This section is from the book "Commercial Gardening Vol3", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
This disease is by no means new nor rare amongst us, although until recently the fungus concerned in its production was known as Glceosporium fructigenum. The last-named, however, is now known to be only one stage in the life-cycle of a higher condition called Glomurella.
The injury to the fruit is best known to growers. It first appears at scattered points - rarely more than two or three on a fruit - as a small brown speck showing through the skin. These spots gradually increase in size, at the same time changing to a darker-brown colour, and the surface of the diseased patch gradually sinks below the general surface of the apple, owing to the collapse of the tissues. At this stage the diseased portions have a very bitter taste; hence the disease is known as "bitter pit" in the United States, where it is quite as prevalent as with us; in fact it is estimated that the loss through this disease in the United States amounted to 10,000,000 dollars in 1900. Numerous summer spores are produced on the sunken patches, which frequently extend to such an extent that they run into each other, and frequently almost cover the surface of the fruit. The disease hastens the period of ripening, and causes the fruit to fall early in the season.
The disease is by no means confined to the fruit, in fact the fruit is infected by spores of the fungus growing on diseased portions of the branches. The fungus, when present on the branches, forms canker-like patches, blackened and more or less sunk below the surface, and the bark is killed for some distance round such diseased patches. These cankered patches, if allowed to remain, produce spores each season in succession, which infect the young fruit. The cankers form on the previous year's fruit spurs, also on branches up to 3 in. in diameter.
Smaller branches and spurs showing the disease should be removed, and cankers should be cut out of the larger branches, and the wounds at once coated with gas tar. When the fruit is attacked when quite young, it usually remains on the branches in a mummified condition until the following season, when fungus spores are produced that infect the young fruit. All such hanging diseased fruit should be removed, as should also all diseased fallen fruit. Pigs are quite capable of accomplishing this object, if for other reasons they can be allowed to enter the orchard.