This section is from the book "The Gardener's Monthly And Horticulturist V20", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
It must be clear to all who have given close observation to the subject, that there are several, if not many causes, which make the fruit of the pear crack - that one cause is the operation of a minute fungus; and we believe this has been made quite clear to our readers, as well as to the readers of the proceedings of the American Pomological Society. The knowledge we have gained in this country on this subject does not, however, seem to have extended to Europe, for we find the following in one of our European exchanges, given as an original discovery:
"M. Prilleux has communicated to the French Academy some observations on the black spots sometimes found on pears, and which are known to the Paris gardeners as tavelures. He has noticed that cracks in the fruit usually originate in these spots. All varieties are not equally subject to them. Doyenne d'hiver suffers most frequently and most severely. Wet seasons favor the appearance of these spots, and standards generally suffer more than wall-trees, and those with a southwest or western more than others with an eastern aspect. Some trees are affected year after year, while others similarly circumstanced escape altogether. The spots he finds to be produced by a small fungus, Cladosporum dendrit-icum, Wallroth, which was first noticed by that naturalist on apple trees. The filamentary spores take root in and penetrate the superficial tissues, swell at the extremities, and divide into small cells, which again divide, forming a mass of minute blackish cells (as may be seen by lifting the epidermis of a leaf thus affected), spreading their sporiferous filaments in all directions. The effects are different on different parts of the plant. On a leaf the part affected blackens and dies, but the rest of the leaf remains sound.
On the bark crevices and nodes are formed, which, however, are not generally conspicuous. On the fruit it is different. The superficial growth is partially checked by the presence of the parasite, whilst that of the minor parts continues; consequently, unless relieved by early excision, the fruit becomes deformed, the dead parts distend, and the exterior cracks, exposing the sound portions within. The existence, sometimes unnoticed, of the fungus on the bark of particular individuals explains its reappearance year after year on their fruit, although it may not be found on their neighbors. The peculiarity may be communicated by grafting".