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 most injurious fungus parasite attacking the Apple, and unfortunately it abounds wherever the Apple is cultivated. It is estimated that the annual loss due to "spotted" fruit caused by this fungus exceeds £1,000,000 sterling.
Until recently the fungus causing scab was known as Fusicladium dendriticum. It has, however, now been discovered that the Fusicladium is only the summer condition of a fungus which produces fruit the spring after its formation. This is the higher or Venturia stage. Naturally the scabs or black spots on the fruit are best known to horticulturists, but its occurrence is by no means confined to the fruit. If we take a tree that has produced scabbed apples for some years, proving that the disease is well established, the following is the yearly course followed by the fungus: The Fusicladium or summer condition is present in quantity on dead shoots and young branches. Its presence is indicated by the bark becoming broken up as if gnawed by some insect. Such . wounds often occur at the base of the youngest shoot, or on fruit spurs, and are also sometimes present on older branches. The fungus itself is not very conspicuous, and appears as a blackish stain on the wood under the loose bark and on the bark itself. It is very important to grasp the fact that when a shoot or branch is once infected the spawn or mycelium of the fungus continues to grow from year to year, or is perennial in the branch and produces spores each season, and naturally the number of spores produced is greater each season as the mycelium spreads in the dead branch. Spores are produced on these diseased branches in the spring, and are washed by rain or conveyed by other means on to the young leaves, which in turn become infected, the presence of the fungus on the leaves being indicated by blackish olive patches of various size. If these patches are examined under a good pocket lens, they will be seen to consist of numerous crowded lines arranged in a dendritic manner, and radiating from the edge of the patch. This appearance proves with certainty that the patch consists of the fungus causing Apple scab. In due course the fungus present on the leaves produces spores, which in turn are carried by some means or other on to the young fruit, which in turn becomes infected. Spores are produced on the diseased shoots throughout the season, and these may also play a part in infecting the fruit.
From the above account it will be seen that when the branches of a tree are once infected, if such branches are not removed, the disease is capable of perpetuating itself from year to year without any infection from outside sources. At the same time such an infected tree represents a nursery of disease from which the fungus is certain to spread to adjoining trees.
Under the circumstances my advice is to remove all dead and decayed shoots, as by so doing you remove the primary cause of the disease. During the spring following the removal of the branches, spray with Bordeaux mixture (see p. 49), commencing when the leaves are unfolding, and again when the fruit is just set. A third spraying when the apples are about the size of marbles is advisable, as some dead shoots bearing spores are certain to be missed when pruning.
It is but fair to state that my view, as given above, is not endorsed by other specialists, who consider that pruning out the dead shoots is impracticable, and that by constant spraying alone the disease can be kept under control. Quite true; but if the primary cause of infection is not removed, the constant spraying must of necessity be repeated each season, and the "russeting" of fruit and scorching of foliage are the frequent results of spraying.