CHAPTER IV.

NEW DISEASE OF POTATOES.

Peziza postuma, Berk. and Wils.

In the beginning of the month of August 1880, Mr. Ambrose Balfe, secretary to the Royal Horticultural Society of Ireland, reported to us a disease then invading certain crops of potatoes in the west of Ireland, in a manner hitherto unknown to him. The potatoes had been bought as "Champions," and planted in land which had been reclaimed from bog eight years previous to the outbreak of the disease. When the ground was reclaimed a coat of clay was spread over and incorporated with the soil. For the first three years potatoes were grown, followed by a year of oats, next the ground was sown with grass and meadowed, and lastly "champion" potatoes were planted. In preparing the ground for the potatoes sea-weed was first spread over the grass, and ten days afterwards it was covered with farmyard manure. The potato sets were laid on the manure, and then covered. Ridge planting was adopted. No doubt the mode of culture was defective, as it is bad in practice to place potato sets in immediate contact with decaying vegetable matter and farmyard manure; such materials always contain an immense number of disease germs both of animal and vegetable origin. The manure used for potatoes should always be old and thoroughly decayed, and it is perhaps best that the cut faces of the sets should be allowed to dry before they are planted. Some planters pass the cut surfaces rapidly across a hot iron with good effect, but others maintain that it is better to place the

freshly-cut sets in the soil immediately after cutting, and whilst the wound is still quite fresh. Planting in rank undecayed material is not only destructive to the material which is stored up within the tuber or set (material which is the food of the future plant), but it is also injurious to the young shoots and rootlets, for any hot, fermenting material acts as a poison to these growths, and diminishes the vigour of the infant plant. The conditions of planting in the instance here adverted to may, however, have had nothing to do with the disease which followed in the summer.

Until the attack now under description, the potato plants, as far as outward appearances went, were free from any taint of the fungus of the potato disease proper, named Peronospora infestans, Mont. The disease was first noticed in the beginning of July, at the time the potato flowers were opening; but there can be little doubt that it was in or upon the plants several weeks previously, as by its nature it would not attract much attention at first. It is strange that other potatoes named "Protestants," growing close to the "Champions," were not attacked. The appearance of the diseased plants was peculiar; they were covered within and without with a thick felt of white fungus spawn or mycelium. The growth of this spawn was so rapid and profuse that in a week or two the whole of the stems and leaves were reduced to tinder, the entire moisture belonging to the stems and leaves being exhausted by the fungus. Leaves are of such vital importance to plants that the destruction of them is synonymous with a cessation of the plant's growth. If the parts of a potato plant which are above ground get seriously injured or destroyed, there will be little or no further growth in the tubers. In the Peziza disease, now under description, the mycelium was not a putrefactive one as in Peronospora. It merely caused a sudden cessation of growth in the tubers.

Immersed in the thick felt of white fungus spawn,

when it had reached its maximum of growth on the potato stems, there were thousands of small black nodular

C

bodies, varying in size from a grain of sand to that of a small bean. An affected potato stem is illustrated, natural size, at A, Fig. 3, and a section through a part of a similar stem is illustrated at B to show the black nodular growths in situ. The small black bodies here drawn are at first white, at length they become externally brown, and ultimately black; they are hard and compact, and, owing to their hardness, they have been termed Sclerotia from skleros, hard. One of these bodies surrounded by spawn threads is shown, twice its natural size, at Fig. 4. These nodular growths when examined

with the microscope are found to consist of highly condensed and compacted spawn cells or mycelium, white in the centre and gradually getting black (through brown) towards the outside. When an excessively thin slice is taken off a cut surface of one of these nodules and magnified 400 diameters, the appearance is similar with the illustration at Fig. 5; here the gradual change of colour from the white internal cells to the black thick-walled outer ones is illustrated, together with the felted statum of white mycelial threads, on the top of illustration, in which the Sclerotia are embedded.

Sclerotia, or compacted masses of fungus spawn in a resting state, are common amongst fungi; some examples

never attain a larger size than a grain of gunpowder, others are as large as peas or small beans. The "Native Bread" or Mylitta of Australia, which often measures several inches across, may be a Sclerotium. The edible Americo-Indian Tuckahoo, which is dug out of the ground in large masses, is not really a fungus, although so esteemed

Enlarged 400 diameters.

by Fries, and named by him Pachyma cocos.

Some Sclerotia

are sphærical in shape, whilst others are elongated irregular ovals. Sclerotia are not confined to one order or genus of fungi, but they possibly occur throughout the entire family. Some forms are much less compact than others, and the looser forms germinate after a comparatively short rest.

By means of Sclerotia certain fungi which would probably perish during drought or severe frost are preserved alive through inclement seasons. The spawn naturally compacts itself into these little hard masses and falls to the ground; it there remains, like a seed, uninjured by continued cold or dryness, whereas the vitality of uncom