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POTATO

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Originally appearing in Volume V22, Page 204 of the 1911 Encyclopedia Britannica.
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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 See also:

bad seasons for a single grower to lose £3o per See also:acre in one See also: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 See also:host plant, and this fungus has the See also:peculiar See also:property of piercing and breaking up the cellular tissues and setting up putrescence in the course of its growth. The See also:parasite, which has a somewhat restricted range of host See also:plants, chiefly invades the potato, Solanum tuberosum; the bittersweet, S. Dulcamara, and other See also:species of Solanum. It is also very destructive to the See also: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 Schiz- anthus. As a See also:rule, although there are a few exceptions, the disease occurs wherever the potato is grown. It is known in See also:South See also:America in the See also:home of the potato plant. In See also:England the disease is generally first seen during the last ten days of See also:July; its See also:extension is greatly favoured by warm and showery See also:weather. To the unaided See also:eye the disease is seen as purplish See also:brown or blackish blotches of various sizes, at first on the tips and edges of the leaves, and ultimately upon the See also: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 See also:

form the dying leaves give out a highly offensive odour. The fungus, which is chiefly within the leaves and stems, seldom emerges through the See also:firm upper See also:surface of the leaf; it commonly appears as a See also:white See also:bloom or See also: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 See also:size by the growth of the fungus they at length become confluent, and so the leaves are destroyed and an end is put to one of the See also:chief vital functions of the host plant. On the destruction of the leaves the fungus either descends the See also:stem by the interior or the spores are washed by the See also:rain to the tubers in the ground. In either See also: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 See also:microscope appears slightly whitish or colourless. The highest See also:powers are required to see all parts of the parasite. The accompanying See also:illustration shows the See also:habit and structure of the fungus. The letters A B show a See also:vertical See also:section through a fragment of a potato leaf, enlarged too diameters; A is the upper surface See also:line, and B the See also:lower; the lower surface of the leaf is shown at the See also:top, A the better to exhibit the nature of the fungus growths. Between A and B the loose cellular See also: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 See also:touch the leaf-cells they See also:pierce or break down the tissue, and so set up decomposition, as indicated by the darker shading.

The lower surface of the potato leaf is furnished with numerous See also:

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 See also:air they See also:branch in a See also: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 See also:wind. One is shown more highly magnified (400 diameters) at F; the contained See also:protoplasm breaks up into a definite number of parts as at G, forming eight See also:minute See also:mobile bodies called " zoospores," each zoospore being furnished with two extremely attentuated vibrating hairs termed " See also:cilia," as shown at H. These zoospores See also:escape and swim about in any film of moisture, and on going to See also:rest take a spherical form, germinate and produce threads of mycelium as at K. The sporangia may also germinate directly without undergoing See also: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 See also: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 See also: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 See also:sun, wind, See also:hail and rain, and injured by small animals and See also: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 See also:general See also:adoption. The means of mitigating the damage done by this disease are (r) the selection of varieties found to resist its attacks; (2) the collection and destruction of diseased tubers so that none are See also:left in the See also:soil to become a menace to future crops; (3) care that no tubers showing traces of the disease are planted; (4) spraying with See also: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, how-ever, 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 See also:

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 See also:majority. They do not appear, however, to maintain the same degree of See also:immunity over a See also:long See also: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 See also:time to time. Many ingenious attempts have been made to obtain a variety perfectly immune. See also:Maule, thinking a hardier See also:blood might be infused into the potato by See also:crossing it with some of the native species, raised hybrids between it and the two See also:common species of Solanum native in this See also: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 See also:record to show whether the product was any hardier than the parents. See also:Dean (See also:Gard. Citron., See also:Sept. 1876, p.

304) succeeded in grafting the potato on the tomato, and Messrs See also:

Sutton have carried out similar experiments on an extensive See also:scale (Journ. See also:Roy. See also:Hort. See also:Soc. 1899, See also:xxiii. Proc. p. 20), but in no case have the See also:variations produced proved disease-See also:proof. Various experimenters, especially Fenn, have asserted that by engrafting an eye of one variety into the tuber of another, not only will See also:adhesion take See also:place but the new tubers will See also:present great variety of See also: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 See also: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 See also: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 See also: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 See also:wheat is a transmissible recessive character reappearing in some of the individuals of the second See also:generation, it would appear that there is great See also:hope of securing an immune variety with the aid of this form.

It is possible, too, that continued cultivation. in the See also:

rich soil of gardens may induce that tendency to vary when seedlings are raised that is so marked a feature of the potato of See also: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 See also:curling of the leaves, which later show See also:black spots due to the See also:production of numerous dark spores in patches on the diseased leaves. The damage is often considerable, as the See also:crop is greatly lessened by the interference with the functions of the leaf. The parasite may be held in check by spraying with Bordeaux mixture See also:early in the season. The fungus passes the See also: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 See also:numbers of Bacillus solanacearum in the tubes through which See also:water is conveyed to the leaves from the roots. Their presence causes the See also:appearance of blackish streaks in the stem and a dark See also: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 See also: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. The tubers frequently show scurfy or scab-like spots upon their surface, thus greatly depreciating their value for See also:

market purposes. The fungus, Sorosporium See also: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 See also: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 See also:solution of 2 See also:pint See also:formalin in 15 gallons of water for two See also: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 See also:recent introduction into Great See also:Britain, but bids See also:fair to be-come 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 See also:damp See also: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 See also:

fruit is succeeded by others which have received different names, and lastly by the mature Nectria which forms minute red See also: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 See also:year, and care should be taken that diseased tubers are not planted. See also:Flowers of See also:sulphur plentifully sprinkled over the potatoes before storing has been found to check the spread of the rot in the heap. (From the See also:Journal of the See also:Board of See also:Agriculture and See also:Fisheries, by permission of the controller of H. M. See also:Stationery See also:Office.) Fin. 4.-Chrysophlyclis endobiotica (Oedomyces leproides) in the Potato. t and 2, Tubers deformed by the fungus. 3, Section through diseased tissue showing dark masses of spores. 4 and 5, Tissue-See also:cell, more highly magnified, showing enclosed spores.

End of Article: POTATO

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POTATO (Solanum tuberosum)

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