488 



ENCYCLOPEDIA OF PRACTICAL HORTICILTURE 



in spring and summer and causes the 

 spots described above. The spores of the 

 fungus are produced in the spots on leaves 

 or fruit in great abundance, and are dis- 

 seminated by the wind, spreading the dis- 

 ease to other leaves and fruit. One finds 

 the scab spots first appearing in the spring 

 on the under side of leaves on the lower 

 branches. Spores produced in these spots 

 are disseminated to other leaves and to 

 the developing fruits. Several genera- 

 tions may thus occur during a single 

 season. 



The mycelium of the fungus present on 

 the leaves which tall to the ground in 

 the autumn does not die, but develops 

 in these leaves during the winter as a 

 saprophyte, and in the spring produces 

 spores quite different both in manner of 

 formation and in the shape and size from 

 those borne in the spots on living leaves 

 and fruit. The spores found in the spots 

 on leaves and fruit are one-celled, rarely 

 two-celled, and are borne on the ends of 

 short threads, while the spores developed 

 on the dead leaves in spring are always 

 distinctly two-celled and are borne in lit- 

 tle cylindrical sacs called asci. A con- 

 siderable number of these sacs are borne 

 in a hollow, more or less pear-shaped re- 

 ceptacle buried in the tissues of the leaf. 

 These receptacles containing the asci are 

 known as perithecia, and when mature 

 project as little black elevations from the 

 surface of the leaf. These are barely 

 visible to the naked eye. At maturity an 

 opening appears in the projecting eleva- 

 tions. The asci elongate and protrude 

 through this opening and forcibly eject 

 the spores, which, wafted by currents of 

 air, may be carried to the young leaves 

 of the apple, where they germinate and 

 produce the first scab spots. The sexual 

 spores are disseminated about the time 

 the blossoms open. So far as is known, 

 all primary Infection of the leaves and 

 fruit in the spring takes place as a con- 

 sequence of disseminations of the sexual 

 spores. Subsequent infections result from 

 the dissemination of the conidial or sum- 

 mer spores. 



The development of the scab fungus is 

 found to be very much influenced by 



weather conditions. Moisture on the sur- 

 face of leaves and fruit is essential to the 

 germination of the spores and the conse- 

 quent infection of the plant. On this ac- 

 count scab is found to spread most rapidly 

 in the spring, early summer and late fall. 

 The disease spreads little, if any, during 

 the summer in dry seasons. In seasons of 



Fig. 2. Scab on Apple Leaf. New Hampshire 

 Experiment Station. 



frequent summer rains, as in 1912, scab 

 may spread all summer, if the trees are 

 not properly protected by spraying. 



Apple scab is not uncommonly found de- 

 veloping in storage. If fruit infested with 

 scab is stored the fungus may continue 

 to develop around the edges of the old 

 spots. New spots due to infection in stor- 

 age may also occur. It is not uncommon 

 to find small black spots of apple scab de- 

 veloped on apples in storage, particularly 

 in the hollow about the stem. This may 

 occur on fruit that was perfect when 

 stored. 



Treatment 



As with all fungi of this nature, treat- 

 ment must be preventive rather than cura- 

 tive. The method of treatment is depend- 



