SECTION XXIX. PREVENTION OF PLANT DISEASE
In the last two sections you have learned something of the nature of those fungi and bacteria that cause disease in animals and plants. Now let us see how we can use this knowledge to lessen the diseases of our crops. Farmers lose through plant diseases much that could be saved by proper precaution.
First, you must remember that every diseased fruit, twig, or leaf bears millions of spores. These must be destroyed by burning. They must not be allowed to lie about and spread the disease in the spring. See that decayed fruit in the bin or on the trees is destroyed in the same manner. Never throw decayed fruit 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.