Potato Diseases
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 Phylophihora 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, Solatium tuberosum; the bittersweet, 5. 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 Schizanl/tus.
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 diseasepatches 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 confluent, 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.
The accompanying illustration shows the habit and structure of the fungus. The letters A B show a vertical section through a fragment ofapotato 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, FIG. 2. Phytophthora infestans. Fungus of Potato Disease.
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 by 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 attentuated 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 i 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 (i) 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 Solatium 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 specie's and, though he succeeded, there is no record to show whether the product was any hardier than the parents. Dean (Card. 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 Solatium Maglia has exhibited the same undesirable trait. The form cultivated in England for some time under the name Solatium 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 ?reat hope of securing an immune variety with the aid of this form. t 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 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, etc., 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 FIG. 3. Portion of Leaf of and branches is characteristic Potato-Plant showing patches o{ disease of the tato of a black mould, Macrosporium , -m i i unj Solani, on the surface. 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 (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.
i 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.
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.
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 devejopment 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 $ 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 penthecia 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.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)
