M. says: "Dear Sir: -Since my return from Europe, where I spent the summer, 1 have been looking over some of the back numbers of the Gardener's Chronicle, and find that the potato rot continues to excite speculation. You have doubtless often seen a lot of English gooseberries, with a mild spring make some nice young wood of several inches long, when one night's frost would come, which would lot only check up the growth of the young moots, but in a few days you would find both the shoots and the young berries covered with a leathery fungus which enclosed them as a coat of mail, and which you, sir, knowing well what was the cause for that fungus being there, would count it idle to inquire further whether it was a spore or a fungus. During my several crossings of the Atlantic I have always spent a few weeks in fatherland (north of Ireland), where, in answer to my many inquiries as to the potato rot and its cause, I was always invariably told that the rot was preceded by a heavy fog, that approached the land from the east, and that as soon as the fog passed away, the potato leaf would be limp, and get discolored, and should the sun come out strong, would throw off a very disagreeable odor.

I was in Ireland during the potato rot fog last August (1875). For some days afterwards you could have smelled the potato fields at a considerable distance. The fog passed over the north of Ireland about the 15th of August, and in a conversation with a friend who plants 12 to 15 acres potatoes annually, he told me that he had passed through his potato fields at early morn, before the fog had passed away, and that every leaf was frozen stiff".

" Since my return home I passed one evening a beautiful flat of Dahlias belonging to my sons. Two days afterwards, upon passing the same flat of Dahlias, it reminded me of an Irish potato field stricken with the potato rot, and there was no perceptible difference in the smell. Frost did it. M."

[Nothing is more clearly proved than the fungoid origin of the potato disease. An examination of the evidence given in back volumes of the Gardener's Monthly and other magazines shows this. The potato disease will often rot a whole cellar full of tubers, when certainly no frost entered there. The point made by our correspondent that last year the disease appeared with most virulence in Ireland after a heavy fog, is a good one, for it has been shown by the observations of Worthington Smith that the form of fungus so destructive last year requires considerable moisture for its development - Ed. G. M.]

Permit me to make a few remarks in answer to your and Mr. Worthington Smith's conclusions respecting potato rot. You are aware, I suppose, of the very marked difference between the old potato rot of 1846, '47 and '48 and what is termed the potato blight of to-day. The rot commenced with the potato, the blight with the leaf. In 1848, in Ireland, potatoes rotted everywhere, with one exception, and that was where they were put in ridges by the spade. Those ridges were generally from five to seven feet wide, and I was assured by a correspondent that wrote me at the time that while the potatoes in the centres of the ridges always rotted yet the rows upon the brows of the ridges, alongside the trench, where no water could lie, were always sound and good. In those days, in this country, I could tell by the thermometer what prospect we had for a crop of potatoes. If we had a succession of warm, showery weather in August, so much that the water would lie between the rows, and when tried by the thermometer it would stand from 80 to 85 degrees, and this state of weather continued for three or four days without dry, windy weather setting in, so as to absorb the heated moisture the potato was enclosed in, the crop was gone.

If the weather blew up suddenly dry, only such potatoes as the water sat around would go to rot. All others were safe. Many is the stalk of potatoes that I have pulled up, and always found the points of such potatoes as were imbedded in the heated moisture going to decay. I have spread many and often stalks of potatoes partially rotted along the tops of the rows to dry, and always found that as soon as perfectly dried that the rot extended no farther, and that such portions of potatoes kept perfectly sound during winter. Such was potato rot; but potato blight is a horse of another and very different color. It attacks the leaves and tender tops. I have never seen the appearance of a more promising crop of potatoes in Ireland than was last year until, one day a cold, chilly fog came floating along, and which was so cold that by the next morn potato leaves were frozen stiff, so that as soon as the sun came out upon them they began to blacken and afterwards to smell, destroying all the late crop. Now, what was it that killed those potato tops? - for at this time examine the tubers and you can notice nothing wrong with them. Mr. Worthington Smith says that it was fungus.

The conclusion that I came to was that a frost that would freeze a tomato, a snap-bean or a potato stiff was enough to kill them without asking any aid from a fungus to help. A few days later - 26th of August - I sailed from Rothsay, Isle of Bute, up the Clyde to Glasgow, and found that where the fog, in place of floating, covered its banks like a blanket, and where there was no frost, of course, that not a potato was injured. 27th of August I strolled up past old Bothwell Castle to near to Corrie Lynn and still no potato blight.

It is certainly true that put diseased potatoes in a cellar they will rot, but that has nothing to do with what made them diseased. I once dumped into a covered cave 800 bushels of sound potatoes. The weather was wet, and a good deal of clay stuck to them. About mid-winter I found them heated and half rotten, which I certainly never would have thought of charging to fungus. The same with 200 or 300 bushels of sound turnips that after being carefully topped were piled away in a corner of the cellar that soon began to grow and then to heat, and which, if they had not been immediately scattered, would all have been lost. I was lazy, Mr. Meehan, about sending you my last communication, and as much so this time, but having written, and my conclusions about potato rot thought to be wrong, I want you and your correspondents to pitch into me, as it is only facts that I am after.

[Can our correspondent explain why no "frosts," "heats," "damps," or "fogs," had these destructive effects previous to 1840? - Ed. G. M.]