Page:The American Cyclopædia (1879) Volume XIII.djvu/795

 POTATO FLY POTATO ROT 771 may also be obtained by dissolving dextrine or gum arabic in the water both, however, much more expensive than the flour." Much has been said against the wholesale use of Paris green for this purpose ; but it is the experience of those who have had most to do with it, that there is no danger in its judicious use as here recommended. It effectually kills the insect without affecting the plant either above or be- low ground; and what little gets into the soil is converted into an insoluble and harmless precipitate with the oxide of iron very gener- ally diffused. There is another insect^ known as the bogus Colorado potato beetle (doryphora juncta, Germar), which has always existed in the southwestern states, feeding on the wild horse nettle (solanum Carolinense). It is so much like the genuine beetle that it is often mistaken for it. Yet it will not touch the cul- tivated potato, and a close examination shows many specific differences between the two spe- cies. Compared with D. IQ-lineata, the eggs of D. juncta are much paler ; the larva is also paler, and has but a single row of black dots d FIG. 7. Doryphora juncta. o, a. Eggs. &, &. Larvae, c. Beetle, d. Wing cover, show- ing character of punctures, e. Enlarged leg. along the sides ; and the beetle has the second and third lines instead of the third and fourth (counting from below) joined, and the inter- vening space brown ; also a spot on the thighs, which is lacking in D. IQ-lineata. POTATO FLY. See CANTHAKIDES. POTATO ROT. In 1845 the potato crop in various parts of Europe, and in the United States and 'British provinces, was attacked by a most destructive disease, which was called the Eotato murrain or potato rot. It has occurred i other years, both before and after 1845, on both continents, and to some extent in 1874; but at no time has it been so severe as in 1845. So sudden and general was the disease in that year that it caused heavy losses to farmers, and in Ireland, where the rural population depend largely upon potatoes for food, their sudden failure brought with it severe famine and dis- tress. At first various causes were assigned for the disease, notable among which was the theory that the potato, having been multiplied for many years by the subdivision of its tu- bers, had become debilitated, and that the dis- ease was due to a depraved constitution. Though this view led to the general improve- ment of our varieties, especially through the 'abors of Mr. Goodrich (see POTATO), its fal- lacy was long ago demonstrated, and it is known that the disease occurs in the potato in FIG. 1. a. Asexual spore. &. The same germinating, c. Ejection of zoospores. d. A zoospore with cilia. its wild state in South America. The presence of aphides or plant lice upon the tubers, and electricity and other meteorological influences, have been among the causes assigned. When the disease exists in a mild form, brown spots appear upon the herbage ; after a while these spread over the plant until it dies, and the tu- bers will be found more or less affected ; the trouble may be confined to a few plants or ex- tend over all in a field. When it comes in its worst form the destruction is complete in a few hours, and a field that was in the morning green and flourishing will be at night a mass of rotten herbage and decaying tubers. The disease appears usually early in August and in damp weather. In 1846 the Rev. J. M. Berkeley, the eminent cryptogamic botanist of England, published his observations on the rot, and found it due to a minute fungus which, though it has been studied with great care by mycologists, still keeps a part of its history concealed, and though generally called perono- spora infestans, there is some doubt whether it FIG. 2.-The Mycelium within the tissues of the leaf ; the aerial portion bearing asexual spores. (After Farlow.) belongs to this genus. The mycelium or vege- tative portion of this fungus (see FUNGI) is s< small that it can find abundant room to li and spread within the tissues of the leaf, 01 )m,