68 BACTERIA IN RELATION TO PLANT DISEASES. 



TREATMENT. 



Little is known regarding the transmission of this disease from one plant to another 

 except by way of the sets. The planters themselves are believed both by Cobb and Tryon 

 to be largely responsible for the spread of the disease. Dr. Cobb says: 



It seems evident that gumming is not a disease that is spread to any great extent through the 

 air * * * . This is shown by an array of facts that cannot be for a moment overlooked. Some 

 stalks in a stool may be badly gummed and others in the same stool fairly healthy ; part of a crop may 

 be gummed, and the rest remain in good condition; one field may be badly gummed, and an adjacent 

 field perfectly healthy; the Upper Clarence is comparatively free from gumming, while the Lower 

 Clarence has suffered severely for several seasons. All these facts are incompatible with the idea that 

 the disease is very infectious. If healthy plants easily caught the disease by receiving the germs of it 

 from elsewhere, borne on the wind, such a case as a healthy crop standing alongside a badly diseased 

 one would be almost an impossibility. [The writer has made precisely similar observations on the 

 black rot of the cabbage. In this connection see Bacterium campestre, vol. II, p. 306.] 



On the other hand, the above facts are in harmony with the idea that the disease originates with 

 the seed the sets. * * * 



I was able to discover three cases on the Lower Clarence in which the crops were almost a total 

 failure on account of gumming, where the planters, now that they know the nature and injuriousness 

 of gumming, can recollect that the sets were badly gummed. They noticed the gum in the sets, 

 which, when bad, is indeed very conspicuous, but not then knowing its nature, went ahead and put in 

 the sets notwithstanding. These three cases are those of very intelligent farmers, to converse with 

 whom was to be convinced that they were quite right in their observations. In another case a farmer 

 purposely took sets from diseased plants in order to see whether they would reproduce the disease. 

 The disease was reproduced. I had an opportunity to examine the resulting plants, and can certify 

 to the result. 



Tryon speaks no less emphatically. According to his observations in Queensland the 

 disease sometimes occurs spontaneously in patches of cane planted in low places onundrained 

 land, but much more often it can be traced directly to the planting of cane-sets taken from 

 diseased fields. I quote from his paper, published in 1895, as follows: 



It is an established fact in connection with this gumming disease that pieces of affected cane, 

 when used for "sets," give rise to it in the resulting crop. The malady may be sooner or later in mani- 

 festing its presence, but, as a rule, its occurrence may be depended upon. Should the sets be badly 

 gummed, however, no crop at all is obtained, as, though they may emit slender sprouts, these soon 

 cease to be further developed. These facts have been demonstrated experimentally both at Winder- 

 mere and Fairymead, and they explain in great measure the present distribution of the disease. 

 * * * . Similarly, when a plantation was discovered in which no disease occurred * * * it 

 was found that drainage had been resorted to, the crops from which the seed cane had been procured 

 were perfectly healthy, or that no seed cane had been used except such as had been yielded by local 

 healthy crops. Many instances could be adduced of this method of propagating the disease, and even 

 just prior to my visit a considerable amount of unhealthy cane had been distributed from one Mary- 

 borough center to be used for starting fresh cane plots. In many cases having observed a stand of 

 diseased cane, and not discovering any explanation of its occurrence in local conditions of growth, a 

 visit was afterwards made to the plantation whence the seed cane from which this was raised had been 

 derived, when it was invariably found that the original stock was also diseased, and that other stands 

 of cane which it had also served to originate were similarly affected. * * * When the disease 

 appeared upon dry ridges, and no cane plants had been procured from beyond the plantation for sev- 

 eral years, it was generally found that it was also present in some low-lying spots, where it had evi- 

 dently originated spontaneously at an earlier date, and that its occurrence in one situation had led 

 through the use of local sets, to its manifestation in the other. 



My own experience tends to confirm Cobb's statement respecting transmission of the 

 disease. It is not readily spread from diseased to healthy plants, at least not in the hot- 

 house. 



Cobb stated to the writer that he had recommended the use of certain varieties whose 

 resistance to the disease he had observed, and wherever his advice had been followed the 

 disease had disappeared. 



