18 BULLETIN ?29, U. S. DEPARTMENT OE AGRICULTURE. 
by some principle present only in dieased plants is responsible for the 
appearance of the disease in formerly healthy individuals. 
TRANSMISSION OF MOSAIC IN DISEASED SEED PIECES. 
Experiments in Porto Eico * and elsewhere have repeatedly demon- 
strated that cuttings from infected stalks invariably give rise to 
infected plants. The young shoots are seen to be mottled as soon as 
they appear. These are referred to as primary infections. The 
fact is one of far-reaching importance, and to it must be attributed the 
spread of the disease to new regions, remote from any infected cane, 
by shipments of cane seed. The use of diseased stalks for propagating 
results in wider distribution of diseased plants on the same planta- 
tion from year to year and insures the survival of the virus, even in 
the absence of secondary infections. Transmission of the disease in 
cuttings is a fact, the importance of which can not be overemphasized 
in view of its obvious bearing on control measures. 
TRANSMISSION OF THE DISEASE BY CARRIERS. 
It can be proved mathematically that by the law of chance the 
percentage of diseased plants in a plantation would tend to remain 
stationary from year to year provided there was no conscious or 
unconscious selection, 2 if the spread of the disease depended wholly 
upon the use of infected cuttings. Nature has provided a far more 
efficient method for the quick dissemination of the malady. Second- 
ary infection, i. e., infection due to the inoculation of healthy plants 
during the growing season, goes on at a more or less rapid rate wher- 
ever the disease has been observed. Secondary infections are easily 
determined as such when the plants are young. In the case of plants 
infected in the greenhouse it has been determined that only the leaves 
which were immature at the time of inoculation and leaves subse- 
quently formed become mottled.* When a plant is found with normal 
leaves up to a certain point on the stalk and mottled leaves above 
that point it is a clear case of secondary infection. Since in older 
plants the lower leaves are gradually sloughed off until only a 
relatively small terminal tuft of the youngest leaves remain when the 
plant approaches maturity, this method is obviously limited to 
young plants or to plants with green leaves still present above and 
below the point of inoculation. 
The rate of spread of the disease, as indicated by these secondary 
infections, varies greatly. Fields are frequently seen in which there 
has been apparently no secondary infection during an entire growing 
i Stevenson, John A. The "mottling" disease of cane. Porto Rico Insular Exp. Sta. Ann. Rpt. 1916-17, 
p. 40-77. 1917. [Literature], p. 76-77. 
2 Selection is employed where the disease is not recognized. During the beginning of the epidemic in 
Porto Rico, when sugar was bringing an unprecendented price, it was learned that the manager of one of the 
mills was instructed to grind the best cane and save the poorest for seed. The '■' poorest ' ' was undoubtedly 
that attacked by mosiac. 
