troduction.

The spots on the leaves and fruits are not deposits of dirt nor are
they caused by mysterious conditions in the atmosphere, as once
supposed, nor is it in the nature of leaves to be spotted and of
fruits to be scabby; nor are the one-sided dwarfed fruits merely
accidents. The organism responsible for these blemishes is less
evident than the codlin-moth; yet what fruit-grower knows the eggs of
the codlin-moth? But the organisms are as definite as are the insects;
no longer are the fungi things without form and without positive
cycles.

On the ground are apple leaves, shed in the autumn. On the leaves are
spots or lesions,--injured or "diseased"--infected with the apple-scab
fungus. Under a good microscope the investigator finds immature
fruiting bodies in these areas. In the early days of Spring, these
bodies or winter-spores mature. A rain discharges them in astonishing
numbers. Rising in the air (for they are incredibly light), these
spores lodge on the unfolding leaves and flowers of the apple, and
there begin to germinate, invading the tissue. The tissue is
penetrated and killed so rapidly that the practiced eye soon discovers
a "spot." The leaf, if badly infected, may not reach full size; it may
curl; it may die and fall; the tree thereby is injured.

From the fungus in the active diseased areas, another kind of spore
develops rapidly. It is the summer-spore, which may be produced in
prodigious numbers, and being discharged carries the disease
elsewhere.

All summer the process of spore-formation and distribution keeps up.
If conditions are favorable, the tree is invaded in foliage and fruit.
The flower-stems in the unfolding buds are attacked by the
winter-spores and the flower falls. The apples become spotted from the
invasion of the summer-spores, perhaps misshapen. Late infections may
not show at picking time, but develop on the fruit in storage. The
affected leaves are cast in the autumn, the winter-spores begin to
form, the snows come and hide the processes, in spring the spores
mature; and so does the round of life go on and on.

There are beautiful forms in these fragile fungus threads that eat
their way into the tissues of the host. There are fascinating
phenomena in the growth and reproduction. Even so and for all that,
man protects his tree by spraying it with poison, and thereby again
does he have dominion.

The spraying for apple-scab is with lime-sulfur to which may be added
arsen