DISEASES OF PLANTS 
the change and can live under the modi- 
fied conditions with greater ease and safe- 
ty. 
If the variation is greater, the growth 
of the plant may be decidedly checked, 
the leaves being small and many more 
of them shed. In still more extreme 
cases the tender leaves may be dried up 
and killed either wholly or in part. Ac- 
cording to Hartig, “It is only when the 
sickly condition leads to the death of 
some part of the plant that we may speak 
of actual disease.” Where a few leaves, 
unable to adapt themselves to a changed 
condition, turn yellow and fall, the 
leaves themselves may be diseased, but 
the plant as a whole is benefited by their 
loss as being relieved of sources of un- 
controlled drain of its water supply. As 
the loss of leaves becomes greater, how- 
ever, we pass from the extremely localized 
disease to a point where the whole or a 
considerable part of the plant is weak- 
ened, either by the direct loss of food that 
should be furnished by the leaves to the 
rest of the plant structure, or by the use 
of reserve food in the reproduction of 
lost parts. It is evident in such cases 
that the border line between health and 
disease is hard to define. The case is 
not much easier if, instead of variations 
produced by moisture and temperature, we 
consider those caused by insects or fun- 
gi. A few leaves eaten from a tree by 
some insect or destroyed by a fungus 
might have no injurious effect on the tree 
as a whole, and might even be an advan- 
tage, but as the number of injured leaves 
increases the tree is weakened and its 
life threatened. Slight doses of certain 
poisons stimulate the cells to more vigor- 
ous growth, acting as a tonic, while a 
little larger dose poisons and destroys 
the cell. Leaving all questions of con- 
sistency of definition, we may practically 
define as diseased all those conditions of 
a plant which directly or indirectly en- 
danger its life or prevent normal devel- 
opment under given conditions of environ- 
ment. Or, as Marshall Ward puts it, “We 
may define disease as dangerous disturb- 
ances in the regularity, or interference 
with the completeness or range of the 
molecular activities constituting normal 
891 
life—that is, health—and it is evident 
that every degree of transition may be 
realized between the two extremes.” 
Prevention 
Successful treatment of plant diseases 
consists in preventing the spread of the 
disease and not in curing the plants al- 
ready affected. The tiny thread-like 
plants—the fungi—which cause diseases, 
grow inside the tissue of the leaves, 
stems, fruit, etc., of the plants, which 
they attack, and after they have gained 
entrance there it is impossible to reach 
them or to treat the tissue which they 
are destroying. These fungi perpetuate 
themselves by producing myriads of tiny 
seed-like reproductive bodies—-spores— 
which are so tiny as to be invisible and 
so light that they float about everywhere 
in the air. They are thus carried from 
one plant to another by the wind, and 
where they lodge on a leaf or stem and 
find conditions favorable they germinate 
and grow. Disease is thus scattered from 
plant to plant and from field to field. 
These spores are always produced on the 
diseased areas of affected plants, and for 
this reason where it is possible to do so 
all diseased parts should be collected and 
burned as soon as the disease appears on 
them. If this could be done with all 
plants and all diseases they could be elim- 
inated at one clean sweep. Unfortunately, 
some of these fungi live over in the soil 
or in fragments of decaying plants, which 
cannot be collected by any practical 
means. In such cases we have to resort 
to other means of controlling them. One 
way of doing this is by using disease 
resistant varieties. Certain individuals 
and certain varieties of plants are more 
resistant to disease than are other indi- 
viduals and varieties. By planting seed 
from such individuals, and by continually 
discarding the plants which succumb to 
the disease, we originate a disease-resist- 
ant strain or variety. In some cases this 
is simple and can be practiced by any 
one; in other cases where the plants, such 
as trees, are long-lived, and we have to 
wait a long time for results, it is objec- 
tionable, and we have to resort to some 
more artificial method, such as spraying. 
