COBB'S DISEASE OF SUGAR-CANE. 21 



ETIOLOGY. 



This disease is caused by Bacterium vascularum (Cobb) Greig Smith, a honey-yellow, 

 one-flagellate organism, which forms the yellow slime always present in the vessels of 

 diseased plants. It is a short rod occurring singly, in pairs, fours, or eights (end to end), 

 and it often exists in practically pure culture in the fibro-vascular bundles of the diseased 

 sugar-cane. The constant presence of this particular yellow organism in the diseased cane 

 goes a long way toward establishing a causal relation, or would if it could be shown that 

 nothing else is constantly or usually present. The writer knows, however, from personal 

 observation, that other bacteria are sometimes present in the vessels, and Dr. Cobb's paper 

 leads one to suppose that "traces" of other organisms are usually present in some part of 

 each diseased cane. There are many parts of the cane, however, where Bad. vascularum 

 exists in pure culture (Cobb, EFS.), and the fact that no other organism is necessary to 

 the production of the disease has been proved conclusively by the writer's successful pure- 

 culture inoculations. 



In October 1902, the writer received from the Richmond River district, Australia, 

 through the courtesy of the Commissioner of Agriculture of New South Wales, three diseased 

 canes, from two of which he had no difficulty in obtaining pure cultures of the organism by 

 means of Petri-dish poured plates. No attempt was made to get cultures from the most 

 badly decayed of these canes, because in places the bacterial ooze was nearly white, indi- 

 cating an extensive mixture of organisms in the stem.* Nine Petri-dish poured plates were 

 made from the interior of each of the other canes. One set of plates gave in addition to 

 Cobb's organism a mixture of other things, mostly white bacteria of several sorts, but also 

 colored colonies and molds; the other set of plates yielded practically pure cultures of 

 Cobb's organism (fig. 14). 



Cuttings from healthy canes were obtained in November 1902 from southern Georgia 

 and were planted in a hot-house in Washington. Before planting, the canes were shortened 

 at each end and inspected critically by the writer. All were free from Cobb's disease and 

 otherwise sound. Shoots from these canes were inoculated early in February 1903, with 

 slime from potato subcultures of the yellow organism derived from the Australian cane. 

 These inoculations succeeded beyond expectation, the canes developing local signs in a few 

 days. Stripes extended up and down from the pricked part of the leaves and in one variety 

 constitutional signs of a pronounced and typical sort appeared a few weeks later, the same 

 being preceded by the enormous multiplication of the honey-yellow bacterium, first in the 

 bundles of the inoculated leaves and later in those of the main axis of the cane and those 

 of the uninoculated leaves. From the interior of the stem of several of these inoculated 

 plants the organism was again obtained in pure culture. Inoculations made with sub- 

 cultures of the organism thus isolated from the artificially infected cane were likewise 

 successful, and the bacterium was again isolated from the diseased cane in pure cultures. 



INOCULATIONS. 



SERIES 1. 1903. 



The first set of inoculations (12 in all) was made on February 6, into common green 

 cane. The inoculations were made by means of needle-pricks on the leaf blades about 

 1 to 2 feet from the stem. Two leaves were inoculated on each plant, those selected being 

 about the tenth or eleventh from the base of the plant. About 20 to 30 fine pricks were 



*Respecting these canes Dr. Cobb makes the following statement in his Third Report: "There can not be the 

 slightest doubt as to the identity of the organism cultivated by Dr. Smith and inoculated by him into American canes. 

 * * * The second shipment was attended to personally by myself after it had been forwarded to me by my friend 

 Mr. A. C. Barry, manager of the Broadwater sugar-mill, Richmond River, who at my request selected the canes and 

 forwarded the same to Sydney. They were there unpacked, and sealed up with sealing wax and repacked at my 

 laboratory. I examined the gum. The organism was the same species as that described in 1803 and reported on 

 subsequently as the cause of the newly discovered disease for which I proposed the name "Gumming of the sugar-cane." 



