Beyond all doubt what is known as fire-blight of the apple, pear, and quince is caused by bacterial growth in the cell-structure of the leaf, blossom, and outer wood. Yet long-continued observation has demonstrated that the color of the soil, elevation, exposure, and soil-covering have much to do with its occurrence and spread in a given climate. Other things being equal, it is found that soil-shading during the heated term by a cover-crop of buckwheat, vetch, rape, cow-peas, or soy-beans, will show less blight with given varieties than the orchard with bare soil during July and August.
It is also true in the prairie States that a given variety of the apple, such as Yellow Transparent, will be free from blight on light-colored ridge soil without shelter from winds, while on a lower level with darker soil and sheltered from winds it will be blackened with the disease. In the same line it is also true that in cooler moister climates like that of west Europe, from whence most of our fruits were derived, fire-blight is not known.
In these cases, and others now well known, we have reason to believe that abnormal heat of soil and air have much to do with the growth and development of this bacterial disease. Professor M. Fremy, of France, is a recognized scientist and a close observer. He says that fire-blight of the apple and pear only occurs in France when the trees are trained on south walls where subjected to abnormal heat. His opinion founded on chemical investigation was decided that ferment of the pectose preceded the introduction and growth of the bacterial spores. He says: "Pectose is accompanied in the vegetable tissues in which it is found by a ferment pectose, sometimes soluble and at others insoluble, which possesses the property of transforming pectose and pectin into pectic and metapec-tic acid successively. Pectic fermentation plays an important part in the conversion of ripe fruits into an ever-ripe, half-rotten, or sleepy state. It also assists in the formation of vegetable jellies. In fact the transformation of the natural juices of fruits into jellies is a result of the metamorphosis of pectin contained in these juices into the pectosic and pectic acids. Pectic fermentation is effected at about 95° Fahr." It may be that with further investigation it will be discovered that a ferment precedes the attack of bacteria. If this proves true it would not disturb the fact that blight can be extended by inoculation.
It is urged by those who yet favor continued orchard culture to conserve moisture that cover-crops rob the fruit trees of needed moisture at the time when fruit needs it most for perfect maturation. The first and even second leguminous cover-crop sown in a bearing orchard will result in diminished supply of soil moisture as compared with the plan of continued culture. But this only favors the perfect ripening of relatively young trees. But the turning under of two or three leguminous crops soon gives a supply of humus and nitrogen that so changes the mechanical texture of the soil that it will hold moisture through the season far better than the cultivated ones that each year become more destitute of needed humus and nitrogen.