13 



seasons entire crops are much depreciated in value and even 

 rendered unsaleable. Scab is a disease, however, which can 

 be well controlled if growers will carry out the necessary 

 treatment for spraying. 



Although the fungi causing apple and pear scab respec- 

 tively are different species they are very closely allied. The 

 general appearance and method of treatment are the same in 

 each case so that separate descriptions of the two diseases are 

 unnecessary. 



Description. On the Leaves. The first active stages of 

 the fungus-attack appear on the leaves during the early spring. 

 The general appearance at this stage varies according to the 

 variety; in every case the part of the leaf which actually 

 bears the fungus becomes a dark olive-green or sooty colour. 

 Sometimes the fungus confines its attack almost entirely to 

 the veins of the leaf, while in other cases it forms distinct 

 and sharply marked patches scattered over the surfaces 

 (Fig. 3). The spores, which are produced on these dark 

 patches in enormous numbers, are distributed by air currents, 

 and in this way spread the infection to other leaves, young 

 fruit and also new wood. It is most important to remember 

 that the early infection of the fruit takes place almost entirely 

 from the leaves, hence unless these are kept healthy a clean 

 crop cannot be expected. 



On the Fruit. Figure 1 depicts the disease as it occurs 

 on the fruit. The spores alighting on the fruit germinate 

 and penetrate the skin, beneath which they form a mass of 

 fungus tissue or " mycelium." THis does not, however, 

 grow deeply into the flesh of the fruit, but spreads immediately 

 beneath the surface. In course of time the covering skin of 

 the fruit is broken, with the result that thousands of dark 

 coloured spores escape from the open wound or scab. At 

 the margin of the scab the silvery fragments of broken skin 

 may often be seen. The extent and nature of the injury 

 depend largely on the date of infection and the variety of 

 apple or pear. When attacked quite young the fruits are 

 often much distorted and may even be injured to such an 

 extent that they fall off. In other cases, especially with pears, 

 more or less severe cracking takes place (Fig. 2). If the attack 

 occurs late in the season the fungus does not make so much 

 headway, small scabs or spots only being formed. 



The spores produced on the fruit, as in the case of those 

 formed on the leaves and wood, may infect leaves, young 

 twigs, or other fruit. From this it will be noted that the 

 fruits of apples 1 and pears are subject to infection right through 

 the growing season. Susceptibility to fresh infection ceases 

 when the growth is completed and the fruits are picked and 

 stored. 



