This last kind of reproductive organs (not yet positively found in the potato mould) appear to be wholly analogous to the oogonia of the “white rusts” already described, producing oospores in like manner. Dr. de Bary avows with regret that his numerous efforts to observe the germination of these oospores were unsuccessful. Nevertheless, he considers that the perfect resemblance between them and the oospores of the “white rusts” will justify him in concluding that the germination in both is very similar. It will be unnecessary to repeat here the observations already made on the growth and development of oogones and zoospores. What has been advanced respecting these organs in _Cystopus_ will apply also to _Peronospora_.
264.—POTATO MOULD. _Peronospora infestans._ ]
265.—LETTUCE MOULD. _Peronospora gangliformis._ ]
POTATO MOULD.—Towards the close of the summer of 1845, in the course of a few weeks, every one became aware of the fact that a new disease had appeared which threatened the entire destruction of the potato crop. Until then it seemed to have been almost, although not entirely unknown. It first appeared in the Isle of Wight about the middle of August, and a week afterwards had become general in the South of England, and the next week there were but few sound samples of potatoes in the London market. Early in September the disease had commenced its ravages in Ireland, and shortly afterwards it was discovered in Scotland. With the same rapidity it seems to have spread throughout Europe and North America, or at least the western portion of the former and the northern districts of the latter. It must not be imagined, however, that the Isle of Wight was the centre from which this disease spread over such an extended area and with such alarming rapidity. From this spot it doubtless made its first appearance that year amongst our own crops, but there is not the least doubt of its existence both on the continent of Europe and in North America in the previous year, and the farmers of Belgium had noted its appearance in the province of Liège as far back as 1842 and 1843. Other diseases had been observed affecting the potato crop before, and one which was also associated with a parasitic fungus had made its appearance in 1815. It is also exceedingly probable that, in a milder form, the murrain was present with us a year or two before it broke out to such an alarming extent. A correspondent to the _Gardeners’ Chronicle_, in 1844, notices it in the Isle of Thanet, and another testifies to its occurrence in districts of Ireland for two or three years previous to its general outbreak. The description of the disease in Canada, in 1844, contained in a letter addressed to Dr. Bellingham, and quoted by the Rev. M. J. Berkeley,[9] leaves no doubt of its identity:—“During the months of July and August (1844), we had repeated and heavy showers, with oppressive heat, and an atmosphere strongly charged with electricity. Towards the close of the month of August I observed the leaves to be marked with black spots, as if ink had been sprinkled over them. They began to wither, emitting a peculiar, offensive odour; and before a fortnight the field, which had been singularly luxuriant, and almost rank, became arid and dried up, as if by a severe frost. I had the potatoes dug out during the month of September, when about two-thirds were either positively rotten, partially decayed and swarming with worms, or spotted with brownish-coloured patches, resembling flesh that had been frost-bitten. These parts were soft to the touch, and upon the decayed potatoes I observed a whitish substance like mould.”