into the garden or orchard, as it may
cause disease the following year.

Second, you can often kill spores on seeds before they are planted and
thus prevent the development of the fungus (see pp. 134-137).

Third, often the foliage of the plant can be sprayed with a poison that
will prevent the germination of the spores (see pp. 138-140).

Fourth, some varieties of plants resist disease much more stoutly than
others. We may often select the resistant form to great advantage (see
Fig. 119).

Fifth, after big limbs are pruned off, decay often sets in at the wound.
This decay may be prevented by coating the cut surface with paint, tar,
or some other substance that will not allow spores to enter the wound or
to germinate there.

Sixth, it frequently happens that the spore or fungus remains in the
soil. This is true in the cotton wilt, and the remedy is so to rotate
crops that the diseased land is not used again for this crop until the
spores or fungi have died.

SECTION XXX. SOME SPECIAL PLANT DISEASES

=Fire-Blight of the Pear and Apple.= You have perhaps heard your father
speak of the "fire-blight" of pear and apple trees. This is one of the
most injurious and most widely known of fruit diseases. Do you want to
know the cause of this disease and how to prevent it?

First, how will you recognize this disease? If the diseased bough at
which you are looking has true fire-blight, you will see a blackened
twig with withered, blackened leaves. During winter the leaves do not
fall from blighted twigs as they do from healthy ones. The leaves wither
because of the diseased twig, not because they are themselves diseased.
Only rarely does the blight really enter the leaf. Sometimes a sharp
line separates the blighted from the healthy part of the twig.

This disease is caused by bacteria, of which you have read in another
section. The fire-blight bacteria grow in the juicy part of the stem,
between the wood and the bark. This tender, fresh layer (as explained on
page 79) is called the _cambium_, and is the part that breaks away and
allows you to slip the bark off when you make your bark whistle in the
spring. The growth of new wood takes place in the cambium, and this part
of the twig is therefore full of nourishment. If this nourishment is
stolen the plant of course soon suffers.

The bacteria causing fire-blight are readily carried from flower to
flower and from twig to twig by insects; therefore to keep these and
ot