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The Cause And Nature Of Plant Disease

Plants have diseases just as animals do; not the same diseases, to be
sure, but just as serious for the plant. Some of them are so dangerous
that they kill the plant; others partly or wholly destroy its usefulness
or its beauty. Some diseases are found oftenest on very young plants,
others prey on the middle-aged tree, while still others attack merely
the fruit. Whenever a farmer or fruit-grower has disease on his plants,
he is sure to lose much profit.

You have all seen rotten fruit. This is diseased fruit. Fruit rot is a
plant disease. It costs farmers millions of dollars annually. A
fruit-grower recently lost sixty carloads of peaches in a single year
through rot which could have been largely prevented if he had known how.

Many of the yellowish or discolored spots on leaves are the result of
disease, as is also the smut of wheat, corn, and oats, the blight of the
pear, and the wilt of cotton. Many of these diseases are contagious, or,
as we often hear said of measles, "catching." This is true, among
others, of the apple and peach rots. A healthy apple can catch this
disease from a sick apple. You often see evidence of this in the apple
bin. So, too, many of the diseases found in the field or garden are

Sometimes when the skin of a rotten apple has been broken you will find
in the broken place a blue mold. It was this that caused the apple to
decay. This mold is a living plant; very small, certainly, but
nevertheless a plant. Let us learn a little about molds, in order that
we may better understand our apple and potato rots, as well as other
plant diseases.

If you cut a lemon and let it stand for a day or two, there will
probably appear a blue mold like that you have seen on the surface of
canned fruit. Bread also sometimes has this blue mold; at other times
bread has a black mold, and yet again a pink or a yellow mold.

These and all other molds are tiny living plants. Instead of seeds they
produce many very small bodies that serve the purpose of seeds and
reproduce the mold. These are called _spores_. Fig. 112 shows how they
are borne on the parent plant.

The single stalk on the left shows how spores are borne]

It is also of great importance to decide whether by keeping the spores
away we may prevent mold. Possibly this experiment will help us. Moisten
a piece of bread, then dip a match or a pin into the blue mold on a
lemon, and draw the match across the moist bread. You will thus plant
the spores in a row, though they are so small that perhaps you may not
see any of them. Place the bread in a damp place for a few days and
watch it. Does the mold grow where you planted it? Does it grow
elsewhere? This experiment should prove to you that molds are living
things and can be planted. If you find spots elsewhere, you must bear in
mind that these spores are very small and light and that some of them
were probably blown about when you made your sowing. When you touch the
moldy portion of a dry lemon, you see a cloud of dust rise. This dust is
made of millions of spores.

If you plant many other kinds of mold you will find that the molds come
true to the kind that is planted; that like produces like even among

You can prove, also, that the mold is caused only by other mold. To do
this, put some wet bread in a wide-mouthed bottle and plug the mouth of
the bottle with cotton. Kill all the spores that may be in this bottle
by steaming it an hour in a cooking-steamer. This bread will not mold
until you allow live mold from the outside to enter. If, however, at any
time you open the bottle and allow spores to enter, or if you plant
spores therein, and if there be moisture enough, mold will immediately
set in.

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