This leaf-curl has been ascribed to Pleospora, Polydesmus, Verticillium, and other parasites, as well as to excessive manuring and other agencies, but it still needs explanation.
Rot of Potato tubers in the soil, or in store, may be brought about by very different agents.
If Phytophthora has obtained access, the fungus hyphae spread between the cells, starting from the haulm, and cause the flesh to turn yellowish and then brown in patches. On the exterior are discoloured patches, depressed, with the flesh beneath brown and soft. The mycelium spreads mostly in the outer layers, which though they turn deep brown remain firm.
Wet rot of potatoes may be due to various fungi, and, in excess of water, to putrefactive bacteria (e.g. Clostridium), which destroy the cell-walls. The flesh becomes soft, then soup-like, and finally putrefies to a liquid mass with a vile smell of butyric acid, etc., in which the starch grains may be seen floating.
Tubers are often found with the cork burst and peeling in shreds, the flesh more or less converted into a putrid and stinking pulp, with a spotted brown boundary of partly destroyed but firmer tissue between the dark utterly rotten and the white and still firm healthy flesh. The principal agent in the destruction of the tissues is Clostridium, an anaerobic bacillus which consumes the cell-walls but leaves the starch intact. Hence a thoroughly decomposed tuber consists of a cork bag full of starch and foetid liquid. In the dried condition the flesh shows a brown marbling; this passes into a soft soupy starchy part, and here and there may be violet grey cavities lined with Spicaria, Hypomyces, etc., the white stromata of the latter often appearing externally. The excavations are filled with loose starch grains, and bounded by cork and cambium formed in the peripheral cells. The cell-walls eventually undergo slimy decomposition.
Spicaria, Fusisporium, various moulds, and bacteria may all be associated with wet-rot.
Dry-rot of Potatoes is also due to various fungi and bacteria, but the destructive action goes on slowly, owing to there being no more moisture than the tissues afford. The flesh becomes excavated here and there, owing to the slow destruction of the cell-walls by Clostridium: the destroyed tissues are brown, and the uninjured starch grains powder them all over. Finally the whole shrunken mass has a crumbly consistency.
When the flesh remains white, but assumes a powdery consistency and dry-rot, with the cork destroyed here and there, Frank refers the damage to Phellomyces. Where the dry-rot is due to Fusarium the chalk-white stromata may often be detected breaking through the periderm; but it must be remembered that the soil-contaminated, broken skin of a potato-tuber is a favourable lurking spot for many fungi, and Periola, Acrostalagmus, and others have been detected therein.
Brown spots, depressed into the flesh, sometimes result from the ravages of Tylenchus, the minute worms being found in the diseased tissues.
In some cases the flesh turns watery and soft, grey, almost glass-like, starting at the haulm end, and this may be owing to the invasion of Rhizoctonia.