Part 14 (1/2)
SECTION x.x.x. 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.
[Ill.u.s.tration: FIG. 119. A RESISTANT VARIETY OF SEA ISLAND COTTON All the other plants in this field died. This one row lived because it could resist the cotton wilt]
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 tr.i.m.m.i.n.g 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.
[Ill.u.s.tration: FIG. 120. FIRE-BLIGHT BACTERIA Magnified]
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, Was.h.i.+ngton, D.C., and can be had by writing for them. They will help your father much in treating fire-blight.
=Oat s.m.u.ts.= 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 _s.m.u.t_.
[Ill.u.s.tration: FIG. 121. LOOSE s.m.u.t OF OATS The glumes at _a_ more nearly destroyed than the glumes at _b_]
When thres.h.i.+ng-time comes you will notice a great quant.i.ty of black dust coming from the grain as it pa.s.ses through the machine. The air is full of it. This black dust consists of the spores of a tiny fungous plant.
The fungous s.m.u.t 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 thres.h.i.+ng-machine.
These spores cling to the grain and at the next planting are ready to attack the sprouting plantlet. A curious thing about the s.m.u.t 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 s.m.u.t 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 s.m.u.t. A substance has been found that will, without injuring the seeds, kill all the s.m.u.t 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 s.m.u.t.
[Ill.u.s.tration: FIG. 122. A CROP FROM OATS TREATED WITH FORMALIN]
Fig. 122 ill.u.s.trates what may be gained by using seeds treated to prevent s.m.u.t. The annual loss to the farmers of the United States from s.m.u.t 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.