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 is undoubtedly the commonest and most generally distributed of diseases attacking plants belonging to the family Rosaceae, to which the majority of our orchard fruit trees belong. Until recently the injury was supposed to be entirely due to a fungus called Monilia fructigena. It has, however, now been proved that the Monilia is only one phase in the life-cycle of a more highly organized fungus called Sclerotinia. It is quite true that the Monilia condition causes all the damage, yet it is well to know that another stage exists, which under certain conditions is capable of tiding the parasite over from one year to another.
As usual, the effect on the fruit is best known to people generally, appealing as it does both to the eye and the pocket. On apples the earliest indication of its presence is the appearance of one or more small, brown patches which gradually increase in size, until in many instances the entire surface of the fruit becomes diseased. Soon afterwards whitish warts, arranged in concentric circles, resembling little fairy rings, burst through the skin covering the diseased parts. These warts are the summer fruits, or Monilia condition of the fungus. If a thin slice of one of these white warts is examined under a microscope, it will be seen to consist of numerous spores arranged in long chains, like strings of beads. These spores become free from each other when ripe, and are scattered by wind, etc, and are capable of infecting other fruits or young shoots. Such diseased apples do not rot and decay soon, but become dry and mummified, and often remain hanging on the tree until the following season (fig. 348).
Whether hanging on the tree or lying on the ground, such diseased apples produce a second crop of Monilia spores the following spring, each one of which is capable of infecting any leaf, shoot, or fruit on which it may alight. It sometimes happens that apples infected with this disease turn almost black all over, and are popularly known as " Black Apples". These become dry and hard, and do not produce Monilia spores until the spring.
When plums and cherries are attacked, the white warts of Monilia fruit are not usually produced in irregular circles as on the apple, but are irregularly scattered over the fruit. When diseased fruit has been lying on the ground for two seasons, and has become more or less buried, it sometimes produces the higher or Sclerotinia kind of winter fruit, under the form of little brown cups supported on slender stalks. The spores of this form are, of course, also able to set up an infection.
Fig. 348. - Brown Fruit Rot A. Apple in mummified condition. B, Chains of spores.
As in the case of " Apple Scab ", the fungus attacks the young shoots, and on these continues to produce spores every season until the branch or shoot is removed, either by decay or by pruning. The disease is readily recognized on the shoots by the whitish, wart-like tufts of spores. These spores are the first to be formed in the spring, and are washed on to the young leaves, which in turn become infected and pass their spores on to the young fruit. When a tree has many infected shoots, and spores are produced in abundance, the blossom is also often attacked. This is more especially the case with the Cherry, where the fungus forms minute, velvety tufts on the flower stalks, sepals, and petals, which in consequence turn brown and die, but often remain hanging on the tree for some time. Such effects are almost invariably attributed to frost.
The only method of getting rid of this disease permanently is to strike at the root of the matter, which means that, in the first instance, all diseased shoots should be removed and burned. If this is not done, and the admitted primary cause of the disease is allowed to remain, then repeated spraying every year becomes a necessity. The next thing to attend to is the removal of all dead, mummified fruit, whether hanging on the trees or lying on the ground. This should be burned or deeply buried, and not given to animals to eat. Next, spray with Bordeaux mixture, first when the leaves are expanding, and again when the fruit is just set. After such precautions, which should be repeated a second season, the disease will be well in hand.