Scab on Gala apples
Pear Scab
Apple scab is the most economically important disease of apples. It can cause serious loss of fruit and weaken trees by causing premature loss of leaves. Pear scab is a closely related and serious disease of pears. Both are most serious in wet regions.
Symptoms
Scab can infect leaves, fruit, blossom, young shoots as well as other parts of the tree. The most obvious symptoms are seen on the leaves and fruit. Infections on leaves appear as blotches of dead leaf tissue which turn brown in the case of apples and black in the case of pears. Infected leaves have stunted growth and are often distorted. They eventually turn yellow and drop prematurely. Heavily infected young leaves may shrivel up completely and fall very quickly. Similar brown or black blotches appear on infected young fruit. As the fruit matures these blotches become corky and the skin cracks giving rise to the typical scab-like fruit symptoms which give the disease its name. Infected fruit is usually also small and misshapen. Although the fruit may be very unsightly it is still perfectly edible as only the skin is affected.
Disease Cycle
Apple scab is caused by a fungus called Ventura inaequalis. Pear scab is caused by a closely related fungus called Ventura pirina. The fungus overwinters in dead infected leaves and fruit on the ground, and less commonly on infected shoots on the tree. In spring around the time of bud burst air borne spores are released which land on young leaves, blossom or fruitlets infecting them. Once these primary infections are established they in turn release more spores which are spread by wind or rain splash to young leaves elsewhere giving rise to secondary infections. This cycle repeats itself several times during the growing season. Spores can only germinate and infect if the leaf surface is wet and the fungus grows best in relatively humid conditions. Consequently the disease develops mainly during wet weather and is most serious in wet regions.
Prevention and Control
Chemical control of scab requires regular spraying from bud burst through the entire growing season with a suitable fungicide such as myclobutanil which is available to amateur gardeners in various formulations such as Bio Systhane Fungus Fighter. If you do not wish to use chemicals a certain amount of control can be achieved by removing sources of primary infection by sweeping up and destroying infected dead leaves and fruit in the autumn. Infected young shoots are another source of primary infection which should be cut off and destroyed in the course of winter pruning. These can be identified by the coarse and cracked appearance of their bark. However, by far the most effective method of organic control is to grow fruit varieties which are resistant to scab. This is particularly important in wet regions where it is important to avoid susceptible varieties altogether. There are many excellent resistant varieties available. These include older established dessert varieties such as Adams Pearmain, Ashmeads Kernel, Discovery, Egremont Russet, Ellison's Orange, Kidds Orange Red, Sunset, Tydeman's Late Orange and cookers such as Lord Derby, Reverend W Wilks. There are also a number of new varieties specifically bred for scab resistance. Of these we would highly recommend two new Czech varieties Rajka and Topaz both for disease resistance, and quality of fruit. You can use our advanced search for a full list of resistant varieties.
Sources of further information on scab