16 BULLETIN 829, U. S. DEPARTMENT OE AGRICULTURE. 



of the disease would be immeasurably complicated if it were to be- 

 come prevalent on such omnipresent weeds. Fortunately, however, 

 our observations appear to indicate that the grasses other than cane 

 become infected only under conditions favorable to the disease and 

 in the near vicinity of infected sugar-cane plants. Infected corn, 

 for instance, has been seen by the writer only in Porto Rico, where 

 it was growing between the rows of diseased cane stubble. Infected 

 rice plants were observed there only once, growing just across a 

 narrow dirt road from a badly attacked cane field. At Audubon 

 Park, La., attacked sorghum was seen in a similar situation, the 

 most remote plants being only about 3 rods from the cane, and the 

 percentage of attacked plants decreased in an inverse ratio to the 

 distance from the cane. The same was true of crab-grass, which 

 was abundant in the sorghum field. These observations are en- 

 couraging and tend to offset the disconcerting facts discussed above. 



NATURE OF THE DISEASE. 



INFECTION PHENOMENA. 



Sugar-cane mosaic is an infectious chlorosis, similar in many re- 

 spects to the mosaic diseases of tobacco, cucumber, bean, tomato, 

 and potato. Evidence of its infectious nature exists in hundreds of 

 field observations and in the infection of experimental plants under 

 controlled conditions. The well-defined epidemic in Porto Rico, in 

 which it has been established that the disease started in a small local 

 area and gradually spread from this focus of infection, diseased plants 

 being confined within the limits of the ever-increasing infested terri- 

 tory and not appearing sporadically at remote points, is convincing. 

 It leads to the inevitable conclusion that some virus or inoculum is 

 responsible for the appearance of new cases and that the only source 

 of inoculum is some plant previously infected with the disease. No 

 other explanation accounts satisfactorily for the observed facts. 

 Climatic conditions were at first suggested, but the epidemic has 

 lasted already for a period of years, during which rainfall, tempera- 

 ture, sunshine, and the other factors that go to make up climate 

 have been normal. The wearing out of soils was regarded as a 

 possible cause, but during the steady progress of the disease it gradu- 

 ally encroached upon every conceivable type of soil, including the 

 richest and most productive in the island. Strong support was 

 given to the idea that it was a case of deterioration or the "running 

 out" of varieties, but when it became evident that all varieties 

 present in the invaded district were affected, this idea was aban- 

 doned. For the same reason the hypothesis that it is a case of bud 

 variations, or "sports," seems highly improbable, and when the 

 regular progress of the epidemic is borne in mind, radiating outward 



