REED: PHYTOPATHOLOGY 163 



tion of bacteria to plant diseases — Wakker (1883-1889), yellow disease of 

 hyacinths; Smith (1897) and Russell and Harding (1898) black rot of cab- 

 bage; Stewart (1897) bacterial wilt of sweet corn; Smith and Townsend 

 (1907) and later publications by Smith and others on crown gall. These, and 

 such other diseases as blight of beans, citrus canker, soft rot, cucurbit wilt, black 

 leg or black rot of potato, red-stripe disease of sugar cane, and wildfire of to- 

 bacco, have all been associated with bacteria. Smith (1905, 1911, 1914) pub- 

 lished three \Arge volumes dealing extensively with the bacterial diseases and in 

 1920 published his summary. Elliott (1930) listed 177 species of bacterial plant 

 pathogens — 13 caused by Aplanohacter, 53 by Bacillus, and 111 by Bacterium. 



It is also interesting to recall the controversy between Dr. Alfred Fischer 

 and Dr. Erwin F. Smith in 1899. The former maintained that bacteria did not 

 cause disease in plants, while Smith affirmed their causal connection. 



Other organisms have also been associated with plant disease. Club root 

 of cabbage, caused by Plasmodiophora brassicae, has been studied by Woronin 

 (1878), Lutman (1913), Kunkel (1918), and others. Root knot or root gall, 

 caused by nematodes, was first observed by Berkeley (1855). Greef (1872) 

 described the nematodes, Frank (1885) and Atkinson (1889) gave further 

 details on the disease and the causal organism. The nematode disease of wheat 

 was found by Johnson (1909) in California and by Fromme (1917) in Vir- 

 ginia. Byers (1918-1920) has made detailed studies. 



5. Virus diseases. A separate chapter in plant pathology deals with the 

 virus diseases of plants. The first scientific studies were concerned with the 

 tobacco mosaic, which has continued to be a favorable subject of many investi- 

 gators. Mayer (1886) discovered the infectious nature of the juice of mosaic 

 tobacco plants, Ivanowski (1892) discovered that the infectious principle could 

 pass through a Chamberland filter, which held back bacteria, and Beiyerinck 

 (1898) extended the work, introducing the term "contagium vivum fluidum." 

 Many plant diseases are caused by a filterable virus, among them aster yellows, 

 curly top of beet, sugar cane Fiji disease, peach yellows, stunt disease of rice, 

 mosaics of sugar cane, cucumber, hop, lily, and potato. We may note in passing 

 that Loefiler and Frosch (1898) estabHshed the first causal connection of a 

 virus to a disease of animals, the foot and mouth disease of cattle. 



Studies have been made on the methods of transmission of the filterable 

 viruses, being distributed by grafting, budding, and on the seed, as in the case 

 of the legume mosaic. A most interesting development is the discovery of insect 

 vectors. Takami (1901) found that the stunt disease of rice which was often 

 destructive in Japan, sometimes resulting in crop failures involving famines, 

 was caused by the feeding of the leaf hopper, although the actual virus was not 

 discovered until 1908-1909. Aphids and leaf hoppers are very common vectors. 

 Usually, there is a high degree of specialization in the carrier, a specific insect. 



