very destructive, is one of the white moulds. The mycelium of this fungus is able to penetrate every part of the plant, discolouring and corroding the green parts, and causing loss of vitality and decay in the tuber. Partial observations of several mycologists had revealed much of its life history and mode of growth during the summer, but it was left for an honorary member of the Woolhope Club to discover how it survived the winter. It has Jong bean known that some funguses, like insects, go through several stages or metamorphoses. The final and perfect stage is easily recognised in mast insects, because that is the only one that has the power of reproduction; but among funguses every stage is able to propagate itself in some way; thus in summer the potato blight throws off from the free ends of its mycelial. threads two kinds of short-lived spores, which, if they fail on the leaf of a potato, germinate and quickly reproduce themselves, killing their victim and perishing with it.
Our friend Mr. Worthington Smith lad the good fortune, while investigating the natural history of this fungus, to discover another kind of spore, called a resting spore, because it hybernates in or on the ground, He watched its mode of formation in the autumn and its growth the following spring, and thus was enabled to prove that this spore was the long sought for means by which Peronospora infestans continues its existence from year to year.
This spore is to he found in the tissues of the decaying plant. It is formed by a process of conjugation not uncommon among funguses. By degrees it acquires a hard protecting coat, and, with the dying plant, falls to the ground, where it remains to take its chance during the winter. On the rectum of warmth, the hard coat bursts, mycelial threads exude, and extend in search of a foster mother. If they do not meet with a potato plant in growth, they speedily exhaust themselves, and die; but if unfortunately successful, they pierce the cuticle, and the work of destruction commences.
Through want of thought and custom, much is done that favours the existence and propagation of this pest—diseased haulm and tubers are left on the surface of the ground when the crop is taken up, and are afterwards dug in to serve as manure. If this happens in a garden or rental potato ground, and the same crop is put in a second year, a vigorous crop of Peronospore is the result, and the cottager scarcely gets his seed back. The potato blight is also extensively propagated in another way. It most houses it is usual to throw away diseased tubers along with parings and other rubbish into dust heaps, which are in due course carted away and used as manure. It is probable that staring potatos in the same buildings or floor year after year, favours the spread of the disease.
Mr. Worthington Smith’s discovery teaches that every part of an infected plant should be burnt; it is the simplest way of effectually destroying the fungus; and also, that under no circumstances should potatos be planted for two consecutive years in the same ground.
