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 
