MOSAIC OF SUGAR CANE AND OTHER GRASSES. 19 



season. As an extreme case illustrating this point, the fields near 

 Cienfuegos, Cuba, may be cited. There the disease has merely sur- 

 vived by the planting of infected seed pieces, and secondary infection, 

 if it. goes on at all, is certainly very limited. Even in Porto Rico, 

 during the height of the epidemic, secondary infection was at a 

 standstill in some localities for a year or more. On the contrary, 

 whole fields of healthy cane became infected in the short space of a 

 month or two. Such a case was the invasion of the variety test field 

 at Santa Rita, Porto Rico, previously mentioned. No doubt the 

 explanation for this great variation in rate of spread by secondary 

 infection must be sought in the mechanics of inoculation. Up to 

 the present no positive proof of the method by which inoculation is 

 accomplished in nature has been brought forward. Reasoning from 

 the fact that new cases often appear at some distance from diseased 

 individuals, it would seem that some agent or carrier is necessary. 

 Mere contact of diseased and healthy plants does not serve to com- 

 municate the infection from the former to the latter. In no case has 

 the planting of healthy cuttings in the same pots with diseased plants 

 resulted in the new plants becoming diseased. The same holds true 

 for plants in the field, where healthy plants are often seen with their 

 leaves mingling freely with the leaves of diseased plants for a time 

 much longer than the incubation period for mosaic, but with no 

 evidence of transference of the inoculum. It is evident that special 

 conditions are necessary in order that the disease can be communi- 

 cated to healthy plants. 



Field observations indicate that acceleration in the spread of the 

 mosaic disease is accompanied with or preceded by severe insect infesta- 

 tions. The cane leafhopper (Tettigonia sp.) in particular has been 

 noticed to accompany the rapid spreading of the disease. This evidence 

 is incomplete, but it is supported by the fact that 10 healthy plants 

 placed in insect-proof cages in the greenhouse at Garrett Park, Md., 

 did not contract the disease, while five control plants outside of the 

 cages, but otherwise under identical conditions, all became infected. 

 Aphids were abundant on the diseased cane in this greenhouse, and 

 a few leafhoppers were present. A great deal of experimental work 

 remains to be done before formal proof of the responsibility of any 

 particular insect or insects for the transmission of the disease can 

 be offered. 



SOIL RELATIONS. 



There has been no indication that the contagion persists in the soil 

 after a crop has been removed and the stubble plowed up. Fields 

 that have been veritable hotbeds of infection after being plowed up 

 and planted with clean seed have only a few scattered cases, which 

 can be accounted for by faulty seed selection. Healthy cuttings 

 planted in the soil of pots from which badly diseased specimens had 



