The Potato-disease is best known by the pale whitish fringe, giving an almost meally appearance to the margins of the brown to black patches in damp weather. In dry weather the brown patches shrivel and dry, and as they are apt to be at the edges and tips of the leaflets, these curl up. The young disease spots are yellowish, and the leaves of badly affected plants are apt to be sickly yellow throughout.
This Potato-disease due to Phytophthora must be distinguished from the curling and puckering, with wilting and browning of the leaves and yellow glassy look of the stems, due to the invasion of the vessels by a fungus which lurks in the tubers, and gains access thence to the shoots.
In the disease traceable to Phytophthora the stock remains green and the leaves plump and plane, and only the brown patches slough out in wet or shrivel in dry weather, and are bordered by the pale whitish zone of conidiophores.
In the leaf-curl the yellow and flaccid appearance of all the leaves of a stalk, or even of the plant, is the striking symptom, and the stem soon droops and blackens just above the soil, a white mould appearing also at the black spots. Subsequently black spots appear higher up, and bacteria gain an entrance. The stolons rot, and eventually the roots and the leaves wither. The tubers appear sound, but are small; they are apt to rot in the store, the vascular zones turning brown.
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.