192i:\ SUGAR CANE PESTS ANT) DISEASES. 119 



THE NATURAL SPREAD OF THE DISEASE. 



Observations in the field in Trinidail diiring the year 1920 indicate 

 that there was practically no spread of the infection from diseased to 

 liealthy plants during dry weather. 



In one plot of about 360 stools at St. Augustine 63 plants were 

 marked as diseased on March 3, 1920, and by July 14 only three additional 

 plants were found to be infected. In another bed in which 14 stools out 

 of about 360 were infected on March 30 there was only one fresh infection 

 by July 14. 



The wet season in 1920 was minsually late in starting and the first 

 heavy rains fell about June 15 to 18, so that for about one month after 

 the rains there was little visible spread. 



This particular experiment was stopped at the end of July in an 

 attempt to exterminate the disease from the Experiment Station, 

 but field observation showed a rapid increase in the disease dviring 

 August and September and after a period of rest there were slight indica- 

 tions of another increase about November. This latter is somewhat 

 doubtful. 



Further indications that the spread of the disease was in some way 

 connected with moisture were found. Thus on one estate in the 

 Couva district the greatest spread wtu? found to hav3 occurred along 

 the banks of a small river. 



On one estate in the Naparimas where the infected stools had been 

 planted in long beds running down a slope and across a small flat moist 

 area at the bottom of the slope, the original infection (due to planting 

 infected cuttings) was evenly distributed on both slope and flat, but the 

 secondary infection was considerably greater on the flat than on the 

 slope, 



Mr. C. M. Roach, who spent most of his time organising the control 

 of the disease among farmers' canes in the St. Augustine district reported 

 independently that he thought the disease more prevalent in hollows 

 •than on slopes. 



Insect Transmission. 



There now seems to be no doubt that the disease is spread from 

 ■ diseased to healthy plants chiefly, if not entirely, through the agency of 

 insects. It remains to determine which insects are capable of trans- 

 mitting the infection. 



E. W. Brandes conclusively proved that in the United States Aphis 

 tnaidis carries the infection (Jonryi. Afjric. Hesearcli, 1920. XIX. 135). 

 E. G. Smyth has obtained in Porto Rico less conclusive evidence 

 that the disease can be carried by the cane fiy (StenocrannH sacchari- 

 vorons, the cane leaf scale [L'nlvinaria iceryi), the yellow cane Aphis 

 {Siphii flova) and a mealy bug (Pscuclococcus calccolaricc) or 

 P.sacchari). {Journ. Dept. Agr. I'orto liico. III. 83-116). 



In Trinidad the insect carrier is still unknown. Aphids do not seem 

 to exist on the sugar cane. I have not seen the leaf scale (Fulviiiaria) 

 and the cane fly (Stenocranus) is extremely rare. 



