying from 6 to 12 in., 8 in. being a good average space for garden crops, with 2 ft. between the rows. The sets may be put in 6 in. deep. The planting of whole tubers instead of the cut sets usually gives a better return. The full-sized tubers are, however, preferable to smaller ones, as their larger buds tend to produce stronger shoots, and where cut sets are used the best returns are obtained from sets taken from the points of the tubers-not from their base. Thomas Dickson of Edinburgh long ago observed that the most healthy and productive crop was to be obtained by planting unripe tubers, and proposed this as a preventive of the disease called the “curl,” which sometimes attacks the young stems, causing them and also the leaves to become crumpled, and few or no tubers to be produced; in this Connexion it is interesting to note that Scottish and Irish seed potatoes give a larger yield than English, probably on account of their being less matured. It has also been noted that the sprouting of the eyes of the potato may be accelerated if, while still unripe, it is taken up and exposed for some weeks to the influence of a scorching sun. The best sets are those obtained from plants grown in elevated and open situations, and it is also beneficial to use sets grown on a different soil.
The earliest crops should, if possible, be planted in a light soil and in a warm situation, towards the end of February, or as early as possible in March. In some cases the tubers for early crops are sprouted on a hotbed, the plants being put out as soon as the leaves can bear exposure.
The main crop should be planted by the middle of March, sprouted sets being used; late planting is very undesirable. Those intended for storing should be dug up as soon as they are fairly ripe, unless they are attacked by the disease, in which case they must be taken up as soon as the murrain is observed; or if they are then sufficiently developed to be worth preserving, but not fully ripe, the haulms or shaws should be pulled out, to prevent the fungus passing down them into the tubers; this may be done without disturbing the tubers, which can be dug afterwards.
Forcing:—The earliest crop may be planted in December, and successional ones in January and February; the varieties specially suited for forcing being chosen. The mode of cultivation adopted by the London market gardeners is thus in substance explained by Cuthill: A long trench, 5 ft. wide and 2 ft. deep, is filled with hot dung, on which soil to the depth of 6 in. is put. The sets employed are middle-sized whole potatoes, which are placed close together over the bed, covered with 2 in. of mould, and then hooped and protected with mats and straw, under which conditions they will sprout in about a month. A bed of the requisite length (sometimes 100 yds.) is then prepared of about 2 ft. thickness of hot dung, soil is put on to the depth of 8 in., and the frames set over all. The potatoes are then carefully taken up from the striking bed, all the shoots being removed except the main one, and they are planted 4 in. deep, radishes being sown thinly over them and covered lightly with mould. When the haulm of the potato has grown to about 6 in. in height the points are nipped off, in order to give the radishes fair play; and, although this may stop growth for a few days, still the potato crop is always excellent. After planting nothing more is required but to keep up the temperature to about 70°, admitting air when practicable, and giving water as required. The crop is not dug up until it has come to maturity.
Potatoes are also grown largely in hooped beds on a warm border in the open ground. The sets after having been sprouted, as above, are planted out in January in trenches 2 ft. deep filled with hot dung, the sets being planted 5 in. deep, and over all radishes are sown. The ridges are then hooped over, allowing about 2 ft. of space in the middle, between the mould and the hoop, and are covered with mats and straw, but as soon as the radishes come up they are uncovered daily, and covered again every night as a protection against possible frosts. This is continued till the potatoes are ready for digging in May.
Potatoes are sometimes grown in pots in heat, sprouted sets being planted in 11-in. pots about two-thirds full of soil, and placed near the glass in any of the forcing-houses, where a temperature of from 65° to 70° is to be maintained. The plants are duly watered and earthed up as they advance in growth.
Potato Diseases
There are few agricultural subjects of greater importance than the culture of the potato and the losses entailed by potato disease. It is not unusual in bad seasons for a single grower to lose £30 per acre in one season. In extreme cases every tuber is lost, as the produce will not even pay the cost of lifting.
The best-known disease of potatoes is caused by the growth of a fungus named Phytophthora infestans, within the tissues of the host plant, and this fungus has the peculiar property of piercing and breaking up the cellular tissues and setting up putrescence in the course of its growth. The parasite, which has a somewhat restricted range of host plants, chiefly invades the potato, Solanum tuberosum; the bittersweet, S. Dulcamara, and other species of Solanum. It is also very destructive to the tomato, Lycopersicum esculentum, and to all or nearly all the other species of Lycopersicum. At times it attacks petunias and even scrophulariaceous plants, as Anthocersis and Schizanthus.
As a rule, although there are a few exceptions, the disease occurs wherever the potato is grown. It is known in South America in the home of the potato plant. In England the disease is generally first seen during the last ten days of July; its extension is greatly favoured by warm and showery weather. To the unaided eye the disease is seen as purplish brown or blackish blotches of various sizes, at first on the tips and edges of the leaves, and ultimately upon the leaf-stalks and the larger stems. On gathering the foliage for examination, especially in humid weather, these dark blotches are seen to be putrid, and when the disease takes a bad form the dying leaves give out a highly offensive odour. The fungus, which is chiefly within the leaves and stems, seldom emerges through the firm upper surface of the leaf; it commonly appears as a white bloom or mildew on the circumference of the disease patches on the under surface. It grows within the tissues from central spots towards an ever-extending circumference, carrying putrescence in its course. As the patches extend in size by the growth of the fungus they at length become confident, and so the leaves are destroyed and an end is put to one of the chief vital functions of the host plant. On the destruction of the leaves the fungus either descends the stem by the interior or the spores are washed by the rain to the tubers in the ground. In either case the tubers are reached by the fungus or its spores, and so become diseased. The fungus is very small in size, and under the microscope appears slightly whitish or colourless. The highest powers are required to see all parts of the parasite.
Fig. 2.—Phytophthora infestans. Fungus of Potato Disease.
The accompanying illustration shows the habit and structure of the fungus. The letters A B show a vertical section through a fragment of a potato leaf, enlarged 100 diameters; A is the upper surface line, and B the lower; the lower surface of the leaf is shown at the top, the better to exhibit the nature of the fungus growths. Between A and B the loose cellular tissue of which the leaf is partly built up is seen in section, and at C the vertical palisade cells which give firmness to the upper surface of the leaf. Amongst the loose tissue of the leaf numerous transparent threads are shown; these are the mycelial threads or spawn of the fungus; wherever they touch the leaf-cells they pierce or break down the tissue, and so set up decomposition, as indicated b the darker shading. The lower surface of the potato leaf is furnished with numerous organs of transpiration or stomata, which are narrow orifices opening into the leaf and from which moisture is transpired in the form of vapour. Out of these small openings the fungus threads emerge, as shown at D, D, D. When the threads reach the air they branch in a tree-like manner, and each branch (sporangiophore) carries one or more ovate sporangia, as shown at E, E, E, which fall off and are carried by the wind. One is shown more highly magnified (400 diameters) at F; the contained protoplasm breaks up into a definite number of parts as at G, forming eight minute mobile bodies called “zoospores,” each zoospore being furnished with two extremely attenuated vibrating hairs termed “cilia,” as shown at H. These zoospores escape and swim about in any film of moisture, and on going to rest take a spherical form, germinate and produce threads of mycelium as at K. The sporangia may also germinate directly without undergoing division. The mycelium from the germinating sporangia or zoospores soon finds its way into the tissues of the potato leaf by the organs of transpiration, and the process of growth already described is repeated over and over again till the entire potato leaf, or indeed the whole plant, is reduced to putridity.
The germinating spores are not only able to pierce the leaves and stems of the potato plant, and so gain an entry to its interior through the epidermis, but they are also able to pierce the skin of the tuber, especially in young examples. It is therefore obvious that, if the tubers are exposed to the air where they are liable to become slightly cracked by the sun, wind, hail and rain, and injured by small animals and insects, the spores from the leaves will drop on to the tubers, quickly germinate upon the slightly injured places, and cause the potatoes to become diseased. Earthing up therefore prevents these injuries, but where practised to an immoderate extent it materially reduces the produce of tubers. The labour entailed in repeated earthing up is also considered a serious objection to its general adoption.
The means of mitigating the damage done by this disease are (1) the selection of varieties found to resist its attacks; (2) the collection and destruction of diseased tubers so that none are left in the soil to become a menace to future crops; (3) care that no tubers showing traces of the disease are planted; (4) spraying with Bordeaux mixture at intervals from midsummer onwards. The last measure prevents the germination of the spores of the fungus on the leaves, and is a most useful mode of checking the spread of the disease; to be successful in its use, however, entails care in the preparation of the spray and thoroughness in its application. In spite of the many efforts in the direction of obtaining a resistant variety no great measure of success has been attained. The earlier varieties of potato appear to escape the disease almost; entirely, as they are usually ready to be lifted before it becomes troublesome; while certain of the later varieties are much less prone to it than the majority. They do not appear, however, to maintain the same degree of immunity over a long period of years, but to become more and more open to the attack as the variety becomes older; nor do they always exhibit the same degree of immunity in different localities. Something may be done to mitigate the loss arising from the disease by selecting comparatively immune varieties from time to time.
Many ingenious attempts have been made to obtain a variety perfectly immune. Maule, thinking a hardier blood might be infused into the potato by crossing it with some of the native species, raised hybrids between it and the two common species of Solanum native in this country, S. Dulcamara and S. nigrum, but the hybrids proved as susceptible as the potato itself. Maule also tried the effect of grafting the potato on these two species and, though he succeeded, there is no record to show whether the product was any hardier than the parents. Dean (Gard. Chron., Sept. 1876, p. 304) succeeded in grafting the potato on the tomato, and Messrs Sutton have carried out similar experiments on an extensive scale (Journ. Roy. Hort. Soc. 1899, xxiii. Proc. p. 20), but in no case have the variations produced proved disease-proof. Various experimenters, especially Fenn, have asserted that by engrafting an eye of one variety into the tuber of another, not only will adhesion take place but the new tubers will present great variety of character; this seems to be the case, but it can hardly be considered as established that the variations in question were the result of any commingling of the essences of the two varieties. The wound may simply have set up that variation in the buds the occasional existence of which has been already noted.
It is possible that the hybridizing of the potato with one or other of the wild types of tuberous Solanums may give rise to a variety which shall be immune, though unfortunately most are themselves liable to the attacks of the fungus, and one of the few crosses so made between the common potato and Solanum Maglia has exhibited the same undesirable trait. The form cultivated in England for some time under the name Solanum tuberosum (which, however, forms tubers and is probably not that known under this name by Lindley) seems so far to have escaped. In view of the fact that Biffen has proved that immunity from the attacks of a certain fungus in wheat is a transmissible recessive character reappearing in some of the individuals of the second generation, it would appear that there is great hope of securing an immune variety with the aid of this form. It is possible, too, that continued cultivation in the rich soil of gardens may induce that tendency to vary when seedlings are raised that is so marked a feature of the potato of commerce, in one or more of the other species of tuberous Solanums.
Another fungus attacking the leaves is Macrosporium Solani (fig. 3), but this attack usually comes earlier in the season than the foregoing. It is characterized by the curling of the leaves, which later show black spots due to the production of numerous dark spores in patches on the diseased leaves. The damage is often considerable, as the crop is greatly 
Fig. 3.—Portion of Leaf of Potato-Plant showing patches of a black mould, Macrosporium Solani, on the surface.lessened by the interference with the functions of the leaf. The parasite may be held in check by spraying with Bordeaux mixture early in the season. The fungus passes the winter on pieces of leaf, &c., left on the ground. All such refuse should be cleared up and burned. A third fungus, Cercospora concors, also forms spots on the leaves and may be kept in check by the same means.
Wilting of the foliage followed by the discoloration of the stem and branches is characteristic of a disease of the potato known as “Blackleg.” This disease is due to the presence of large numbers of Bacillus solanacearum in the tubes through which water is conveyed to the leaves from the roots. Their presence causes the appearance of blackish streaks in the stem and a dark ring some little distance below the surface in the tissues of the tuber. Tubers showing any trace of such a ring should not be used for seed, and rotation of crops should be observed as a means of preventing the infection of the crop with the germ. Biting and sucking insects have been found to carry the bacilli from one plant to another.
 |
| (From the Journal of the Board of Agriculture and Fisheries, by permission of the controller of H. M. Stationery Office.) |
| Fig. 4.—Chrysophlyctis endobiotica (Oedomyces leproides) in the Potato. |
| 1 and 2, Tubers deformed by the fungus. |
| 3, Section through diseased tissue showing dark masses of spores. |
| 4 and 5, Tissue-cell, more highly magnified, showing enclosed spores. |
The tubers frequently show scurfy or scab-like spots upon their surface, thus greatly depreciating their value for market purposes. The fungus, Sorosporium scabies, which is the cause of the scab, does not penetrate into the flesh of the tuber, nor detract from its edible properties. Excess of lime in the soil is said to favour the development of the fungus. Similar spots are produced on potatoes in America by the fungus Oospora scabies, and in both cases, if affected “seed” potatoes are steeped in a solution of 12 pint formalin in 15 gallons of water for two hours before planting, the attack on the resulting crop is materially lessened. The fungus, Oedomyces leproides, produces large, blackish, irregular warts which sometimes involve the whole surface of the tuber. This disease is of recent introduction into Great Britain, but bids fair to become very troublesome. The spores of the fungus pass the winter in the soil and the delicate mycelium attacks the young shoots in the summer. These become brown, finally blackish and greatly contorted until a large scab is formed on the developing tuber, whence the name by which the disease is known—“black scab.” Diseased potatoes left in the soil and even slightly diseased “sets” are a source of infection of succeeding crops. Rotation must be observed and no diseased sets planted.
The rotting of tubers after lifting may be due to various causes, but the infection of the tubers by the Phytophthora already mentioned is a frequent source of this trouble, while “Winter Rot” is due to the fungus Nectria Solani. This fungus finds conditions suitable for growth when the potatoes are stored in a damp condition; rotting from this cause rarely occurs when they are dried before being placed in heaps. The first signs of this fungus is the appearance of small white tufts of mycelium bursting through the skin of the tuber, the spores of the fungus being carried at the tips of the threads forming these tufts. This form of fruit is succeeded by others which have received different names, and lastly by the mature Nectria which forms minute red flask-shaped perithecia on parts of the rotted potatoes that have dried up. The intermediate forms are known as Monosporium, Fusarium and Cephalosporium. The pieces of dried-up potato with the spores of Nectria upon them are a source of infection in the succeeding year, and care should be taken that diseased tubers are not planted. Flowers of sulphur plentifully sprinkled over the potatoes before storing has been found to check the spread of the rot in the heap.
- ↑ Although these six are the only species admitted as such by Baker, it is well to note some of the varieties. The S. etuberosum of Lindley, differing from the common S. tuberosum in not producing tubers, was found in Chile, and is probably not specifically distinct, although exceptional, for it is by no means very unusual to find even cultivated plants produce no tubers. S. Fernandezianum is, according to Baker, a form of S. tuberosum, but if so its habitat in the mountain woods of Juan Fernandez is climatically different from that in the dry mountains of central Chile, where the true S. tuberosum grows. S. otites was found more recently by André on the summit of Quindiu in Colombia, at a height of 11,483 ft. It produces tubers of the size of a nut. S. Andreanum, found by André at Cauca (6234 ft.), was considered by the traveller to be the true S. tuberosum, but this view is not shared by Baker, who named it after the discoverer. Its tubers, if it produces any, have not been seen. S. immite is probably only a slight variety of S. tuberosum, as are also the Venezuelan S. colombianum, S. verrucosum, S. demissum and S. utile. S. Fendleri, a native of the mountains of New Mexico and Arizona, was considered by Asa Gray to be likewise a form of S. tuberosum.