Nov. 15, 1924 
Resistance to the Sugar Cane Mosaic 
503 
SELECTING FOR MOSAIC TOLERANCE 
The work on the problem of sugar¬ 
cane mosaic which is considered by the 
writers to be the most important is the 
attempt to obtain more resistant or 
tolerant strains of our common com¬ 
mercial varieties. It is recognized 
that it is difficult to change or to im¬ 
prove a plant that is propagated 
vegetatively, yet there seeemed to be 
two possibilities worthy of testing. (1) 
Will sugar-cane varieties which show a 
moderate susceptibility or a moderate 
tolerance to mosaic vary enough, 
possibly by bud variation, to produce 
some plants or strains which will be¬ 
come more tolerant than the rest? 
Previous work on vegetatively prop¬ 
agated plants, such as potatoes, has 
shown that but little can be done in 
improving a variety, and it seemed 
very doubtful if any selection would 
improve the resistance of sugar cane. 
(2) Will a plant that is affected with 
mosaic, if it is propagated for a number 
of years, gradually acquire more re¬ 
sistance or possibly an immunity to 
the disease? The acquiring of im¬ 
munity is a common thing in human 
and animal diseases, but no example 
of a plant acting in a similar manner 
has ever been demonstrated. It would 
seem, however, if a plant could acquire 
an immunity to any disease it would be 
to a mosaic. 
Experiments to test these two 
possibilities were begun in the fall of 
1920. At planting time selections 
were made in fields of D 74 and Purple 
cane in a region that had had for 
several years a 100 per cent infection of 
mosaic. Stools showing the least in¬ 
jury from mosaic were selected. No 
cane showing a total freedom from the 
disease was used, for healthy cane was 
not desired. What was wanted was 
infected cane showing a minimum 
amount of damage. This cane was 
planted by the side of unselected cane 
in a field showing a maximum infection. 
In the fall of 1921 the best cane from 
the selected plat was used for planting, 
and from this in the fall of 1922 the 
cane showing the least damage from 
mosaic was again planted. The results 
for the three years 1921, 1922, and 1923 
are now available. Counts of the 
mosaic were made at frequent intervals 
and the comparative yields were ob¬ 
tained for the last two years. The 
results are shown in tabular form. 
RESULTS WITH D 74 CANE 
On the basal or younger portion of a 
D 74 leaf the mosaic shows in the 
rather usual form of somewhat lighter 
1339—25f-3 
colored stripes or areas, but as the 
leaves grow out portions of these 
stripes tend to fade out and appear in 
the form of light-colored, sometimes 
nearly white, dots or slightly elongated 
spots (PI. 1, A, a, b). On canes show¬ 
ing least damage from mosaic these 
spots may be so reduced during the 
latter part of the growing season that 
it is difficult to say whether the mosaic 
symptoms are present (PI. 1, B, a). 
With this variety canes bearing the 
smallest and fewest light-colored dots 
or areas were selected for planting. 
The results obtained from the selection 
work with the D 74 variety are given 
in Table III. 
The contrast between the selected 
and the unselected rows in the field was 
much more striking than appears in 
the figures in the table. The selected 
rows were darker green in color 
and the canes were not only larger but 
had stooled more. An examination of 
the individual plants showed that those 
in the selected rows did not have the 
mosaic symptoms in nearly so marked 
a form as in the unselected. Although 
all of the plants were known to be af¬ 
fected at the time some of the examina¬ 
tions were made, it was absolutely 
impossible to be certain that mosaic 
symptoms were present on some of 
them. While it is difficult to show 
sugar-cane mosaic in a black-and-white 
illustration, an attempt has been made 
in Plate 1, A and B, to bring out the 
contrast between the leaves from the 
selected and the unselected rows. 
These illustrations were made from 
photographs taken on September 15, 
1922. Plate 1, A, shows the white 
dotted appearance very common with 
this variety and also the smaller dots 
on the leaf from the selected row. In 
Plate 1, B, which illustrates the more 
common condition of the leaves in 
these tests, the white dots from the 
selected row (a) are very few while the 
mosaic symptoms on the leaves from 
the unselected rows (b and c) are very 
obvious. These leaves do not show an 
exaggerated comparison, but are some¬ 
where near the average. 
How much of the 32 and 34 per cent 
increases in yield shown in 1922 and 
1923 is due to the reduced effect of the 
mosaic is a question. It may be 
claimed that other factors of health 
and vigor were unconsciously con¬ 
sidered when the selections were made 
each year. For the 1923 test no stool 
was used unless it contained at least 
four good stalks. There is little doubt, 
however, that a portion of the in¬ 
creased yield was due to the less 
serious damage caused by the mosaic. 
