THE OEIGIN AND PEOPAGATION OF DISEASE. 233 



offensive odor, and the disease spread so rapidly that the wliole vine 

 would be blighted in a few days, and a field, which had before been cov- 

 ered with a luxuriant growth, at the end of a fortnight was merely a 

 scene of desolation, and looked as if it had been struck by a severe frost. 

 If the potatoes were immediately dug out of the ground, many of them 

 were found already partially decayed, or touched with brownish and soft- 

 ened spots." 



The disease broke out again in 1854 and 1855, and was destructive in 

 the State of New York, in Ehode Island, Massachusetts, Ohio, Illinois, 

 and at various other points ;* and about 1865, or ten years later, it made 

 its appearance for a third time. I am told by an old and experienced 

 farmer of Washington County, iSTew York, that in 1864 and in 1865 the 

 230tato-crop in that region was practically destroyed 5 so that often in a 

 twenty-acre field there would not be a single good potato. Potatoes 

 were usually to be had at that place for seventy-five cents per bushel, 

 but in those years they were in some cases sold at eight dollars per 

 bushel, for farmers' consumption. 



This destructive malady was at last found to be due to the ravages of 

 a microscopic fungus, called, from its mode of fructifi-cation and its in- 

 jurious effects, the Fero7iospora infestans. 



The fungus has a mycelium of fine, cylindrical, ramifying tubes. Its 

 fructifying part consists of filaments which stand up vertically from the 

 mycelium, dividing at the end into four or five branches, and each branch 

 bears upon it several successive swellings, making a kind of sausage- 

 like chain, whence its name of " PeroriosporaJ^ At the end of each 

 chain there is a complete oval spore, and the spore, when ripe, detaches 

 itself and germinates, to produce again a new mycelium. 



When the Feronospora is placed in contact with the leaves of a potato 

 vine, its filaments j)enetrate into and through the epidermic cells, and 

 so reach the intercellular tissue of the leaf and stem ; and there they con- 

 tinue to grow, producing a rapid withering and blight. When the parasite 

 has attained a certain growth, it begins to fructify. Its upright fila- 

 ments burst through the pores of the leaves, and are crowned with 

 the characteristic chain of spores. Each spore, when ripe, if supplied 

 with moisture, produces six or seven secondary zoospores, armed with 

 long vibrating cilia, and capable of a rapid spontaneous motion. After 

 moving about for a short time, the zoospore becomes quiescent, throws 

 out an elongated filament, and germinates afresh. 



It is no doubt in this way that the germ of the i3arasite reaches the 

 tuber of the potato at the root of the vine. For if sound potatoes be 

 placed in the ground, and the surface of the soil be sprinkled with the 

 spores of Feronospora^ and then watered from time to time, the potatoes 

 are found to be infested with the disease in about ten days.t 



* "Patent-Office Eeports, Department of Agriculture," 1856; "Massachusetts State 

 Board of Agriculture," 1856. 



i Eobin, "Trait6 du Microscope," Paris, 1871, p. 967. 



