In the first place, the potato-vines suffer by excessive exhalation; in the second, by sudden reduction of temperature, and, though not frozen, their functions are much deranged, and their vitality greatly enfeebled. To use a common expression, the plant "has caught a violent cold that has settled on the lungs."
The leaves (which are the lungs of plants) now fail to perform their functions properly. The points of many of the leaves turn brown, curl up, and die.
The ascending sap, not being fully elaborated by the diseased leaves, oozes out through the skin of the stalk in a thick, viscous state, and the plant to all appearance is in a state of consumption.
At this stage the ever-present minute spores of the Botrytis infestans eagerly pounce on the sickly plant, fastening themselves on its most diseased parts. The Botrytis infestans is a cryptogamous plant, and is included in the Mucidineous family, (moulds.) It is a vegetable parasite preying upon the living potato plant, like lice or other animal parasites upon the animal species.
At first this mould forms webby, creeping filaments, known in botanical language as mycelium. These root-like fibres then branch out, sending out straight or decumbent articulated stems. These bead-like joints fill up successively with seeds or spores, which are discharged at the proper time to multiply the species.
Under favorable conditions of warmth and moisture, the mycelium spreads very rapidly. Spores are soon formed and matured, to be carried to plants not yet infected. Rains also wash the seminal dust down the plant, causing it to fasten and grow on the vine near the ground. The roots of the parasite penetrate and split up the stalk even to the medullary canal.
These roots exude a poisonous substance, which is carried by the elaborated descending sap down to the tubers, and as the largest tubers require the largest amount of elaborated sap for their development, they will, consequently, receive the greatest quantity of the vitiating principle, and will, on digging, be found a mass of rottenness, when the smaller ones are often but slightly affected. The Botrytis infestans can not gain a lodgment on vines that are truly healthy and vigorous, high authority to the contrary notwithstanding.
Healthy varieties, growing in a sheltered situation on dry, new soil, to which no nitrogenous manures have been applied, can not be infected, though brushed with other vines covered with the fungus. Different varieties, and sometimes different members of the same variety, are not always alike affected by the disease, though growing in the same hill.
As will be noticed, the potato disease is rather an effect than a cause, and appears to have been designed to prevent members enfeebled by accident or otherwise from propagating their species by putting such members out of existence. Ozone, supposed to be a peculiar form of oxygen, is exhaled from every part of the green surface of plants in health, and effectually repels the attacks of mildew; but it is found that when the atmosphere is very dry, or, on the other hand, very humid, plants cease to evolve ozone, and are therefore unprotected. Winds from the ocean are strongly ozonic, and it is ascertained that plants growing on soil to which salt has been applied evolve more ozone than others. Hence the benefit derived from the use of salt on potato lands.
The "Black knot," another species of fungus that attacks the branches of the plum and Morello cherry, operates very similarly to the potato mildew. The roots of the parasite penetrate and split up the cellular tissue of the branch on which it fastens, and if the limb be not promptly amputated, the descending sap carries the deleterious principle through the whole system, and the following year the disease appears in a greatly aggravated form in every part of the whole tree. The remedy in this case is prompt amputation of the part diseased on its first appearance, and a judicious application of salt to the soil.