SECTION XXVII. 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.
Fig. 112. Tangled Threads of Blue Mold
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 contagious.
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.
Fig. 113. Magnified Rose Mildew
Fig. 114. A Mildewed Rose
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 molds.
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.
Fig. 115. A Highly Magnified Section of Diseased Pear Leaf
Showing how spores are borne
The little plants which make up these molds are called fungi. Some fungi, such as the toadstools, puffballs, and devil's snuff-box, are quite large; others, namely the molds, are very small; and others are even smaller than the molds. Fungi never have the green color of ordinary plants, always reproduce by spores, and feed on living matter or matter that was once alive. Puffballs, for example, are found on rotting wood or dead twigs or roots. Some fungi grow on living plants, and these produce plant disease by taking their nourishment from the plant on which they grow; the latter plant is called the host.
The same blue mold that grows on bread often attacks apples that have been slightly bruised; it cannot pierce healthy apple skin. You can plant the mold in the bruised apple just as you did on bread and watch its rapid spread through the apple. You learn from this the need of preventing bruised or decayed apples from coming in contact with healthy fruit.
Fig. 116. Spores of the Pear Scab
The spores are borne on stalks
Just as the fungus studied above lives in the apple or bread, so other varieties live on leaves, bark, etc. Fig. 113 represents the surface of a mildewed rose leaf greatly magnified. This mildew is a fungus. You can see its creeping stems, its upright stalk, and numerous spores ready to fall off and spread the disease with the first breath of wind. You must remember that this figure is greatly magnified, and that the whole portion shown in the figure is only about one tenth of an inch across. Fig. 114 shows the general appearance of a twig affected by this disease.
Mildew on the rose or on any other plant may be killed by spraying the leaves with a solution of liver of sulphur; to make this solution, use one ounce of the liver of sulphur to two gallons of water.
The fungus that causes the pear-leaf spots has its spores in little pits (Fig. 115). The spores of some fungi also grow on stalks, as shown in Fig. 116. This figure represents an enlarged view of the pear scab, which causes so much destruction.
You see, then, that fungi are living plants that grow at the expense of other plants and cause disease. Now if you can cover the leaf with a poison that will kill the spore when it comes, you can prevent the disease. One such poison is the Bordeaux (bôr-dō') mixture, which has proved of great value to farmers.
Since the fungus in most cases lives within the leaves, the poison on the outside does no good after the fungus is established. The treatment can be used only to prevent attack, not to cure, except in the case of a few mildews that live on the outside of the leaf, as does the rose mildew.
EXERCISE
Why do things mold more readily in damp places? Do you now understand why fruit is heated before it is canned? Try to grow several kinds of mold. Do you know any fungi which may be eaten?
Transfer disease from a rotten apple to a healthy one and note the rapidity of decay. How many really healthy leaves can you find on a strawberry plant? Do you find any spots with reddish borders and white centers? Do you know that this is a serious disease of the strawberry? What damage does fruit mold do to peaches, plums, or strawberries?
Write to your experiment station for bulletins on plant diseases and methods for making and using spraying mixtures.