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.
