506 
Journal of Agricultural Research 
Vol. XXIX, No. 10 
season when the symptoms were most 
pronounced the mosaic could only be 
found by examining the basal or young¬ 
er portion of the leaves. As the leaves 
grew out the light-colored stripes dis¬ 
appeared and the whole leaf surface 
took on a normal dark green color. 
In Plate 1, C, are shown leaves from 
the selected and the unselected rows on 
September 15, 1922. All the leaves 
are from mosaic-affected plants, but 
none of the symptoms are apparent on 
the leaf taken from the selected row. 
The selected rows also showed a some¬ 
what larger yield and it is probable 
that this increase was largely due to 
the decreased damage from mosaic. 
DISCUSSION 
The tests with the D 74 and the 
Purple canes have demonstrated that in 
fields with a 100 per cent infection 
plants can be selected that show a 
marked tolerance to the mosaic. How 
this has come about is as yet not 
clear. It is uncertain whether this is 
due to slight variations of the host 
with the variants more resistant, or 
whether the plant is gradually ac¬ 
quiring an* immunity somewhat similar 
to acquired immunity in man and 
animals. As far as usefulness in the 
control of the mosaic is concerned, it is 
immaterial which explanation is cor¬ 
rect. 
If resistant strains are developing 
in nature, it is only a matter of time 
until the low-yielding, highly sus¬ 
ceptible plants will be eliminated. 
Each year the percentage of stalks from 
tolerant canes would be greater and 
naturally a greater percentage of such 
would be used for seed. There is some 
evidence that this may be taking place 
in some sections of Louisiana where 
the oldest infections occur. 
By discarding the susceptible and 
selecting only the most tolerant canes 
for seed, it is possible that our present 
varieties can be developed to a tolerant 
condition in a much shorter space of 
time than if natural selection is al¬ 
lowed to take its course. 
This paper is not an argument for 
allowing the disease to take its natural 
course in all sugar countries. In those 
regions where the disease can be kept 
under control by roguing or by the use 
of clean seed it would be folly to let the 
mosaic go and try to select resistant 
varieties, but in places like Louisiana 
where no clean seed is available and 
roguing has been a failure this offers a 
possible solution of the problem, at least 
until satisfactory resistant varieties 
become available. 
