Fungous Disi-asls of Plants, Puach Vlllows H9 



lands for two decades. Extensive investigation, Harry Ardell 

 Allard lias since written."' proved the disease to be 



communicable by artificial inoculation ; that the sap of healthy plants was 

 not infectious; that an incubation period of ten or twelve days preceded 

 the first observable symptoms; that the disease was persistent, appearing 

 ultimately in all immature growing parts of affected plants; that diseased 

 sap filtered once through filter paper still retained its virulence; that sap 

 sufficiently heated lost its virulence; and that the seed of affected plants 

 did not carry the disease to the next generation. Mayer failed to produce 

 the disease in other solanaceous plants. Further, he found that liming, 

 crowding the plants, sudden atmospheric changes, unfavorable conditions 

 of whatever sort afi'ecting the roots, i. e., mechanical injury, nematodes, 

 parasitic fungi, etc., were not in any way responsible for the origin of the 

 disease. . . . He recognized the sporadic occurrence of the disease in the 

 field, and finally concluded that its spread must be through the soil, both 

 in the field and in the seed bed. He recognized the fact, however, that a 

 transfer of the disease by means of the soil had never been proved. 



During the 1880's the study of fungous diseases had dominated 

 research in plant pathology, and not infrequently maladies, later 

 shown to be caused by bacteria, were attributed to fungi. Slowly 

 now the study of bacterial diseases was beginning to gain the 

 ascendancy it attained during the next decade, and the basis for 

 a third major classification of plant diseases — those caused by 

 viruses — was beginning to acquire outlines. For years the failures 

 by research students to isolate specific pathogenic organisms from 

 virus plant diseases (that is, by pure culture, reinoculation into 

 healthy plants, and reproduction of the disease) had the effect of 

 discouraging study of these maladies: or, if studies were made 

 and no bacteria or fungi were found, of too often causing them to 

 attribute the disorders to physiological disturbances. Occasionally, 

 furthermore, these maladies were ascribed to a bacterial origin, 

 and this notwithstanding no parasite had as yet been isolated. 



Tobacco mosaic is today believed to have been the first disease 

 in plants (or in animals for that matter) to be positively proved 

 to be caused by a virus. Mayer's conclusion, however, was that 

 It is of bacterial origin, and Smith's "rather literal rendering" of 

 the European's first points read as follows: 



The " mosaic disease " of tobacco is a bacterial disease, the infective 



" A review of investigations of the mosaic disease of tobacco, together with a 

 bibliography of the moie important contributions, Bull. Torrey Botanical Club 

 41 (9): 435-458, 1914. 



