This section is from "The Horticulturist, And Journal Of Rural Art And Rural Taste", by P. Barry, A. J. Downing, J. Jay Smith, Peter B. Mead, F. W. Woodward, Henry T. Williams. Also available from Amazon: Horticulturist and Journal of Rural Art and Rural Taste.
Members generally had found these two maladies to go together, but not invariably. The leaf-blight more frequently attacked young plants in the seed bed, and sometimes larger orchard trees. When on bearing trees, it always produced cracking; but the fruit was often known to crack while the trees were unaffected with leaf blight and in the thriftiest state of growth. L. E. Berkmans informed the meeting that the leaf blight in Belgium was unknown, while cracking of the fruit was frequent; but the climate was so moist, that twenty days in a summer without rain, would be called a dry one. Other members had observed cracking caused exclusively by wet weather.
Cracking seemed in many cases to depend on the soil, and an instance was mentioned where trees of the Virgalien, on the grounds of T. 6. Yeomans, of Wayne County, where the fruit of this variety is always ruined by cracking, were removed to the grounds of a neighbor, and afterwards bore fair and excellent fruit. But the disease could not be caused by exhaustion of the soil, several instances being mentioned where it had occurred on young trees, on new soil, and in one case the first crop, out of nine or ten, was the only one affected.
As it had been found that young seedlings once affected, were more apt to be troubled with leaf blight the following year, the opinion was entertained that it was a very small fungus, whose extremely minute seed were carried through the sap-pores to all parts of the plant, and were ready to germinate and develop themselves whenever the wet weather favored their growth on the surface of the leaves. It had been proved that the seed of the little fungus that produces rust in wheat, were carried from the grain or seed up the stalk in the sap, these seed being immeasurably smaller than the pores; and it was in accordance with analogy to suppose that the leaf blight was similarly propagated.