304 



BACTERIA IN RELATION TO PLANT DISEASES. 



one to discriminate. The lapse of a little time between inspections will also help one to 

 judge, since in its further progress the black rot is quite different from the other foliar 

 diseases. In this connection see plate 18. 



ETIOLOGY. 



The cause of this disease is a yellow one-flagellate micro-organism, Bacterium campestre 

 (Pammel) BFS. Conclusive proof of the infectious nature of this organism was obtained 



on turnips by Pammel in 1893 and pub- 

 lished in 1895. He performed 20 direct 

 inoculations successfully. Subsequently he 

 isolated the organism. His pure cultures 

 were derived from turnips and rutabagas, 

 and 8 successful inoculations were made 

 into rutabagas, as many plants being held 

 for control. The signs of the disease ap- 

 peared in the course of a few days, finally 

 involving the whole plant; this same or- 

 ganism was subsequently isolated from the 

 diseased tissues, i. c., from the blackened 

 bundles and advancing margin of the rot. 



In 1896 97 the writer repeated and con- 

 firmed Pammers experiments on turnips 

 and rutabagas and extended the inocula- 

 tions to cabbages, cauliflower, kale, rape, 

 radish, and black mustard. Two strains of 

 the organism were used for most of these 

 infections, viz., pure culture isolations from 

 diseased turnips obtained in Maryland and 

 similar cultures from diseased cabbages 

 from Wisconsin, but some cross-infections 

 were also made with the organism obtained 

 from charlock in Wisconsin. Microscopic 

 examinations, bacteriological cultures and 

 cross-inoculations showed the disease to be 

 identical in all of these plants.* He also 

 confirmed and considerably extended Pam- 

 mel's description of the organism. The 

 writer at this time had obtained altogether 

 more than 60 successful infections resulting 

 in typical cases of the disease. From dis- 

 eased plants at long distances from the 

 point of inoculation he several times re- 

 isolated the organism and obtained addi- 

 tional infections with such cultures. Only 

 one of his many control plants contracted 

 the disease and this under circumstances 

 which pointed clearly to neighboring inocu- 

 lated plants as the source of the infection 

 and to mollusks (Agriolimax agrestis)as the 

 Fig. io2.| carriers of the bacteria. 



*Punctures with a sterile needle never induced any disease in my experiments. Hecke also mentions this fact 

 particularly. 



fFiG. 102. Cross-section of petiole of cabbage, showing every bundle blackened by Bad. campestre, the paren- 

 chyma being free. Slide 8, plant No. 42. 



