Chapter XXXIV: 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 other bacteria away from your trees you must see to it that all the trees in the neighborhood of your orchard are kept free from mischievous enemies. If harmful bacteria exist in near-by trees, insects will carry them to your orchard. You must therefore watch all the relatives of the pear; namely, the apple, hawthorn, crab, quince, and mountain ash, for any of these trees may harbor the germs.

When any tree shows blight, every diseased twig on it must be cut off and burned in order to kill the germs, and you must cut low enough on the twig to get all the bacteria. It is best to cut a foot below the blackened portion. If by chance your knife should cut into wood containing the living germs, and then you should cut into healthy wood with the same knife, you yourself would spread the disease. It is therefore best after each cutting to dip your knife into a solution of carbolic acid. This will kill all bacteria clinging to the knife-blade. The surest time to do complete trimming is after the leaves fall in the autumn, as diseased twigs are most easily recognized at that time, but the orchard should be carefully watched in the spring also. If a large limb shows the blight, it is perhaps best to cut the tree entirely down. There is little hope for such a tree.

A large pear-grower once said that no man with a sharp knife need fear the fire-blight. Yet our country loses greatly by this disease each year.

It may be added that winter pruning tends to make the tree form much new wood and thus favors the disease. Rich soil and fertilizers make it much easier in a similar way for the tree to become a prey to blight.

=EXERCISE=

Ask your teacher to show you a case of fire-blight on a pear or
apple tree. Can you distinguish between healthy and diseased wood?
Cut the twig open lengthwise and see how deep into the wood and how
far down the stem the disease extends. Can you tell surely from the
outside how far the twig is diseased? Can you find any twig that
does not show a distinct line of separation between diseased and
healthy wood? If so, the bacteria are still living in the cambium.
Cut out a small bit of the diseased portion and insert it under the
bark of a healthy, juicy twig within a few inches of its tip and
watch it from day to day. Does the tree catch the disease? This
experiment may prove to you how easily the disease spreads. If you
should see any drops like dew hanging from diseased twigs, touch a
little of this moisture to a healthy flower and watch for results.

Cut and burn all diseased twigs that you can find. Estimate the
damage done by fire-blight.

Farmers' bulletins on orchard enemies are published by the
Department of Agriculture, Washington, D.C., and can be had by
writing for them. They will help your father much in treating
fire-blight.

=Oat Smuts.= Let us go out into a near-by oat field and look for all the blackened heads of grain that we can find. How many are there? To count accurately let us select an area one foot square. We must look carefully, for many of these blackened heads are so low that we shall not see them at the first glance. You will be surprised to find as many as thirty or forty heads in every hundred so blackened. These blackened heads are due to a plant disease called _smut_.

When threshing-time comes you will notice a great quantity of black dust coming from the grain as it passes through the machine. The air is full of it. This black dust consists of the spores of a tiny fungous plant. The fungous smut plant grows upon the oat plant, ripens its spores in the head, and is ready to be thoroughly scattered among the grains of the oats as they come from the threshing-machine.

These spores cling to the grain and at the next planting are ready to attack the sprouting plantlet. A curious thing about the smut is that it can gain a foothold only on very young oat plants; that is, on plants about an inch long or of the age shown in Fig. 121.

When grain covered with smut spores is planted, the spores develop with the sprouting seeds and are ready to attack the young plant as it breaks through the seed-coat. You see, then, how important it is to have seed grain free from smut. A substance has been found that will, without injuring the seeds, kill all the smut spores clinging to the grain. This substance is called _formalin_. Enough seed to plant a whole acre can be treated with formalin at a cost of only a few cents. Such treatment insures a full crop and clean seed for future planting. Try it if you have any smut.

Fig. 122 illustrates what may be gained by using seeds treated to prevent smut. The annual loss to the farmers of the United States from smut on oats amounts to several millions of dollars. All that is needed to prevent this loss is a little care in the treatment of seed and a proper rotation of crops.

=EXERCISE=

Count the smutted heads on a patch three feet square and estimate
the percentage of smut in all the wheat and oat fields near your
home. On which is it most abundant? Do you know of any fields that
have been treated for smut? If so, look for smut in these fields.
Ask how they were treated. Do you know of any one who uses
bluestone for wheat smut? Can oats be treated with bluestone?

At planting time get an ounce of formalin at your drug store or
from the state experiment station. Mix this with three gallons of
water. This amount will treat three bushels of seeds. Spread the
seeds thinly on the barn floor and sprinkle them with the mixture,
being careful that all the seeds are thoroughly moistened. Cover
closely with blankets for a few hours and plant very soon after
treatment. Try this and estimate the per cent of smut at next
harvest-time. Write to your experiment station for a bulletin on
smut treatment.

=Potato Scab.= The scab of the white, or Irish, potato is one of the commonest and at the same time most easily prevented of plant diseases. Yet this disease diminishes the profits of the potato-grower very materially. Fig. 123 shows a very scabby potato, while Fig. 124 represents a healthy one. This scab is caused by a fungous growth on the surface of the potato. Of course it lessens the selling-price of the potatoes. If seed potatoes be treated to a bath of formalin just before they are planted, the formalin will kill the fungi on the potatoes and greatly diminish the amount of scab at the next harvest. Therefore before they are planted, seed potatoes should be soaked in a weak solution of formalin for about two hours. One-half pint of formalin to fifteen gallons of water makes a proper solution.

One pint of formalin, or enough for thirty gallons of water, will cost but thirty-five cents. Since this solution can be used repeatedly, it will do for many bushels of seed potatoes.

=Late Potato Blight.= The blight is another serious disease of the potato. This is quite a different disease from the scab and so requires different treatment. The blight is caused by another fungus, which attacks the foliage of the potato plant. When the blight seriously attacks a crop, it generally destroys the crop completely. In the year 1845 a potato famine extending over all the United States and Europe was caused by this disease.

Spraying is the remedy for potato blight. Fig. 128 shows the effect of spraying upon the yield. In this case the sprayed field yielded three hundred and twenty-four bushels an acre, while the unsprayed yielded only one hundred bushels to an acre. Fig. 127 shows the result of three applications of the spraying mixture on the diseased field. Figs. 129 and 130 show how the spraying is done.

=EXERCISE=

Watch the potatoes at the next harvest and estimate the number that
is damaged by scab. You will remember that formalin is the
substance used to prevent grain smuts. Write to your state
experiment station for a bulletin telling how to use formalin, as
well as for information regarding other potato diseases. Give the
treatment a fair trial in a portion of your field this year and
watch carefully for results. Make an estimate of the cost of
treatment and of the profits. How does the scab injure the value of
the potato? The late blight can often be recognized by its odor.
Did you ever smell it as you passed an affected field?

=Club Root.= Club root is a disease of the cabbage, turnip, cauliflower, etc. Its general effect is shown in the illustration (Fig. 131). Sometimes this disease does great damage. It can be prevented by using from eighty to ninety bushels of lime to an acre.

=Black Knot.= Black knot is a serious disease of the plum and of the cherry tree. It attacks the branches of the tree; it is well illustrated in Fig. 132. Since it is a contagious disease, great care should be exercised to destroy all diseased branches of either wild or cultivated plums or cherries. In many states its destruction is enforced by law. All black knot should be cut out and burned some time before February of each year. This will cost little and save much.

=Peach Leaf Curl.= Peach leaf curl does damage amounting to about $3,000,000 yearly in the United States. It can be almost entirely prevented by spraying the tree with Bordeaux mixture or lime-sulphur wash before the buds open in the spring. It is not safe to use strong Bordeaux mixture on peach trees when they are in leaf.

=Cotton Wilt.= Cotton wilt when it once establishes itself in the soil completely destroys the crop. The fungus remains in the soil, and no amount of spraying will kill it. The only known remedy is to cultivate a resistant variety of cotton or to rotate the crop.

=Fruit Mold.= Fruit mold, or brown rot, often attacks the unripe fruit on the tree, and turns it soft and brown and finally fuzzy with a coat of mildew. Fig. 133 shows some peaches thus attacked. Often the fruits do not fall from the trees but shrivel up and become "mummies" (Fig. 134). This rot is one of the most serious diseases of plums and peaches. It probably diminishes the value of the peach harvest from 50 to 75 per cent. Spraying according to the directions in the Appendix will kill the disease.