CHAPTER VI.

SMUT OF POTATOES.

Tubercinia scabies, B.

THE smut which produces one form of scab in potatoes is caused by an olive-green or brownish fungus, of subterranean habit, named Tubercinia scabies, B.; from tuber, an ancient Roman name for a fungus; cineres, ashes; and scabies, the itch. This is the same with Rhizosporium Solani, Rab., and is the Protomyces of Martius. The smut fungus, which is very common upon potatoes, is supposed to be allied to the bunt of wheat, Tilletia caries, Tul.; and the smut of corn, Ustilago carbo, Tul. It was described and illustrated by Martius, Die Kartoffel-Epidemie, p. 23, tab. 2, Figs. 9-13, tab. 3, Figs. 36-38, and afterwards by the Rev. M. J. Berkeley in the Journal of the Royal Horticultural Society, vol. i., 1846, page 33, Figs. 30 and 31. The spores are compound, and are composed of minute cells, which together form a hollow globe with one or more apertures. In this character Tubercinia agrees well with the allied genus Urocystis (formerly Polycystis, in reference to the spores being composed of many cells), to which the well-known smuts of rye and violets belong. The fungus grows beneath the bark of the tuber, where it forms a thin dark greenish-brown stratum, often extending over the greater part of the external surface of the potato. The presence of the fungus may be detected by discoloured blotches on the bark. As in the disease caused by the Peronospora, it often happens that no trace of the fungus is to be seen at the time of harvesting. It frequently shows itself during the winter in stored potatoes, which,

at the time of digging, were apparently quite sound. In bad cases small discoloured spots first appear on the bark of the tuber; these increase in size and become confluent, till at length the entire skin of the potato is discoloured. The cuticle then bursts in many places, and the olive-green spores are liberated. Like many other plant diseases, potato smut is in some seasons rare, at other times it is very common and destructive. No potatoes showing traces of smut should be planted as seed.

CHAPTER VII.

SCAB AND CRACKING OF POTATOES.

THE brownish scab on the skin or bark, and the cracking of the bark in potatoes, are probably due to one and the same cause, and that a mechanical one. Scab and cracking are often confounded with smut in potatoes, but whereas the latter ailment is invariably caused by a fungus, there is seldom any fungus present (unless it be one of the species which so commonly follow injuries) in scab and cracking. Cracked and scabbed potatoes are not marketable, and as the fissures in the bark expose the inner substance of the potato to the earth it is generally said that tubers so injured possess an earthy and disagreeable taste foreign to uncracked and unscabbed examples.

Scabbing and cracking begin at a very early stage of growth in the tuber, and both are at first seen as small corroded spots, or minute open pustules; in bad cases the spots and cracks become confluent, and the whole bark of the potato presents an unsightly appearance. When the inner substance of the potato is once exposed minute insects, and sometimes fungi, add to the injury. If such fungi as Peronospora or Fusisporium light on the exposed places, destruction of the tuber is soon complete.

Scab and cracking are said, in the first instance, to be due to the presence of some irritating or corrosive substance in the soil. Continued drought, and possibly sudden and superabundant moisture, will also cause one form of scab. A natural effort is made by the potato to repair the injury, and so a hard scab originates; when

insects and fungi light on the injured parts, repeated efforts are made to repair the damage, and so the bark is brought into a scabbed and cracked condition. Lime rubbish, builders' refuse, refuse from ashpits, and other materials of the same class, are said to cause corrosion, scab, and cracking of the bark of potato tubers by contact. When the bark is carefully removed small depressions are clearly seen, answering to the scabs removed with the bark. In bad cases the pits beneath the scabs are excavated deeply into the substance of the potato, and when the bark is removed the substance of the tuber, though frequently slightly discoloured, is left intact.

It generally happens that a portion only of a crop of potatoes is scabbed, and this portion can be often distinctly traced to one part of the field whence the potatoes were derived. On visiting this position the irritating substances in the soil will usually be seen. When scab and cracking can be thus traced the remedy is obvious.

CHAPTER VIII.

NEW DISEASE OF ONIONS.

Puccinia mixta, Fl.

THERE are few kitchen-garden crops more liable to disease than onions, and in the best managed fields and kitchen gardens, and in dry as well as wet seasons, whole crops of onions, and all varieties alike, are liable to be swept off by the attacks of fungi.

During the summer of 1883 great attention was directed to a fungus named Puccinia mixta, Fl., found growing on chives, near Shrewsbury. The name of the genus Puccinia was given in honour of Puccini, a Florentine professor. When we remember how completely Puccinia malvacearum, Mont., has, during the last few years, destroyed all our best garden hollyhocks, we may well feel some anxiety as to the course this new pest of onions may pursue. Mr. William Phillips, F.L.S., of Shrewsbury, was the first to detect the onion parasite, named Puccinia mixta, Fl., growing in a garden. Mr. Phillips recorded its occurrence in the Gardeners' Chronicle for 14th July 1883, and there stated that the parasite was growing on chives, Allium Schoenoprasum, L., and the crop, he said, was in a deplorable condition of disease, the leaves and scapes, or naked flower-stems, being covered with yellow and brown spots, and presenting a miserable appearance. Mr. Phillips was good enough to forward some specimens to us at the time of finding, and from these examples the illustrations have been made.

Chives are perennial and indigenous to Britain. They are grown to no great extent in England, but in Scotland